WHY WE SHOULD WORK ON NEAR OR DE-ENERGIZED EQUIPMENT?
Working on or near de-energized equipment.
The definition of the term de-energized can be found in IEEE Std 100-1996 and in several other documents. It is defined as "free from any electrical connection to a source of potential difference and from electric charge; not having a potential different from that of the earth".
At first thought, some people might think that they are safe if the electrical equipment on which they are going to work is de-energized. However, things are not always as they appear.
The unexpected happens. A person should think, "What if...?." What if the wrong disconnect switch was opened? Or, since you can't watch the switch and work at the same time, what if someone turns the switch back on while you are busy working?
What if a source of voltage from another circuit somehow gets accidentally connected onto the conductors on which you are going to work? What if a very large induced voltage is present? The point is that there are several things to consider to ensure a person's safety while working. De energizing is only one part of creating an electrically safe work condition.
Establishing an electrically safe work condition
In the past, the methods that electrical personnel followed to protect themselves were lumped into a term called clearance procedures. In some cases, clearance simply meant permission to work on a particular system, whether it was energized or not.
In other cases, clearance meant taking measures to ensure that equipment is de-energized, and to reinforce those measures with formal safeguards against altering that de-energized status for as long as clearance is required. The latter use of the word clearance is closer to the hazardous energy control requirements in place today.
The term clearance is falling out of use in modern electrical safety terminology because it does not mean safety. Clearance (for work) is defined in 29 CFR 1910.269 as "authorization to perform specified work or permission to enter a restricted area."
Today, for safety purposes, the phrase "establish an electrically safe work condition" is preferred. An electrically safe work condition is defined in Part II of NFPA 70E- 1995. Section 2-3.1.3 of that document states
An electrically safe work condition shall be achieved and verified by the following process:
a) Determine all possible sources of electrical supply to the specifc equipment. Check applicable up-to-date drawings, diagrams, and identifcation tags.
b) After properly interrupting the load current, open the disconnecting device(s) for each source.
c) Where it is possible, visually verify that all blades of the disconnecting devices are fully open, or that drawout type circuit breakers are withdrawn to the fully disconnected position.
d) Apply lockout/tagout devices in accordance with a documented and established policy.
e) Use an adequately rated voltage detector to test each phase conductor or circuit part to verify that it is de-energized. Before and after each test, determine that the voltage detector is operating satisfactorily.
f) Where the possibility of induced voltages or stored electrical energy exists, ground the phase conductors or circuit parts before touching them. Where it could be reasonably anticipated that the conductors or circuit parts being de-energized could contact other exposed energized conductors or circuit parts, apply ground connecting devices rated for the available fault duty.
When nondrawout, molded-case circuit breakers are being used as the disconnecting device mentioned in item b), visual verification of an open circuit, as suggested in item c), cannot be made.
One technique that could be used to verify true opening is to have a voltmeter, or other voltage indicating device, safely applied somewhere away from the breaker enclosure itself on the load side of the breaker before the breaker is opened.
Always try to place the voltmeter at a point where exposure to energized conductors is minimized. Then, have someone watch the meter as the breaker is being opened. Simultaneous opening of the breaker and disappearance of voltage is generally a good indicator of disconnection.
If that can't be done, the next best way is to measure load-side voltage (using safe practices and appropriate protective and test equipment), remove the meter, open the breaker, and measure again immediately. With multiple pole systems, all load-side poles should be verified to have voltage prior to disconnection.
Again, apply a voltmeter to one of the poles. After the breaker is opened and the first pole is verified, move the meter, as safely and quickly as possible, to verify deenergization of the other poles.
Showing posts with label Shock. Show all posts
Showing posts with label Shock. Show all posts
HOW TO MAKE ACCURATE SINGLE LINE DIAGRAM FOR POWER SYSTEM TUTORIALS
What are the elements of an accurate single line diagram?
A reliable single-line diagram of an industrial or commercial electrical power distribution system is an invaluable tool. It is also called a one-line diagram. The single-line diagram indicates, by single lines and standard symbols, the course and component parts of an electric circuit or system of circuits. The symbols that are commonly used in one-line diagrams are defined in IEEE Std 315-1975.
The single-line diagram is a road map of the distribution system that traces the ßow of power into and through the system. The single-line drawing identifiers the points at which power is, or can be, supplied into the system and at which power should be disconnected in order to clear, or isolate, any portion of the system.
Characteristics of an accurate diagram
The following characteristics should help to ensure accuracy as well as ease of interpretation:
a) Keep it simple
A fundamental single-line diagram should be made up of short, straight lines and components, similar to the manner in which a block diagram is drawn. It should be relatively easy to get the overall picture of the whole electrical system.
All, or as much as possible, of the system should be kept to one sheet. If the system is very large, and more than one sheet is necessary, then the break should be made at voltage levels or at distribution centers.
b) Maintain relative geographic relations
In many cases, it is possible to superimpose a form of the one-line diagram onto the facility plot plan. This is very helpful toward a quick understanding of the location of the system's major components for operating purposes.
It may, however, be more difficult to comprehend the overall system operation from this drawing. Such a drawing could be used for relatively simple systems. For more complex systems, however, it should be used in addition to the fundamental single-line diagram.
c) Maintain the approximate relative positions of components when producing the single-line diagram
The drawing should be as simple as possible and should be laid out in the same relationship as an operator would view the equipment. The diagram does not need to show geographical relationships at the expense of simplicity.
NOTE: A site plan with equipment locations may be required to accompany the single-line diagram.
d) Avoid duplication
Each symbol, figure, and letter has a definite meaning. The reader should be able to interpret each without any confusion. In this regard, equipment names should be selected before publishing the document; then, these names should be used consistently.
e) Show all known factors
All details shown on the diagram are important. Some of those important details are as follows:
Ñ ManufacturersÕ type designations and ratings of apparatus;
Ñ Ratios of current and potential transformers and taps to be used on multi-ratio transformers;
Ñ Connections of power transformer windings;
Ñ Circuit breaker ratings in volts, amperes, and short-circuit interrupting rating;
Ñ Switch and fuse ratings in volts, amperes, and short-circuit interrupting rating;
Ñ Function of relays. Device functions used should be from IEEE Std C37.2-1991;
Ñ Ratings of motors, generators, and power transformers;
Ñ Number, size, and type of conductors;
Ñ Voltage, phases, frequency, and phase rotation of all incoming circuits. The type
of supply system (wye or delta, grounded or ungrounded) and the available
short-circuit currents should be indicated.
f) Future plans
When future plans are known, they should be shown on the diagram or explained by notes.
g) Other considerations
Refer to IEEE Std 141-1993 for further discussion of singleline diagrams.
A reliable single-line diagram of an industrial or commercial electrical power distribution system is an invaluable tool. It is also called a one-line diagram. The single-line diagram indicates, by single lines and standard symbols, the course and component parts of an electric circuit or system of circuits. The symbols that are commonly used in one-line diagrams are defined in IEEE Std 315-1975.
The single-line diagram is a road map of the distribution system that traces the ßow of power into and through the system. The single-line drawing identifiers the points at which power is, or can be, supplied into the system and at which power should be disconnected in order to clear, or isolate, any portion of the system.
Characteristics of an accurate diagram
The following characteristics should help to ensure accuracy as well as ease of interpretation:
a) Keep it simple
A fundamental single-line diagram should be made up of short, straight lines and components, similar to the manner in which a block diagram is drawn. It should be relatively easy to get the overall picture of the whole electrical system.
All, or as much as possible, of the system should be kept to one sheet. If the system is very large, and more than one sheet is necessary, then the break should be made at voltage levels or at distribution centers.
b) Maintain relative geographic relations
In many cases, it is possible to superimpose a form of the one-line diagram onto the facility plot plan. This is very helpful toward a quick understanding of the location of the system's major components for operating purposes.
It may, however, be more difficult to comprehend the overall system operation from this drawing. Such a drawing could be used for relatively simple systems. For more complex systems, however, it should be used in addition to the fundamental single-line diagram.
c) Maintain the approximate relative positions of components when producing the single-line diagram
The drawing should be as simple as possible and should be laid out in the same relationship as an operator would view the equipment. The diagram does not need to show geographical relationships at the expense of simplicity.
NOTE: A site plan with equipment locations may be required to accompany the single-line diagram.
d) Avoid duplication
Each symbol, figure, and letter has a definite meaning. The reader should be able to interpret each without any confusion. In this regard, equipment names should be selected before publishing the document; then, these names should be used consistently.
e) Show all known factors
All details shown on the diagram are important. Some of those important details are as follows:
Ñ ManufacturersÕ type designations and ratings of apparatus;
Ñ Ratios of current and potential transformers and taps to be used on multi-ratio transformers;
Ñ Connections of power transformer windings;
Ñ Circuit breaker ratings in volts, amperes, and short-circuit interrupting rating;
Ñ Switch and fuse ratings in volts, amperes, and short-circuit interrupting rating;
Ñ Function of relays. Device functions used should be from IEEE Std C37.2-1991;
Ñ Ratings of motors, generators, and power transformers;
Ñ Number, size, and type of conductors;
Ñ Voltage, phases, frequency, and phase rotation of all incoming circuits. The type
of supply system (wye or delta, grounded or ungrounded) and the available
short-circuit currents should be indicated.
f) Future plans
When future plans are known, they should be shown on the diagram or explained by notes.
g) Other considerations
Refer to IEEE Std 141-1993 for further discussion of singleline diagrams.
BEST LOCATIONS FOR PANEL BOARDS AND SWITCH BOARDS OF ELECTRONIC EQUIPMENT BASIC TUTORIALS
Where should panel boards be located?
Switchboards and panelboards that support electronic load equipment and related loads should be properly designed and installed. Recommended practice is to use panelboards specifically listed for nonlinear loads if they serve electronic load equipment.
As a minimum, panelboards should be rated for power or lighting applications, and should not be a lighterduty type. Special attention should be given to the location and installation methods used when installing panelboards.
In addition, protective devices shall adequately protect system components, neutral buses should be sized to accommodate increased neutral currents due to harmonic currents from nonlinear electronic load equipment, and equipment ground buses should be sized to accommodate increased numbers of equipment grounding conductors due to the recommended practices of using insulated equipment grounding conductors and dedicated circuits for electronic load equipment.
Surge protective devices may also be installed external to, or internal to, the switchboards or panelboards.
Location
Panelboards that serve electronic load equipment should be placed as near to the electronic load equipment as practicable, and should be bonded to the same ground reference as the electronic load equipment.
Other panelboards located in the same area as the electronic load equipment that serve other loads such as lighting, heating, ventilation, air conditioning, and process cooling equipment should also be bonded to the same ground reference as the electronic load equipment.
Panelboards should be directly mounted to any building steel member in the immediate area of the installation. Isolation of a panelboard from the metallic building structure by an electrically insulating material, as an attempt to prevent flow of high frequency current through the panelboard, is not recommended practice.
The panelboard and metallic building structure, separated by a dielectric material, become capacitively coupled. The capacitive coupling presents a low impedance at high frequency defeating the original purpose.
NFPA 780-1997 requires effective grounding and bonding between objects such as structural building steel and a panelboard located within side-flash distance (approximately 1.8 m (6 ft), horizontally) of each other. Insulation materials, commonly used in an attempt to separate a panelboard from building steel, are rarely capable of withstanding lightningi nduced arcing conditions.
Switchboards and panelboards that support electronic load equipment and related loads should be properly designed and installed. Recommended practice is to use panelboards specifically listed for nonlinear loads if they serve electronic load equipment.
As a minimum, panelboards should be rated for power or lighting applications, and should not be a lighterduty type. Special attention should be given to the location and installation methods used when installing panelboards.
In addition, protective devices shall adequately protect system components, neutral buses should be sized to accommodate increased neutral currents due to harmonic currents from nonlinear electronic load equipment, and equipment ground buses should be sized to accommodate increased numbers of equipment grounding conductors due to the recommended practices of using insulated equipment grounding conductors and dedicated circuits for electronic load equipment.
Surge protective devices may also be installed external to, or internal to, the switchboards or panelboards.
Location
Panelboards that serve electronic load equipment should be placed as near to the electronic load equipment as practicable, and should be bonded to the same ground reference as the electronic load equipment.
Other panelboards located in the same area as the electronic load equipment that serve other loads such as lighting, heating, ventilation, air conditioning, and process cooling equipment should also be bonded to the same ground reference as the electronic load equipment.
Panelboards should be directly mounted to any building steel member in the immediate area of the installation. Isolation of a panelboard from the metallic building structure by an electrically insulating material, as an attempt to prevent flow of high frequency current through the panelboard, is not recommended practice.
The panelboard and metallic building structure, separated by a dielectric material, become capacitively coupled. The capacitive coupling presents a low impedance at high frequency defeating the original purpose.
NFPA 780-1997 requires effective grounding and bonding between objects such as structural building steel and a panelboard located within side-flash distance (approximately 1.8 m (6 ft), horizontally) of each other. Insulation materials, commonly used in an attempt to separate a panelboard from building steel, are rarely capable of withstanding lightningi nduced arcing conditions.
WORKS REQUIRING PERMITS BASIC INFORMATION AND TUTORIALS
What are the activities/ works that requires work permit?
The main types of permit and the work to be covered by each are identified below. Appendix 6.4 illustrates the essential elements of a permit form with supporting notes on its operation.
General permit
The general permit should be used for work such as:
S alterations to or overhaul of plant or machinery where mechanical, toxic or electrical hazards may arise
S work on or near overhead crane tracks
S work on pipelines with hazardous contents
S work with asbestos-based materials
S work involving ionising radiation
S work at height where there are exceptionally high risks
S excavations to avoid underground services.
Confined space permit
Confined spaces include chambers, tanks (sealed and open-top), vessels, furnaces, ducts, sewers, manholes, pits, flues, excavations, boilers, reactors and ovens.
Many fatal accidents have occurred where inadequate precautions were taken before and during work involving entry into confined spaces. The two main hazards are the potential presence of toxic or other dangerous substances and the absence of adequate oxygen.
In addition, there may be mechanical hazards (entanglement on agitators) and raised temperatures. The work to be carried out may itself be especially hazardous when done in a confined space, for example, cleaning using solvents, cutting/welding work.
Should the person working in a confined space get into difficulties for whatever reason, getting help in and
getting the individual out may prove difficult and dangerous.
Stringent preparation, isolation, air testing and other precautions are therefore essential and experience shows that the use of a confined space entry permit is essential to confirm that all the appropriate precautions
have been taken.
Work on high voltage apparatus (including testing)
Work on high voltage apparatus (over about 600 volts) is potentially high risk. Hazards include:
S possibly fatal electric shock/burns to the people doing the work
S electrical fires/explosions
S consequential danger from disruption of power supply to safety-critical plant and equipment.
In view of the risk, this work must only be done by suitably trained and competent people acting under the terms of a high voltage permit.
Hot work
Hot work is potentially hazardous as a:
S source of ignition in any plant in which highly flammable materials are handled
S cause of fires in all locations, regardless of whether highly flammable materials are present.
Hot work includes cutting, welding, brazing, soldering and any process involving the application of a naked
flame. Drilling and grinding should also be included where a flammable atmosphere is potentially present.
In high risk areas hot work may also involve any equipment or procedure that produces a spark of sufficient energy to ignite highly flammable substances.
Hot work should therefore be done under the terms of a hot work permit, the only exception being where hot work is done in a designated area suitable for the purpose.
The main types of permit and the work to be covered by each are identified below. Appendix 6.4 illustrates the essential elements of a permit form with supporting notes on its operation.
General permit
The general permit should be used for work such as:
S alterations to or overhaul of plant or machinery where mechanical, toxic or electrical hazards may arise
S work on or near overhead crane tracks
S work on pipelines with hazardous contents
S work with asbestos-based materials
S work involving ionising radiation
S work at height where there are exceptionally high risks
S excavations to avoid underground services.
Confined space permit
Confined spaces include chambers, tanks (sealed and open-top), vessels, furnaces, ducts, sewers, manholes, pits, flues, excavations, boilers, reactors and ovens.
Many fatal accidents have occurred where inadequate precautions were taken before and during work involving entry into confined spaces. The two main hazards are the potential presence of toxic or other dangerous substances and the absence of adequate oxygen.
In addition, there may be mechanical hazards (entanglement on agitators) and raised temperatures. The work to be carried out may itself be especially hazardous when done in a confined space, for example, cleaning using solvents, cutting/welding work.
Should the person working in a confined space get into difficulties for whatever reason, getting help in and
getting the individual out may prove difficult and dangerous.
Stringent preparation, isolation, air testing and other precautions are therefore essential and experience shows that the use of a confined space entry permit is essential to confirm that all the appropriate precautions
have been taken.
Work on high voltage apparatus (including testing)
Work on high voltage apparatus (over about 600 volts) is potentially high risk. Hazards include:
S possibly fatal electric shock/burns to the people doing the work
S electrical fires/explosions
S consequential danger from disruption of power supply to safety-critical plant and equipment.
In view of the risk, this work must only be done by suitably trained and competent people acting under the terms of a high voltage permit.
Hot work
Hot work is potentially hazardous as a:
S source of ignition in any plant in which highly flammable materials are handled
S cause of fires in all locations, regardless of whether highly flammable materials are present.
Hot work includes cutting, welding, brazing, soldering and any process involving the application of a naked
flame. Drilling and grinding should also be included where a flammable atmosphere is potentially present.
In high risk areas hot work may also involve any equipment or procedure that produces a spark of sufficient energy to ignite highly flammable substances.
Hot work should therefore be done under the terms of a hot work permit, the only exception being where hot work is done in a designated area suitable for the purpose.
PROJECT RISK CONTROL - HIERARCHY OF RISK CONTROLS BASIC INFORMATION AND TUTORIALS
When assessing the adequacy of existing controls or introducing new controls, a hierarchy of risk controls should be considered. The Management of Health and Safety at Work Regulations 1999 Schedule 1 specifies the general principles of prevention which are set out in the European Council Directive.
These principles are:
1. avoiding risks
2. evaluating the risks which cannot be avoided
3. combating the risks at source
4. adapting the work to the individual, especially as regards the design of the workplace, the choice
of work equipment and the choice of working and production methods, with a view, in particular, to alleviating monotonous work and work at a predetermined work-rate and to reducing their effects on health
5. adapting to technical progress
6. replacing the dangerous by the non-dangerous or the less dangerous
7. developing a coherent overall prevention policy which covers technology, organization of work, working conditions, social relationships and the influence of factors relating to the working environment
8. giving collective protective measures priority over individual protective measures and
9. giving appropriate instruction to employees.
These principles are not exactly a hierarchy but must be considered alongside the usual hierarchy of risk control which is as follows:
S elimination
S substitution
S engineering controls (e.g. isolation, insulation and
ventilation)
S reduced or limited time exposure
S good housekeeping
S safe systems of work
S training and information
S personal protective equipment
S welfare
S monitoring and supervision
S reviews.
These principles are:
1. avoiding risks
2. evaluating the risks which cannot be avoided
3. combating the risks at source
4. adapting the work to the individual, especially as regards the design of the workplace, the choice
of work equipment and the choice of working and production methods, with a view, in particular, to alleviating monotonous work and work at a predetermined work-rate and to reducing their effects on health
5. adapting to technical progress
6. replacing the dangerous by the non-dangerous or the less dangerous
7. developing a coherent overall prevention policy which covers technology, organization of work, working conditions, social relationships and the influence of factors relating to the working environment
8. giving collective protective measures priority over individual protective measures and
9. giving appropriate instruction to employees.
These principles are not exactly a hierarchy but must be considered alongside the usual hierarchy of risk control which is as follows:
S elimination
S substitution
S engineering controls (e.g. isolation, insulation and
ventilation)
S reduced or limited time exposure
S good housekeeping
S safe systems of work
S training and information
S personal protective equipment
S welfare
S monitoring and supervision
S reviews.
POWER CIRCUIT BREAKER TYPES FOR SAFETY INFORMATION BASICS
The five general types of high-voltage circuit breakers are as follows.
1 Oil circuit breakers use standard transformer oil, an effective medium for quenching the arc and providing an open break after current has dropped to zero. There are two general types of oil circuit breakers: dead-tank for the higher voltage ranges and live-tank for lower voltages.
Oil circuit breakers have been improved by adding such features as oil-tight joints, vents, and separate chambers to prevent the escape of oil.
Also, improved operating mechanisms prevent gas pressure from reclosing the contacts, making them reliable for system voltages up to 362 kV. However, above 230 kV, oil-less breakers are more economical.
2 Air-blast circuit breakers were developed as alternatives to oil circuit breakers as voltages increased. They depend on the good insulating and arc-quenching properties of dry and clean compressed air injected into the contact region.
3 Magnetic-air circuit breakers use a combination of strong magnetic field with a special arc chute to lengthen the arc until the system voltage is unable to maintain the arc any longer. They are used principally in power distribution systems.
4 Gas circuit breakers take advantage of the excellent arc-quenching and insulating properties of sulfur hexafluoride (SF6) gas. These outdoor breakers can interrupt system voltages up to 800 kV.
These circuit breakers are typically included in gasinsulated substations (GISs) that offer space-saving and environmental advantages over conventional outdoor substations. Gas (SF6) circuit breakers are made with ratings up to 800 kV and continuous cur rent up to 4000 A.
They are alternatives to oil and vacuum breakers for metal-clad and metal-enclosed switchgear up to 38 kV.
5 Vacuum circuit breakers, more accurately termed vacuum-bottle interrupters, are generally used for voltages up to 38 kV and continuous current ratings to 3000 A.
They are used for higher system voltage, current, and interrupting ratings, and are typically specified for metal-clad and metal-enclosed switchgear in distribution systems.
1 Oil circuit breakers use standard transformer oil, an effective medium for quenching the arc and providing an open break after current has dropped to zero. There are two general types of oil circuit breakers: dead-tank for the higher voltage ranges and live-tank for lower voltages.
Oil circuit breakers have been improved by adding such features as oil-tight joints, vents, and separate chambers to prevent the escape of oil.
Also, improved operating mechanisms prevent gas pressure from reclosing the contacts, making them reliable for system voltages up to 362 kV. However, above 230 kV, oil-less breakers are more economical.
2 Air-blast circuit breakers were developed as alternatives to oil circuit breakers as voltages increased. They depend on the good insulating and arc-quenching properties of dry and clean compressed air injected into the contact region.
3 Magnetic-air circuit breakers use a combination of strong magnetic field with a special arc chute to lengthen the arc until the system voltage is unable to maintain the arc any longer. They are used principally in power distribution systems.
4 Gas circuit breakers take advantage of the excellent arc-quenching and insulating properties of sulfur hexafluoride (SF6) gas. These outdoor breakers can interrupt system voltages up to 800 kV.
These circuit breakers are typically included in gasinsulated substations (GISs) that offer space-saving and environmental advantages over conventional outdoor substations. Gas (SF6) circuit breakers are made with ratings up to 800 kV and continuous cur rent up to 4000 A.
They are alternatives to oil and vacuum breakers for metal-clad and metal-enclosed switchgear up to 38 kV.
5 Vacuum circuit breakers, more accurately termed vacuum-bottle interrupters, are generally used for voltages up to 38 kV and continuous current ratings to 3000 A.
They are used for higher system voltage, current, and interrupting ratings, and are typically specified for metal-clad and metal-enclosed switchgear in distribution systems.
RECOGNIZING HAZARDS IN ELECTRICAL WORKS BASIC INFORMATION AND TUTORIALS
The first step is to recognize and identify the existing and potential hazards associated with the work you need to perform. A task and hazard analysis and pre-job briefing are two of the tools you can utilize to ascertain the risks involved in your work for the day.
It’s a good idea to include everyone who will be involved in the task or associated work to discuss and plan for the hazards. Sometimes a coworker will think of hazards that you have overlooked, and it will ensure that everyone involved will be on the same page.
Careful planning of safety procedures reduces the risk of injury. Determine whether everyone has been trained for the job they need to do that day. Do you need to present a safety training focused on specific risks that are present today?
Decisions to lockout and tagout circuits and equipment and any other action plans should be made part of recognizing hazards. Here are some other topics to address:
n Is the existing wiring inadequate?
n Is there any potential for overloading circuits?
n Are there any exposed electrical parts?
n Will you be working around overhead power lines?
n Does any of the wiring have damaged insulation that will produce a shock?
n Are there any electrical systems or tools on the site that are not grounded or double insulated?
n Have you checked the condition of any power tools that will be used to confirm that they are not damaged and that all guards are in place?
n What PPE is required for the tasks to be performed?
n Have you reviewed the MSDS for any chemicals present on the site or that will be used that could be harmful?
n Will any work need to be performed from ladders or scaffolding and are these in good condition and set-up properly? Is there any chance of ladders coming in contact with energized circuits?
n Are the working conditions or equipment likely to be damp or wet or affected by humidity?
It’s a good idea to include everyone who will be involved in the task or associated work to discuss and plan for the hazards. Sometimes a coworker will think of hazards that you have overlooked, and it will ensure that everyone involved will be on the same page.
Careful planning of safety procedures reduces the risk of injury. Determine whether everyone has been trained for the job they need to do that day. Do you need to present a safety training focused on specific risks that are present today?
Decisions to lockout and tagout circuits and equipment and any other action plans should be made part of recognizing hazards. Here are some other topics to address:
n Is the existing wiring inadequate?
n Is there any potential for overloading circuits?
n Are there any exposed electrical parts?
n Will you be working around overhead power lines?
n Does any of the wiring have damaged insulation that will produce a shock?
n Are there any electrical systems or tools on the site that are not grounded or double insulated?
n Have you checked the condition of any power tools that will be used to confirm that they are not damaged and that all guards are in place?
n What PPE is required for the tasks to be performed?
n Have you reviewed the MSDS for any chemicals present on the site or that will be used that could be harmful?
n Will any work need to be performed from ladders or scaffolding and are these in good condition and set-up properly? Is there any chance of ladders coming in contact with energized circuits?
n Are the working conditions or equipment likely to be damp or wet or affected by humidity?
OHSA FREQUENT VIOLATION CATEGORIES BASIC INFORMATION
OSHA relies heavily on data and statistics to formulate its regulations and focus its attention on workplace safety. The most frequently violated OSHA construction industry standards include the following categories:
Aerial lifts (OSHA 1926.453)
Electrical general requirements (OSHA 1926.403)
Electrical wiring design and protection (OSHA 1926.404)
Electrical wiring methods, components, and equipment for general use (OSHA 1926.105)
Eye and face protection (OSHA 1926.102)
Fall protection practices (OSHA 1926.502) and fall protection training requirements (OSHA 1926.503)
General duty requirements (OSHA 5 A 1)
General safety and health regulations (OSHA 1926.20)
Head protection (OSHA 1926.100)
Ladder safety (OSHA 1926.1053)
Recordkeeping requirements (OSHA 1926.1101)
Scaffolding safety practices (OSHA 1926.451) and scaffolding training requirements (OSHA 1926.21)
There are many safety compliance issues for the average small company to digest. But OSHA is not some big, bad wolf that lurks in the shadows waiting to pounce on unsuspecting employers.
OSHA seeks to identify clear and realistic priorities and to provide employers with the tools and opportunity to protect their workers by emphasizing safety and health. OSHA’s purpose is to save lives, prevent workplace injuries and illnesses, and protect the health of all American workers.
Whenever possible, OSHA’s primary emphasis is on the implementation of hazard control strategies that are based on prevention, and reducing hazardous exposures at their source. For these reasons, OSHA focuses the majority of its field activities on workplaces and job sites where the greatest potential exists for injuries and illnesses.
Aerial lifts (OSHA 1926.453)
Electrical general requirements (OSHA 1926.403)
Electrical wiring design and protection (OSHA 1926.404)
Electrical wiring methods, components, and equipment for general use (OSHA 1926.105)
Eye and face protection (OSHA 1926.102)
Fall protection practices (OSHA 1926.502) and fall protection training requirements (OSHA 1926.503)
General duty requirements (OSHA 5 A 1)
General safety and health regulations (OSHA 1926.20)
Head protection (OSHA 1926.100)
Ladder safety (OSHA 1926.1053)
Recordkeeping requirements (OSHA 1926.1101)
Scaffolding safety practices (OSHA 1926.451) and scaffolding training requirements (OSHA 1926.21)
There are many safety compliance issues for the average small company to digest. But OSHA is not some big, bad wolf that lurks in the shadows waiting to pounce on unsuspecting employers.
OSHA seeks to identify clear and realistic priorities and to provide employers with the tools and opportunity to protect their workers by emphasizing safety and health. OSHA’s purpose is to save lives, prevent workplace injuries and illnesses, and protect the health of all American workers.
Whenever possible, OSHA’s primary emphasis is on the implementation of hazard control strategies that are based on prevention, and reducing hazardous exposures at their source. For these reasons, OSHA focuses the majority of its field activities on workplaces and job sites where the greatest potential exists for injuries and illnesses.
THE SAFETY-RELATED CASE FOR ELECTRICAL MAINTENANCE
The relationship between safety and preventive maintenance is not a difficult one to establish. Properly designed equipment that is properly installed is well capable of doing its job when it is new.
As equipment ages however, several factors begin to take their toll on electrical equipment.
● Dust, dirt, and other contaminants collect on equipment causing the equipment to overheat and bearings and other moving parts to bind.
● Vibration causes hardware to loosen. Subsequent operations of equipment can cause joints and equipment to fail explosively.
● Heat and age can cause insulation to fail, resulting in shock hazards to personnel.
● Increased loads, motor starting surges, and power quality issues such as harmonics combine to increase the aging process and set the stage for equipment failure.
Unfortunately, the ultimate failure of unmaintained equipment usually occurs when the equipment is needed the most—during electrical faults. Such failures result in arc and blast events that can and do harm workers in the area.
They also result in significant downtime, loss of equipment, and construction cost incurred in rebuilding the equipment. The only way to ensure that electrical equipment continues to operate in an optimal manner is to maintain it so that it stays in factory-new-operating condition.
Regulatory
As discussed above and in previous chapters, the catastrophic failure of electrical equipment creates severe hazards for personnel working in the area. Recognizing this the
Standard for Electrical Safety in the Workplace (NFPA 70E)3 requires that electrical equipment be properly maintained to minimize the possibility of failure.
Relationship of Improperly Maintained Electrical Equipment to the Hazards of Electricity
Improperly maintained equipment may expose workers to any of the three electrical hazards. For example:
1. Improperly maintained tools or flexible cord sets (extension cords) can have frayed insulation which exposes the energized conductors and allows them to contact the worker or the metallic tool the worker is using. The result is an electric shock.
2. Improperly maintained protective devices, such as circuit breakers or fuses, can fail when interrupting an overcurrent. Such a failure is likely to be explosive; consequently, the worker is exposed to electrical arc and electrical blast.
3. Improperly maintained connections can overheat resulting in any of the following:
a. melted insulation, exposed conductors, and the attendant electrical shock hazard
b. fire
c. failed connections resulting in electrical arc and blast
4. Improperly maintained switchgear, motor control centers, or panelboards can fail explosively when an arc occurs internally. This exposes workers to the effects of electrical blast and possibly electrical arc.
As equipment ages however, several factors begin to take their toll on electrical equipment.
● Dust, dirt, and other contaminants collect on equipment causing the equipment to overheat and bearings and other moving parts to bind.
● Vibration causes hardware to loosen. Subsequent operations of equipment can cause joints and equipment to fail explosively.
● Heat and age can cause insulation to fail, resulting in shock hazards to personnel.
● Increased loads, motor starting surges, and power quality issues such as harmonics combine to increase the aging process and set the stage for equipment failure.
Unfortunately, the ultimate failure of unmaintained equipment usually occurs when the equipment is needed the most—during electrical faults. Such failures result in arc and blast events that can and do harm workers in the area.
They also result in significant downtime, loss of equipment, and construction cost incurred in rebuilding the equipment. The only way to ensure that electrical equipment continues to operate in an optimal manner is to maintain it so that it stays in factory-new-operating condition.
Regulatory
As discussed above and in previous chapters, the catastrophic failure of electrical equipment creates severe hazards for personnel working in the area. Recognizing this the
Standard for Electrical Safety in the Workplace (NFPA 70E)3 requires that electrical equipment be properly maintained to minimize the possibility of failure.
Relationship of Improperly Maintained Electrical Equipment to the Hazards of Electricity
Improperly maintained equipment may expose workers to any of the three electrical hazards. For example:
1. Improperly maintained tools or flexible cord sets (extension cords) can have frayed insulation which exposes the energized conductors and allows them to contact the worker or the metallic tool the worker is using. The result is an electric shock.
2. Improperly maintained protective devices, such as circuit breakers or fuses, can fail when interrupting an overcurrent. Such a failure is likely to be explosive; consequently, the worker is exposed to electrical arc and electrical blast.
3. Improperly maintained connections can overheat resulting in any of the following:
a. melted insulation, exposed conductors, and the attendant electrical shock hazard
b. fire
c. failed connections resulting in electrical arc and blast
4. Improperly maintained switchgear, motor control centers, or panelboards can fail explosively when an arc occurs internally. This exposes workers to the effects of electrical blast and possibly electrical arc.
ELECTRICAL EMERGENCIES GUIDE - WHAT TO DO IN CASE OF ELECTRICAL EMERGENCIES?
Strong winds, ice or unintentional contact with equipment may cause trees or tree limbs to fall into powerlines. This may cause wires to break and fall to the ground. Should this happen, notify the electric utility company immediately.
A fallen wire can create hazards for workers and the general public. Objects touched by a fallen wire - fences, vehicles, buildings or even the surrounding ground - must be considered energized and should not be touched.
It is impossible to tell simply by looking whether a downed wire is energized. Consider all downed wires energized and dangerous until the electric utility personnel notify you otherwise.
Where a power line has fallen across a vehicle, occupants should remain within the vehicle. If they must leave the vehicle because of a life-threatening situation, such as fire or potential explosion, they should jump clear of the vehicle without touching either the vehicle or wire and the ground at the same time.
Once clear of the vehicle, they should shuffle away, taking small steps and keeping both feet in contact with the ground.
Remember, electricity can be transmitted from the victim to you. If a switch is accessible, shut off the power immediately. Otherwise, stand on a dry surface and pull the victim away with a dry board or rope. Do not use your hands or anything metal.
Use a C02 or dry chemical extinguisher to put out an electrical fire. Water should be used only by trained firefighting personnel. In an emergency involving power lines or electrical equipment, call the electric utility company immediately.
Training Workers
Ensure that workers assigned to operate cranes and other boomed vehicles are specifically trained in safe operating procedures. Also ensure that workers are trained (1) to understand the limitations of such devices as boom guards, insulated lines, ground rods, nonconductive links, and proximity warning devices, and (2) to recognize that these devices are not substitutes for de-energizing and grounding lines or maintaining safe clearance.
Workers should also be trained to recognize the hazards and use proper techniques when rescuing coworkers or recovering equipment in contact with electrical energy. CSA guidelines list techniques that can be used when equipment contacts energized power lines [CSA 1982]. All employers and workers should be trained in cardiopulmonary resuscitation (CPR).
A fallen wire can create hazards for workers and the general public. Objects touched by a fallen wire - fences, vehicles, buildings or even the surrounding ground - must be considered energized and should not be touched.
It is impossible to tell simply by looking whether a downed wire is energized. Consider all downed wires energized and dangerous until the electric utility personnel notify you otherwise.
Where a power line has fallen across a vehicle, occupants should remain within the vehicle. If they must leave the vehicle because of a life-threatening situation, such as fire or potential explosion, they should jump clear of the vehicle without touching either the vehicle or wire and the ground at the same time.
Once clear of the vehicle, they should shuffle away, taking small steps and keeping both feet in contact with the ground.
Remember, electricity can be transmitted from the victim to you. If a switch is accessible, shut off the power immediately. Otherwise, stand on a dry surface and pull the victim away with a dry board or rope. Do not use your hands or anything metal.
Use a C02 or dry chemical extinguisher to put out an electrical fire. Water should be used only by trained firefighting personnel. In an emergency involving power lines or electrical equipment, call the electric utility company immediately.
Training Workers
Ensure that workers assigned to operate cranes and other boomed vehicles are specifically trained in safe operating procedures. Also ensure that workers are trained (1) to understand the limitations of such devices as boom guards, insulated lines, ground rods, nonconductive links, and proximity warning devices, and (2) to recognize that these devices are not substitutes for de-energizing and grounding lines or maintaining safe clearance.
Workers should also be trained to recognize the hazards and use proper techniques when rescuing coworkers or recovering equipment in contact with electrical energy. CSA guidelines list techniques that can be used when equipment contacts energized power lines [CSA 1982]. All employers and workers should be trained in cardiopulmonary resuscitation (CPR).
THE USE OF GROUND FAULT CURRENT INTERRUPTER (GFCI) IN SAFE ELECTRICAL SYSTEM
A groundfault circuit interrupter (GFCI) is an electrical device which protects personnel by detecting potentially hazardous ground faults and immediately disconnecting power from the circuit. Any current over 8 mA is considered potentially dangerous depending on the path the current takes, the amount of time exposed to the shock, as well as the physical condition of the person receiving the shock.
GFCIs should be installed in such places as dwellings, hotels, motels, construction sites, marinas, receptacles near swimming pools and hot tubs, underwater lighting, fountains, and other areas in which a person may experience a ground fault.
A GFCI compares the amount of current in the ungrounded (hot) conductor with the amount of current in the neutral conductor. If the current in the neutral conductor becomes less than the current in the hot conductor, a ground fault condition exists.
The missing current is returned to the source by some path other than the intended path (fault current). A fault current as low as 4 mA to 6 mA activates the GFCI and interrupts the circuit.
Once activated, the fault condition is cleared and the GFCI manually resets before power may be restored to the circuit. GFCI protection may be installed at different locations within a circuit.
Direct-wired GFCI receptacles provide a ground fault protection at the point of installation. GFCI receptacles may also be connected to provide GFCI protection at all other receptacles installed downstream on the same circuit. GFCI CBs, when installed in a load center or panelboard, provide GFCI protection and conventional circuit overcurrent protection for all branch-circuit components connected to the CB.
Plug-in GFCls provide ground fault protection for devices plugged into them. Plug-in devices are generally utilized by personnel working with power tools in an area that does not include GFCI receptacles.
HOW GFCI WORKS? THE OPERATING PRINCIPLE OF GFCI
A GFCI compares the amount of current in the ungrounded (hot) conductor with the amount of current in the neutral conductor.
GFCI operation diagram is found below:
GFCIs should be installed in such places as dwellings, hotels, motels, construction sites, marinas, receptacles near swimming pools and hot tubs, underwater lighting, fountains, and other areas in which a person may experience a ground fault.
A GFCI compares the amount of current in the ungrounded (hot) conductor with the amount of current in the neutral conductor. If the current in the neutral conductor becomes less than the current in the hot conductor, a ground fault condition exists.
The missing current is returned to the source by some path other than the intended path (fault current). A fault current as low as 4 mA to 6 mA activates the GFCI and interrupts the circuit.
Once activated, the fault condition is cleared and the GFCI manually resets before power may be restored to the circuit. GFCI protection may be installed at different locations within a circuit.
Direct-wired GFCI receptacles provide a ground fault protection at the point of installation. GFCI receptacles may also be connected to provide GFCI protection at all other receptacles installed downstream on the same circuit. GFCI CBs, when installed in a load center or panelboard, provide GFCI protection and conventional circuit overcurrent protection for all branch-circuit components connected to the CB.
Plug-in GFCls provide ground fault protection for devices plugged into them. Plug-in devices are generally utilized by personnel working with power tools in an area that does not include GFCI receptacles.
HOW GFCI WORKS? THE OPERATING PRINCIPLE OF GFCI
A GFCI compares the amount of current in the ungrounded (hot) conductor with the amount of current in the neutral conductor.
GFCI operation diagram is found below:
HOW ELECTRIC SHOCK IS RECEIVED?
Whenever you work with power tools or on electrical circuits, there is a risk of electrical hazards, especially electrical shock. Anyone can be exposed to these hazards at home or at work.
Workers are exposed to more hazards because job sites can be cluttered with tools and materials, fast-paced, and open to the weather. Risk is also higher at work because many jobs involve electric power tools.
Electrical trades workers must pay special attention to electrical hazards because they work on electrical circuits. Coming in contact with an electrical voltage can cause current to flow through the body, resulting in electrical shock and burns. Serious injury or even death may occur.
As a source of energy, electricity is used without much thought about the hazards it can cause. Because electricity is a familiar part of our lives, it often is not treated with enough caution. As a result, an average of one worker is electrocuted on the job every day of every year!
An electrical shock is received when electrical current passes through the body. Current will pass through the body in a variety of situations.
Whenever two wires are at different voltages, current will pass between them if they are connected. Your body can connect the wires if you touch both of them at the same time. Current will pass through your body.
In most household wiring, the black wires and the red wires are at 120 volts. The white wires are at 0 volts because they are connected to ground. The connection to ground is often through a conducting ground rod driven into the earth. The connection can also be made through a buried metal water pipe. If you come in contact with an energized black wire—and you are also in contact with the neutral white wire—current will pass through your body. You will receive an electrical shock.
If you are in contact with a live wire or any live component of an energized electrical device—and also in contact with any grounded object—you will receive a shock. Plumbing is often grounded. Metal electrical boxes and conduit are grounded.
Your risk of receiving a shock is greater if you stand in a puddle of water. But you don’t even have to be standing in water to be at risk. Wet clothing, high humidity, and perspiration also increase your chances of being electrocuted. Of course, there is always a chance of electrocution, even in dry conditions.
You can even receive a shock when you are not in contact with an electrical ground. Contact with both live wires of a 240-volt cable will deliver a shock. (This type of shock can occur because one live wire may be at +120 volts while the other is at -120 volts during an alternating current cycle—a difference of 240 volts.). You can also receive a shock from electrical components that are not grounded properly. Even contact with another person who is receiving an electrical shock may cause you to be shocked.
ELECTROCUTION AND ELECTRICAL FATALITIES BASIC INFORMATION
The term electrocution refers to an electrical event with electrical current exposure that results in death. The implication is that the current flow has caused an electrical shock with subsequent death.
“Electrical accident fatality” is a general use phrase seen in news reports meaning either electrocution, or death resulting at the time of the electrical accident. This phrase may include fatalities associated shock or other forms of energy released at the time of the electrical accident, in particular those causing physical changes including burns, blast effects, and radiation damage.
“Electrical injury mortality” is a medical statistics phrase which suggests that persons who were injured in an electrical accident lived long enough to receive medical care for their injuries, but the medical care was not followed by survival.
It’s important to appreciate that an electrical event can produce a fatality or injury even when there is no electrical current flow to the victim or electrical shock. This might be the situation, for example, when a victim is caught in an electrical ignition fire, explosion, or blast.
In this type of scenario, the “root cause” of the accident is electrical, but the mechanism of death or injury is from thermal, acoustic, radiation, or blast exposure related to electrothermal chemical (ETC) combustion.
Another way employees can be killed or injured after an electrical event is that they are surprised by an energized source, either through a spark, like a static “zap” to exposed skin, or through a noise, like a sharp “gunshot” type sound close to the head.
The surprise can lead to an unintended body movement which might be characterized as a “startle response.” If the startle occurs at the top of a ladder or scaffold, the direct mechanism of death or injury can be through a fall.
If the startle occurs in proximity to other energized equipment that is moving, the direct mechanism of death or injury can be with a body part being caught in or by the moving equipment.
Fatal and non-fatal electrical incidents share three characteristics:
1. The unintentional exposure of employees to electrical energy;
2. Compliance failure in at least one aspect of electrical design, installation, policies, procedures, practices, or personal protection; and
3. Energy transfer to exposed employees in some combination of electrical, thermal, radiation, acoustic (pressure), mechanical, light, kinetic, or potential energy.
What is the difference between fatal and non-fatal electrical incidents? The answer depends in part on whether the question is asked hypothetically, like in a “what if ” planning scenario; or whether the question is asked retrospectively after a traumatic accident has occurred.
Hypothetically, based on human physical and biological characteristics, we know that a fatal electrical event transfers a greater amount of energy to its victim than a non-fatal situation. This knowledge about the fatal risk of energy transfer underlies the use of equipment designs (for example, required doors, specified space clearances, venting systems on equipment to discharge combustion products, “umbilical corded” controls, infrared monitoring ports for doors closed heat monitoring) and barrier protection (such as PPE, including leather gloves, flash suits, safety glasses, face shields, long sticks, extended handles, and flame resistant clothing).
By reducing the amount of possible energy transfer during an unintentional electrical exposure, strategies including equipment design and barrier protection can increase the likelihood of survival after an electrical incident.
Retrospectively, if two people are present in an electrical incident when one dies and the other survives, the difference in survival may come down to nuances in the victims’ innate individual differences and their spatial and temporal relationship to the electrical hazard at the time of the energy release, transformation, and transfer. Medical and legal privacy protections tend to reduce accessibility to accident details, so systematic information is lacking about how various scenarios unfold.
Generally, there is a lethal exposure “dose” for different forms of energy that can result in death. When multiple forms of energy are involved in an electrical event, multiple lethal or sublethal doses of energy may flow from the event, transformed from the electrical hazard source, and transferred to nearby employees may result in highly variable damage to the body.
“Electrical accident fatality” is a general use phrase seen in news reports meaning either electrocution, or death resulting at the time of the electrical accident. This phrase may include fatalities associated shock or other forms of energy released at the time of the electrical accident, in particular those causing physical changes including burns, blast effects, and radiation damage.
“Electrical injury mortality” is a medical statistics phrase which suggests that persons who were injured in an electrical accident lived long enough to receive medical care for their injuries, but the medical care was not followed by survival.
It’s important to appreciate that an electrical event can produce a fatality or injury even when there is no electrical current flow to the victim or electrical shock. This might be the situation, for example, when a victim is caught in an electrical ignition fire, explosion, or blast.
In this type of scenario, the “root cause” of the accident is electrical, but the mechanism of death or injury is from thermal, acoustic, radiation, or blast exposure related to electrothermal chemical (ETC) combustion.
Another way employees can be killed or injured after an electrical event is that they are surprised by an energized source, either through a spark, like a static “zap” to exposed skin, or through a noise, like a sharp “gunshot” type sound close to the head.
The surprise can lead to an unintended body movement which might be characterized as a “startle response.” If the startle occurs at the top of a ladder or scaffold, the direct mechanism of death or injury can be through a fall.
If the startle occurs in proximity to other energized equipment that is moving, the direct mechanism of death or injury can be with a body part being caught in or by the moving equipment.
Fatal and non-fatal electrical incidents share three characteristics:
1. The unintentional exposure of employees to electrical energy;
2. Compliance failure in at least one aspect of electrical design, installation, policies, procedures, practices, or personal protection; and
3. Energy transfer to exposed employees in some combination of electrical, thermal, radiation, acoustic (pressure), mechanical, light, kinetic, or potential energy.
What is the difference between fatal and non-fatal electrical incidents? The answer depends in part on whether the question is asked hypothetically, like in a “what if ” planning scenario; or whether the question is asked retrospectively after a traumatic accident has occurred.
Hypothetically, based on human physical and biological characteristics, we know that a fatal electrical event transfers a greater amount of energy to its victim than a non-fatal situation. This knowledge about the fatal risk of energy transfer underlies the use of equipment designs (for example, required doors, specified space clearances, venting systems on equipment to discharge combustion products, “umbilical corded” controls, infrared monitoring ports for doors closed heat monitoring) and barrier protection (such as PPE, including leather gloves, flash suits, safety glasses, face shields, long sticks, extended handles, and flame resistant clothing).
By reducing the amount of possible energy transfer during an unintentional electrical exposure, strategies including equipment design and barrier protection can increase the likelihood of survival after an electrical incident.
Retrospectively, if two people are present in an electrical incident when one dies and the other survives, the difference in survival may come down to nuances in the victims’ innate individual differences and their spatial and temporal relationship to the electrical hazard at the time of the energy release, transformation, and transfer. Medical and legal privacy protections tend to reduce accessibility to accident details, so systematic information is lacking about how various scenarios unfold.
Generally, there is a lethal exposure “dose” for different forms of energy that can result in death. When multiple forms of energy are involved in an electrical event, multiple lethal or sublethal doses of energy may flow from the event, transformed from the electrical hazard source, and transferred to nearby employees may result in highly variable damage to the body.
EFFECT OF CURRENT and ITS DURATION TO THE HUMAN BODY DURING ELECTRIC SHOCK
WHAT KILLS A PERSON? CURRENT OR VOLTAGE?
To answer the question, we need to put things in context. That means, there is no absolute. Current kills, but it needs to be present for a certain period of time.
The amount of energy delivered to the body is directly proportional to the length of time that the current flows; consequently, the degree of trauma is also directly proportional to the duration of the current. Three examples illustrate this concept:
1. Current flow through body tissues delivers energy in the form of heat. The magnitude of energy may be approximated by:
J = I2Rt
where J = energy, joules
I = current, amperes
R = resistance of the current path through the body, ohms
t = time of current flow, seconds
If sufficient heat is delivered, tissue burning and/or organ shutdown can occur. Note that the amount of heat that is delivered is directly proportional to the duration of the current (t).
2. Some portion of the externally caused current flow will tend to follow the current paths used by the body’s central nervous system. Since the external current is much larger than the normal current flow, damage can occur to the nervous system.
Note that nervous system damage can be fatal even with relatively short durations of current; however, increased duration heightens the chance that damage will occur.
3. Generally, a longer duration of current through the heart is more likely to cause ventricular fibrillation. Fibrillation seems to occur when the externally applied electric field overlaps with the body’s cardiac cycle. The likelihood of this event increases with time.
Also, we need to understand how much current is significant.
The magnitude of the current that flows through the body obeys Ohm’s law, that is,
I = E/R
where I = current magnitude, amperes (A)
E = applied voltage, volts (V)
R = resistance of path through which current flows, ohms (Ω)
Parts of the Body.
Current flow affects the various bodily organs in different manners. For example, the heart can be caused to fibrillate with as little as 75 mA.
The diaphragm and the breathing system can be paralyzed, which possibly may be fatal without outside intervention, with less than 30 mA of current flow. The specific responses of the various body parts to current flow are covered in later sections.
Nominal Human Response to Current Magnitudes
To answer the question, we need to put things in context. That means, there is no absolute. Current kills, but it needs to be present for a certain period of time.
The amount of energy delivered to the body is directly proportional to the length of time that the current flows; consequently, the degree of trauma is also directly proportional to the duration of the current. Three examples illustrate this concept:
1. Current flow through body tissues delivers energy in the form of heat. The magnitude of energy may be approximated by:
J = I2Rt
where J = energy, joules
I = current, amperes
R = resistance of the current path through the body, ohms
t = time of current flow, seconds
If sufficient heat is delivered, tissue burning and/or organ shutdown can occur. Note that the amount of heat that is delivered is directly proportional to the duration of the current (t).
2. Some portion of the externally caused current flow will tend to follow the current paths used by the body’s central nervous system. Since the external current is much larger than the normal current flow, damage can occur to the nervous system.
Note that nervous system damage can be fatal even with relatively short durations of current; however, increased duration heightens the chance that damage will occur.
3. Generally, a longer duration of current through the heart is more likely to cause ventricular fibrillation. Fibrillation seems to occur when the externally applied electric field overlaps with the body’s cardiac cycle. The likelihood of this event increases with time.
Also, we need to understand how much current is significant.
The magnitude of the current that flows through the body obeys Ohm’s law, that is,
I = E/R
where I = current magnitude, amperes (A)
E = applied voltage, volts (V)
R = resistance of path through which current flows, ohms (Ω)
Parts of the Body.
Current flow affects the various bodily organs in different manners. For example, the heart can be caused to fibrillate with as little as 75 mA.
The diaphragm and the breathing system can be paralyzed, which possibly may be fatal without outside intervention, with less than 30 mA of current flow. The specific responses of the various body parts to current flow are covered in later sections.
Nominal Human Response to Current Magnitudes
ELECTRIC SHOCK HAZARD ANALYSIS
WHAT HAPPENS WHEN WE GET ELECTRIC SHOCK?
Electric shock is the physical stimulation that occurs when electric current flows through the human body. The distribution of current flow through the body is a function of the resistance of the various paths through which the current flows. The final trauma associated with the electric shock is usually determined by the most critical path called the shock circuit. The symptoms may include a mild tingling sensation, violent muscle contractions, heart arrhythmia, or tissue damage.
Common effect are the following:
Burning.
Burns caused by electric current are almost always third-degree because the burning occurs from the inside of the body. This means that the growth centers are destroyed. Electric-current burns can be especially severe when they involve vital internal organs.
Cell Wall Damage.
Research funded by the Electric Power Research Institute (EPRI) has shown that cell death can result from the enlargement of cellular pores due to high-intensity electric fields. This research has been performed primarily by Dr. Raphael C. Lee and his colleagues at the University of Chicago. This trauma called electroporation allows ions to flow freely through the cell membranes, causing cell death.
HOW YOUR PHYSICAL CONDITION IS A FACTOR DURING ELECTRIC SHOCK
Physical Condition and Physical Response.
The physical condition of the individual greatly influences the effects of current flow. A given amount of current flow will usually cause less trauma to a person in good physical condition.
Moreover, if the victim of the shock has any specific medical problems such as heart or lung ailments, these parts of the body will be severely affected by relatively low currents. A diseased heart, for example, is more likely to suffer ventricular fibrillation than a healthy heart.
Electric shock is the physical stimulation that occurs when electric current flows through the human body. The distribution of current flow through the body is a function of the resistance of the various paths through which the current flows. The final trauma associated with the electric shock is usually determined by the most critical path called the shock circuit. The symptoms may include a mild tingling sensation, violent muscle contractions, heart arrhythmia, or tissue damage.
Common effect are the following:
Burning.
Burns caused by electric current are almost always third-degree because the burning occurs from the inside of the body. This means that the growth centers are destroyed. Electric-current burns can be especially severe when they involve vital internal organs.
Cell Wall Damage.
Research funded by the Electric Power Research Institute (EPRI) has shown that cell death can result from the enlargement of cellular pores due to high-intensity electric fields. This research has been performed primarily by Dr. Raphael C. Lee and his colleagues at the University of Chicago. This trauma called electroporation allows ions to flow freely through the cell membranes, causing cell death.
HOW YOUR PHYSICAL CONDITION IS A FACTOR DURING ELECTRIC SHOCK
Physical Condition and Physical Response.
The physical condition of the individual greatly influences the effects of current flow. A given amount of current flow will usually cause less trauma to a person in good physical condition.
Moreover, if the victim of the shock has any specific medical problems such as heart or lung ailments, these parts of the body will be severely affected by relatively low currents. A diseased heart, for example, is more likely to suffer ventricular fibrillation than a healthy heart.
WHAT IS ELECTRICAL SHOCK - HOW ELECTRIC SHOCK HAPPENS?
Most fatal electrical shocks happen to people who are actually knowledgeable regarding electrical shock safety precautions. A majority of the electrical shock accidents occur when an employee in a rush disregards safety precautions that they know should be followed.
The following are some electromedical facts intended to make employees think twice before taking chances. It's not the voltage but the current that kills. People have been killed by 100 volts AC in the home and with as little as 42 volts DC.
The real measure of a shock's intensity lies in the amount of current (in milliamperes) forced through the body. Any electrical device used on a house wiring circuit can, under certain conditions, transmit a fatal amount of current.
Currents between 100 and 200 milliamperes (0.1 ampere and 0.2 ampere) are fatal. Anything in the neighborhood of 10 milliamperes (0.01) is capable of producing painful to severe shock. Current values and their effects are summarized below.
The severity of the shock increases as the current increases. Below 20 milliamperes, breathing becomes labored, and it can cease completely even at values below 75 milliamperes. As the current approaches 100 milliamperes ventricular fibrillation occurs. This is an uncoordinated twitching of the walls of the heart's ventricles.
Since you don't know how much current went through the body, it is necessary to perform artificial respiration to try to get the person breathing again. If the heart is not beating, cardio pulmonary resuscitation (CPR) is necessary.
Electrical shock occurs when a person comes in contact with two conductors of a circuit or when the body becomes part of the electrical circuit. In either instance, a severe shock can cause the heart and lungs to stop functioning. Additionally, severe burns may occur where current enters and exits the body
Prevention is the best medicine for electrical shock. Employees should respect all voltages and possess the knowledge of the principles of electricity. It is important to always follow safe work procedures.
Do not take unnecessary chances. All electricians should be encouraged to take a basic course in CPR so they can aid a coworker in emergency situations.
Portable electric tools should always be well maintained and in safe operating condition. Make sure there is a third wire on the plug for grounding in case of shorts. The fault current should flow through the third wire to ground instead of through the operator's body to ground if electric power tools are grounded and if an insulation breakdown occurs.
The following are some electromedical facts intended to make employees think twice before taking chances. It's not the voltage but the current that kills. People have been killed by 100 volts AC in the home and with as little as 42 volts DC.
The real measure of a shock's intensity lies in the amount of current (in milliamperes) forced through the body. Any electrical device used on a house wiring circuit can, under certain conditions, transmit a fatal amount of current.
Currents between 100 and 200 milliamperes (0.1 ampere and 0.2 ampere) are fatal. Anything in the neighborhood of 10 milliamperes (0.01) is capable of producing painful to severe shock. Current values and their effects are summarized below.
The severity of the shock increases as the current increases. Below 20 milliamperes, breathing becomes labored, and it can cease completely even at values below 75 milliamperes. As the current approaches 100 milliamperes ventricular fibrillation occurs. This is an uncoordinated twitching of the walls of the heart's ventricles.
Since you don't know how much current went through the body, it is necessary to perform artificial respiration to try to get the person breathing again. If the heart is not beating, cardio pulmonary resuscitation (CPR) is necessary.
Electrical shock occurs when a person comes in contact with two conductors of a circuit or when the body becomes part of the electrical circuit. In either instance, a severe shock can cause the heart and lungs to stop functioning. Additionally, severe burns may occur where current enters and exits the body
Prevention is the best medicine for electrical shock. Employees should respect all voltages and possess the knowledge of the principles of electricity. It is important to always follow safe work procedures.
Do not take unnecessary chances. All electricians should be encouraged to take a basic course in CPR so they can aid a coworker in emergency situations.
Portable electric tools should always be well maintained and in safe operating condition. Make sure there is a third wire on the plug for grounding in case of shorts. The fault current should flow through the third wire to ground instead of through the operator's body to ground if electric power tools are grounded and if an insulation breakdown occurs.
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