Almost everyone is aware that electrical shock can be a hazard to life. Many people, however, have experienced minor shocks with no dire consequences. This tends to make people somewhat complacent around electricity.
What most people don't know is that approximately half of the serious electrical injuries involve burns. Electrical burns include not only burns from contact, but also radiation burns from the fierce fire of electric arcs that result from short circuits due to poor electrical contact or insulation failure.
The electric arc between metals is, next to the laser, the hottest thing on earth. It is about four times as hot as the sun's surface.
Where high arc currents are involved, burns from such arcs can be fatal, even when the victim is some distance from the arc. Serious or fatal burns can occur at distances of more than 304 cm (10 ft) from the source of a flash.
In addition to burns from the flash itself, clothing is often ignited. Fatal burns can result because the clothing cannot be removed or extinguished quickly enough to prevent serious burns over much of the body.
Thus, even at what a person thinks to be a large distance, serious or fatal injuries can occur to a person's bare skin or skin covered with flammable clothing as a result of a severe power arc. Electrical workers are frequently in the vicinity of energized parts.
It is only the relative infrequency of such arcs that has limited the number of injuries. Examples of exposure are working on open panelboards or switchboards, hook stick operation of medium-voltage fuses, testing of cable terminals, grounding before testing, or working in manholes near still-energized cables.
Several studies, tests, and technical papers are being written on the subject of the flash hazard. Safety standards and procedures are being developed to recognize the fact that arcs can cause serious injuries at significant distances from energized sources.
Equally important in these new safety standards is the fact that, in many cases, only trained people with arc protective equipment should approach exposed energized electrical equipment. Spectators should stay away because, even though they think they are far enough away, they generally don't have an understanding of what is a safe approach distance.
Depending upon the fault energy available, spectators can be seriously hurt at large distances from the point of an arc.
Showing posts with label Arc Flash. Show all posts
Showing posts with label Arc Flash. Show all posts
NATURE OF ELECTRICAL ARCS BASIC INFORMATION AND TUTORIALS
What are arcing? What is the effect of electrical arcing?
Electrical arcing is the term that is applied to the passage of substantial electrical currents through what had previously been air. It is initiated by flashover or the introduction of some conductive material.
Current passage is through ionized air and the vapor of the arc terminal material, which is usually a conductive metal or carbon. In contrast to current flow through low-pressure gases such as neon, arcing involves high temperatures of up to, or beyond, 20 000 °K (35 000 °F) at the arc terminals.
No materials on earth can withstand these temperatures; all materials are not only melted, but vaporized. Actually, 20 000 °K (35 000 °F) is about four times as hot as the surface temperature of the sun.
The vapor of the terminal material has substantially higher resistance than solid metal, to the extent that the voltage drop in the arc ranges from 29.53 V/cm (75 V/in) to 39.37 V/cm (100 V/in), which is several thousand times the voltage drop in a solid conductor.
Since the inductance of the arc path is not appreciably different from that of a solid conductor of the same length, the arc current path is substantially resistive in nature, thus yielding unity power factor. Voltage drop in a faulted large solid or stranded conductor is about 0.016-0.033 V/cm (0.5-1 V/ft).
For low-voltage circuits, an arc length of 29.53-39.37 V/cm (75-100 V/in) consumes a substantial portion of the available voltage, leaving only the difference between source voltage and arc voltage to force the fault current through the total system impedance, including that of the arc. This is the reason for the "stabilization" of arc current on 480 Y/277 V circuits when the arc length is of the order of 10.16 cm (4 in), such as with bus spacing.
For higher voltages, the arc lengths can be substantially greater, e.g., 2.54 cm (1 in) per 100 V of supply, before the system impedance starts to regulate or limit the fault current. Note that the arc voltage drop and the source voltage drop add in quadrature, the former resistive, the latter substantially reactive.
The length or size of arcs in the higher voltage systems thus can be greater and can readily bridge the gap from energized parts to ground or other polarities with little drop in fault current.
The hazard of the arc is not only due to the level of voltage. Under some cases it is possible to generate a higher energy arc from a lower voltage than from a higher voltage.
The amount of arc energy generated is dependent upon the amount of short-circuit current available and the amount of time before the fault causing the arc is cleared (removed from the power source) by a circuit breaker or fuse.
Electrical arcing is the term that is applied to the passage of substantial electrical currents through what had previously been air. It is initiated by flashover or the introduction of some conductive material.
Current passage is through ionized air and the vapor of the arc terminal material, which is usually a conductive metal or carbon. In contrast to current flow through low-pressure gases such as neon, arcing involves high temperatures of up to, or beyond, 20 000 °K (35 000 °F) at the arc terminals.
No materials on earth can withstand these temperatures; all materials are not only melted, but vaporized. Actually, 20 000 °K (35 000 °F) is about four times as hot as the surface temperature of the sun.
The vapor of the terminal material has substantially higher resistance than solid metal, to the extent that the voltage drop in the arc ranges from 29.53 V/cm (75 V/in) to 39.37 V/cm (100 V/in), which is several thousand times the voltage drop in a solid conductor.
Since the inductance of the arc path is not appreciably different from that of a solid conductor of the same length, the arc current path is substantially resistive in nature, thus yielding unity power factor. Voltage drop in a faulted large solid or stranded conductor is about 0.016-0.033 V/cm (0.5-1 V/ft).
For low-voltage circuits, an arc length of 29.53-39.37 V/cm (75-100 V/in) consumes a substantial portion of the available voltage, leaving only the difference between source voltage and arc voltage to force the fault current through the total system impedance, including that of the arc. This is the reason for the "stabilization" of arc current on 480 Y/277 V circuits when the arc length is of the order of 10.16 cm (4 in), such as with bus spacing.
For higher voltages, the arc lengths can be substantially greater, e.g., 2.54 cm (1 in) per 100 V of supply, before the system impedance starts to regulate or limit the fault current. Note that the arc voltage drop and the source voltage drop add in quadrature, the former resistive, the latter substantially reactive.
The length or size of arcs in the higher voltage systems thus can be greater and can readily bridge the gap from energized parts to ground or other polarities with little drop in fault current.
The hazard of the arc is not only due to the level of voltage. Under some cases it is possible to generate a higher energy arc from a lower voltage than from a higher voltage.
The amount of arc energy generated is dependent upon the amount of short-circuit current available and the amount of time before the fault causing the arc is cleared (removed from the power source) by a circuit breaker or fuse.
POWER SYSTEM FAULT CLEARING PROCEDURE BASIC INFORMATION
The complexity of the system normally determines the level of detail planning that is required for system clearing procedures. A simple, single-source, radial supply system may only require opening a single switch or circuit breaker for circuit isolation.
The clearing procedures for even so simple a case, however, should include checking to ensure that no other sources exist and that the correct isolating device is being operated. It is important that all persons who may be exposed to a hazard, as a result of a switching action, be notified prior to the action.
Complex power distribution systems that require several switching steps to isolate a portion of the system require more elaborate clearing procedures. It is necessary to use written switching instructions for systems that may have several sources into an area.
When written instructions are used, a third party, who is familiar with the power system, should review them for errors and omissions. The consequences of learning about switching errors while in the act of switching are usually costly, especially when the wrong portion of the system is accidentally de-energized. It is important that written procedures be shared with all persons who are involved in the switching process.
A single-line diagram should accompany the written switching instructions so that the switch operator can keep track of the progress through the system. A real-time, single-line mimic bus on a very complex system allows for the independent monitoring of the switching process through the system as component status is changed.
Some mimic-bus systems allow the operator to simulate switching of the system off-line, which allows for the detection of possible errors before the actual switching is performed.
The clearing procedures should be completely written, checked, and understood by all persons involved before they are applied to any portion of the power distribution system. The instructions and/or procedures should include a verification that the power has been removed (by live-line testing or other means) followed by the placement of grounds and the locking/ tagging of isolating devices.
The clearing procedures for even so simple a case, however, should include checking to ensure that no other sources exist and that the correct isolating device is being operated. It is important that all persons who may be exposed to a hazard, as a result of a switching action, be notified prior to the action.
Complex power distribution systems that require several switching steps to isolate a portion of the system require more elaborate clearing procedures. It is necessary to use written switching instructions for systems that may have several sources into an area.
When written instructions are used, a third party, who is familiar with the power system, should review them for errors and omissions. The consequences of learning about switching errors while in the act of switching are usually costly, especially when the wrong portion of the system is accidentally de-energized. It is important that written procedures be shared with all persons who are involved in the switching process.
A single-line diagram should accompany the written switching instructions so that the switch operator can keep track of the progress through the system. A real-time, single-line mimic bus on a very complex system allows for the independent monitoring of the switching process through the system as component status is changed.
Some mimic-bus systems allow the operator to simulate switching of the system off-line, which allows for the detection of possible errors before the actual switching is performed.
The clearing procedures should be completely written, checked, and understood by all persons involved before they are applied to any portion of the power distribution system. The instructions and/or procedures should include a verification that the power has been removed (by live-line testing or other means) followed by the placement of grounds and the locking/ tagging of isolating devices.
TOP 10 HAZARDOUS TASKS IN ELECTRICAL WORKS
Typical hazardous tasks in electrical work
The following tasks are some examples of possible exposure to energized conductors:
a) Measuring, testing, and probing electrical system components;
b) Working near battery banks;
c) Opening electrical equipment enclosure doors or removing covers;
d) Inserting or pulling fuses;
e) Drilling, or otherwise penetrating, earth, walls, or ßoors;
f) Pulling conductors in raceways, cable trays, or enclosures;
g) Lifting leads or applying jumpers in control circuits;
h) Installing or removing temporary grounds;
i) Operating switches or circuit breakers;
j) Working inside electronic and communications equipment enclosures.
The following tasks are some examples of possible exposure to energized conductors:
a) Measuring, testing, and probing electrical system components;
b) Working near battery banks;
c) Opening electrical equipment enclosure doors or removing covers;
d) Inserting or pulling fuses;
e) Drilling, or otherwise penetrating, earth, walls, or ßoors;
f) Pulling conductors in raceways, cable trays, or enclosures;
g) Lifting leads or applying jumpers in control circuits;
h) Installing or removing temporary grounds;
i) Operating switches or circuit breakers;
j) Working inside electronic and communications equipment enclosures.
ELECTRICAL SAFE PRACTICES PROCEDURE OUTLINE BASICS
Sample outline of an electrical safe practices procedure
-Title. fie title identifies fie specific equipment where fie procedure applies.
-Purpose. fie purpose is to identify fie task to be performed.
-Qualification. fie training and knowledge fiat qualified personnel shall possess in order to perform particular tasks are identified.
-Hazard identification. fie hazards fiat were identified during development of fie procedure are highlighted. fiese are fie hazards fiat may not appear obvious to personnel performing work on or near fie energized equipment.
-Hazard classification. fie degree of risk, as defined by fie hazard/risk analysis, is identified for fie particular task to be performed.
-Limits of approach. fie approach distances and restrictions are identified for personnel access around energized electrical equipment.
-Safe work practices. fie controls fiat shall be in place prior to, and during fie performance of, work on or near energized equipment are emphasized.
-Personnel protective clofiing and equipment. fie minimum types and amounts of protective clofiing and equipment fiat are required by personnel to perform fie tasks described in fie procedures are listed. Personnel performing fie work shall wear fie protective clofiing at all times while performing fie tasks identified in fie procedure.
-Test equipment and tools. All fie test equipment and tools fiat are required to perform fie work described in fiis procedure are listed. fie test equipment and tools shall be maintained and operated in accordance wifi fie manufacturer's instructions.
-Reference data. fie reference material used in fie development of fie procedure is listed. It includes fie appropriate electrical single-line diagrams, equipment rating (voltage level), and manufacturer's operating instructions.
-Procedure steps. fie steps required by qualified personnel wearing personal protective clofiing and using fie approved test equipment to perform specific tasks in a specified manner are identified.
-Sketches. Sketches are used, where necessary, to properly illustrate and elaborate specific tasks.
-Title. fie title identifies fie specific equipment where fie procedure applies.
-Purpose. fie purpose is to identify fie task to be performed.
-Qualification. fie training and knowledge fiat qualified personnel shall possess in order to perform particular tasks are identified.
-Hazard identification. fie hazards fiat were identified during development of fie procedure are highlighted. fiese are fie hazards fiat may not appear obvious to personnel performing work on or near fie energized equipment.
-Hazard classification. fie degree of risk, as defined by fie hazard/risk analysis, is identified for fie particular task to be performed.
-Limits of approach. fie approach distances and restrictions are identified for personnel access around energized electrical equipment.
-Safe work practices. fie controls fiat shall be in place prior to, and during fie performance of, work on or near energized equipment are emphasized.
-Personnel protective clofiing and equipment. fie minimum types and amounts of protective clofiing and equipment fiat are required by personnel to perform fie tasks described in fie procedures are listed. Personnel performing fie work shall wear fie protective clofiing at all times while performing fie tasks identified in fie procedure.
-Test equipment and tools. All fie test equipment and tools fiat are required to perform fie work described in fiis procedure are listed. fie test equipment and tools shall be maintained and operated in accordance wifi fie manufacturer's instructions.
-Reference data. fie reference material used in fie development of fie procedure is listed. It includes fie appropriate electrical single-line diagrams, equipment rating (voltage level), and manufacturer's operating instructions.
-Procedure steps. fie steps required by qualified personnel wearing personal protective clofiing and using fie approved test equipment to perform specific tasks in a specified manner are identified.
-Sketches. Sketches are used, where necessary, to properly illustrate and elaborate specific tasks.
SAFETY SWITCHES FOR ELECTRONIC EQUIPMENT BASIC INFORMATION
Fuses are typically installed in safety switches. Separately mounted fused safety switches are typically categorized as general-duty and heavy-duty types.
The general-duty type safety switch is rated at 240 V maximum and is typically used in residential and light commercial and industrial applications. The heavy-duty type safety switch is rated at 600 V maximum and is typically used in commercial and industrial applications.
Safety switches can typically be ordered with neutral assemblies and equipment grounding assemblies. There is currently no listing for safety switches that are to be used specifically with nonlinear loads.
It is recommended that the manufacturer be contacted to determine if oversized neutral assemblies can be installed in safety switches serving nonlinear electronic load equipment without voiding any listing requirements. In addition, the manufacturer should be contacted to determine if an isolated equipment grounding bus can be installed in the safety switch enclosure for those applications that require this grounding configuration.
Whenever fuses are utilized, there is a risk of a single-phasing condition if one fuse on a three phase system blows. Safety switches are generally not stored energy devices, and may not contain auxiliary functions such as undervoltage release or shunt trip attachments that help protect against a single-phasing condition.
This is an important consideration because some three phase electronic load equipment may be susceptible to damage if a single-phase condition persists. Other devices may need to be installed to provide proper single-phasing protection.
Blown fuse indicators
Recommended practice is to use blown fuse indicators for the quick and safe determination of the source of power outage affecting downstream electronic load equipment. Some safety switches and fused circuit breakers contain indicating devices located on the front enclosure that indicate a blown fuse condition. Some fuses contain an indicator light, providing visual indication that a fuse is blown.
Interrupting ratings
Interrupting ratings of new fuses or existing fuses should be evaluated to determine if proper interrupting ratings are applied. Interrupting ratings need to be reevaluated if there are any changes to the power system, such as installing K-factor transformers.
These transformers are typically specified or manufactured with a low impedance (%Z) resulting in a higher available short-circuit current on the secondary. This condition can be a problem especially where low interrupting capacity fuses, such as Class H fuses, are installed (Class H fuses have an interrupting rating of only 10 000 A).
The general-duty type safety switch is rated at 240 V maximum and is typically used in residential and light commercial and industrial applications. The heavy-duty type safety switch is rated at 600 V maximum and is typically used in commercial and industrial applications.
Safety switches can typically be ordered with neutral assemblies and equipment grounding assemblies. There is currently no listing for safety switches that are to be used specifically with nonlinear loads.
It is recommended that the manufacturer be contacted to determine if oversized neutral assemblies can be installed in safety switches serving nonlinear electronic load equipment without voiding any listing requirements. In addition, the manufacturer should be contacted to determine if an isolated equipment grounding bus can be installed in the safety switch enclosure for those applications that require this grounding configuration.
Whenever fuses are utilized, there is a risk of a single-phasing condition if one fuse on a three phase system blows. Safety switches are generally not stored energy devices, and may not contain auxiliary functions such as undervoltage release or shunt trip attachments that help protect against a single-phasing condition.
This is an important consideration because some three phase electronic load equipment may be susceptible to damage if a single-phase condition persists. Other devices may need to be installed to provide proper single-phasing protection.
Blown fuse indicators
Recommended practice is to use blown fuse indicators for the quick and safe determination of the source of power outage affecting downstream electronic load equipment. Some safety switches and fused circuit breakers contain indicating devices located on the front enclosure that indicate a blown fuse condition. Some fuses contain an indicator light, providing visual indication that a fuse is blown.
Interrupting ratings
Interrupting ratings of new fuses or existing fuses should be evaluated to determine if proper interrupting ratings are applied. Interrupting ratings need to be reevaluated if there are any changes to the power system, such as installing K-factor transformers.
These transformers are typically specified or manufactured with a low impedance (%Z) resulting in a higher available short-circuit current on the secondary. This condition can be a problem especially where low interrupting capacity fuses, such as Class H fuses, are installed (Class H fuses have an interrupting rating of only 10 000 A).
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.
OVER CURRENT PROTECTIVE DEVICES FOR GENERATORS BASIC INFORMATION AND TUTORIALS
What are the basic overcurrent protection devices for generators?
As with other motors, NEC 445.11 requires a generator to have a nameplate giving the manufacturer’s name, the rated frequency, power factor, number of AC phases, the subtransient and transient impedances, the rating in kilowatts or kilovolt amperes, a rating for the normal volts and amps, rated revolutions per minute, insulation system class, any rated ambient temperature or temperature rise, and a time rating.
The size and type of OCPD will be based on this critical data. NEC 445.12 defines the basic overcurrent protection standards for various types of generators. A constant-voltage generator must be protected from overloads by either the generator’s inherent design or circuit breakers, fuses, or other forms of overcurrent protection that are considered suitable for the conditions of use.
This is true except for AC generator exciters.
Two-wire, DC generators are allowed to have overcurrent protection in only one conductor if the overcurrent device is triggered by the entire current that is generated other than the current in the shunt field. For this reason, the overcurrent device cannot open the shunt field.
If the two-wire generator operates at 65 V or less and is driven by an individual motor then the overcurrent device protection device needs to kick-in if the generator is delivering up to 150% of its full-load rated current.
When a two-wire DC generator is used in conjunction with balancer sets it accomplishes the neutral points for the three-wire system. This means it requires an overcurrent device that is sized to disconnect the three-wire system if an extreme unbalance occurs in the voltage or current.
For three-wire DC generators, regardless of whether they are compound or shunt wound, one overcurrent device must be installed in each armature lead, and must be connected so that it is activated by the entire current from the armature.
These overcurrent devices need to have either a double-pole, double-coil circuit breaker or a four-pole circuit breaker connected in both the main and equalizer leads, plus two more overcurrent devices, one in each armature lead.
The OCPD must be interlocked so that no single pole can be opened without simultaneously disconnecting both leads of the armature from the system.
The ampacity of the conductors that run from the generator terminals to the first distribution device that contains overcurrent protection cannot be less than 115% of the nameplate current rating for the generator per NEC 445.13.
All generators must be equipped with at least one disconnect that is lockable in the open position that will allow the generator and all of its associated protective devices and controls to be disconnected entirely from the circuits that are supplied by the generator.
As with other motors, NEC 445.11 requires a generator to have a nameplate giving the manufacturer’s name, the rated frequency, power factor, number of AC phases, the subtransient and transient impedances, the rating in kilowatts or kilovolt amperes, a rating for the normal volts and amps, rated revolutions per minute, insulation system class, any rated ambient temperature or temperature rise, and a time rating.
The size and type of OCPD will be based on this critical data. NEC 445.12 defines the basic overcurrent protection standards for various types of generators. A constant-voltage generator must be protected from overloads by either the generator’s inherent design or circuit breakers, fuses, or other forms of overcurrent protection that are considered suitable for the conditions of use.
This is true except for AC generator exciters.
Two-wire, DC generators are allowed to have overcurrent protection in only one conductor if the overcurrent device is triggered by the entire current that is generated other than the current in the shunt field. For this reason, the overcurrent device cannot open the shunt field.
If the two-wire generator operates at 65 V or less and is driven by an individual motor then the overcurrent device protection device needs to kick-in if the generator is delivering up to 150% of its full-load rated current.
When a two-wire DC generator is used in conjunction with balancer sets it accomplishes the neutral points for the three-wire system. This means it requires an overcurrent device that is sized to disconnect the three-wire system if an extreme unbalance occurs in the voltage or current.
For three-wire DC generators, regardless of whether they are compound or shunt wound, one overcurrent device must be installed in each armature lead, and must be connected so that it is activated by the entire current from the armature.
These overcurrent devices need to have either a double-pole, double-coil circuit breaker or a four-pole circuit breaker connected in both the main and equalizer leads, plus two more overcurrent devices, one in each armature lead.
The OCPD must be interlocked so that no single pole can be opened without simultaneously disconnecting both leads of the armature from the system.
The ampacity of the conductors that run from the generator terminals to the first distribution device that contains overcurrent protection cannot be less than 115% of the nameplate current rating for the generator per NEC 445.13.
All generators must be equipped with at least one disconnect that is lockable in the open position that will allow the generator and all of its associated protective devices and controls to be disconnected entirely from the circuits that are supplied by the generator.
ARC FLASH BOUNDARY SAFE DISTANCE BASIC INFORMATION AND TUTORIALS
Arc-flash boundaries need to be established around electrical equipment such as switchboards, panelboards, industrial control panels, motor control centers, and similar equipment if you plan to work on or in the proximity of exposed energized components.
Parts are considered exposed if they are energized and not enclosed, shielded, covered, or otherwise protected from contact. Work on these parts includes activities such as examinations, adjustment, servicing, maintenance, or troubleshooting.
Equipment energized below 240 V does not require arc-flash boundary calculation unless it is powered by a 112.5 KVA transformer or larger.
The arc-flash boundary is the limit at which a person working on energized parts can be standing at the time of an arc-flash without risking permanent injury unless they are wearing flame-resistant clothing. Permanent injury results from an arc-flash that causes an incident energy of 1.2 calories/centimeter2 (cal/cm2) or greater and causes a minimum of second-degree burns.
This distance can only be effectively determined by calculating the destructive potential of an arc.
First you must determine the magnitude of the arc based on the available short circuit current, then estimate how long the arc will last based on the interrupting time of the fuse or circuit breaker.
Finally, you will need to calculate how far away an individual must be to avoid being exposed to an incident energy of 1.2 cal/cm2. It may sound like a lot of math and factoring in of potentials, but believe me the extra time you take to determine the arc flash boundary is well worth your safety and well-being.
Calculating flash protection boundaries for systems over 600 V requires performing a flash hazard analysis coupled with either the NFPA 70E Hazard Risk Category/PPE tables or the Incident Energy Formula.
Additionally, Section 4 of IEEE 1584 Guide for Arc Flash Hazard Calculations states that the results of the arc flash hazard analysis are used to identify the flash-protection boundary and the incident energy at assigned working distances throughout any position or level in the overall electrical system.
The purpose is to establish safe work distances and the PPE required to protect workers from injury. A flash-hazard analysis is comprised of the following three different electrical system studies:
1. A short circuit study
2. A protective device time-current coordination study
3. The flash-hazard analysis and application of the data
Parts are considered exposed if they are energized and not enclosed, shielded, covered, or otherwise protected from contact. Work on these parts includes activities such as examinations, adjustment, servicing, maintenance, or troubleshooting.
Equipment energized below 240 V does not require arc-flash boundary calculation unless it is powered by a 112.5 KVA transformer or larger.
The arc-flash boundary is the limit at which a person working on energized parts can be standing at the time of an arc-flash without risking permanent injury unless they are wearing flame-resistant clothing. Permanent injury results from an arc-flash that causes an incident energy of 1.2 calories/centimeter2 (cal/cm2) or greater and causes a minimum of second-degree burns.
This distance can only be effectively determined by calculating the destructive potential of an arc.
First you must determine the magnitude of the arc based on the available short circuit current, then estimate how long the arc will last based on the interrupting time of the fuse or circuit breaker.
Finally, you will need to calculate how far away an individual must be to avoid being exposed to an incident energy of 1.2 cal/cm2. It may sound like a lot of math and factoring in of potentials, but believe me the extra time you take to determine the arc flash boundary is well worth your safety and well-being.
Calculating flash protection boundaries for systems over 600 V requires performing a flash hazard analysis coupled with either the NFPA 70E Hazard Risk Category/PPE tables or the Incident Energy Formula.
Additionally, Section 4 of IEEE 1584 Guide for Arc Flash Hazard Calculations states that the results of the arc flash hazard analysis are used to identify the flash-protection boundary and the incident energy at assigned working distances throughout any position or level in the overall electrical system.
The purpose is to establish safe work distances and the PPE required to protect workers from injury. A flash-hazard analysis is comprised of the following three different electrical system studies:
1. A short circuit study
2. A protective device time-current coordination study
3. The flash-hazard analysis and application of the data
CARE OF RUBBER GLOVES FOR ELECTRICAL WORKS FOR SAFETY
Issue rubber gloves to each lineman and inform the lineman of the following safety precautions and procedures:
• Store rubber gloves in a canvas bag when they are not in use.
• Give rubber gloves an air test each day before starting work and when encountering an object that may have damaged the gloves.
• Wash gloves with warm water and mild detergent.
Be careful when applying or removing rubber protective equipment. Do not work over energized conductors or equipment or get in a position that may result in unwanted contact with the equipment.
Dry rubber protective equipment before putting it away if possible.
Do not carry rubber protective equipment in compartments on trucks or in tool bags with tools or other equipment.
Roll, never fold, rubber blankets when putting them away.
Place rubber hoods and hoses flat in truck compartments.
• Store rubber gloves in a canvas bag when they are not in use.
• Give rubber gloves an air test each day before starting work and when encountering an object that may have damaged the gloves.
• Wash gloves with warm water and mild detergent.
Be careful when applying or removing rubber protective equipment. Do not work over energized conductors or equipment or get in a position that may result in unwanted contact with the equipment.
Dry rubber protective equipment before putting it away if possible.
Do not carry rubber protective equipment in compartments on trucks or in tool bags with tools or other equipment.
Roll, never fold, rubber blankets when putting them away.
Place rubber hoods and hoses flat in truck compartments.
ARC FLASH HAZARD ANALYSIS BASICS
To perform an arc flash hazard analysis, you need to start by gathering information on the building’s power distribution system. This data should include the arrangement of components on a one-line drawing with nameplate specifications of every device on the system and the types and sizes of cables.
The local utility company should be contacted so that you can get the minimum and maximum fault currents entering the facility.
Next you will want to perform a short circuit analysis and a coordination study. You will need this information to put into the equations provided in NFPA 70E or the IEEE Standard 1584. These equations will give you the flash protection boundary distances and incident energy potentials you will need to determine your minimum PPE requirements.
In many ways an arc fault analysis is actually a study in risk management. You can be very conservative in your analysis and the results will almost always indicate the need for category 4 PPE.
On the other hand, you can perform the analysis and make adjustments to reduce the arc fault conditions resulting in reduced PPE requirements.
However, use caution when adjusting your calculations. Reducing the bolted fault current can reduce the arc fault current, but it can actually result in a worse situation.
For example, if you reduce the current applied to a motor from 4000 to 1800 A, the arc fault energy is increased from 0.6 to 78.8 cal/cm2. This is the exact opposite outcome that you might expect to achieve before doing the math.
Keep in mind that you are risking OSHA violations and fines if you choose nominal compliance. On the other hand, you can actually be increasing the risk of injury if you force workers to unnecessarily wear cumbersome PPE.
This can also result in little or no high voltage maintenance being performed, which will eventually compromise safety and proper equipment operation. It might prove beneficial to get a registered professional engineering firm to perform arc flash hazard calculations on your behalf and have them recommend appropriate actions and the lowest appropriate category of PPE.
The local utility company should be contacted so that you can get the minimum and maximum fault currents entering the facility.
Next you will want to perform a short circuit analysis and a coordination study. You will need this information to put into the equations provided in NFPA 70E or the IEEE Standard 1584. These equations will give you the flash protection boundary distances and incident energy potentials you will need to determine your minimum PPE requirements.
In many ways an arc fault analysis is actually a study in risk management. You can be very conservative in your analysis and the results will almost always indicate the need for category 4 PPE.
On the other hand, you can perform the analysis and make adjustments to reduce the arc fault conditions resulting in reduced PPE requirements.
However, use caution when adjusting your calculations. Reducing the bolted fault current can reduce the arc fault current, but it can actually result in a worse situation.
For example, if you reduce the current applied to a motor from 4000 to 1800 A, the arc fault energy is increased from 0.6 to 78.8 cal/cm2. This is the exact opposite outcome that you might expect to achieve before doing the math.
Keep in mind that you are risking OSHA violations and fines if you choose nominal compliance. On the other hand, you can actually be increasing the risk of injury if you force workers to unnecessarily wear cumbersome PPE.
This can also result in little or no high voltage maintenance being performed, which will eventually compromise safety and proper equipment operation. It might prove beneficial to get a registered professional engineering firm to perform arc flash hazard calculations on your behalf and have them recommend appropriate actions and the lowest appropriate category of PPE.
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