When an electrical distribution system is designed and constructed, a fault-current coordination study should be conducted, and circuit protective devices should be sized and set according to the results of the study. In time, however, the electrical system configurations are often changed due to the changing needs of the end users.
If the coordination and capability of the electrical equipment are not reviewed at the time of the changes, faults could result in unnecessary tripping of a main breaker or, even worse, an explosion of equipment that was thought to be in good condition.
When system conditions change, the results that were obtained in the original fault-current coordination study may no longer apply to the current system. Unnecessary tripping, known as lack of selectivity, could be caused by poor coordination.
An equipment explosion could result from the interrupting capability of the circuit breaker being exceeded. Both indicate a clear need for an updated fault-current coordination study.
Utility systems delivering higher fault currents
The demand for electricity, particularly in the industrial and commercial environment, has been steadily increasing. Consequently, utility systems have grown much larger and have become capable of delivering much higher fault-currents at service points than in the past.
Therefore, protective devices that were properly applied at the time they were installed may have become inadequate after system changes, and the protective system may no longer be coordinated. When available fault current increases to the point at which it exceeds protective device interrupting and withstand ratings, violent failure is possible, regardless of how well the devices are maintained.
Protection in an electrical distribution system
System and equipment protective devices are a form of insurance. This insurance pays nothing as long as there is no fault or other emergency.
When a fault occurs, however, properly applied protective devices reduce the extent and duration of the interruption, thereby reducing the exposure to personal injury and property damage. If, however, the protective system does not match system needs, just as an insurance policy should keep up with inflation, it is no help at all.
It is the responsibility of the system operator to ensure proper system protection and coordination.
Protective equipment set to sense and remove short circuits
In medium-voltage systems, the protective equipment for feeder conductors is often set to sense and remove short circuits, but not necessarily to provide overload protection of circuits. Device settings sometimes are purposely chosen low enough to sense and provide a degree of overload protection.
Operators should be aware of this so that a protective device that is set lower than necessary for coordination does not cause a false tripping action during system switching procedures. System protection coordination is an important consideration in switching systems with loop feeds and alternate sources.
To avoid false tripping action, operators should be aware of the settings and any probable temporary overloads or circulating currents during switching.
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.
NEC FLAMMABLE CONDITIONS BASIC INFORMATION AND TUTORIALS
The National Electrical Code addresses hazardous conditions that create the potential for fires to occur. Environments that pose fire or combustion hazards are listed in Articles 500-510. Requirements covering specific types of facilities that pose additional hazards, such as bulk storage plants or motor fuel dispensing locations, are explained in Articles 511-516.
NEC Section (C)(2)(1) describes Class II, Division 2 locations classifications. These are listed as:
1. Locations where some combustible dust is normally in the air but where abnormal operations may increase the suspended dust to ignitable or explosive levels.
2. Locations where combustible dust accumulations are normally not concentrated enough to interfere with the operation of electrical equipment unless an “infrequent equipment malfunction” occurs that increases the level of dust suspended in the air.
3. Locations where combustible dust concentrations in or on electrical equipment may be sufficient to limit heat dissipation or that could be ignited by failure or abnormal operation of electrical equipment.
A variety of airborne environmental conditions that require classification are listed in Article 500. Class I covers locations specified in Sections [500.5(B)(1)] and [500.5 (B)(2)] where flammable gases or vapors are present, or could exist in the air in high enough quantities that they could produce explosive or ignitable mixtures. Section [500.5(B)(1) FPN 1] provides examples of locations usually included in Class I as the following:
1. Where volatile flammable liquids or liquefied flammable gases are transferred from one container to another.
2. Interiors of spray booths and areas in the vicinity of spraying and painting operations where volatile flammable solvents are used.
3. Locations containing open tanks or vats of volatile flammable liquids.
4. Drying rooms or compartments for the evaporation of flammable solvents.
5. Locations with fat and oil extraction equipment that uses volatile flammable solvents.
6. Portions of cleaning and dyeing plants where flammable liquids are used.
7. Gas generator rooms and other portions of gas manufacturing plants where flammable gas may escape.
8. Pump rooms for flammable gas or for volatile flammable liquids that are not adequately ventilated.
9. The interiors of refrigerators and freezers where flammable materials are stored in open or easily ruptured containers.
10. All other locations where ignitable concentrations of flammable vapors or gases are likely to occur in the course of normal operations.
NEC Section (C)(2)(1) describes Class II, Division 2 locations classifications. These are listed as:
1. Locations where some combustible dust is normally in the air but where abnormal operations may increase the suspended dust to ignitable or explosive levels.
2. Locations where combustible dust accumulations are normally not concentrated enough to interfere with the operation of electrical equipment unless an “infrequent equipment malfunction” occurs that increases the level of dust suspended in the air.
3. Locations where combustible dust concentrations in or on electrical equipment may be sufficient to limit heat dissipation or that could be ignited by failure or abnormal operation of electrical equipment.
A variety of airborne environmental conditions that require classification are listed in Article 500. Class I covers locations specified in Sections [500.5(B)(1)] and [500.5 (B)(2)] where flammable gases or vapors are present, or could exist in the air in high enough quantities that they could produce explosive or ignitable mixtures. Section [500.5(B)(1) FPN 1] provides examples of locations usually included in Class I as the following:
1. Where volatile flammable liquids or liquefied flammable gases are transferred from one container to another.
2. Interiors of spray booths and areas in the vicinity of spraying and painting operations where volatile flammable solvents are used.
3. Locations containing open tanks or vats of volatile flammable liquids.
4. Drying rooms or compartments for the evaporation of flammable solvents.
5. Locations with fat and oil extraction equipment that uses volatile flammable solvents.
6. Portions of cleaning and dyeing plants where flammable liquids are used.
7. Gas generator rooms and other portions of gas manufacturing plants where flammable gas may escape.
8. Pump rooms for flammable gas or for volatile flammable liquids that are not adequately ventilated.
9. The interiors of refrigerators and freezers where flammable materials are stored in open or easily ruptured containers.
10. All other locations where ignitable concentrations of flammable vapors or gases are likely to occur in the course of normal operations.
TYPES OF SAFETY LIFELINES FOR CONSTRUCTION AND WORKS BASIC INFORMATION AND TUTORIALS
There are three basic types of lifelines:
1) vertical
2) horizontal
3) retractable
All lifelines must be inspected daily to ensure that they are
- free of cuts, burns, frayed strands, abrasions, and other defects or signs of damage
- free of discolouration and brittleness indicating heat or chemical exposure.
1) Vertical Lifelines
Vertical lifelines must comply with the current edition of the applicable CSA standard and the following minimum requirements:
- Only one person at a time may use a vertical lifeline.
- A vertical lifeline must reach the ground or a level above ground where the worker can safely exit.
- A vertical lifeline must have a positive stop to prevent the rope grab from running off the end of the lifeline.
Vertical lifelines are typically 16-millimetre (5/8-inch) synthetic rope (polypropylene blends).
2) Horizontal Lifelines
The following requirements apply to any horizontal lifeline system:
- The system must be designed by a professional engineer according to good engineering practice.
- The design can be a standard design or specifically engineered for the site.
The design for a horizontal lifeline system must
- clearly indicate how the system is to be arranged, including how and where it is to be anchored
- list and specify all required components
- clearly state the number of workers that can safely be attached to the lifeline at one time
- spell out instructions for installation, inspection, and maintenance
- specify all of the design loads used to design the system.
The system must be installed, inspected, and maintained in accordance with the professional engineer’s design. Before each use, the system must be inspected by a professional engineer or competent worker designated by a supervisor. A complete and current copy of the design must be kept on site as long as the system is in use.
3) Retractable Lifelines
Retractable lifelines consist of a lifeline spooled on a retracting device attached to adequate anchorage. Retractable lifelines must comply with CAN/CSAZ259.2.2- M98.
In general, retractable lifelines
- are usually designed to be anchored above the wo rker
- employ a locking mechanism that lets line unwind off the drum under the slight tension caused by a user’s normal movements
- automatically retract when tension is removed, thereby preventing slack in the line
- lock up when a quick movement, such as that caused by a fall, is applied
- are designed to minimize fall distance and the forces exerted on a worker’s body by fall arrest.
Always refer to the manufacturer’s instructions regarding use, including whether a shock absorber is recommended with the system.
Any retractable lifeline involved in a fall arrest must be removed from service until the manufacturer or a qualified testing company has certified it for reuse.
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