POWER SYSTEM PROTECTION COORDINATION BASICS

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.

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.