POWER SYSTEM PROTECTION COORDINATION AND SAFETY ENGINEERING

THE IMPORTANCE OF SYSTEM PROTECTION COORDINATION WITH REGARDS TO PERSONNEL SAFETY

How system protection coordination serve its safety purpose?



System protection coordination
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.

ELECTRICAL SAFETY HAZARDS THAT NEEDS TO BE IDENTIFIED ON ELECTRICAL SAFETY WORKS

WHAT ARE THE DIFFERENT ELECTRICAL SAFETY HAZARDS?

Identifying electrical safety hazards.



When electrical systems break down what are the primary hazards and what are the consequences to personnel? Electric shock Exposure to Arc-Flash Exposure to Arc-Blast Exposure to excessive light and sound energies

Secondary hazards may include burns, the release of toxic gases, molten metal, airborne debris and shrapnel. Unexpected events can cause startled workers to lose their balance and fall from ladders or jerk their muscles possibly causing whiplash or other injuries.

Electric Shock
When personnel come in contact with energized conductors they receive a shock with current flowing through their skin, muscles and vital organs. The severity of the shock depends on the current’s path through the body, the current intensity, and the duration of the contact.

They may only experience a mild tingling sensation or it could result in serious injury or death. As voltage levels increase, the effects of electric shock escalate. Current may also cause an erratic heartbeat known as ventricular fibrillation.

If fibrillation occurs even briefly and goes untreated, the effects are usually fatal. A clear understanding of how electric current travels through the body can help minimize injury if such contact occurs. The table below outlines the effects that various values of electrical current have on the human body.

There are three basic pathways electric current travels through the body;

1) Touch Potential (hand/hand path)

2) Step Potential (foot/foot path)

3) Touch/Step Potential (hand/foot path)



1) In a touch potential contact, current travels from one hand through the heart and out through the other hand. Because the heart and lungs are in the path of current, ventricular fibrillation, difficulty in breathing, unconsciousness, or death may occur.

2) In a step potential contact, current travels from one foot through the legs, and out of the other foot. The heart is not in the direct path of current but the leg muscles may contract, causing the victim to collapse or be momentarily paralyzed.

3) In a touch/step potential contact, current travels from one hand, through the heart, down the leg, and out of the foot. The heart and lungs are in the direct path of current so ventricular fibrillation, difficulty in breathing, collapse, unconsciousness, or death may occur.

Even though there may be no external signs from the electrical shock, internal tissue or organ damage may have occurred. Signs of internal damage may not surface immediately; and when it does, it may be too late.

Any person experiencing any kind of electrical shock should seek immediate medical attention. Using the correct personal protective equipment (PPE) and following safe work practices will minimize risk of electrical shock hazards.

Arc-Flash and Arc Blasts 

An Arc-Flash is an unexpected sudden release of heat and light energy produced by electricity traveling through air, usually caused by accidental contact between live conductors. Temperatures at the arc terminals can reach or exceed 35,000 degrees Fahrenheit (F), or four times the temperature of the sun’s surface.

The air and gases surrounding the arc are instantly heated and the conductors are vaporized causing a pressure wave called an Arc Blast. Personnel directly exposed to an Arc-Flash and Arc-Blast events are subject to third degree burns, possible blindness, shock, blast effects and hearing loss. Even relatively small arcs can cause severe injury.

The secondary effect of arcs includes toxic gases, airborne debris, and potential damage to electrical equipment, enclosures and raceways. The high temperatures of the arc and the molten and vaporized metals quickly ignite any flammable materials.

While these fires may cause extensive property damage and loss of production, the hazards to personnel are even greater. Any energized electrical conductor that makes accidental contact with another conductor or with ground will produce an Arc-Flash.

The arcing current will continue to flow until the overcurrent protective device used upstream opens the circuit or until something else causes the current to stop flowing. The arc current can vary up to the maximum available bolted fault current

CIRCUIT PROTECTION CHECKLIST ESSENTIAL FOR ELECTRICAL SAFETY

THE IMPORTANCE OF HAVING A CIRCUIT PROTECTION CHECKLIST IN ELECTRICAL SAFETY

Before a system is designed or when unexpected events may occur, circuit designers should ask themselves the following questions:

What is the normal or average current expected?

What is the maximum continuous (three hours or more) current expected?

What inrush or temporary surge currents can be expected?

Are the overcurrent protective devices able to distinguish between expected inrush and surge currents and open under sustained overloads and fault conditions?

What kind of environmental extremes are possible? Dust, humidity, temperature extremes and other factors need to be considered.

What is the maximum available fault current the protective device may have to interrupt? Is the overcurrent protective device rated for the system voltage?

Will the overcurrent protective device provide the safest and most reliable protection for the specific equipment?

Under short-circuit conditions, will the overcurrent protective device minimize the possibility of a fire or explosion?

Does the overcurrent protective device meet all the applicable safety standards and installation requirements?

Answers to these questions and other criteria will help to determine the type of overcurrent protective device to use for optimum safety and reliability.