Engineering Tutorials

The primary goal of all electrical power distribution systems is to provide power to electrical equipment with the utmost safety. System protection is designed to add the remaining goals of equipment/conductor protection and service continuity at the most reasonable cost.

Protective equipment, such as molded case circuit breakers, during Overcurrent conditions, must quickly isolate the affected section of the power system to maintain service to other sections. They must also minimize equipment damage and limit the extent or duration of outages. We will first discuss what overcurrent conditions are and then talk about system coordination.

Overcurrent Conditions

Overcurrent: This is a current that is higher than the amount of current a conductor or piece of equipment can carry safely. An overcurrent condition left unchecked can cause insulation and/or equipment damage as a result of excessive temperature and/or dynamic stresses. The cable insulation is the most vulnerable to overcurrent conditions. The conductor itself may be able to withstand extremely high heat, but the insulation around the conductor cannot.

There are three types of overcurrent conditions:

Overloads are the result of placing excessive loads on a circuit, beyond the level the circuit was designed to handle safely. Insulation deterioration in electrical conductors is most often the result of such overload conditions. When an overload condition exists, a temperature buildup occurs between the insulation and the conductor.

Short Circuits: Short Circuits, frequently called Faults, are usually caused by abnormally high currents that flow when insulation on a conductor fails. When the insulation that protects one phase from another or one phase from ground breaks down, short circuit currents can be expected to flow. The short circuit condition must be eliminated quickly to protect against damage to the system.

A large dam is built and feeds a controlled amount of water into a small river. Downstream, a small town is built along the river's banks. The amount of water permitted to enter the river safely is independent of the amount of water behind the dam. Should the dam break and suddenly release the water behind it, the town could be severely damaged or even washed away. This sudden rush of water is like the flow of current in a circuit under fault conditions. The amount of damage done depends on the amount of water stored behind the dam or the amount of current available to feed the fault.

Ground Faults: A Ground Fault is a particular type of short circuit. It is a short circuit between one of the phases and ground. It is probably the most common low level fault experienced, especially on lower voltage circuits.

Ground fault currents are often not large in magnitude and can go undetected for a period of time. This type of fault might occur in the electrical outlets located in the bathroom or in other areas where water could be present.

Because of the overcurrent conditions that can occur in distribution systems, thought has to be put into properly coordinating that system. There are three types of system coordinations: Fully Rated, Selectively Coordinated and Series Rated.

Fully Rated Systems: In a fully rated system, all circuit breakers are rated to operate independently. They all have an Interrupting Rating adequate for the maximum Fault Current available at their point of application. All of the breakers are equipped with long time delay and instantaneous overcurrent trip elements.