Engineering and Consulting Services

Coordination Study

After a Short Circuit Study has indicated the magnitudes of short-circuit current that would attend a failure on the system, the next appropriate step is to perform a Coordination Study. There are two objectives that can be achieved with a coordination study.

Selectivity
The most generally recognized is to determine the correct selection, ratings, and adjustments of protective devices so that if an overload or short-circuit were to occur, the protective device nearest the problem would operate, leaving the rest of the power system undisturbed. This is also known as "selectivity", when a minimum portion of the system is affected. Without selectivity, large and important portions of the system would unnecessarily suffer a loss of electric power. The least impact of this occurrence would be an inconvenience to users of the system. The greater impacts might be the loss of power to vital operations such as computer systems, elevators, air conditioning, refrigeration or manufacturing processes where a delay can cause a product or equipment to be damaged.

Protection
A second objective of a coordination study can be to minimize damage to the electrical system or building if an arcing short-circuit were to occur. Designers, because of a general unawareness of the way in which a protective device can be adjusted to minimize damage, often do not recognize this benefit.

If the energy in an arcing short-circuit is limited to less than 10,000 kilowatt-cycles, (on a 480 volt system) the arc will not burn through the steel enclosure. A graphical method of determining the "go" or "no-go" limits of arc energy has been developed and is used to assure that ground fault and circuit breaker settings are correct for selectivity and also limit the arc energy to less than 10,000 kilowatt-cycles. Obviously, it is necessary to first perform a short-circuit study in order to know the magnitude of short-circuit currents to consider in a coordination study. For this reason, it is usually advantageous to perform both types of studies together; a Short Circuit and Coordination Study.