If a building has 65,000 amperes of fault current available at the service entrance, every circuit protection device must also be rated at 65,000 amperes. Below is the one-line of what this would look like. A system such as this provides excellent equipment protection and is highly reliable.
Service continuity is a little less than a selectively coordinated system, but the initial cost is also less. When compared to a series-combination system, the fully rated system provides the same level of service continuity with a higher initial cost.
Selectively Coordinated Systems:
As with the fully rated system, all circuit breakers are fully rated to interrupt the maximum fault current available at their point of application. The selectively coordinated system maximizes service continuity because only the breaker nearest the fault operates to isolate the faulted circuit.
Each upstream breaker in the power distribution system incorporates short time delay tripping. The upstream breaker must be capable of withstanding the thermal and magnetic stresses delivered by the fault current for the time period required by the breaker nearest the fault to trip.
The selectivity of the system can be based, up to a point, on:
• Magnitude of the fault current providing current selectivity.
• Fault withstand time providing time selectivity.
• Both current and time providing complete selectivity.
The selectively coordinated system is the most costly of the three basic systems. However, it provides the best overall protection of equipment and maximum continuity of service.
Series Rated System: The series rated system states that the main upstream circuit protection device must have an interrupting rating equal to or greater than the available fault current of the system, but downstream devices connected in series can be rated at lower values. Under fault conditions, both the main device and the downstream device would open to clear the fault. Series rated breaker combinations must be tested in series in order to be UL Listed.
In the Work Place
For example, a building with 42,000 amperes of available fault current might have the breaker at the service entrance rated at 42,000 amperes and the additional downstream breakers rated at 18,000 amperes.
Series rated systems are intended for use on systems where the branch circuits are primarily lighting and other resistive loads. If the branch circuits supply motor loads, fully rated or selectively coordinated systems should be used. The major advantage to this system is it allows you to use lower-cost branch breakers. However, because both breakers trip on a major fault, service continuity may not be as high as the other systems.
The three basic systems offer protection of electrical conductors and equipment with equal effectiveness. The initial cost and continuity of service are the varying factors. The decision of which protection system to use should be based on the application variables and needs.
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