I was teaching an electrical safety class and a question was raised on selective coordination and series rating while we discussed the basics of overcurrent protection. Understanding when and where to apply these two different systems is imperative!

Applying a series-rated system for an emergency power system and not having selective coordination for the emergency power system could be a disaster! I will explain the two systems and where each could and should be used.

Selective coordination

Selective coordination is defined in Article 100 as the “localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the selection and installation of overcurrent protective devices and their ratings or settings for the full range of available overcurrent, from overload to the available fault current, and for the full range of overcurrent protective device opening times associated with those overcurrents.” 

Selectivity occurs when the device immediately ahead of a short circuit, a ground fault or an overload operates without involving the next device ahead of it. It will ensure that the remainder of the electrical system will continue to operate, with only the immediate overcurrent device ahead tripping off
or blowing the fuse.

Selectivity occurs when the device immediately ahead of a short circuit, a ground fault or an overload operates without involving the next device ahead of it.

For example, a fault on a branch circuit would open only the branch circuit breaker and not any device ahead of it. That selectivity will ensure that the remainder of the electrical system will continue to operate, with only the immediate overcurrent device ahead tripping off or blowing the fuse. In other words, the main goal of selective coordination is to isolate the faulted portion of the electrical circuit quickly, while maintaining power to the remainder of the system.

Selective coordination would be used for any extremely important system, such as emergency systems in high-rise buildings [700.32], life safety branch and critical branch in hospitals [517.31(G)], elevators where more than one elevator is supplied from a single feeder [620.62] and other similar critical applications and locations. Anywhere that requires continuity of operation of lighting and life-safety equipment necessary for safe occupant evacuation must have selective coordination. This requirement minimizes the possibility that an overload, short circuit or ground fault in a 20A branch circuit would cause the feeder protective device or the main supplying the branch-circuit panelboard to open. Coordination must be carried through each level of distribution that supplies power to the emergency system.

Series-rated systems

To understand a series-rated system, one must first understand a fully rated system, which is where each overcurrent protective device is completely rated for the available fault current at the device’s line side. Available fault current is defined as the largest amount of current capable of being delivered at a point on the system during a short circuit condition. In other words, if the available fault current is 65,000A—commonly shown as 65 kiloamperes (kA)—then the overcurrent protective device is fully rated at 65 kA.

In a series-rated system, the overcurrent devices will act together to interrupt the fault current. A common example of a series-rated system is where a 50-kA main circuit breaker, a 22-kA feeder circuit breaker and a 10-kA branch circuit breaker will act together to interrupt a series fault condition when the series fault occurs on the load side of the 10-kA circuit breaker. The available fault current is 50,000A (50 kA) on the line side of the main circuit breaker.

As the fault current is sensed, the 50-kA circuit breaker starts to open, creating impedance in the arc between the contacts. This arc at the main circuit breaker will decrease the amount of arc fault current downstream to no more than the 22 kA at the line side of the 22-kA circuit breaker as it starts to open. The arc that occurs in the 22-kA circuit breaker will have enough impedance to reduce the arc value to no more than 10 kA at the branch circuit breaker, thus allowing all the circuit breakers to withstand the amount of fault current without sustaining damage to any circuit breakers and allowing all the breakers to interrupt the appropriate available fault current on their line side.

This means all the circuit breakers will ultimately be open at the end of the fault. This would not be acceptable for emergency circuits where a fault occurs on a branch circuit, resulting in all upstream devices to be open with no other emergency circuits operational. Another example would be in a hospital with power to an intensive care unit where a single fault in a branch circuit could shut down power for the entire hospital.

Next month’s article will expand on series-rated systems.