Last month’s article dealt with series-rated systems, with some information on selective coordination. I believe it provided a good description of series-rated systems, but not a thorough explanation of where they can be found in the National Electrical Code, or where they might be used. This month’s article will dig into that, and go into further detail about selective coordination by looking at specific texts.

Series-rated systems are covered in 240.86(A) or (B), and both (A) and (B) must comply with 240.86(C). The introductory text is as follows: “Where a circuit breaker is used on a circuit having an available fault current higher than the marked interrupting rating by being connected on the load side of an approved overcurrent protective device having a higher rating, the circuit breaker must meet the requirements specified in 240.86(A) or (B), and (C).”

Series-rated combination devices

Section 240.86(A) applies to engineered system series-rated combination devices. These devices “must be selected by a licensed professional engineer engaged primarily in the design or maintenance of electrical installations. The selection must be documented and stamped by the professional engineer. This documentation must be available to those authorized to design, install, inspect, maintain, and operate the system. This series combination rating, including identification of the upstream device, must be field marked on the end use equipment.” It must specifically identify it as a specialized system.

Section 240.86(A) continues: “For calculated applications, the engineer must ensure that the downstream circuit breaker(s) that are part of the series combination remain passive during the interruption period of the line side fully rated, current-limiting device.” In other words, in an engineered system, the upstream device must operate before the downstream device for any fault that exceeds the interrupting rating of the downstream device in the series circuit.

Section 240.86(B) pertains to tested combination devices that can be applied in any new or existing installations. This means that these devices have been tested and listed by the manufacturer and a testing laboratory so that the upstream device or devices will open before the downstream device fault current is exceeded.

However, 240.86(C) does not permit 240.86(A) or (B) installations where motors are connected between the line-side (protecting device) and the load-side (protected) circuit breaker, or where the sum of motor full-load currents exceed 1% of the interrupting rating of the lower-rated circuit breaker. In a fault condition where there is a motor load, the motor may provide additional fault current that could exceed the rating of the series-rated system.

For example, where a 22,000A series-rated circuit breaker is ahead of a 10,000A downstream series-rated device with motor contribution exceeding 100A (1% of 10,000A), there could be major issues with circuit breakers. Some power companies are now delivering 22,000A of fault current to single-family dwelling services of 100A or more. These homes could have panelboards installed that are series-rated at 22,000A at the main with branch circuit breakers series-rated at 10,000A.

Selective coordination

Last month’s article stated that 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, ground fault or an overload operates without involving the next device ahead of that overcurrent device. Two sections in Article 240 provide general requirements for selective coordination. Section 240.11 states: “If one or more feeder overcurrent protective devices are required to be selectively coordinated with a service overcurrent protective device by other requirements in this Code, all feeder overcurrent protective devices supplied directly by the service overcurrent protective device must be selectively coordinated with the service overcurrent protective device.”

240.12 covers orderly shutdown and states that where an orderly shutdown is required to minimize the hazard(s) to personnel or equipment, a system of coordination is permitted based on the following conditions: that there is coordinated short-circuit protection and that there is overload indication based on monitoring systems or devices.

The purpose of selective coordination is to ensure that only the device immediately ahead of the fault will open, thereby ensuring that all other circuits will continue to operate. This selective coordination is very important to the design for any critical loads, but especially when nonselective loss of power would be an issue.