A 20-ampere circuit breaker with an interrupting rating of 10,000 amperes can be installed at a point with available fault current of 65,000 amperes only if it is series rated with a lineside device that has a series rating of at least what value?

A 20-ampere circuit breaker with an interrupting rating of 10,000 amperes can be installed at a point with available fault current of 65,000 amperes only if it is series rated with a lineside device that has a series rating of at least what value?

• A. 10,000
• B. 32,000
• C. 65,000
• D. 100,000

Answer: (C)

When you want to use a circuit breaker with a lower interrupting rating in a system where a high fault current might occur, you can do so only if the upstream lineside device has a series rating that can handle that fault level. The series rating represents the maximum short-circuit current the combination can endure without damaging the devices, effectively allowing the downstream breaker to operate safely at higher fault currents than its own rating would suggest. In this scenario, the available fault current is 65,000 amperes. To use the 20-amp breaker in series with a lineside device, that lineside device must have a series rating of at least 65,000 amperes. Therefore, the minimum required value is 65,000 A. Choosing a smaller value would not provide enough protection, while a larger value would still work but isn’t necessary for meeting the requirement.

When you want to use a circuit breaker with a lower interrupting rating in a system where a high fault current might occur, you can do so only if the upstream lineside device has a series rating that can handle that fault level. The series rating represents the maximum short-circuit current the combination can endure without damaging the devices, effectively allowing the downstream breaker to operate safely at higher fault currents than its own rating would suggest.

In this scenario, the available fault current is 65,000 amperes. To use the 20-amp breaker in series with a lineside device, that lineside device must have a series rating of at least 65,000 amperes. Therefore, the minimum required value is 65,000 A. Choosing a smaller value would not provide enough protection, while a larger value would still work but isn’t necessary for meeting the requirement.