Many electrical engineers, electrician and inspectors are under the common misconception that any fuse can be used to protect circuit breakers that have an insufficient AIC rating as long as the fuse has an interrupting rating equal to or higher than the Available Fault Current (AFC). This is incorrect.

Each different fuse type has different trip and let through characteristics. The let through value is the maximum amount of current that the fuse can let through to downstream components being protected before the fuse blows during a fault.

When protecting passive components, sometimes a fuse is selected instead of a circuit breaker simply because it has the right ampere rating and other times a fuse is chosen over a circuit breaker because of the specific current limiting characteristics of the fuse. To protect passive components, the only way to properly select a fuse because of its current limiting abilities is to examine the fuse chart for the specific class of fuse.

Protection of defensive equipment such as other overcurrent devices is different. In a new installation, fuses are commonly used as the fully rated device in a series combination but only when part of a tested combination. Just using the fuse chart to examine let through characteristics of a fuse being used in a series combination doesn’t cut it. The fuse must be part of a tested series combination that was put to the test by an approved testing laboratory.

If the combination of the fuse and lower rated circuit breaker was never actually tested to see how they behave together, then during a fault condition occurring downstream of the lower rated circuit breaker, its anyone’s guess as to which overcurrent device will open first or how fast. The longer it takes for either of the overcurrent devices to trip, the more amount of AFC gets through to the lower rated circuit breaker. At some point, more AFC than the lower rated circuit breakers AIC rating can occur, causing the lower rated breaker to explode.