How Many Fuses Will Open on a Short Circuit?

By Mersen

In electronics and electrical engineering, a fuse is an electrical safety device that operates to provide over-current protection of an electrical circuit. Its essential component is a metal wire or strip that melts when too much current flows through it, thereby stopping or interrupting the current. It is a sacrificial device; once a fuse has operated it is an open circuit, it must be replaced or rewired, depending on type.

In a three phase (3-phase) system typically only two fuses will open on a line-to-line short circuit. Since all three line currents are offset from each other (see chart below), each fuse will see the full fault at different times. Therefore the fuses will open at different times, once the first two fuses open, the circuit is disconnected and the third one typically never sees the full fault current. The third line can only conduct
current directly to ground.

Three-Phase Line Currents


How many fuses will open on an overload?


Similar to a short circuit typically two fuses will open on an overload. Typically, one fuse opening will not be adequate to disconnect all three phases so the two remaining phases will conduct the overcurrent until one of them opens. At this point, the last fuse will only be able to conduct current directly to ground so it most likely will not open.


Is It OK to replace only the open fuses?


It is always recommended to replace all three fuses. In both short circuit and overload conditions the third fuse might not open but there is no way to tell how much of the element may have melted due to the overcurrent. Not replacing the third fuse can lead to issues in the future such as nuisance openings which can result in costly downtime.

Mersen Fuses


Is there a life expectancy on my fuse?

A fuse does not have a “mean time between failures” because theoretically a fuse only needs to be replaced once it opens on an overcurrent. Fuses are 100% tested before leaving the factory to ensure that they will perform as intended. In the real world,
factors such as temperature and humidity can cause a fuse to need replacement. Mersen suggests using ten years as a guideline for replacing both fuses installed and in inventory.

Mersen fuses are 100% tested before leaving the factory to ensure they
will perform as intended.

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