Keep the Fireworks Out of the Fusebox

Fuses are one of the simpler devices that we work with, yet techs do not really know some important fuse specifications. What you don’t know could hurt you if you use the wrong type of fuse as a replacement. Fuses have five important ratings: voltage, amps, interrupting, one-time or time delay, and finally whether or not it is current limiting.  Most techs are familiar with the volt rating and the amp rating, but a lot are unfamiliar with the interrupting rating.

Although all fuses are designed to open when the current exceeds their amp rating, this does not happen instantly. In the case of a dead short, the fuse will be subjected to a much higher level of current for a fraction of a second. For a fraction of a second, the fuse can be exposed to 100,000 amps, causing it to explode like a bomb! The amount of current the fuse can withstand and not explode like a bomb is called the interrupting rating, listed as IR on the fuse body. Inexpensive fuses have an interrupting rating of 10,000 amps – which is comparable to most breakers. Better quality fuses have much higher interrupting ratings – such as 200,000 amps.

Two fuses can have the same volt and amp rating but have vastly different interrupting ratings. If you replace a fuse which has a 200,000 amp interrupting rating with one which has only a 10,000 amp interrupting rating, you are creating a bomb. If you take look at the two fuses pictured here, you can see that they are both 600 volt, 30 amp fuses and are physically interchangeable. However, the one on the top has a 200,000 interrupting rating but the interrupting rating of the fuse on the bottom is only 50,000. They are not functionally interchangeable.

Further, the fuse on the top is designed to be a current limiting fuse. This means that it limits the amount of current that can pass downstream of the fuse. I can hear you saying “all fuses are current limiting.” Not really. During that fraction of a second after a direct short, thousands of amps pass downstream through the fuse. A current limiting fuse limits this spike. Typically, the spike is limited to 10,000 amps. The importance of this is that it can reduce or even prevent an arc flash from happening in the equipment down-stream of the fuse. This is why industrial and commercial services usually protect their service panels with current limiting fuses which have a high interrupting rating. Keep the fireworks out of the fusebox. Always replace fuses with ones that meet ALL the fuse specifications, not just the volt and amp ratings.