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February 17, 2010
Bad Contacts Equal Failure


I once heard if said that when a fuse "fails" it actually works...

In most cases, I suspect that would be a true statement...however there are times when a fuse opens when it is suppose to remain close...

The primary reason a fuse opens is because there is too much heat in the circuit and the fuse element melts.  The heat needed to open the fuse can come from an overcurrent such as an overload or short-circuit.  When the fuse opens in this situation, it performs it duty and sacrifices itself for the protection of personnel and equipment.

The heat can also come from a faulty connection.  If the connection is loose, is dirty or has been oxidized, then the flow of current in the circuit can be impeded.  This additional resistance creates additional heat as described by the following equation:


Heat = Resistance x Current²

If there is a problem with the connection, then the heat will increase by the square of the current.

For example, in a normal circuit with little contact resistance operating at 10A, the resulting calculation might be:

Heat = (0.003 Ω) x 10² = 0.30 Watts

If the contacts are loose, dirty, and/or corroded, then the resistance could be as high as 100 times the normal contact resistance.  The resulting calculation is:

Heat = (0.3 Ω) x 10² = 30 Watts

This additional heat is transferred and distributed to the fuseholder, fuse end-cap and fuse element.  The resulting heat then turns into an avalanche effect as heated metal adds even more resistance to the circuit.

Soon, there is just too much heat for the circuit to absorb and the fuse element finally melts...causing a nuisance trip (as no over-current situation ever existed).

It is very important to keep maintain clean and tight contact between the fuse and the fuse clips in order to avert any future problems that will cause you to replace fuses unnecessarily.

Thanks you for your continued support of OptiFuse as we try to save you money by not having to replace fuses needlessly.

Jim Kalb

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