Reasons not to use wirewound resistors beyond their ratings

At first glance, the readily available wirewound power resistor appears to be a good solution to inexpensive high voltage resistor requirements. The breakdown voltage is typically several kilovolts per inch of length, so a 100 watt resistor 6 inches long will probably work at 20 kV, particularly for an application where the substantial inductance isn't a problem. However, in systems where there is a lot of stored energy, the failure mode for these resistors can cause catastrophic damage to other components.

A wirewound resistor is made by winding a very fine resistance element (e.g. Nichrome) on a ceramic form. If a small open develops (the typical failure mode, either due to a physical impact deforming or severing the wire, or due to excessive power disspation) an arc will form at the gap, melting the wire back, making the arc bigger. This is a continuing process that can be quite spectacular.


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