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What is common mode voltage?

All too often customers find their relays failing early in their life times that is often caused by the existence of common mode voltages.  Common mode voltages usually arise from line voltages in the area or nearby a given circuit.  If there is stray capacitance in the line it can become charged to the peak of the line voltage.  If the line voltage is 240 VRMS this translates to potential peaks as high as 400 volts.  Switching this voltage even though the stray capacitance is only, say, in the order of 50 picofarads, it will cause metal transfer on the contacts.  This will eventually cause early life failures.  Better grounding can eliminate common mode voltages.  Reducing the stray capacitance will help.  Also, adding some resistance in series with the contacts will reduce the inrush.  Keep in mind all the damage occurs in the first 50 nanosecond upon contact closure.

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