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What would cause the trip unit to make a humming sound with only the 24 Vdc applied to the trip unit (bus not energized)?

A trip unit humming with only DC power to the trip unit would indicate that the trip unit is detecting a fault condition and sending a signal to the mitop. It is the energizing of the mitop that is making the humming sound.
The display would always be blank because the mitop is being energized (the trip unit keeps send momentary pulses to the mitop). This sounds like a trip unit not seated properly, no T1/T2 jumper installed or a bent pin between trip unit and performer plug.

CTA-ID : 2035340
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