Capacitor Discharge: The Hazard That Remains After Unplugging

Equipment with significant capacitance in the primary circuit, typically an EMC filter or a large reservoir capacitor, can retain a hazardous voltage after the supply is disconnected. The requirement is that the voltage falls below a defined level within a defined time, or that the user is warned and protected.

The numbers

The usual pattern is a discharge to a safe level within a few seconds of disconnection. The exact time and level depend on the standard and on the accessibility of the capacitor: a capacitor behind a cover that requires a tool may be treated differently from one behind a removable panel. What matters is that the requirement is stated for the specific construction and verified by measurement.

How it fails

A bleeder resistor that is present in the design but omitted in production. A bleeder sized for the nominal voltage that is too small to discharge a capacitor charged to the peak of the tolerance range. A discharge path that depends on a relay being in its normal position, which is not the position it is in when the fault occurs. All three are construction or component issues that a design review cannot see.

Verification

Measure the voltage at the accessible points at the specified time after disconnection, at the maximum rated supply voltage, with the equipment in the state it would be in after normal use. Record the decay curve, because a single point measurement at the limit time hides a capacitor that is discharging through an unintended path and will change behaviour as components age.

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