Capacitor Discharge
Capacitor discharge is the controlled removal of stored electrical energy from a capacitor or capacitor bank to a safe level before work on or near equipment. It is a required step in hazardous energy control, as capacitors retain charge after power is removed. Discharge involves connecting terminals to a rated discharge path, such as a resistor or grounding stick, to dissipate energy to a non-hazardous voltage, typically ≤50 V, within a defined time, often ≤5 minutes.
On the shop floor, capacitor discharge is a specific step in lockout/tagout (LOTO) for equipment like VFDs, servo drives, power supplies, welders, UPS, and power factor correction banks. The workflow includes identifying capacitor hazards, de-energizing and locking out all sources, executing discharge via bleeder resistors, grounding discharge sticks, or resistor-assisted manual discharge, verifying with a voltmeter that voltage is ≤50 V, and maintaining a safe state by shorting and grounding terminals during work. This ensures compliance with WorkSafeBC and OSHA standards for controlling stored electrical energy.
- Treating 'power off' as 'safe' without explicit capacitor discharge, leading to shocks or arc events when touching allegedly dead circuits.
- Relying solely on internal discharge devices without external short-circuiting and grounding, risking failure or re-charge from induced voltages.
- Improper discharge technique, such as using ad-hoc tools like screwdrivers to short high-energy capacitors, or using under-rated resistors, causing arcing, overheating, or fire.
What makes capacitor discharge a 'hazardous energy control' issue under OSHA / WorkSafeBC?
Capacitors store electrical energy (E = ½CV²) that can deliver hazardous shock or arc current even after supply is disconnected. WorkSafeBC and OSHA require controlling this stored energy as part of hazardous energy control, similar to bleeding hydraulic pressure. Discharge is a required step before equipment is considered safe for contact.
What is a 'safe' discharge endpoint (voltage / energy) for work?
Many standards define safe voltage as ≤50 V at accessible terminals before work. For stored energy >5 J, additional controls like automatic mechanical discharge devices or discharge impedance ≤500 A are required. On the shop floor, drives and power factor correction banks are discharged until terminal voltage is at or below 50 V, confirmed with a properly rated meter.
How long must we wait for automatic discharge, and how is 'time constant' used?
For a capacitor with discharge resistor R, the time constant τ = R × C seconds. After 5τ, voltage decays to <1% of initial. Manufacturers commonly specify 5–30 minutes discharge time. If unknown, OSHA suggests a minimum 15-minute wait. Professional practice uses 5τ as the design benchmark and verifies with a voltmeter.