Capacitors & Stored Electrical Energy
A capacitor stores electrical energy like a small, fast battery. Many machines keep a dangerous charge for minutes after the power is turned off and locked out. If you don’t release and verify that stored energy, you can get shocked or burned working on “dead” equipment.
- Electricians
- HVAC
- Maintenance
Photo: variable frequency drive with a warning label showing the required wait time before opening, and a technician measuring DC bus voltage
Where you’ll find them
- Variable frequency drives (VFDs) on motors: large DC bus capacitors at several hundred volts
- HVAC units: start and run capacitors on compressors and fan motors
- Power factor correction capacitor banks in plants and substations
- Power supplies, UPS units and welders
- Microwave ovens, photo flashes, X-ray and medical equipment
- Long cable runs, which can hold a charge after high-voltage testing
Releasing stored energy
- Lock out all sources of power. See Lockout/Tagout.
- Wait the time on the equipment label or in the manual. Many drives require 5 to 15 minutes.
- Measure the voltage across the capacitor or DC bus with a properly rated meter, wearing the PPE for live work. See Testing for Absence of Voltage.
- Discharge any remaining voltage with the maker’s discharge tool or a rated resistor, never by shorting with a screwdriver.
- Short and ground large capacitors and banks as the procedure requires, and leave the grounds in place while working.
- Test again before touching.
Don’t short capacitors with a screwdriver. A large capacitor can release its energy in a bang that vaporizes metal, throws sparks into your eyes and burns your hands. Use a proper discharge tool and wear eye protection.
Rules that apply
- OSHA’s lockout and electrical work practice rules require stored energy to be released before work, including capacitors.
- Capacitors can recharge on their own (dielectric absorption) after a quick discharge, so keep shorting devices on large ones while you work.
- Damaged, bulging or leaking capacitors should be handled with gloves; very old capacitors may contain PCBs.
Checklist
- All power sources locked out
- Label wait time observed
- Voltage measured with a rated meter (live-dead-live)
- Remaining charge discharged with a proper tool
- Grounds or shorting devices applied where required
- Safety glasses and the right gloves on
Standards & references
- 29 CFR 1910.333(b)(2)(ii)(B): Releasing stored electric energy, including capacitors
- 29 CFR 1910.147(d)(5): Stored energy in lockout/tagout
- See also: HVAC Work · Battery Safety
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