Injury Caused by High Voltage Capacitor Discharge

May 31, 2005

What Happened?

A campus employee working in an electronics shop was repairing a power supply unit. The cooling fan had not been working properly, causing the unit to overheat. The employee replaced the defective cooling fan and then reached into the open top of the power supply unit to check the airflow from the replacement fan. The employee either made contact with a charged capacitor or was close enough (within 1/4") to allow electricity to arc to his hand causing an electric shock that entered his left hand and exited through his right hand.

Immediately after the incident the employee was transported by a co-worker to the local hospital emergency room and remained overnight for observation. No serious physical injuries were found. The Office of Environment, Health & Safety (EH&S) was notified of the incident the day it happened.

What was the Cause?

The power supply unit receives electricity from a 208 volt (V) wall outlet and converts that voltage to 5 kilovolts (kV) that are stored in the capacitor, which is capable of discharging 200 milliamps (mA) of current. Physical contact or close proximity to the open power supply caused a discharge from the capacitor that resulted in an electric shock. Capacitors can discharge current even when not energized because they hold a charge for some time after the power is turned off.

What corrective action was taken?

During the investigation of the incident, it was noted that there were no written procedures in the shop for the hazards of working with electrical equipment. A Job Safety Analysis (JSA) is a means of providing a written procedure to the personnel performing these operations. A complete JSA would be based in part on applicable principles found in the Energy Isolation: Lock Out/Tag Out program document. This document outlines campus procedures for those working on equipment that could be unexpectedly energized or release stored energy.

As described in the document, work on equipment that cannot be shut down requires supervisor authorization, qualified employees performing the work, an alternate means of protecting the employee, and an EH&S review of the process. Additionally, a complete JSA on this subject includes recognition of hazards, engineering, or other procedural controls that eliminate hazards, and appropriate personal protective equipment or barriers. A JSA for the shop where this incident occurred has been developed.

EH&S offers training on both the Lockout/Tagout Program and the JSA process.

Lessons Learned

Avoid working on energized electrical equipment; only qualified employees are permitted to repair electrical equipment. A review of this Lessons Learned document alone does not qualify you to work on energized electrical equipment. Use lockout/tagout procedures, in accordance with the Fact Sheet mentioned above. If work on energized equipment is absolutely necessary, a knowledgeable supervisor must authorize the work.

In this instance, the fan could have been tested without placing a hand inside the equipment from the outside, using a piece of tissue paper. The capacitor could have been guarded with a non-conductive material. It is important to ensure that capacitors are discharged prior to working on or near them.

  • Capacitors can be discharged with a bleeder (de-energizing) resistor, or by discharging the capacitors with proper equipment.
  • Capacitors should always be tested with a suitable voltmeter and, if necessary, discharged before working on or near them.
  • A loose high voltage capacitor should have a wire connected across the terminals once it has been discharged.
  • If an employee needs to probe, solder, or otherwise touch circuits with power off, discharge (across) large power supply filter capacitors with a 2 W or greater resistor of 100 to 500 ohms/V approximate value (e.g., for a 200 V capacitor, use a 20K to 100K ohm resistor).
  • Use only resistors and insulated wires with ratings for the voltages involved.
  • Protect operator with suitable insulators during the discharge process.
  • Monitor while discharging and/or verify that there is no residual charge on the capacitor with a suitable voltmeter.

The term “bleeder resistor” implies that the resistor is permanently installed inside the equipment to force the capacitor to discharge when the equipment is not powered. If a bleeder resistor is in place, it should not be assumed that the resistor is adequately discharging the capacitor until it has been tested.

Personnel should be trained on JSAs and manufacturer’s manuals that identify the potential hazards as well as the correct procedures for avoiding potential injuries from work performed in the electronics shop.


Power supply left in place following the incident

Capacitor, the source of the discharge