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Arc flash incident, bad racking?
There are many reasons causes arc flash incidents. Some of arc flash explosions occur when substation personnel insert or remove (rack in or out) circuit breakers from low or high voltage switchgear cubicles. Manual racking in and racking out circuit breakers poses potentially lethal arc flash exposure.
Assumptions and complacency are two of electrical safety‘s worst enemies. If the following story hits uncomfortably close to home for you, it will have served its purpose.
In January 1993, two employees were killed and three were seriously injured by an electrical arc flash at a Texas power station.
The dead employee (not confirmed) had racked-in a circuit breaker and sent the close command, but the breaker failed to close. With the breaker indicator showing that it was still open, the operator began racking-out the breaker to troubleshoot the problem with his supervisor looking on. Neither knew that the breaker had received the close command. The breaker had unlatched and begun to close, but the mechanical problem prevented it from completing the operation.
As a consequence, the operator attempted to rack out the circuit breaker even as it was receiving a close command. When he was finally able to move the breaker, the mechanical bind was relieved and the breaker closed while it was partially racked-out. The result was an electrical arc flash and explosion that critically burned the operator and the supervisor and slammed them into the wall. Both were killed.
The arc flash then travelled around a corner and burned three other workers. All of this carnage occurred within milliseconds.
Download PDF guides on arch flash protection and assesment (from cablejoints.co.uk):
- Arc Flash Risk Asssessment – DuPont Arc Guide
- Arc Flash Protection – LV – HV Switching Suits for Racking In & Out
- CATU Electrical Safety Equipment for Low & High Voltages
Reference: Ben Johnson and Jack Wells – Steering Committee Cochairs – IEEE/NFPA Collaboration on Arc Flash Occurence, Feb 2007