A common question that I often hear in my NFPA 70E and IEEE 1584 Arc Flash Studies training courses is:
“Can simply operating a device such as a fusible switch or circuit breaker cause an arc flash?”
I respond to the question with a very specific answer which is:
Learning the Hard Way
A good friend of mine here in Arizona has a client that found the answer the hard way. An electrical contractor was performing electrical work at their facility. The work involved creating an electrically safe work condition at the 277Y/480 Volt main service switchboard following NFPA 70E 120.5 Process of Establishing and Verifying an Electrically Safe Work Condition.
The main service switchboard contained four separate mains as permitted by NEC 230.71 – commonly referred to as the Six Disconnect Rule. One of the mains was a 1200 Amp bolted pressure switch with 1200 Amp fuses that fed a downstream distribution switchboard in another room. As required by NEC 240.95, ground fault protection was also provided on the main since the disconnect exceed 1000 amps and was a solidly grounded 277Y/480V system.
The work began by interrupting the load by opening each of the smaller fusible disconnects at the downstream switchboard. Once the load was interrupted, the 1200 Amp main was opened along with the three other mains. However, the line side of the mains in the switchboard remained energized – and still hazardous.
To completely de-energize the switchboard, Continue reading