Circuit Breaker Safety

[fraymond]

Knowing that anything is possible can an arc flash occur on the line side of a breaker with the breaker open? If not could the line side of the CB be protected to prevent shock hazard to allow work? We have been installing disconnects between the power distribution panel and the machines circuit breaker. Was wondering if an another avenue is available...

 

[big john]

I've seen faults that originated on the breaker lineside as the result of equipment failure.

And if you are in a panel with a feeder energized up to the line side, your PPE needs to be based on the incident energy available from that feeder.

IMO totally enclosing the breaker lineside and conductors with a substantial, permanent guard would render that panel safe to work in. That said, an upsteam disconnect is definitely the safest option.

 

[kwired]

Knowing that anything is possible can an arc flash occur on the line side of a breaker with the breaker open? If not could the line side of the CB be protected to prevent shock hazard to allow work? We have been installing disconnects between the power distribution panel and the machines circuit breaker. Was wondering if an another avenue is available...

Conductive airborne deposits would be sort of rare but not impossible to have a seemingly sudden arc flash incident because of their presence.

 

[Jraef]

The most serious arc flash I have ever been exposed to happened on the line side of an open disconnect switch. The 500MCM cables were pulled, terminated, then tested. But the soft starter needed to be removed and replaced because the motor came in with a higher HP than ordered, which required removing the back panel and the cables had to be pulled back into the conduit to do that. When I got there to commission it, the cables had been pushed back out of the conduit from the other end and reterminated, but apparently not retested. As I was closing the cabinet, the electrician in the switchgear room closed the feeder breaker and one of the cables went to ground, likely because the insulation was nicked. It vaporized and there was a huge flashover and explosion. Luckily I had at least already engaged the door interlock for the disconnect and it held enough to direct the blast up and down instead of into my face. The heat burned my eyebrows off and the bottom flame torched my shoelaces, but nothing worse.

Also lucky for me I had just used the outhouse so I didn't pee my pants...