Arc Flash Hazard

If the circuit is loaded, breaking of the current path causes the arc flash.

With no-load (no current) the interruption under energized conditions cannot produce arc flash.

Do realize that energized long transmission lines even with no load at the far end draw a considerable amount of charging current hence there would be an arc flash on circuit interruption.

Rather than depending on procedures, make sure that the interlocks between disconnect switch and the circuit breakers is reliably functioning.

I find it hard to believe that someone would design a system with three load break switches in a row. Either that or when the FS₆ became faulty it should have been removed when the new swithes were put in.

Even though you say they are metal clad around each phase I I would still consider them to be non load break switches, especially since they are hand crank design. I would be getting the FS₆ breakers back into service for safety reasons.

See NFPA 70-E

Mark684,

The post #1 is important. The NLB switches on either side of the SF₆ breakers are strictly for use as isolators, to allow safe work on the cubicle in which the SF₆ sits. They are suitable for operation while energized, but not while carrying a load current. You should have interlock keying that physically prevents their operation unless the SF₆ breaker is open.

To safely interrupt a load current, they must 1) have blades that open swiftly via a spring and toggle mechanism, 2) have arcing contacts that are separate from the main contacts and open after the main contacts have opened, so the main contacts are not damaged by the arc, and 3) have a means of quenching the arc that is produced, such as arc quenching chutes and magnetic blast coils. Your description implies that none of these essential load-break features are present. If any one of them develops an arc across the opening contacts, it can quickly transfer itself into an arc from the energized contact to the nearby metal enclosure wall, and thus become a ground fault arc that can be extremely damaging to the equipment and a risk of harm to anyone nearby.

At one job site, I regularly work around 12.5kV switchgear. Some of the transformers have line-side isolation switches rated 600A continuous and 100A breaking. They are interlock keyed so they can only be opened when the secondary disconnect of the transformer is opened (so they are interrupting the magnetizing current only). I suspect these switches have a more robust rating than the ones you describe.

--JMM