Disconnect Interlock Bypass

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Brandon Loyd

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On your standard 208-480VAC, 3 phase, fusible or non-fusible disconnect, you often find an bypass screw/switch to defeat the mechanical interlock. This allows you to open the cover without disconnecting power for inspection/testing/troubleshooting purposes. On a lot of older equipment you cannot find these bypass screws/switches, as I assume they were not designed that way. Does the NEC currently require you to have a way to defeat these interlocks? I would imagine this is a common problem these days for Infrared inspections or gathering field data for Arc Flash analysis.
 
Brandon Loyd said:
On your standard 208-480VAC, 3 phase, fusible or non-fusible disconnect, you often find an bypass screw/switch to defeat the mechanical interlock. This allows you to open the cover without disconnecting power for inspection/testing/troubleshooting purposes. On a lot of older equipment you cannot find these bypass screws/switches, as I assume they were not designed that way. Does the NEC currently require you to have a way to defeat these interlocks? I would imagine this is a common problem these days for Infrared inspections or gathering field data for Arc Flash analysis.
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I don't recall seeing anything like that without a bypass mechanism. But maybe there is a lower cost option that does not have it that some people use.
 
We certainly run into them from time to time. Mostly older equipment, but also some newer. I have a customer asking if this is a code violation, but can't seem to find a definitive answer.
 
That bypass screw / mechanism (aka "vault mechanism") is a requirement of NEMA/UL Type 12, that's why you don't see it all the time. Part of "Type 12", besides being dust tight and drip proof, is the term "industrial use", which is based on the assumption that in an industrial setting, qualified people must be able to perform testing without killing power. So the vault mechanism allows you to defeat the handle interlock, yet STILL require another deliberate action to open the box while still live. This is something from bygone days of course, long before Arc Flash rules came to basically make this impossible to do without a mountain of paperwork anyway, but the vault mechanism is left over still.
 
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