UL 508a and door interlocks

[sbyrne]

UL 508A states that if a panel contains 120vac then the incoming power must be interlocked so that the door cannot be opened with the power on. Is this correct?

Simple question, is there a way around this? Is there a VA rating that this only applies to? Or is there some stipulation on using a tool? Any signage that could be put in place?

Thanks!

[mdprice55]

It is true that the door of an enclosure that gives access to live parts operating at 50 volts rms ac or 60vdc or more are to be interlocked with the disconnect such that non of the doors can be opened unless the power is disconnected (66.1.5). But UL508A goes on to state in 66.1.5.1 that this interlocking means shall be provided with all of the following:
1) a means to defeat the interlock w/o removing power and which requires the use of a tool to operate;
2) means to prevent restoring power while the enclosure doors are open unless a defeat mechanism is operated; and
3) reactivated automatically when all the doors are closed.

So on most disconnect handles there is a defeat mechanism that allows entry to a "hot" panel with the use of a tool. Of course, these defeat mechanisms are designed for entry by competent and trained personnel.

As far as signage, you'd probably want a sign that states the general hazards (voltage, arc falsh, approprate PPE for entry, etc.) And by UL standards there must be a sign on or in the panel that states Manufacturer, SCCR rating, FLA rating, voltage, etc.).

[Jraef]

Originally Posted by mdprice55

It is true that the door of an enclosure that gives access to live parts operating at 50 volts rms ac or 60vdc or more are to be interlocked with the disconnect such that non of the doors can be opened unless the power is disconnected (66.1.5). But UL508A goes on to state in 66.1.5.1 that this interlocking means shall be provided with all of the following:
1) a means to defeat the interlock w/o removing power and which requires the use of a tool to operate;
2) means to prevent restoring power while the enclosure doors are open unless a defeat mechanism is operated; and
3) reactivated automatically when all the doors are closed.

So on most disconnect handles there is a defeat mechanism that allows entry to a "hot" panel with the use of a tool. Of course, these defeat mechanisms are designed for entry by competent and trained personnel...

And if it is on a machine, NFPA 79 now requires that the disconnect device have an integral operating mechanism that provides a means to open the switch if the door is open without needing a tool. It used to be that you could use a wrench or pliers to operate a rotary disconnect after you opened the door, but now the disconnect must have a handle on the inside as well as the outside. This is a moot point on flange mounted disconnects of course, because the handle is always in control on those.