Control Circuit Only UL 508 Disconnect Switches

[uwryguy]

Does a control panel with only control circuits located inside require a disconnect switch? UL508a defines a control panel as two or more control or power components.

Section 39.1 states that "a control circuit intended to be supplied from a separate source shall be provided with a disconnecting means that complies to Section 30 disconnect switches"

With that statement wouldn't any system require a disconnect switch, even simply a pushbutton box? My system has 120 or 230 Vac single phase input to a 24VDC power supply which is connected to a PLC and some control relays.

I don't see how not using a disconnect switch can be avoided

 

[jraef]

Do you have an objection to using one? It's a good practice whether or not it is a requirement. You want to ensure that if anyone opens the enclosure door, the power is turned off before they can get access. Even "just control power" is dangerous.


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[uwryguy]

No I do not have any objection to use one, I completely understand the safety reasons. I just need some justification to go back to my customer and explain to them that it is in fact required per the specifications. From your interpretation is it or isn't it required per UL508a, it does not specifically talk about a system with just control power.

 

[jraef]

No. I don't believe it is required per UL508 as long as the enclosure is "lockable". But it may be required per the NEC or OSHA when someone goes to install it. OSHA 29CFR1910.147 LOTO (Lock Out / Tag Out) provisions are definitely being enforced and may now also be part of the NEC(?) for potentials over 50V. So if you don't put it in the control panel, the guy INSTALLING the control panel will have to go through more machinations to get approval for it on-site. He may be able to do it with a padlock attachment for the breaker feeding power to your panel, but that is not always acceptable. So a good "justification" might be to state that the box will be LOTO compliant without additional field work.




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