UL508A and small, low voltage panels.

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SamBowery

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Location
Georgia, USA
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Control engineer.
Hi, my company does not have a copy of UL specifications so I am trying to glean some information on the web.

We have some small plc panels which use 24VDC to provide the control signals to run some lights, water valves, and a circulation fan.
The actual lights, and fans are fed separately for the power to run them, we simply send 0-10vdc to control speed, intensity. the water vales are 24VDC.

I find an awful lot of infor for machinery, and 3 phase motor circuits but nearly nothing on small applications like ours.

We have a small breaker inside the panle to protect the 24VDC power supplies, thats it. We feel this can be a supplimental protective device, and the breaker feeding the boxes from the building power would be the primary breaker. Supplimental protective devices are not rated as strictly under 508A, and are less expensive by quite a margin.

Ultimately the question is will the breaker from the building power panel suffice as the primary branch
circuit protection alllowing us to use the less expensive panle device inside the panlel.

Thank you in advance for any feedback. I am pressing our company to buy a copy of the UL508 standards but need an answer quickly on this particular application.
 
If you don't have a copy of uL508a, then you probably are not authorized by UL to build panels that are listed to UL508a. So what difference does it make if you follow UL508a or not?

Having said that, UL508a does not require that you put branch circuit protection within the Industrial Control Panel at all. It can be external as long as the enclosure is marked in accordance with 60.1.

60.1 An industrial control panel provided with a power circuit where the disconnecting means, branch
circuit protection and/or motor overload protection is omitted shall be marked to indicate that these devices
shall be provided by the installer. The marking for field installed branch circuit protection shall include the
size and type of protection when required as a result of a component marking as indicated in 31.2.2.
Click to expand...

If it mattered to me, I would argue that the circuit in question is not a power circuit so this provision would not apply.

Personally, I have not found that supplementary breakers are all that much cheaper than branch circuit rated breakers.

We pay < $15 for a QOU110. Your price might be different.
 
petersonra said:
If you don't have a copy of uL508a, then you probably are not authorized by UL to build panels that are listed to UL508a. So what difference does it make if you follow UL508a or not?

Having said that, UL508a does not require that you put branch circuit protection within the Industrial Control Panel at all. It can be external as long as the enclosure is marked in accordance with 60.1.



If it mattered to me, I would argue that the circuit in question is not a power circuit so this provision would not apply.

Personally, I have not found that supplementary breakers are all that much cheaper than branch circuit rated breakers.

We pay < $15 for a QOU110. Your price might be different.
Click to expand...
Could you also share the screeshot of that 31.2.2 article? Thank you for taking time to reply!
 
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