UL508A EXCLUSIONS

[WMBELEDES]

Under UL508A, Resistors are listed as components in the power circuit that are excluded from SCCR evaluation.
How do I interpret this in regards to heating elements?
What is the cutoff point for calculating SCCR of a heating power circuit?

Regards,
Bill

 

[Jraef]

Under UL508A, Resistors are listed as components in the power circuit that are excluded from SCCR evaluation.
How do I interpret this in regards to heating elements?
What is the cutoff point for calculating SCCR of a heating power circuit?

Regards,
Bill

Are you referring to heaters that the panel is feeding, meaning they are outside of the panel? If so, they are irrelevant to the SCCR of the panel. Or are you referring to space heaters inside of the panel that are connected at less than line voltage, meaning they are fed from a CPT or from Line to Neutral? Again, that would make them irrelevant. Or are you referring to space heaters inside of the panel that are fed with line voltage? If so, are you nuts? If so (to both), it might be applicable so rethinking that design might be best in any account.

 

[WMBELEDES]

Heater exclusion

Heater exclusion

Are you referring to heaters that the panel is feeding, meaning they are outside of the panel? If so, they are irrelevant to the SCCR of the panel. Or are you referring to space heaters inside of the panel that are connected at less than line voltage, meaning they are fed from a CPT or from Line to Neutral? Again, that would make them irrelevant. Or are you referring to space heaters inside of the panel that are fed with line voltage? If so, are you nuts? If so (to both), it might be applicable so rethinking that design might be best in any account.

Ok that answers my question. They are heaters in an industrial oven (outside panel panel).
May I ask why are they excluded?

 

[drktmplr12]

Ok that answers my question. They are heaters in an industrial oven (outside panel panel).
May I ask why are they excluded?

The interruption capacity is the ability of a device to withstand a fault current. I suggest that there is nothing downstream of a motor or heater that can subject the motor or heater to a fault, except a short inside the motor where, it kind of doesn't matter what the rating is. UL does not test interruption capacity of motors as far as I know.

That said, UL508A is a certification that a control panel is constructed according to the 508 standard. The point of inspection is at the factory. Anything outside of the panel falls outside of the scope of UL508A. The interruption capacity is determined by the weakest link in the components connected to line voltage, at any time, inside the panel. Think power distribution blocks, circuit breakers, contactors, overloads, fuses, etc.

 

[WMBELEDES]

SCCR HEAT

SCCR HEAT

The interruption capacity is the ability of a device to withstand a fault current. I suggest that there is nothing downstream of a motor or heater that can subject the motor or heater to a fault, except a short inside the motor where, it kind of doesn't matter what the rating is. UL does not test interruption capacity of motors as far as I know.

That said, UL508A is a certification that a control panel is constructed according to the 508 standard. The point of inspection is at the factory. Anything outside of the panel falls outside of the scope of UL508A. The interruption capacity is determined by the weakest link in the components connected to line voltage, at any time, inside the panel. Think power distribution blocks, circuit breakers, contactors, overloads, fuses, etc.

Ok so the heat resistor is exempt because it's outside the panel but you still calculate SCCR the power circuit to the heat in the panel. Why didn't UL just day that!:slaphead:Thanks, I couldn't get my head around the exemption rule now I understand. There is really no exemption in the panel other than control circuits. All power circuits need to have SCCR calculations done.

 

[petersonra]

Ok so the heat resistor is exempt because it's outside the panel but you still calculate SCCR the power circuit to the heat in the panel. Why didn't UL just day that!:slaphead:Thanks, I couldn't get my head around the exemption rule now I understand. There is really no exemption in the panel other than control circuits. All power circuits need to have SCCR calculations done.

Only power circuits inside the panel are evaluated for SCCR under UL508a.

 

[Jraef]

Ok so the heat resistor is exempt because it's outside the panel but you still calculate SCCR the power circuit to the heat in the panel. Why didn't UL just day that!:slaphead:Thanks, I couldn't get my head around the exemption rule now I understand. There is really no exemption in the panel other than control circuits. All power circuits need to have SCCR calculations done.

I think you are missing it in UL 508a, because in the beginning, UL 508a explains that it pertains to the PANEL itself, not what it is connected to.

If there needs to be a separate UL listing of the entire MACHINE, then the listing of the heaters would come into play, but not with regard to SCCR. That ONLY applies to the panel (and whatever feeds it). As mentioned, anything that is a load is the last device in the circuit, so if it shorts, it by definition has not survived, so what happens to it during the time it takes to clear the fault is irrelevant.

 

[petersonra]

Why didn't UL just day that!

they do. Quite explicitly.

1.4 An industrial control panel does not include an evaluation of the controlled equipment such as motors,
heaters, lighting, and other loads connected to power circuits. Unless specifically noted on the wiring
diagram of the industrial control panel, an industrial control panel does not include equipment mounted
remotely from the panel and connected via a wiring systems or equipment field installed on or within the
industrial control panel.