OT: Short Circuit Current Rating

Hi all. I've been looking at the latest version of UL508A, and am finding that my understanding is basic, at best. What I've been trying to find, or validate, is where the requirement for providing SCCR ratings ends. For example, the common consensus here where I work is that power requires SCCR rating, controls does not. My question is, where or how is that line defined? This all started with a panel design I've just done that is nothing more than 120V single phase from a 120/208 panel fed to a controls panel. The 120V is distributed to a few various pieces of equipment, where it passes through a set of alarm contacts, and returns to my control panel. The control panel consists of alarm panel lights that provide remote notification of equipment alarm. Simple, hardwired control signalling. This falls under the UL definition of a control circuit, but I'd like to be able to explain why then this would not require an SCCR rating on my control panel, which houses the remote alarm lights. On all of my more complex panels, the incoming power was of higher potential, and the use of power supplies created a clear break from line power to control power. I'm looking for a more solid understanding of the definitions, and a reference point that shows control power does not require SCCR rating. I know this is valid, because on product documentation, many of the control components state that SCCR rating is not required, so I just would like to be able to refer to what this is based on. Also, any suggestions on seminars, webinars, etc. for this subject are welcome also.

Thanks, Russ

I believe the SCCR applies to the main circuit and its branch circuit. 115VAC does not comes into play.

LP

Struggling with definitions

Ok. So your interpretation would be that my load and control panel power feed are not defined as a branch circuit? It has been my interpretation that because I have a single breaker as overcurrent protection to my panel, that my panel represents a branch circuit. I'm also still getting familiar with the new NEC. Article 100 has this definition:

Branch Circuit, Individual. A branch circuit that supplies only one utilization equipment.

I believe this most accurately describes my circuit. If your interpretation is correct, I'm still looking for the article that states that SCCR only applies to feeder and branch circuits.

Our local Mc & Mc distributer had a seminar on this a while back. See if your local electrical or PLC distributor can help you.

sccr

This is used to co ordinate the installation of service equipment, disconnects, MCCs. If the available fault current available is 40,000 amps, the disconnect and the fuses must be able to safely open the circuit at that fault current. You would not install a disconnect rated only for 20,000 amps. Your fuses must also be rated over 40,000 in this case.
Control circuits are under section 16 in the Canadian code. There is a similar section in the NEC. These are class one and class two circuits. Energy limiting. Their output is limited to 1000Va and 100Va. In Canada, class one is control and the output is limited to 1000va max. In class 2 circuits there are control transformers that limit output by core saturation. These must be labeled as class 2 control transformers.
There is a place to start looking...

The keyword (in the spec?) you want to look for is "control power"

"control power" circuits do not need to be SCCR rated. And there is a strict definition of what compromises a control power circuit. I am fuzzy about that definition but I think it includes a isolation/power transformer, no field wiring, fused less than 10A...something like that.

I have yet to see that SCCR stuff show up on inspections so I'm not sure how quickly people will be worrying about it.

-John

Here are a couple of definitions from the standard (bold underline is mine, not part of the standard):

2.3 BRANCH CIRCUIT – The conductors and components following the last overcurrent protective
device protecting a load.

2.11 CONTROL CIRCUIT – A circuit that carries the electric signals directing the performance of a
controller, and which does not carry the main power circuit. A control circuit is, in most cases, limited to
15 amperes.

2.31 LOAD – A device external to the industrial control panel that is connected to the power circuit.

2.38 POWER CIRCUIT – Conductors and components of branch and feeder circuits.

Click to expand...

Disclaimer: This is only my interpretation of what I read, not a definitive judgment of how your inspector or an AHJ will interpret the code.

From 2.3, a Branch Circuit protects a load and from 2.31 a Load is external to the control panel. Ergo, if everything is inside the panel there is no load so there is no need for a branch circuit.

But then again, from the section on control circuits (underlining is again mine, not part of the code)...

40.2 Location of overcurrent protective devices

40.2.1 A branch circuit protective device complying with 40.1.1 shall be installed in each ungrounded
conductor to a control circuit that is supplied from a separate source voltage (not an isolated secondary).
Exception: An industrial control panel intended to be connected to the load side of a branch circuit
protective device installed in the field shall be marked with the required size and type of branch circuit
protection sized in accordance with 40.3.1. See 60.2 for marking.

Click to expand...

What do you expect.... it is written by committee

The NEC has two applicable sections.
110.9 says that any device intended to interrupt faults (i.e. breaker or fuse) must be rated for the amount of available short circuit amps on its line side terminals. This is why protective devices have an AIC rating.
110.10 says that all equipment (i.e. conductors, panels, and terminal blocks) must be able to withstand fault current flowing through it. This is the SCCR or withstand rating of non-protective equipment.

There are no exceptions to these rules.

Control circuit components must have an SCCR.
UL508 lists the minimum SCCR of devices as 2kA. Then an assumption is made, that circuits fed from a control power transformer always have an SCA <2kA. This has created the myth of control circuits being exempt from SCCR.

A circuit breaker must meet UL489 to be considered as a branch circuit protective device if not, it is considered a supplemental protective device and may not feed circuits that leave the control panel.

I think I get it

You are correct about the myth being out there, as it has existed here. Essentially, what I always need to do is provide some sort of control transformer prior to my electronic equipement. Doing so will ensure that the available short-circuit current is acceptable to the equipment downstream, and will also allow the use of supplementary protection on the load side of the transformer.

Am I correct in stating that the only caveat to this would be if my power feed was so far away from my panel that my supply conductors impedance factored in enough to diminish the available SSC to something below 2kA? Also, I've never done the calculations, so I'm not sure if that is really practical due to the resistive losses incurred by such a supposedly long feeder run. Thanks for the help all.

Russ

russrmartin said:

Am I correct in stating that the only caveat to this would be if my power feed was so far away from my panel that my supply conductors impedance factored in enough to diminish the available SSC to something below 2kA? Also, I've never done the calculations, so I'm not sure if that is really practical due to the resistive losses incurred by such a supposedly long feeder run. Thanks for the help all.

Click to expand...

You are correct, a short circuit analysis of the circuit would be required in order to determine the SCA available at you equipment. But the calculation requires you to know how much SCA you are staring with, so it would be custom for each location. In practice a small CPT is easier to apply.

In reviewing my copy of UL508A supplement SB, my concept was correct, but the exact value of 2kA may not always be applicable.