Motor Protection Breaker SCCR Sizing

[EngBM]

For a motor circuit were you install a a breaker for short circuit protection in series with a thermal overload. How do you come up with the SCCR Short circuit Current rating of the breaker.

In other words how do you calculate the expected short circuit current for the circuit. eg for 40hp squirell cage motor 550Vac. Motor is installed 20 meters from the mcc panel.

 

[dpc]

You calculate it by determining the short circuit current at the motor starter and adding the motor cable - BUT - how do you know the fault will be at the motor?

Starter has a short circuit rating based on the combination of the circuit breaker, starter and overload relay, based on UL test requirements. The UL test standard is a whole other story.

BTW, don't confuse the terms "Rating" and "Duty". The rating is what the equipment is rated to interrupt. The duty is the actual fault current for this installation.

 

[jraef]

The short version of the rule is, the SCCR is the lowest level of any STAND ALONE device in the circuit, or the TESTED combination of any string of devices. To address your specific issue, 99.9% of all overload relays are only rated for 5kA (without fuses) and they are usually the lowest level device in the string. If you want to use circuit breakers, you end up with a 5kA SCCR capacity. Or an alternative is that must use a TESTED and LISTED combination of devices, i.e. circuit breaker, contactor, overload relay. For most of the major manufacturers, this will mean that you must use only THEIR devices in that combination,and exactly as they were tested. They are typically not going to test their devices with someone else's.* Any manufacturer who has done this will have a list of tested and listed combinations. You can get that list from them, or in many cases you can download that list from the UL.com website. The only thing is, UL.com's list may not be as up-to-date as the one the manufacturer maintains.

So for example, if you look at a Siemens contactor, Siemens OL, and a Square D breaker, the breaker is (let's say) 65kAIC, the contactor has a 10kA SCCR and the OL relay has a 5kA SCCR. The entire unit, if you build it that way, can only have a 5kA SCCR label applied. If you used a Siemens breaker and it was on the list as approved to use with that contactor and overload, the entire lineup could be labeled at 65kA SCCR. Conversely f you used a Square D breaker and contactor and OL relay in their tested listed combination, the same is likely true.

* In some cases, device manufacturers who do not make circuit breakers will pay for the testing of their devices with someone else's circuit breakers, but you still must use the EXACT combination as tested. There s no extrapolation allowed. This is often true of big ticket power devices such as VFDs and Soft Starters.

 

[itsmoked]

Nice lesson jraef. Thanks.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

To calculate the short circuit current rating required you calculate the current that would flow when you short the incoming power leads to the circuit breaker.

 

[dpc]

The UL testing for MCCs has always been an issue for me. With the new circuit breakers having much higher interrupting ratings than in years past it is becoming a moot point, I suppose. But in the past it was common to take a circuit breaker rated for 14,000 A and end up with a combination starter rated for 25,000 A as a tested combination. This because the UL test is based on the fault being external to the MCC bucket with at least 3 feet of wire to the fault. Also, I don't believe the UL test requires that the starter still be functional after the fault. But the door has to stay on!

But if the fault is at the load side terminals of the MCCB, this combination rating is just not valid.

 

[jraef]

Yes, one of those "quirks" of the rating systems. Just like the issue of an OL relay being the thermal overload protection device for the cables going FROM a mag-only circuit breaker TO the overload. It assumes that those wires can NEVER go to ground or short in between the main upstream SCPD and the overload.

 

[davidbeach]

jraef, that's not a bad assumption at all. If the wires are not tapped between the mag-only breaker and the TO there can not be an overload ahead of the TO. Sure, there can be a fault, but the TO is never intended to clear faults, that is the function of the mag-only breaker and it can still do the fault protection.

 

[jraef]

davidbeach,
I know, I was just pointing out that the rules have a bit of leeway, usually based on what is know from the real world, even though you can't apply the same principals everywhere.

 

[resqcapt19]

The permission to use the mag-only device only applies when that device is a part of a listed combination starter, so the oportunity for a short circuit or ground fault between the breaker and the overload relay is somewhat limited.

 

[dpc]

so the oportunity for a short circuit or ground fault between the breaker and the overload relay is somewhat limited.

Limited but certainly not impossible. Especially with someone's big mitts sticking voltage probes in the starter to check voltage. I've seen quite a few burned out buckets.

 

[jraef]

The permission to use the mag-only device only applies when that device is a part of a listed combination starter...

You'd be surprised at how many people do not understand that, even Professional Engineers. I have had that discussion dozens of times just in the last year, one even turning into a full blown argument. Not on my side, because I was just the last resort for a panel builder friend of mine (plus I knew I was right anyway) when he refused to do it per spec and the PE was disallowing the ECO. It ended with my having to show it to the PE in the NEC.