Short-circuit ratings

Hi,

I'm a little bit confused on how understand the short-circuit ratings on some components in my cabinet and hoping for some advise. I have looked in the manual for the ratings and I got for my motor groups:

Motor Protection Circuit breaker - 15kA
Contactor - only found for fuses in the manual

Motor Protection Circuit breaker - 15kA
Contactor - only found for fuses in the manual
Soft starter 5kA

Motor Protection Circuit breaker - 15kA
VFD - 100kA


Ok, so I got some numbers. So if my soft starter can handle 5kA I would say that this is the highest shorrt circuit current I would rate my cabinet for. But it is protected by a MPCB so I think it's wrong to do it this way?

So I looked at the SCCR tool at Allen Bradleys webpage. There they say that if I use a MPCB and a contactor I can have 65kA. How is this possible since in the technical specs for the MPCB it's rated 15kA?

Further more, when I choose the SMC-3 soft starter, there is only options with UL class J fuses and not with MPCB?

Even further more, the load switch is only rated for 15kA and a fuse that I have is only rated for 10kA.

So basicly the more I write I guess it comes down to my soft starter. If this can only handle 5kA then I must rate my cabinet for this, because I can't get a test report from rockwell on MPCBs for the SMC-3? With Class J fuses it can handle 65kA.

So if I use Class J fuses all my motor groups would be 65kA but the cabinet is still limited to 10kA for the fuses I use. And if I switch the fuses to higher kA then it would be rated for 15kA because that is what my load switch is

Is my thinking somewhat correct?

/Tim

First thing you have to do is calculate the short circuit current that will appear in case of a real short circuit. The current will still be limited by the resistance of the cables between the power supply and the switching element or the fuse you're calculating the short circuit current for. Your power supply will also impose a limitation.

Then you have to make sure the short circuit current rating for that device is higher than the calculated short circuit current. If not, the device is unable to cu the short circuit current and your protection will fail with a disaster.

This basic power circuit design. Further calculations include selectivity of the system.

I only need to design my cabinet and provide a short circuit rating for it. Then the customer needs to adjust to it or demand a cabinet with higher short circuit rating. The calculation of the real short circuit will be done at place by others.

It's the technical specifications from Allen Bradley that is contredicting each other and needs a bit of explanation. Unfortunally the support for Allen Bradley isn't the best in Sweden, hence why I'm asking here.

/Tim

Devices that have a lower SCCR as stand-alone can be listed at a higher SCCR when TESTED AND LISTED in series combination with SPECIFIC protective devices. So even though the soft starter may say 5kA stand alone, if they say it is 65kA when fed from a Class J fuse, then its 65kA. But not all protective devices are created equal and the testing is very expensive. So that’s why you may not find an MPCB listed in series with that soft starter, because A-B chose not to pay for the extra testing.

As it happens, that’s actually somewhat common on soft starters in general, because if subjected to a large Fault event, the SCRs tend to pass current on their own (self commutate) and/or short out much much faster than an electro-mechanical device can act to interrupt it. That’s why most mfrs will tell you that it needs high speed fuses, such as Class J.

You will also find that if a device has a low SCCR, but is available in a series listing at a higher value with a CB or MPCB, it will ONLY be tested with the same mfr of both devices. So you cannot use an A-B contactor and protect it with a Schneider MPCB for example.

Thanks alot for the clarification!