Short Circuit Current

The rating on the circuit breaker or fuse is the amount of short circuit current that the device can interrupt properly. This rating must be greater than the calculated short circuit current available. In your example, the MCCB is inadequate.

Tornado:

Thank you. If I have one MCCB with SC current rated at 30kA and a plan to buy a brand new one at 20kA,

Which is the better way? Can a 30kA MCCB is edequate to trip against such a low SC current in the system?

By the way, MCCb's overcurrent protection does nothing against the SC case?

Regards,

In MCCB if the rating is indicated as 3O kA for one sec, this device will withstand a current flow of 30 kA for one sec without any damge to the device.

Setting of MCCB is different, MCCBs will have fixed setting for shortcircuit or variable setting.

In the case fixed settings, the MCCB will act when the current flows 10 times the nominal current.

Generally the Fixed setting would be 10 times the nominal rating.

Supposing you have 100 A MCCB with fixed short circuit settings.The Nominal current of the MCCB is 100 Amps

This MCCB will trip only when the Short circuit current is More than 10 x 100=1000 A.

In the case of variable settings short circuit current settings of the MCCB can be set depending on the circuit .

Ramvinod:

Thank you.

I understand that it can withstand the 30kA SC current e.g in 1 sec..

But if I choose the MCCB with SC current rated at 200kA (the biggest) as in Tornado's comment. So, there would be any acting from this one if my system is short circuited at 17kA?

Supposing this MCCB comes with fixed setting at 200kA. It is just supposition only because the 200kA SC would be very big in size.

Regards,

Almost none of that "information" was correct, although a thermal-magnetic circuit breaker works sort of like that.

please point out the mistakes so that I can get corrected

Nice answar, can u pls send me some soft copy or suggest some book on Short circuit current, casecading effect of switch gear,

my email id is spunit@godrej.com

i am from india & working in a power intensive plant

regards

punit

Thank you for your interest.

Unfortunately, I do not have this information in any one book to recommend. I once designed UL-508 industrial control panels, and I got information from a variety of sources. Bussmann Fuse literature is really good. The US Navy book on Basic Electricity likewise. The NEC. Most manufacturer's catalogs for motor starters, circuit breakers, transformers, wiring supplies, etc. have a technical section. I have pieced together lots of stuff from those. I tend to forget details, but the more interesting principles seem to stick.

The OP seems to be new at some of this, but is asking excellent questions. With a little guidance, he is likely to learn well. Two of the real CR4 electrical experts would be JRaef and msamad. Anyone would do well to study their posts carefully.

I read this thread till Tornado's comment # 3 and found it was very well answered, and it was me who first voted GA for it. For curiosity, I just took a look into it now and definitely Tornado's further explanation is also excellent.

- MS

Thank you kindly. I am not an EE or other high-level expert, but I try to understand theory. Some of that may seem counterintuitive at first, so when I attempt to explain something, I look for common-sense descriptions at a non-technical level, yet still technically correct.

Your answer is an excellent explanation on how a CB behaves.

The one thing that the questioner asks, and doesn't seem to understand, is that if the fault current is larger than the rated interrupting of the CB then on the chance that an SC happens, the CB will need to be disposed of because of damage. So he would have a VERY expensive fuse.

So to probably say what has already been said, the MCCB must be rated to withstand the maximum anticipated fault current or you have defeated the purpose of having a circuit breaker as it should not (and may not be able to) be reset after a fault current trip (as opposed to the other trips described).

Yes I understood the problem.I should have given more elaborate answer.

What I meant was that in the case of MCCBS where only instantaneous releases are provided the setting value of the release matters.

In the case of MCCBs where LSI releases are provided the tripping of MCCB will follow inverse time curve.

Dear all,

I appreciate your effort supporting me especially Tornado's although EE is not his specialty.

But, I back at here with a confusion. All of us know that with 22kA MCCB and 17kA SC in the system, it totally can handle the SC current safely.

Things would be different if I choose 200kA MCCB. Because with such a high interupting capacity. The SC current in the system is much lower than the tripping range of the MCCB. I think that, it is somewhat difficcult for this MCCB to sense the SC. It might got wrong function and let the CS get through (God knows), or might the SC lasts long enough to make this MCCB tripped or the other MCCB in the system (with lower SC current rating) acted. All of us know that this is a coordination problem.

It is worth for consideration in a while.

Regards,

Hien

The short circuit withstand rating has nothing to do with the trip rating.

You can get a 200kA circuit breaker and put a trip rating plug of 400A in it. It will trip at some level above 400A depending on how long the overcurrent condition exists and at what level (inverse time rating). It will however, trip instantaneously (for all intents and purposes) if you give it a bolted fault. With 17kA current available, this circuit breaker will just yawn at this level of current and can be reset after the fault is corrected with little need for concern.

However, if you have a 10kA circuit breaker in the same service with a 400A trip rating, it too will trip above 400A after a period of time inverse to the load passing through it that is above 400A. This device will not safely withstand a bolted fault. With 17kA available current available, a bolted fault will damage the circuit breaker, probably beyond serviceability.

The short circuit withstand capability is NOT functionally related to the trip current of the device. Unless you want to assume the trivial case that the trip rating must always be below the short circuit withstand rating.

This may be a bit tricky to understand, but a circuit breaker or fuse will "sense" and interrupt any current above its nominal rating, provided that it can also interrupt the maximum current that system can deliver.

For instance, a 50A circuit breaker or fuse with an interrupting capacity of 200kA will (according to its current-time curve) open at any sustained current over 50A. If the current is only a little over 50A, this will take some considerable time. The greater the current is, the less time it will take for the CB or fuse to trip.

A 50A CB with 20kA interrupting capacity and a 50A CB with 200kA capacity will behave about the same for currents up to 20kA. But above 20kA, the first of these may fail to interrupt the current, probably because its dimensions are too small to provide sufficient arc separation. But there will be no problem with a CB of large interrupting capacity placed into a lower ampacity system. It will still interrupt lesser currents just fine.

You are asking excellent questions in a topic that is subtle to explain. With patience and time, you will become an expert in this.

For understanding you have to assume MCCBs into 3 catagories

1) MCCB with over current trip device

2) MCCB with trip device instantaneous setting only

3) MCCB with out trip device.

Let us assume a circuit where the Sc level is 17 kA.

a)when you connect the MCCB without trip device of 20 kA rating - nothing will happen to the circuit and to the MCCB.

b)When you connect the MCCB without trip device of 10 kA rating- the MCCB will get damaged because it is rated for 10 kA only and the MCCB can not be reused even after repair.

c) If the MCCB of 20 kA with instantaneous trip device is coneected with setting of say 10 kA, during the fault (fault current is 17 kA) this will trip instantaneously.No damage occurs to the device.

d) If the MCCB of 20 kA with over current trip device is connected with a setting,during the fault the breaker will trip after some time (follows inverse time characterstics),no damage occurs to the device