Short Circuit Current 2

Happy new year to you too....

Well, you can use any breaker anywhere, provided that at least one of them has a higher capacity than what the system can deliver.

i know from your past postings that you are no newcomer to this....

It may be very informative for you to study discrimination, cascading, time based, energy based, current based ... there is a wealth of knowledge available out there. Specially ECT 167 from Schneider Electric...

So you mean that will be correct to have main breaker MCCB 10kA and MCB can be 14kA???.....

Not normal. The upstream CB should have higher SC capacity.

But technically not wrong. Remember that MCBs are almost always current-limiting. Having lower inertia than a 3-pole MCCB, the MCB will act faster than the MCCB, and isolate the faulty feeder before the MCCB acts. Good discrimination achieved. Provided that the system fault level is <10kA at any point, no harm is using an MCB with a higher capacity isn't it? That's what the OP asked.

well I understood. so we can drink 100ml water in cup or in a big barrel. Not normal, but technically not wrong. isnt it?..

Well, yes. However, a 14kA MCB is likely to clear a 10kA fault better than a 10kA MCB.

Just like the Ferrari here will do much better than the others around it once it reaches the DND flyway at Delhi...at the moment it is crawling @ 10kmph

Kvsridhar:

The car looks very cool, is that your car?

Thank you for your comment, here I am considering of the coordination. Let's say 14kA MCB is better than 10kA MCB. But since I am having one main 10kA MCCB, so if I choose the branch 14kA MCB, is it good for coordination? If I choose the MCB a bit lower, is there any advantage?

That Ferrari ? Mine ? i have a 25:1 scale model. That is the closest i can get to one

As i said before, you need to study coordination systems, very interesting.

Let us say that you use a 14kA MCB downstream of the 10kA MCCB. A short-circuit happens in one of the feeders. Remember that it can't be >10kA, preferably much less (You would be prudent to use a 10kA CB if your system is not more than 7.5kA, otherwise your system is at risk).Now let us say the fault is 5kA. The 14kA MCB will clear it easily, and the MCCB should not trip. So you don't lose power to the healthy feeders. This is good coordination.

An MCB with lower capacity will be perhaps cheaper, no technical advantage. Nevertheless, you are not getting any great advantage using a 14kA MCB either.

What is the prospective SC level in your system anyway?

10kA, 5kA, 14kA these are the SC capacities the MCB or MCCB can handle without breaking down. These are not the tripping settings for the flowing currents. To select the correct one that will serve your purpose, you need to work out the SC current of the circuit downstream of the MCB (your question...). In any case, if the upstream MCCB is already rated 10KA, a 14kA will seem over-rated just because it will cost more only! It does not help selective tripping or affect the tripping setting of the MCB. Therefore, you select the appropriate MCB with the correct kA capacity that will exceed the maximum SC current on that branch of the circuit. Over sizing only hit your pocket. Undersizing can be desastrous in SC situation.

Ok, I will try to get straight to the point and we will have more serious sondieration. Let's not say about B,C or BC curves.

If your house has 02 floors and power is provided by one 60A-10kA MCB at the gate (presume the maximum SC current would be 10kA in this case). Then, you will install the same 10kA for MCBs, one for the first floor and one for the second floor.

Situation will occur like this. You have a SC at the very very end of your household power system at the 2nd floor and the MCB of the second floor trips. But thing would be Ok if you just need to check and the fault is allocated then you switch back the MCB. But, if the main MCB at the gate also tripped. This would annoy you by making a phone to the Power company (here we say power company) to switch back the power for you.

What does this situation say? If this is a bad coordination, then how we could improve it?

i repeat, you need to read up on discrimination, cascading etc.

Coming to this specific case of one residential building with two floors, you can expect "Partial discrimination" as opposed to full discrimination up to 10kA. Let me try to explain with this image...

Left curve - If the fault level is ≤IB, only the downstream MCB will trip, the other floor will not suffer a blackout. If the fault is higher, both will trip, entire building will have blackout.

Right curve - The threshold of the "partial discrimination" is much higher.

Anyway, when you have a short circuit at the end of a residential circuit, the impedances restrict the fault to low values. In my 3-floor, 6-flat building, the prospective fault at any flat's incoming is ≈ 2kA.

Have they used oversized conductors???.....

What is the kA rating of MCB used in your flat?? 14kA?

Kvsridhar:

Thank you for your kind explanation. If you dont mind my questions, I will keep on asking that if the SC current ~2KA due to adding up the impedance. So, there would be no difference if I choose 5kA or 10kA or even 14kA MCB for each floor?

I mean it there is a fault and its SC current is at ~2kA, 5kA, 10kA, ot 14KA MCBs will work pretty fine on this. Ok, it is clear. But it will not affect to the upstream 60A MCB at the gate? We will not get blackout on the total area?

Additioanlly, if we have much stronger SC current let;s say 10KA, ( I dont know how if we can get this SC current while our system is rated at 2kA). It is sure that downstream MCB will trip but we will not sure that if the upstream MCB will trip or not? Is my though correct?

Thank you so much in advance,

There will be some difference when you use MCBs of different capacities, not significant to the user though.

i mentioned "Partial discrimination"....this means that only the downstream CB will trip up to some low value of SC current, and above that, both may trip, may not trip. It may be that the upstream MCCB may try to open, but since the downstream MCB clears the fault, may re-close. All explained elegantly in the Schneider papers. Do you want the link?

If it is resistive or semiconductor load go for 5kA; higher the inductive load go for higher KA.

Happy new year to all.

Apart from selection of short circuit rating as already discussed, selection of charactiristic curve of MCB is very improtant depending of load. For example for lighting load B curve is correct; for motor load( airconditioner ) C curve is the choice etc.

if you want to connect light and motor, use BC curve.

It would be most unusual for lighting and motors to be wired downstream of the same breaker.