Prospective fault current

[newfutile]

During a recent inspection on a 3 phase board I have some MCBs rated at 6KA
And some rated at 10 KA .

The ZE is about 0.04 to 0.05 depending on phase , form my calculations this works out to 9.95 KA ( I can only measure single phase pfc)

On a consumer unit we allow 16KA if its backed up by a BS1361
Does this apply to a 3 phase board or will a lot of MCBs need changing?

 

[Rockingit]

My maths says you've got 5-6KA single phase if it's a single phase breaker

 

[newfutile]

Mine does as well for a single phase fault ,but what about a 3 phase fault although it would have to happen very close or in the board itself , 1.73 is the factor usually given

 

[Rockingit]

One of us is missing something here. 6KA is effectively the minimum reliability for a CPD to handle in a fault situation. How is a 1ph device on a 1ph circuit going to suddenly encounter a 3ph fault?

434.5.1 Tells us that we need to consider the breaking current of a device at it's point of installation

 

[newfutile]

if it comes into contact with another phase , or would the fault current then be shared with effectively 2 mcbs in series?

 

[Rockingit]

I think you're over-thinking this one

 

[newfutile]

There is a bit of a back story , 2 mcbs were found to supplying the same circuit, lucky they were on the same phase . But your right it made me think what would have happened.

 

[Pete999]

What at does the Electrical Engineering community think on this subject ?

 

[Rockingit]

There is a bit of a back story , 2 mcbs were found to supplying the same circuit, lucky they were on the same phase . But your right it made me think what would have happened.

Apart from being forbidden by regs, in practice actually very little - the load gets split across them and as soon as an overload condition starts then they'll instantaneously all trip in a random order. I know this because years ago I set this up purely to see and find out!

But... bad, bad, bad........

 

[Lucien Nunes]

I think the point of the question is this:

For MCBs where the local PSCC exceeds Icu, is it always valid to rely on the conditional 16kA Icc rating afforded by coordination with BS1361 or BS88-3 fuses, regardless of the system conditions giving rise to that PSCC. In this case, we might have EN60898 MCBs of 6kA rating, supplied and type-tested with a single-phase CU for 16kA Icc. Fitted here in a 3-phase DB, they are still OK with 5kA PSCC for a single-phase fault, but the bolted 3-phase fault current (estimated as twice the single-phase current) exceeds Icu; can we assume the coordination is still valid and that the fuselink still provides backup at the prevailing (possibly different) I²t value?

I think the answer is yes but one might have to check with the manufacturers or study IEC 60947 closely. FWIW the 2x PSCC rule of thumb tends to overestimate the current. We can see intuitively that for a 3-phase symmetrical fault we have one fuse and one MCB pole in series with half the loop impedance and 230V, while for a line-line fault we have two of each in series with the full loop impedance and 400V. Neither is more onerous than the 2x rule of thumb suggests.

How is a 1ph device on a 1ph circuit going to suddenly encounter a 3ph fault?

The SP device in this case is installed in a TP board, so a foreseeable type of fault would be one line conductor insulation chafing / melting into contact with a busbar of a different phase. It is harder to see how an L-L-L fault or L-L-L-N/E fault could occur though.

 

[Rockingit]

The SP device in this case is installed in a TP board, so a foreseeable type of fault would be one line conductor insulation chafing / melting into contact with a busbar of a different phase. It is harder to see how an L-L-L fault or L-L-L-N/E fault could occur though.

For definite anything downstream of the SPD isn't going to have a happy time with that but would it actually affect the device itself? I'm not so sure it would as it just becomes a parallel path.

 

[Farmelectrics]

my calculations deem it acceptable 230 divided by 0.04=5.75ka and 5.75ka x 2 =11.5ka i always double it for 3 phase even though its over the top. think 1.73 is the correct value which would give you a lower fault current