MCB kA Rating 3 phase Board?

[Piratepete]

Hi Guys

I don't do much 3 phase but am used to seeing them populated by 10kA MCBs.

I'm dealing with a board which has a mixture of 6 and 10 kA 3 phase and 1 phase MCbs.

Z line measures 0.13 Ohms and Isc line is 1.6 kA. The supply fuses are 200 Amp.

So 6 kA MCbs are ok in this situation then?

Pete :yes:

 

[Pete999]

Hi Guys

I don't do much 3 phase but am used to seeing them populated by 10kA MCBs.

I'm dealing with a board which has a mixture of 6 and 10 kA 3 phase and 1 phase MCbs.

Z line measures 0.13 Ohms and Isc line is 1.6 kA. The supply fuses are 200 Amp.

So 6 kA MCbs are ok in this situation then?

Pete :yes:

Do you mean Zs is 0.13 and PSC is 1.6 KA ?

 

[Piratepete]

Do you mean Zs is 0.13 and PSC is 1.6 KA ?

Yes, that's correct.

 

[Leesparkykent]

Yes as a general rule of thumb you double PSCC or if you want to be a bit more precise you times by 1.732.

 

[Piratepete]

Yes as a general rule of thumb you double PSCC or if you want to be a bit more precise you times by 1.732.

Thanks. Just wondered. My sense of consistency was offended by the mixture!

Pete

 

[davesparks]

What exactly have you measured here? The OP suggests you've measured the L - L loop impedance and short circuit current, but your reply to Pete suggests that you've measured the earth loop impedance?
Make your mind up!


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[Piratepete]

What exactly have you measured here? The OP suggests you've measured the L - L loop impedance and short circuit current, but your reply to Pete suggests that you've measured the earth loop impedance?
Make your mind up!
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Whatever. It doesn't help that the installation test sheets refer to Ze at a consumer unit as Zs. So excuse my ambiguity. My meter was connected to Line, Neutral and Earth. I tested each phase for worse case. I understand that if I take the single phase value and double it, this gives me worst case scenario.
Pete

 

[Simonslimline]

Whatever. It doesn't help that the installation test sheets refer to Ze at a consumer unit as Zs. So excuse my ambiguity. My meter was connected to Line, Neutral and Earth. I tested each phase for worse case. I understand that if I take the single phase value and double it, this gives me worst case scenario.
Pete

Where is the board fed from?

 

[Lucien Nunes]

My meter was connected to Line, Neutral and Earth.

So you didn't do any tests from one line to another line? Which is how I would interpret 'Z line'. If the resistance of all the conductors is equal, then the PSCC for a line-line fault will be approx sqrt(3) x that for a line-neutral fault. However there can be situations (e.g. with a reduced-neutral cable) where it is higher by a greater factor than the ratio of the voltages. I always do the L-L tests rather than assume.

 

[Leesparkykent]

So you didn't do any tests from one line to another line? Which is how I would interpret 'Z line'. If the resistance of all the conductors is equal, then the PSCC for a line-line fault will be approx sqrt(3) x that for a line-neutral fault. However there can be situations (e.g. with a reduced-neutral cable) where it is higher by a greater factor than the ratio of the voltages. I always do the L-L tests rather than assume.

I do as well but the problem is not all MFT's can do L-L tests.

 

[Piratepete]

Where is the board fed from?

Transormer on a post about 15 metres away via the meter and the DNO's 200 Amp fuses

 

[Piratepete]

I do as well but the problem is not all MFT's can do L-L tests.

Yup. Mine doesn't.
Pete

 

[Piratepete]

So you didn't do any tests from one line to another line? Which is how I would interpret 'Z line'. If the resistance of all the conductors is equal, then the PSCC for a line-line fault will be approx sqrt(3) x that for a line-neutral fault. However there can be situations (e.g. with a reduced-neutral cable) where it is higher by a greater factor than the ratio of the voltages. I always do the L-L tests rather than assume.

The DB is right beside the DNO's meter and 200 Amp cutouts and the Neutral Line is huge as is the Earth. The Neutral is the same size as the phase cables. Can't tell you what as I've never installed this size.
Pete

 

[Lucien Nunes]

I do as well but the problem is not all MFT's can do L-L tests.

Ah, good point, didn't think about that because we've only ever had ones that do 400V tests because most of the stuff we work on is 3-phase. In this case it sounds like the resistances will be the same and the MCBs well in for Icn

 

[netblindpaul]

What is the pollution category of the devices?

 

[happysteve]

Assuming same size neutral cable as lines cables... my understanding was that a line-line fault current would be sqrt(3) times the L-N PSCC; but a symmetrical fault would be 2 times the L-N PSCC. However I've never seen a convincing set of maths to verify this. There are hints of it in this link... but "Z" is ill-defined there, and there seems to be a factor of 2 out.

Question: would a single phase circuit breaker in a 3-phase board ever have to deal with a symmetric fault? A 3-phase breaker, sure (perhaps a broken star-delta starter?). Because I've seen situations where sqrt(3) x PSCC is on the "ok" side of the line, whereas 2 x PSCC is more on the "explody fireball of doom" side...

 

[davesparks]

Whatever. It doesn't help that the installation test sheets refer to Ze at a consumer unit as Zs. So excuse my ambiguity. My meter was connected to Line, Neutral and Earth. I tested each phase for worse case. I understand that if I take the single phase value and double it, this gives me worst case scenario.
Pete

It's not whatever, it's an important technical detail. This is a technical trade and anyone practicing it should be capable of using a reasonable degree of technical accuracy in their communication.

It's no good just saying that your meter was connected to three points, especially when you should be carrying out a test which only needs two test leads, so that in itself suggests that you have carried out a three wire low current test instead of a high current test so your results are quite possible wrong as a result of this.
So what were you measuring? L-E or L-N?

There is no such thing as Ze at a consumer unit, Ze is a measurement of the external loop and is only at the supply terminals, before the consumer unit.





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[davesparks]

What is the pollution category of the devices?

I know where you are going with this, but I think there are somewhat bigger concerns here!


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[davesparks]

I do as well but the problem is not all MFT's can do L-L tests.

Then perhaps one should consider whether such a meter is suitable for testing a three phase supply of it cannot perform all of the tests?


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