loop impedence test

[D Skelton]

This is what Howard wrote in reply to me on HHDs thread about why his RCD would not test.

If the answer is that the earth fault loop impedance value must be confirmed as compliant, then 7666 ohms on a TN system would be compliant for a 30mA RCD.

Whether or not you would actually be able to test this is another issue, and not one that I'm terribly familiar with as I've never given it much thought before???

 

[spark 68]

If the answer is that the earth fault loop impedance value must be confirmed as compliant, then 7666 ohms on a TN system would be compliant for a 30mA RCD.

Whether or not you would actually be able to test this is another issue, and not one that I'm terribly familiar with as I've never given it much thought before???

Sorry D, it may trip at that, but not necessarily in the compliant times, as I said you would need to be below 334 ohms just to be able to test @ 5 X Idn to get your below 40ms trip times for a 30mA device.

 

[D Skelton]

Sorry D, it may trip at that, but not necessarily in the compliant times, as I said you would need to be below 334 ohms just to be able to test @ 5 X Idn to get your below 40ms trip times for a 30mA device.

I get that, and can't disagree as I'm not too au fait with the best solutions available to test an RCD where the EFLI is higher than 1667, but where the regs are concerned, 7667 is an acceptable value for a TN system's EFLI.

 

[spark 68]

I refer you to 411.4.9

 

[D Skelton]

I refer you to 411.4.9

The crux of that reg being "...may be applied..."

 

[spark 68]

Anyway the point is rather moot lol, why would you have a Zs of 7.6k on a TN system anyway ? Rhetorical question BTW lol

 

[D Skelton]

Anyway the point is rather moot lol, why would you have a Zs of 7.6k on a TN system anyway ? Rhetorical question BTW lol

Ah come on man, don't give me that! I was rather enjoying the debate! lol

And yes, I agree entirely

 

[spark 68]

OK then,

I was taught that the reason we use the max TT Zs values as the worst case for TN systems where RCDs are concerned, because even where the reason for the RCD is for say a high-ish Zs (just over the OCPD threshold) a TN system should always still have a lower Zs than most common TT systems (E54's TT excluded lol).

So long as you complied within the worst case TT values it would be ok with TN systems, which by default should have a much lower Zs anyway than those given in the TT table, and if you do have happen to have values approaching the max given in the TT tables on a TN system then you have other more serious faults. lol

 

[jaypp]

Hi guys I was wondering if someone can help me with a question on my 2394 paper it says:

Why is it necessary to carry out an earth fault loop impedence test before carrying out an RCD test

My answer:

Because an earth fault loop impedance test will prove that there is a fault path to earth and without a fault path to earth an RCD cannot operate as they only operate when a live to earth or neutral to earth fault occurs.

If you could tell me if im correct or not in what im saying and if im correct do i need to elaborate a bit more

Thanks

2394 initial verification. As I understand it, you shouldn't be energising anything until you've confirmed there's an EFLI that complies.

 

[spark 68]

2394 initial verification. As I understand it, you shouldn't be energising anything until you've confirmed there's an EFLI that complies.

You do realise that the EFLI is a live test ?

 

[D Skelton]

OK then,

I was taught that the reason we use the max TT Zs values as the worst case for TN systems where RCDs are concerned, because even where the reason for the RCD is for say a high-ish Zs (just over the OCPD threshold) a TN system should always still have a lower Zs than most common TT systems (E54's TT excluded lol).

So long as you complied within the worst case TT values it would be ok with TN systems, which by default should have a much lower Zs anyway than those given in the TT table, and if you do have happen to have values approaching the max given in the TT tables on a TN system then you have other more serious faults. lol

All correct from a working point of view, but as far as actual requirements go (which is what exam questions will want) 7667 ohms is the max for a 30mA RCD on a TN system, as ridiculous as that value may be lol.

I'll say it again though, I don't even think that anything we have talked about is anywhere near an answer that someone marking a 2394 paper would want to see anyway!

 

[jaypp]

You do realise that the EFLI is a live test ?

heh heh. taken at incoming with main sw off and main earth disconnected. installation is not energized

edit. the point i think is it's an Initial Verification question and therefore, it's more to do with the order in which you're 'supposed' to carry out tests.
I'll check though, could be wrong!!!

 

[spark 68]

I would still say that the limiting factor is Ut, as this what almost all the other related regs are designed to limit in the event of a high Zs, and why your meter inhibits the test on say the RCD test etc.

 

[jaypp]

ok so I have misunderstood the depth required by the question and, the possible answer I gave was already given in post #2 by Richard, with more to it, but basically my view of what was being asked. Apologies