High earth impedance

[ericmark]

Out of interest I went to Electrical Safety Council website and looked at the Best Practice Guides.

They say where the ELCB-v works as designed and there are no parallel paths then Code 3, if there are parallel paths then code 2.

I don't agree with their advice, but never the less in the light of a so called safety organisation giving that advice one can hardly chastise any electrician from following it.

But the question remains. How can one test a device when the test equipment required is no longer available?

At 100 Ω and 45 V the current required to trip would be 450 mA I seem to remember the one I tested was more like 250 Ω which means 180 mA but the problem is our tester is designed to work on 230 V not 45 V and to apply the 45 V means completely disconnecting and these devices were the main isolator so it would require drawing on DNO fuse to test.

As already stated from 1974 when the supply regulations were changed with a TN supply it was likely safer without any ELCB than with an ELCB-v.

The more I think about the more I reach the conclusion that unless you have the equipment required you should refuse the job. The report should simple state "We do not have the equipment required to test the now discontinued ELCB-v installed in the installation. Either it need replacing with a RCD or you need to find some one with the antiquated test equipment and even if you do find some one able to test it they will still issue at least a Code 3." unless you have the test equipment of course.

With the exception of incorrect polarity all faults getting Code 1 have immediate danger and clearly having an ELCB-v poses no danger in its self so unless there is physical damage can't be Code 1. So the selection is Potential dangerous or Improvement recommended. One would recommend improvement so the consideration is if it's potential dangerous.

First on the ESC BPG is "Absence of a reliable and effective means of earthing for the installation" and I would consider having an ELCB-v ticks that box. Lack of RCD protection also flags up a Code 2 where cables are buried less than 50mm etc.

But it quotes

Reliance on a voltage-operated earth-leakage
circuit-breaker for fault protection (protection against indirect contact), subject to the device being proved to operate correctly. (If the
circuit-breaker relies on a water pipe not permitted by Regulation 542.2.6 as the means of earthing, this would attract a Code C2 classification.

Under heading for Code 3.

There are a few things I don't agree with in the document it says Code 2 means the item needs require further investigation being remedied or investigated respectively as a matter of urgency. This means of course any Code 3 does not need correcting as a matter of urgency. I am sure it's not intended to be read that way but that's what it says.

The RCD question means that where cables are buried then all circuits get a Code 3 or 2 because of no RCD so any other Code 3 faults get hidden as the RCD issue is repeated again and again.

The Absence of supplementary bonding for installed Class II equipment where required (such as in a location containing a bath or shower), in case the equipment is replaced with Class I equipment in the future. Gets Code 3 to my mind having this in the notes yes but it should not be coded. We have some scheme operators saying you can use the three core earth wire for power with Class II tank thermostats, and others saying it a failure not to have the earth wire available.

So we have a situation where one scheme provider will give an installation a clean bill of health and another one with give the same installation a Code 3.

In one breath we say BS7671 is not retrospective and in the next we Code all the faults which were not faults when installed. Bring back Code 4.

 

[JohnW2]

Out of interest I went to Electrical Safety Council website and looked at the Best Practice Guides. They say where the ELCB-v works as designed and there are no parallel paths then Code 3, if there are parallel paths then code 2.

I suppose I can understand that. If it works and there are no parallel paths, then it's providing more-or-less as good protection against (negligible impedance, per BS7671) L-E faults as would anything else. If there are significant parallel paths then it won't provide such protection but, as you go on to say, it is not an 'immediate danger' (only becomes a danger if an L-E fault arises), so doesn't get a Code 1 - so gets a Code 2.

But the question remains. How can one test a device when the test equipment required is no longer available? ... At 100 Ω and 45 V the current required to trip would be 450 mA I seem to remember the one I tested was more like 250 Ω which means 180 mA but the problem is our tester is designed to work on 230 V not 45 V and to apply the 45 V means completely disconnecting and these devices were the main isolator so it would require drawing on DNO fuse to test.

As I said, since its current that actually causes a VOELCB to operate, I would think that you can probably forget about voltages and just regard it as if it were (in your latter example) an RCD with an IΔn of 180 mA and test it as such - it should trip when the tester creates a 180mA L-E fault current. However, as I said, I'm not sure whether a 'modern' RCD tester would work in the presence of a loop impedance that high. The manual for my Fluke says:

Note: When measuring trip time for any type of RCD, the tester first does a pretest to determine if the actual test will cause a fault voltage exceeding the limit (25 or 50 V).

[one can select whether one wants the limit to be 25V or 50V]. It therefore sounds as if, with the limit set at 50V, such a meter quite probably could test your "180mA VOELCB".

In one breath we say BS7671 is not retrospective and in the next we Code all the faults which were not faults when installed. Bring back Code 4.

I'm not sure that it is unreasonable to 'code' things which are non-compliant with current regs, but which were compliant when installed, so long as the code given (3, a hypothetical 4, or whatever) only represents an indication/recommendation of what would have to be done to bring an installation up to current standards (should one so wish).

Kind Regards, John

 

[securespark]

He is a NAPIT registered electrician so hopefully knows what he's doing...

Well, this is wrong for a start...

This is what he put in the report:

unusually high Zs reading on the main earth 119 Ohms

It should have read Ze....

And if it is TT and not TN, then it is not unsually high, but higher than the recommended 100 ohms.

 

[JohnW2]

And if it is TT and not TN, then it is not unsually high, but higher than the recommended 100 ohms.

Indeed - and, as has been noted, it could possibly be a correct measurement on a TN installation if he, incorrectly, undertook the measurement on the consumer's side of the VOELCB. If he made that same mistake with TT, 119Ω would almost certainly be too low

Kind Regards, John

 

[securespark]

Best advice from our Co. was to get sub 100 on TT. I always strived for that. In theory 200 is the limit, but we had been told (by ECA, I think) to aim for 100 or less.

 

[JohnW2]

Best advice from our Co. was to get sub 100 on TT. I always strived for that. In theory 200 is the limit, but we had been told (by ECA, I think) to aim for 100 or less.

Indeed - I think that's what most people (and most guidances) say and believe - and I hope that nothing I wrote made you believe that I thought otherwise. Mine usually varies in the range 50-75Ω, depending on prevailing weather conditions!

[My reference to 119Ω being 'too low' for TT had the electrician incorrectly measured it from the consumer's side of the VOELCB, is a reflection of the fact that if he were measuring rod plus VOELCB coil, and if the latter were 100-300Ω, then 119Ω would not really be credible.]

Kind Regards, John

 

[securespark]

Gotcha!

 

[dj1471]

The issue has been investigated by another electrician and..... it's a TT system!

As mentioned above the electrician stated that an impedance of up to 200Ω is acceptable.

Thanks all for your advice, had it actually been TN-S this advice could have made all the difference...

If nothing else I now know a bit about how VOELCBs work and that replacing the CU should be the first thing I arrange once I get the keys!

 

[JohnW2]

The issue has been investigated by another electrician and..... it's a TT system!

As we suspected! I presume you must mean 'with an earth rod'?!! So much for the first 'electrician'!

As mentioned above the electrician stated that an impedance of up to 200Ω is acceptable.

Yes, but as also stated, most people strive for 100Ω or less. If you wanted to achieve that, you could put in an additional rod to get the resistance down.

If nothing else I now know a bit about how VOELCBs work and that replacing the CU should be the first thing I arrange once I get the keys!

Indeed - and the sooner you get that VOELCB replaced by RCD(s) (which will obviously part of 'replacing the CU'), the better!

Kind Regards, John

 

[dj1471]

As we suspected! I presume you must mean 'with an earth rod'?!! So much for the first 'electrician'!

Yes, with an earth rod.

Yes, but as also stated, most people strive for 100Ω or less. If you wanted to achieve that, you could put in an additional rod to get the resistance down.

Is the high resistance still an issue with a modern RCD?

According to the wiki page on earthing types, the distributor is obliged to provide a TN-C-S or TN-S supply if requested and available; assuming this doesn't only apply to new installations, is this worth looking into? Presumably they'd charge for this?

 

[westie101]

According to the wiki page on earthing types, the distributor is obliged to provide a TN-C-S or TN-S supply if requested and available; assuming this doesn't only apply to new installations, is this worth looking into? Presumably they'd charge for this?

ESQCR states

(4) Unless he can reasonably conclude that it is inappropriate for reasons of safety, a distributor shall, when providing a new connection at low voltage, make available his supply neutral conductor or, if appropriate, the protective conductor of his network for connection to the protective conductor of the consumer’s installation.

So only, as I've said, applies to new installations. However the DNO may do it at their discretion on an existing supply, so it is worth enquiring.
(or just report it as a poor earth value and say nothing else))

 

[JohnW2]

Yes, but as also stated, most people strive for 100Ω or less. If you wanted to achieve that, you could put in an additional rod to get the resistance down.

Is the high resistance still an issue with a modern RCD?

In some senses, it's obviously not. The earth fault loop resistance (the vast majority of which will be attributable to the earth rod in a TT installation) can be as high as 1,667Ω for the RCD to operate when there is a fault such that the potential of the installation's earth system (hence exposed earthed parts) rises to 50V or more - which is the requirement. The main concern about earth rod resistances >100Ω is that they may be 'unstable', hence potentially liable to rise at times.

Kind Regards, John

 

[securespark]

(or just report it as a poor earth value and say nothing else)

Please sir, you told me off, sir, when I said something similar in another thread, sir!