High earth impedance

[dj1471]

I'm trying to understand an issue my electrician has flagged up but have been finding conflicting information online. I would ask my electrician but he's flagged anything that doesn't meet current regs as "potentially dangerous" (which is nonsense when it would have met the regs in force at the time of installation). But I digress...

He's noted a high impedance on the main earth of 119Ω. I've seen some suggestion that such a high impedance may prevent protective devices from operating as sufficient voltage won't flow to earth. Which makes sense.

It's a TN-S system. There are no RCDs but there is a voltage operated ELD. Wikipedia says that VOELDs are designed for environments with high earth impedance (though it doesn't quantify what "high" means), hence my confusion.

I will get this properly investigated, but I want to understand the issue and figure out whether this needs to be sorted ASAP or whether it can wait a few months (when I'll be getting the CU replaced).

 

[RF Lighting]

VOELCBs normally do not work

If you really have a TN-S supply, then ring the emergency number for your distribution network operator and they will send someone out straight away.

You must not leave this as it is. A fault anywhere on the installation will bring everything live in your house, and the risk of death or serious injury is high.


Post a couple of clear focused well lit photos of your incoming mains supply, cutout and metering arrangement, and someone will confirm your supply type.

 

[DetlefSchmitz]

Is this the house you haven't quite bought yet?

 

[dj1471]

Is this the house you haven't quite bought yet?

Yes

Didn't mention that simply to keep my question clear and to the point.

The electrician stated on his report that it's a TN-S supply and I understand that this is most common, so I have no reason to doubt his assessment.

Given the advice above I'll get the vendors to get this seen to ASAP. Many thanks.

 

[flameport]

The electrician stated on his report that it's a TN-S supply and I understand that this is most common, so I have no reason to doubt his assessment.

You have every reason to doubt them since VOELCBs have not been fitted for 30+ years, and would not normally be fitted to a TN-S supply, since one of those on a TN-S would serve no purpose at all.

Therefore if it really is a TN-S, that device has been there for decades, meaning the earth has been defective for decades as well, and presumably no other work has been done since.
More likely it is a TT which has not been updated.

Then there is:

I would ask my electrician but he's flagged anything that doesn't meet current regs as "potentially dangerous" (which is nonsense

which is nonsense as you say.

 

[JohnW2]

.... VOELCBs have not been fitted for 30+ years, and would not normally be fitted to a TN-S supply, since one of those on a TN-S would serve no purpose at all. ... More likely it is a TT which has not been updated.

I don't really understand that, so perhaps you can explain. The 'voltage' which operates a VOELCB is actually that due to a fault current flowing through the VOELCB's coil (IIRC, usually 100Ω - 300&#937 to earth, and I would have expected that to be at least as effective (probably more) with a low-impedance TN-S earth than with a TT one.

Speaking of "100Ω - 300Ω coils" (and recalling the report of a 119Ω earth impedance!), I just wonder whether this electrician (or 'electrician') may perhaps have measured earth resistance (or EFLI or whatever) on the consumer side of the VOELCB!

Kind Regards, John

 

[JohnW2]

Duplicate deleted.

 

[flameport]

It would certainly trip at least once, but there would be no reason to have one fitted on a TN-S system.

It could be used as a fusewire saving device, as on L-E faults it would trip before any fuse would blow and if it was a Crabtree one might do the same for L-N faults as well.

Speaking of "100Ω - 300Ω coils" (and recalling the report of a 119Ω earth impedance!), I just wonder whether this electrician (or 'electrician') may perhaps have measured earth resistance (or EFLI or whatever) on the consumer side of the VOELCB!

That seems very likely, and if true means everything the 'electrician' wrote should be disregarded.

 

[dj1471]

Speaking of "100Ω - 300Ω coils" (and recalling the report of a 119Ω earth impedance!), I just wonder whether this electrician (or 'electrician') may perhaps have measured earth resistance (or EFLI or whatever) on the consumer side of the VOELCB!

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

This is what he put in the report:

unusually high Zs reading on the main earth 119 Ohms

When I asked him what further action he recommended, he said:

With regard to the high impedance on the earth we would need to check all the connections on the main earth, failing that it may be a supply company issue.

Other than marking it as C2 ("Urgent remedial action required") on the report, he hasn't otherwise suggested that there's any urgency in getting this investigated. He also flagged lots of other stuff as C2 where it clearly isn't, so the classic boy who cried wolf problem...

In any case, the issue will be properly investigated (by another electrician!).

 

[dj1471]

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

And also a member of the Electrical Safety Register and the Electric Safe Register, apparently.

 

[JohnW2]

It would certainly trip at least once, but there would be no reason to have one fitted on a TN-S system. ... It could be used as a fusewire saving device, as on L-E faults it would trip before any fuse would blow ...

That's true. However, with a TN system, even the VOELCBs we had would operate in response to much higher impedance (but still potentially dangerous) L-E faults than would result in any OPDs operating, wouldn't they (hence being partly on the way to a modern RCCB/RCD)?

... and if it was a Crabtree one might do the same for L-N faults as well.

I think you need to explain that one to me!

Speaking of "100Ω - 300Ω coils" (and recalling the report of a 119Ω earth impedance!), I just wonder whether this electrician (or 'electrician') may perhaps have measured earth resistance (or EFLI or whatever) on the consumer side of the VOELCB!

That seems very likely, and if true means everything the 'electrician' wrote should be disregarded.

Quite. Mind you, that would presumably mean that it probably is TN - since it would seem exceeding unlikely that would get as low as 119Ω if measuring on the consumer's side of a VOELCB in a TT installation.

Kind Regards, John

 

[JohnW2]

Speaking of "100Ω - 300Ω coils" (and recalling the report of a 119Ω earth impedance!), I just wonder whether this electrician (or 'electrician') may perhaps have measured earth resistance (or EFLI or whatever) on the consumer side of the VOELCB!

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

Unfortunately, that hope is not always realised in practice.

This is what he put in the report:

unusually high Zs reading on the main earth 119 Ohms ... With regard to the high impedance on the earth we would need to check all the connections on the main earth, failing that it may be a supply company issue.

Other than marking it as C2 ("Urgent remedial action required") on the report, he hasn't otherwise suggested that there's any urgency in getting this investigated.

In a sense it doesn't matter whether it's TN-S or TT - since, in either case, if the "Zs of the main earth" is 119Ω then the installation is positively dangerous without a modern (current operated) RCD protecting it. If it is TN-S and the (correctly measured) impedance is 119Ω then this would indicate a serious supply-side fault requiring urgent attention by the DNO. In either case, one (at least I!) might perhaps have hoped that he would have stressed the urgency of the situation.

In any case, the issue will be properly investigated (by another electrician!).

That would seem very wise.

Kind Regards, John

 

[RF Lighting]

The problem with VOELCBs is they can be very easily bypassed by accident and they will cease to function.

The favourite is a water or gas supply earthed before the VOELCB either by bonding or just by being buried in the ground being connected to the installation earth after the VOELCB by a central heating boiler or immersion heater. This provides a parallel path for the fault voltage which the VOELCB will not see as there is no PD between it's input and output terminals.

 

[JohnW2]

The problem with VOELCBs is they can be very easily bypassed by accident and they will cease to function.

Indeed so - their functionality can be totally undermined by the presence of parallel paths to earth. I can but presume that they were conceived in an era in which the water supply pipe often was the earth and gas supply pipes were neither bonded nor usually connected to water plumbing (hence earth) via CH boilers - hence usually no parallel paths.

The other thing I should have mentioned is that if a VOELCB were used in a TN installation, then one would become totally reliant on that VOELCB for L-E fault protection, since the Zs of all final circuits would inevitably be far too high for OPDs to offer any such protection. I suppose the 'saving grace' of that situation is that if there were parallel paths to earth of low enough impedance to undermine VOELCB operation, those same parallel paths would quite probably lower Zs to the extent that the OPDs would afford at least some protection against L-E faults.

Kind Regards, John

 

[ericmark]

Getting a 119Ω reading with a ELCB-v would on output side would be good as it shows the coil is not by-passed.

I have personally one tested one in resent years where I can remember the results. I was rather surprised. Power from DNO went into an old stone cottage rarely used it was out of the sun so cold and damp in middle of a 12 acre woodland. This had the ELCB-v fitted.

Power from this went to timber frame house which had a ELCB-c (RCD 30ma) fitted and I had first tested this house. The RCD trip times were OK but the earth loop impedance was off the scale so wanted to fit new earth rods.

This lead me to the stone building where the boxes were tucked under a slate slab and very hard to work on.

Result they were left and I fitted a new consumer unit where I could easy work on it supplied from the fuses in the original consumer unit. I also left the ELCB-v in place. When I installed the new earth rod clearly the ELCB-v stopped working as a trip but still worked as an isolator. Once bypassed my ELI readings were very good for an earth rod around 10Ω and I had linked original and new rod really it did not need the new rod.

This also was reflected in the main timber frame house which now also had a really good reading. In fact better than the old house which had me scratching my head. But then it dawned on me the underground feed must have been TN not TT and I tested rods before connecting to house not after so this was it seems a ELCB-v fitted to a TN supply.

As to why I would guess it was due to the long run through the woodland and worry as to the value of the earth due to distance. It was over 10 years ago before Part P so not too sure of readings but it was such a unusual job I remember it well.

I can fully understand an electrician who has never worked with an ELCB-v flagging up 119Ω with a TN system as a fault. That is exactly what I did. Difference was I had to correct the fault as well. Had I not had to correct it I may have not realised what a huge resistance was in the coil of the ELCB-v.

In all my time as an electrician only twice have I come across ELCB-v devices. Once in a caravan park and once in the woodland the caravan park was also a big problem. I had been sent to fit earth wires to each caravan as some one had touched a caravan and got a shock. Most caravans were not occupied in the day so I could not test just fitted earth wires. Then foreman went that evening to test. I had it seems disabled all the ELCB-v devices and then I had to work weekend to replace with ELCB-c in each of the effected caravans.

So if I with 63 turns to the coil have only found them twice what chance would a 30 year old have of understanding what the results showed?