High earth impedance

[bernardgreen]

The original idea behind "voltage operated" Earth Leakage Breakers was that if the CPC potential was more than 50 volts above ground potential the device would operate and disconnect the supply.

When the CPC was isolated from the true earth then an ELB-v coil took all the leakage current and was designed to trip when that current caused a nominal 50 volts across its coil. ( although called a voltage operated it was like all solenoid devices operated by a current flowing through it )

As has been said it is too easy for the CPC to have another path to ground that is parallel connected across the coil in the ELB-v. If the parallel path has a resistance of 1 ohm and the coil in the ELB-v requires 50 volts to operate then an earth fault current of 50 amps can flow through the parallel path before the ELB-v will see 50 volts and trip.

 

[westie101]

It seems to be in the absence of any comment to say there is an earth connection to the DNO equipment that this (with the VOELCB) is an originally water/gas service earth. So a form of TT.

If there is no earth provided by the DNO they are under no legal obligation to supply one, which any competent electrical contractor should know, so the installation should be made TT.
That the electrical contractor has walked away but just left advice I find difficult to understand as leaving a dangerous installation is foolhardy at the least. If the DNO does attend and confirms the figures they are well within their rights to disconnect the supply and advise that a competent electrical contractor is retained to make the situation safe.

 

[JohnW2]

It seems to be in the absence of any comment to say there is an earth connection to the DNO equipment that this (with the VOELCB) is an originally water/gas service earth. So a form of TT.

The presence of the VOELCB certainly would support that suggestion, but the electrician who undertook the inspection described it, rightly or wrongly, as TN-S.

That the electrical contractor has walked away but just left advice I find difficult to understand as leaving a dangerous installation is foolhardy at the least. If the DNO does attend and confirms the figures they are well within their rights to disconnect the supply and advise that a competent electrical contractor is retained to make the situation safe.

Quite so. However, as we've been discussing, it's at least possible that the electrician erroneously measured the "Zs of the main earth" from the consumer's side of the VOELCB - in which case the 119Ω could be credible for TN-S (but would be surprisingly low for TT). It's also possible that he is wrong in saying that it is TN-S and that he correctly measured the Ze as 119Ω on the correct side of the VOELCB in what was actually a TT installation. There are therefore a number of ways in which the installation might actually be safe, but all would be consequences of errors on the part of the electrician. However, as you and I have both said, if he believed that it was TN-S and believed that he had correctly measured the Ze as 119Ω, then it is more than a little surprising that he 'walked away'.

It will be interesting to hear what the new electrician reports.

Kind Regards, John

 

[dj1471]

It seems to be in the absence of any comment to say there is an earth connection to the DNO equipment that this (with the VOELCB) is an originally water/gas service earth. So a form of TT.

He stated that the gas meter isn't currently bonded, so presumably not being used as the earth (the two meters are co-located)? The water supply comes in on the opposite side of the property, he said he couldn't establish whether it was bonded or not (there's no meter).

It sounds like you'd expect to find TT earthing in more remote locations? The property is a c200 year-old terrace in the centre of a major town.

That the electrical contractor has walked away but just left advice I find difficult to understand as leaving a dangerous installation is foolhardy at the least.

Actually he didn't even provide advice, I had to specifically ask what follow-up action he recommended. Plus as noted before I'm not the homeowner (yet), so far as I know he didn't notify them of the issue though they were present for the inspection...

I will let you know the outcome when I hear back from the owners.

 

[ericmark]

This I see as a problem with an EICR where the house is occupied. If you find a fault in commercial premises you can in fact you must make safe which could mean switching off the supply. But in a dwelling one is duty bound, before making a house uninhabitable, to find alternative accommodation for the occupants.

This is a problem as often one has to remove the supply before completing tests and once removed to re-energise is a real problem.

I have in the past left the supply isolated telling the occupants I am not allowed to re-energise but I can't stop them from re-energising knowing full well they will.

I have viewed premises in the past where on entering with the estate agent the agent has switched on the power so we have lights and switched it off again as we leave. Clearly had an electrician left the power off then next time the estate agent arrives they will turn it on again.

As to locking off well that electrician may never be asked to return so that is hardly an option. Using a cable tie will require a tool to remove but question is how does one fit a cable tie most CU's don't have any option for fitting one which includes of course locking off.

If I was inspecting an empty house which failed to reach safe standards I would fit a failed label to consumer unit and leave switched off. I would also take a photo to show label was there. With a occupied house I would do the same plus inform the occupants in writing that the installation had not reached the required standards. I would not fit any locks. If it was easy to fit a cable tie I would but would not go to any great lengths.

As to an ELCB-v being a cause for failure that is up to debate. If it does not trip then clearly it fails but how does one test a ELCB-v other than press the test button.

That is an interesting question I never though about how to test a ELCB-v it has always been a case of it needs replacing but for an EICR then one would need to state if it works or not and I have no idea how to test this device. I know a RCD tester will not trip it but they are designed to work with current not voltage. I seem to remember mine would not even attempt the test.

So all those years ago how were they tested.

 

[JohnW2]

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

...unless, as he presumably should have done, the electrician disconnected the earth connection from the consumer's side of the VOELCB when trying to measure Ze (if thatis, as he implies, what he was trying to measure).

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. ... 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?

That could be an explanation as to how one could could erroneously get a 119Ω reading with TN-S, but not, IMO, a really good excuse. There are probably still a good few VOELCBs out there, and one would hope that a competent electrician would be aware of the consequences (and the reed to replace them with RCDs). However, as we're all agreed. if the electrician believed that he had measured a Ze of 119Ω on a TN-S installation, to 'walk away' does not seem very appropriate.

Kind Regards, John

 

[JohnW2]

As to an ELCB-v being a cause for failure that is up to debate. If it does not trip then clearly it fails but how does one test a ELCB-v other than press the test button. ... That is an interesting question I never though about how to test a ELCB-v it has always been a case of it needs replacing but for an EICR then one would need to state if it works or not and I have no idea how to test this device. I know a RCD tester will not trip it but they are designed to work with current not voltage. I seem to remember mine would not even attempt the test. ... So all those years ago how were they tested.

I suspect that, 'all those years ago', they were not tested other than by pressing the test button. However, if one considers the fact that they were (in terms of L-E faults) essentially the TT equivalent of the OPDs (probably fuses in those days) providing that protection in a TN installation, to even have a test button to press meant that the L-E fault protection would have been more testable with a VOELCB than without one (since fuses/MCBs obviously can't be tested).

Despite the name, VOELCBs were/are in effect current-operated - it's the current through the device's coil which causes it to operate (IIRC the coil impedances correctly, probably tripping at fault currents in the range 110-300 mA). A modern RCD tester is capable of providing such test currents, and higher (for testing 100/300mA RCDs) when loop impedance is fairly low, but I rather doubt that they are able to do so in the presence of a very high loop impedance (due to the VOELCB coil). However, if the tester were able to generate such test currents, it would presumably work on VOELCBs in just the same way as it does with RCDs. I just don't know (but doubt) whether they can.

Kind Regards, John

 

[JohnW2]

That the electrical contractor has walked away but just left advice I find difficult to understand as leaving a dangerous installation is foolhardy at the least.

Actually he didn't even provide advice, I had to specifically ask what follow-up action he recommended. Plus as noted before I'm not the homeowner (yet), so far as I know he didn't notify them of the issue though they were present for the inspection...

As eric has implied, if this property is occupied, that's even more worrying. If your electrician's findings are all correct, rather than erroneous, (I suspect the latter) the present occupiers are living with a very dangerous situation which really requires immediate investigation and rectification.

Kind Regards, John

 

[westie101]

It sounds like you'd expect to find TT earthing in more remote locations? The property is a c200 year-old terrace in the centre of a major town.

Finding TT or no earth is not uncommon in the centre of towns in the situation you describe

 

[flameport]

... 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!

Some of the Crabtree ones also had a magnetic overload trip inside which would disconnect on L-N faults. That would probably disconnect before some rewireable fuses would fail.

 

[JohnW2]

As has been said it is too easy for the CPC to have another path to ground that is parallel connected across the coil in the ELB-v. If the parallel path has a resistance of 1 ohm and the coil in the ELB-v requires 50 volts to operate then an earth fault current of 50 amps can flow through the parallel path before the ELB-v will see 50 volts and trip.

Indeed so. As I've said, I suspect that they were conceived in the days when such parallel paths to earth were much less common - e.g. when the water supply pipe was 'the earth' and gas supply pipes were often not bonded or otherwise connected to CPCs on the consumer's side of the VOELCB.

I inherited (but very rapidly got rid of) a VOELCB when I acquired my present house (TT supply) in 1987. I have no idea as to what the impedance to earth of the water supply pipe was back then, but for the last several years it has been about 0.35Ω. If it had been remotely that low 'back then', the VOELCB would obviously have been totally useless. [in reality, it probably wasn't that low, since I strongly suspect that the reason for its very low impedance now is that some of the houses in my village now have TN-C-S, such that my water supply pipe is probably now 'enjoying' that TN-C-S earth via bonding in the TN-C-S houses].

Kind Regards, John

 

[JohnW2]

Some of the Crabtree ones also had a magnetic overload trip inside which would disconnect on L-N faults. That would probably disconnect before some rewireable fuses would fail.

Ah, I see - the prototype 'MCB'! Did that actually work very well? Given that it was installation wide, it presumably had to have an In much higher than that of any of the final circuit fuses - so I would have thought that it would have been a bit of a challenge for it to 'beat' any of those fuses, even if there was a very high PSCC.

Kind Regards, John

 

[ericmark]

I looked at the IET forum to see if there was a testing procedure when we used ELCB-v devices.

Regulation E.5 in 1966 14th (I haven't gone back or forward to ascertain history of earth-leakage circuit-breaker test requirements) requires that its effectiveness shall be tested in accordance with Item (3) of appendix 6. This says: For the purpose of this test a voltage not exceeding 45 volts, obtained from a double-wound transformer connected to the mains supply, shall be applied across the neutral and earth terminals (or neutral and frame terminals of a voltage-operated earth-leakage circuit-breaker) and the circuit breaker shall trip instantaneously. There is more text.

And..........of course, Regulation E.11 required a completion certificate to be issued, accompanied by an inspection certificate with test results.

For the purpose of this test a voltage not exceeding 45 volts, obtained from a double-wound transformer connected to the mains supply, shall be applied across the neutral and earth terminals (or neutral and frame terminals of a voltage-operated earth-leakage circuit-breaker) and the circuit breaker shall trip instantaneously.

Correct, also there was a point that the ELCB should also trip within 70% of 50mA also, the figure of 50mA arose because this is the maximimum a person can normally endure before fibrillation can take place. On the old testers the maximum was 35mA and if the ELCB did not trip within this parameter it had to be replaced.

Also where RCD's are supposed to be tested every three months, the ELCB had to be tested every month using the test button.

From those two replies it would seem there were machines made to test the ELCB-v See link for what was said on the subject what I wonder is how today we can test the device as clearly the machines are no longer manufactured.

So question is when going to a property with an ELCB-v should you.
1) Explain you have not got the equipment to test the old system and tell owner it will need changing to modern system before you can test.
2) Do all the inspecting and testing you can do with your equipment and note the ELCB-v has not been tested and could cause a danger.
3) None of the above.

I can't see how one can fail a property because it complied with a pre-1974 standard for an ELCB-v, but follow the ESC directive for a lighting without an earth which was pre-1966. There is nothing wrong with ELCB-v protection if all the safety tests are made including electrical isolators on incoming water, gas and other services.

What is a problem is the "TOPTRONIC Power Point Tester" which was used to test the ELCB-v is no longer available and even when it was few electricians had access to one.

 

[ericmark]

Must have dates wrong I have a "CITB Study Notes IEE Regulations 15th Edition Installation Electrician" Republished 1984 which details the use of an ELCB-v giving wiring diagrams.

So it would seem still in use in 1984. After 1984 the 15th had an amendment per year so I suspect it was discontinued in one of the amendments or when 16th came out.

 

[JohnW2]

Regulation E.5 in 1966 14th ... Item (3) of appendix 6. This says: For the purpose of this test a voltage not exceeding 45 volts, obtained from a double-wound transformer connected to the mains supply, shall be applied across the neutral and earth terminals (or neutral and frame terminals of a voltage-operated earth-leakage circuit-breaker) and the circuit breaker shall trip instantaneously.

That's obviously just 'testing from first principles'. The crucial question is presumably whether or not the consumer-side 'earth' of the installation (including possible parallel paths to earth) was disconnected from the VOELCB for the purpose of that test.

So question is when going to a property with an ELCB-v should you.
1) Explain you have not got the equipment to test the old system and tell owner it will need changing to modern system before you can test.
2) Do all the inspecting and testing you can do with your equipment and note the ELCB-v has not been tested and could cause a danger.
3) None of the above.

I can't find it anywhere at present (the Index doesn't help) but I must say that I thought there was a statement somewhere in the current regs that VOELCBs were no longer acceptable (even if they were acceptable and compliant when installed) - that being an exception to the general rule that the regs are not retrospective. Even if I dreamed that, I would think the first thing you should do would be to measure the EFLI from final circuits. If (as is likely) they were appreciably below the (assumed) impedance of the VOELCB's coil, indicating the presence of appreciable parallel paths to earth, I think you could confidently conclude that the installation was unsatisfactory and dangerous, because it could not posibly have effective protection against L-E faults, even if the VOELCB were working 'per spec'.

I can't see how one can fail a property because it complied with a pre-1974 standard for an ELCB-v, ... There is nothing wrong with ELCB-v protection if all the safety tests are made including electrical isolators on incoming water, gas and other services.

See my above comment about what I though was in the current regs, even though I can't currently find it. In any event, in practice, isn't the reality that you will only fairly uncommonly come across properties that have no parallel paths to earth (although it's probably becoming more common, with the increasing prevalence of plastic supply pipes and outdoor meters)?

Kind Regards, John