Earth Loop Impedance

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IET Wiring matters issue 72 said:
: Values of Ze for 230 V single-phase TN and TT supplies not exceeding 100 A according to ENA ER P23/1:1991

Supply arrangement

TN-C-S (Note 1) 0.35 Ω (Note 2)

TN-S 0.8 Ω (Note 2)

TT 21 Ω (Note 3)

  • Note 1: In ENA ER P23/1:1991, this value was quoted for both Protective Multiple Earthing (PME) and Protective Neutral Bonding (PNB) earthing arrangements. Higher values may apply where the consumer was supplied from small capacity pole-mounted transformers and/or long lengths of low voltage overhead line.
  • Note 2: The external earth fault-loop impedance for TT systems consists of the resistance of the neutral to earth plus the impedance of the transformer winding and line conductor, but does not include the resistance of the consumer's earth electrode.
There seems to be a slight change, however I have always wondered what the pass limit is when incoming is under the stated figures? If one measures 0.20 Ω at incomer then should the earth loop impedance for a B32 MCB be 1.22 Ω to allow for the incomer to rise to 0.35 Ω or is this what the 95% is for, so 1.37 Ω is used?

We are told in one breath we should not rely on the RCD yet with 21 Ω plus what ever the premises earth rod is, clearly we do rely on the RCD. OK in the main with a B type MCB the volt drop not earth loop impedance is the limiting factor for cable length so in real terms we are not that close anyway.

Most of my work the supply was over 100A so the figures quoted did not apply, with the step down transformer on site, even if we had no access to it, we would clearly know if the supply impedance changed, so we could use actually readings. So this is some thing which did not really matter for me.

So what does the team think?
 
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either the Ze and/or the PFC might increase at the origin, so not only can you not rely on disconnection times, any CPCs calculated from I2t might melt in a fault. I suppose this is why we are supposed to get testing done every 5 years, then any changes would get picked up.
 
There seems to be a slight change, however I have always wondered what the pass limit is when incoming is under the stated figures? If one measures 0.20 Ω at incomer then should the earth loop impedance for a B32 MCB be 1.22 Ω to allow for the incomer to rise to 0.35 Ω or is this what the 95% is for, so 1.37 Ω is used?
Don't we assess the adequacy of fault protection of a circuit on the basis of the measured Zs at the time of measurement, despiute the fact Zs of the circuit might change at some point in the future if Ze were to change? Indeed, is the measurement of Ze (or Zdb) necessarily a routine part of testing an installation?
We are told in one breath we should not rely on the RCD yet with 21 Ω plus what ever the premises earth rod is, clearly we do rely on the RCD.
I don't think that anyone (other than a complete idiot) would ever suggest that a TT installation has any choice but to 'rely on an RCD' for fault protection, would they? I think what you are probably referring to is the fact that one should not rely on an RCD to provide fault protection in a TN installation just because one is not inclined to take steps to get the Zs low enough for OPD-based fault protection to be satisfactory.[/QUOTE]

Kind Regards, John
 
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Just "or", surely (not "and/or"? If Ze were to rise, PFC would fall, and vice versa - they cannot both increase.
not if the incoming supply has more than two separate conductors, e.g. L, N, E.
worst case fault current is the highest possible current involving a phase conductor
worst case disconnection time is the highest possible impedance path involving a phase conductor

Yes for single phase PME the PEFC and PSSC would be the same as there are only two incoming conductors.
 
either the Ze and/or the PFC might increase at the origin,
Is it just a strange way to phrase it?

As
PSCC is directly related to the Neutral loop (I call it Zn) and
PEFC is directly related to Ze and,
as you say, PFC is greater of PSCC and PEFC.

So,
PSCC goes up and own with Zn and
PEFC goes up and down with Zs.

In efect you are saying that "either Ze and/or Zn might increase.

Perhaps it is the 'either' confusing things. You can't have 'either - and'.
 
not if the incoming supply has more than two separate conductors, e.g. L, N, E. ... worst case fault current is the highest possible current involving a phase conductor ... worst case disconnection time is the highest possible impedance path involving a phase conductor
I'm struggling a bit here ...

If I understand you correctly, you appear to be talking about a situation in which the L-E loop impedance increased AND (simultaneously) the L-N loop impedance decreased. What I'm struggling with is trying to think of a (single) 'change' (or fault) which could result in such a situation arising - am I missing something 'obvious'.

Kind Regards, John
 
With a ring main (not ring final) the supply can be split and any section isolated in order to work on it, this can completely alter the supply loop impedance, also the supply can be repaired and it seems to say they can turn a TN-S into a TN-C-S without informing the user which in turn could alter the loop impedance both line - earth and line - neutral.

With domestic we are except Scotland only required to inspect and test every 10 years, and even that is voluntary, and we are permitted to find Ze by enquiry, in fact only way with a system which has not been as yet connected to mains supply.

Nothing says we have to use a loop impedance meter, we can use a low ohm ohm meter using at least 250 mA and test the whole house with not supply as yet to it, I know one time when the supplier said on the phone it would be TN and tails were left ready to connect, but on connection they were supplied with TT and it was some months before the electrician returned and found the error.

*2 Ze at origin. When the maximum value declared by the distributor is used, the effectiveness of the earth must be confirmed by a test. If measured the main bonding will need to be disconnected for the duration of the test.

However since the electrician had not been told house had been finished and supply connected this did not happen.

*3 Prospective fault current (PFC). The value recorded is the greater of either the short-circuit current or the earth fault current. Preferably determined by enquiry of the distributor.

Fact standard forms want you to measure Ze but enquire about PFC which are clearly linked by ohms law, shows the IET is well aware it could change, however there must be a pass through limit on the supply fuse so really unlikely with domestic PFC ever exceeds 4.7 kA so for domestic does not really matter. However it is same form for commercial.

I am guilty of using the phone, I know for health and safety it must be in writing, but I have phoned up the DNO and asked what the Ze and PFC will be and I have never had written confirmation, maybe it was sent to occupier, but I didn't see it. If they say 0.35 Ω over the phone I assumed it would be that or better. In the main the DNO have required an electrician to be in attendance before connecting, however in that one house at least that didn't happen. The electrician was going through a divorce, and stopped sole trading and that job did not have power when he left his house and went to live in a narrow boat which resulted in problems for people phoning or sending letters, this country is not very good with people of no fixed abode. He had a few jobs where the client was not ready for him to finish the work due to building work in progress and this was one.

However the question remains, with supplies of 100 amp or less should we allow for the Ze to raise to the 0.35 and 0.8 ohms figures given for TN-C-S and TN-S supplies?
 
I'm struggling a bit here ...

If I understand you correctly, you appear to be talking about a situation in which the L-E loop impedance increased AND (simultaneously) the L-N loop impedance decreased. What I'm struggling with is trying to think of a (single) 'change' (or fault) which could result in such a situation arising - am I missing something 'obvious'.

Kind Regards, John
If a cable has been repaired this clearly can happen be it copper replacing aluminium or other cable where there is a poor earth connection or simply a change in cross sectional area, how is not important, what is important is should we allow for the change?
 
If I understand you correctly, you appear to be talking about a situation in which the L-E loop impedance increased AND (simultaneously) the L-N loop impedance decreased. What I'm struggling with is trying to think of a (single) 'change' (or fault) which could result in such a situation arising - am I missing something 'obvious'.
I think you may be missing that (except TN-C-S) the Ze could rise but the PFC may remain the same. Edited - deleted the wrong part.

Just the original wording was confusing.
 
Last edited:
I think you may be missing that (except TN-C-S) the Ze could rise but the PFC may remain the same or vice versa or both could rise if, for some reason, the Line impedance rose.
As you imply, we're effectively talking just about TN-S, since such things clearly cannot happen with TN-C-S, and the discussion is irrelevant in terms of TT.

However, I remain confused. Yes, as you say, Ze could rise and PFC could remain the same (if it was PSCC) - that's surely the common situation of what goes wrong with TN-S supplies (and gets them converted into TN-C-S!). However, John wasn't talking about PFC 'remaining the same' when Ze increased - he was suggesting that it could actually increase ("Ze and/or PFC might increase"). I still don't see what (single) change or fault could bring that about ... if Ze increased then PEFC would obviously decrease, so any increase in PFC would have to be due to an increase in PSCC - but, as I keep asking, what (single) change can make the L-E loop impedance rise and the L-N loop impedance fall?

could remain the same.
 
If I understand you correctly, you appear to be talking about a situation in which the L-E loop impedance increased AND (simultaneously) the L-N loop impedance decreased
No i never mentioned simultaneously, in this context we are talking about any changes that may happen between tests on the installation which could be 30 years plus on a domestic.
The whole AND thing is a red herring, and i added it just before hitting post just to make sure i covered all the possibilities.
 
In efect you are saying that "either Ze and/or Zn might increase.
Not quite, i meant in the time between tests on the installation Ze may increase or Zn may decrease or both may happen. Or vice versa i suppose.
 
I still don't see what (single) change or fault could bring that about ...
Yes, I suppose you are correct in saying that.

However unlikely, other John did not say 'single' change.
If impedances can increase, then they can also decrease when the cause is rectified.


Not quite, i meant in the time between tests on the installation Ze may increase or Zn may decrease or both may happen. Or vice versa i suppose.
Yes, perhaps it would also have been better to say:
"Ze and/or PFC might change".
 
No i never mentioned simultaneously, in this context we are talking about any changes that may happen between tests on the installation which could be 30 years plus on a domestic.
OK, fair enough. As you will understand (I kept using the word "single"), I thought you were talking about a single change/fault. Once one considers two or more changes/faults over a period of time then 'anything' is obviously possible.
The whole AND thing is a red herring, and i added it just before hitting post just to make sure i covered all the possibilities.
Again, fair enough.

As for the rest of your original comment, there would obviously only be a theoretical risk of the CPC of a final circuit 'melting' (in the event of an L-CPC fault) if the Ze that had reduced (you were talking about 'increased'), regardless of what had happened to the PSCC. In practice, unless we were talking about a very short circuit (length!) wired in high CSA cable, I think that the risk of a CPC melting (as a result of a fault) would probably be almost non-existant, even if the Ze (hypothetically) fell to zero, since the R1+R2 of the circuit would be a significant portion of the total Zs.

Kind Regards, John
 

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