Following up on the recent thread about Earth Resistance measurements

[sophiecentaur]

The PV installers wouldn’t have seen a problem - 130Ω is within spec for TT. Don’t forget the RCD requirement.

I can’t explain why you’re not TN-S - perhaps the earth loop impedance (as it’s called, although I was scoffed at on here for doing so!) was above the 0.8Ω max due to poor connections or long cable run.

I had an interesting chat with a guy in UK Power Networks. He saw my problem.

He did say that the supply may never have been changed since 1930 and that could explain why TT is there. He will send engineers to look at the system and they may well fit a termination to the armoured sheath. However, it appears that not all installations 100 years ago used a metal (lead) sheath so I can't get my hopes up too high.

Another thing. I was talking to my neighbour and she let me look at her installation. Despite having been re-wired not long ago, it still has a TT system. But the installers presumably measured an 'acceptable' earth impedance so no one wanted to fix it. She was worried, of course but I told her she's legal and that I'm just fussy.

The UPN service is FREE though!

 

[Guineafowl]

I had an interesting chat with a guy in UK Power Networks. He saw my problem.

He did say that the supply may never have been changed since 1930 and that could explain why TT is there. He will send engineers to look at the system and they may well fit a termination to the armoured sheath. However, it appears that not all installations 100 years ago used a metal (lead) sheath so I can't get my hopes up too high.

Another thing. I was talking to my neighbour and she let me look at her installation. Despite having been re-wired not long ago, it still has a TT system. But the installers presumably measured an 'acceptable' earth impedance so no one wanted to fix it. She was worried, of course but I told her she's legal and that I'm just fussy.

The UPN service is FREE though!

Keep us posted!

 

[sophiecentaur]

Keep us posted!

There has been a development. Men from ' National Power Networks' arrived today and took a look. They stripped off the insulation at the end of the sheath and found no continuity between that and the Neutral conductor.
They told me that it's probably a legacy from the original DC System that was common in our area. There are two conductors coming into the house so at least it wasn't Earth Return.

They told me I have a TT earthing system (which we knew) and that I would need to get a 'better' earthing connection if I want to improve things.

The guy didn't want to commit to what the lead sheath is connected to at the other end but I can't imagine it's not continuous back to the substation so I have to ask WTF. I'll follow this up with Power Networks but I imagine that even making connections to all substation transformers could cost them a fair bit. So "just put up with it mate" is probably going to be the answer. I am surprised that there have been no small disasters in the system if there are widespread TT systems around our area (and many others, probably. Our lives are obviously subject to the need for 100% reliability of RCDs.

I'll look into the history of power distribution and, of course, would be grateful for any input from PF.

 

[Guineafowl]

There has been a development. Men from ' National Power Networks' arrived today and took a look. They stripped off the insulation at the end of the sheath and found no continuity between that and the Neutral conductor.
They told me that it's probably a legacy from the original DC System that was common in our area. There are two conductors coming into the house so at least it wasn't Earth Return.

They told me I have a TT earthing system (which we knew) and that I would need to get a 'better' earthing connection if I want to improve things.

The guy didn't want to commit to what the lead sheath is connected to at the other end but I can't imagine it's not continuous back to the substation so I have to ask WTF. I'll follow this up with Power Networks but I imagine that even making connections to all substation transformers could cost them a fair bit. So "just put up with it mate" is probably going to be the answer. I am surprised that there have been no small disasters in the system if there are widespread TT systems around our area (and many others, probably. Our lives are obviously subject to the need for 100% reliability of RCDs.

I'll look into the history of power distribution and, of course, would be grateful for any input from PF.

Remember that of the three earthing options, only TN-S and TN-C-S are the DNO’s responsibility. TT is entirely the customer’s.

I’d say they had three options:
1. Convert to TN-S. No, since there’s no N-sheath continuity. Cable is probably too old to trust the integrity of the sheath long-term anyway.

2. Convert to TN-C-S. The old cable is probably not suitable, being more at risk of the dreaded broken PEN. Modern supplies use split concentric cable, where the N is split into multiple wires to reduce the risk. A TN-C-S system for you might also require changing the final distribution to PME, where the N is earthed at multiple points, so, a lot of work.

[Note: for both the above, you’d likely have to upgrade your side of the supply as well; earthing/bonding conductors on TT can be tiny compared to the 10mm2 or so needed for the others, which might have to handle the full prospective earth fault current (PEFC) in the kA range.]

3. Leave you on TT.

If they’re talking about a converted DC supply, then surely your cable must be at least 1960s vintage? In which case, past its sell-by date. Is it the old paper-wrapped stuff? Perhaps you could try ‘please replace my antiquated supply, and while you’re at it, give me a decent earth’. Reasonable, given the electrification of modern life - heat pumps, EVs, etc. They’ll have to do it at some point.

In the meantime, why not bang in your own earth rod, or get one put in? You can couple two 4’ ones together. You can also get your RCDs tested for trip time and level under various conditions. At the very least, you can use the test button quarterly (as everyone does, of course!).

If it comes to it, TT isn’t that bad. It’s even the earthing of choice these days in some cases, eg outbuildings, hot tubs, EV chargers.

 

[sophiecentaur]

If they’re talking about a converted DC supply, then surely your cable must be at least 1960s vintage?

Perhaps you could try ‘please replace my antiquated supply, and while you’re at it, give me a decent earth’.

Thanks.
It could be nearly 100 years old.
I'll tell them that my 60A supply fuse is not enough for my electric shower, planned EV and garden extension. If it's as bad as it could be, there could be a lot of digging trenches in our road. The neighbours would all benefit and would need to give approval for upgrades.

 

[Guineafowl]

Thanks.
It could be nearly 100 years old.
I'll tell them that my 60A supply fuse is not enough for my electric shower, planned EV and garden extension. If it's as bad as it could be, there could be a lot of digging trenches in our road. The neighbours would all benefit and would need to give approval for upgrades.

Feeble! Your electric shower would draw 40A or so, for a start. My house has 100A*, and it’s just a rural croft house.

I’d say you’ve got a pretty good case for an upgrade. Try to catch them for a quote before the winter, when they’ll all be chasing storm damage.

* The fuse carrier is rated 100A, but without breaking the company seal and looking inside, I can’t be sure. Sometimes they fit a smaller fuse than rated.

 

[sophiecentaur]

@Guineafowl
I got back to UKPN and it's apparent that they have a structure with several different 'departments' and you have to ask the right question of the right department. When I talked about an 'upgrade' they put me through to a lady who assumed I wanted a three phase supply - because I was talking to the upgrade department. Nobody ( none of the the young women) would have known about the 20th Century but I got an email back with a copy of what I had said about the historical DC service. That particular woman was very helpful and she has booked a 'surveyor' to give an estimate of the possible cost to me!! of any upgrade. She was talking in terms of holes in the road, road closure or traffic control. Top end could be tens of £k.

I've done some trawling around and opinions are not high about T-T systems. Unless my bloke comes and accepts that some installations are just too old to be acceptable and that UKPN would pay for any change, I can see that the Ombudsman might have to be involved. The Ombudsman's website stresses that they cannot deal with 'whatif' situations and users need something to appeal against.

I remember in the 1950s, my parents moved into a remote cottage with no supply and it took several months of oil lamps etc. before there was a hearing and we got connected. Several poles were involved, iirc. (Not the European ones)

I should be a lot better informed by next Tuesday.

 

[Guineafowl]

Men from ' National Power Networks' arrived today and took a look. They stripped off the insulation at the end of the sheath and found no continuity between that and the Neutral conductor.

I’ve just read this bit again - am I missing something, or did those engineers just walk away from a fault? At least part of your cable sheath is floating. What you need is not so much an upgrade, as an urgent repair.

If there were a phase-sheath/earth short after the break, would there not be step potentials? What if you were using an earthed appliance outside, and simultaneously touched something conductive sticking out of the ground, eg a metal railing, near the fault?

 

[sophiecentaur]

did those engineers just walk away from a fault?

Almost. They left an 'assessment form' with the comment "No earth reading. Advisedto get a TT Earth by a qualified electrician"
So they would say that the advice didn't constitute actually walking away from a fault. There might be a problem if the installation was judged to be a hazard. In a possibly similar situation when gas is smelled, the gas company can disconnect you.

 

[Averagesupernova]

1. TT.
You get L and N only, and fit your own earth rod. Earth loop perhaps 100Ω, too much to pop MCBs and fuses reliably, so the whole installation must be protected by RCDs. Mainly older, remote rural properties.

Based on the above, I don't see why the power company should be responsible for the scenario at hand. If this was an accepted practice at one time then it should be now. The ground rod may in fact be missing or previously removed, etc. Of course this is not correct and needs to be dealt with but since the original plan was that the customer provides the earthing then it should be reasonable that the customer is expected to replace or repair what is involved in earthing this system.

 

[sophiecentaur]

If this was an accepted practice at one time then it should be now.

There are many accepted practices in the past which are not acceptable now. It's only cost that affects that.

The presentEarth Loop resistance is 130Ω which is in spec. But that is not inherently safe because it relies on active protection. The standards for T-T earthing are surely only there because of the cost of replacement with safer methods. It's understandable in certain circumstances but, in an urban situation with dense population, 'proper earthing' should be a matter of course.

IMO, this is as much a political matter as anything and it's only because people don't understand the relevance of earthing and the possible dangers and costs of sorting out the problem. Underground services are all potential time bombs. Sewerage, water supply and gas need attention and so does the electricity supply. I feel it's worth while my trying for a solution to my personal problem here.

 

[Guineafowl]

I don't see why the power company should be responsible for the scenario at hand.

That’s right, a TT earth is the customer’s responsibility, and perfectly acceptable if in spec. The OP requested an engineer visit to assess for TN-S connection to avoid the drawbacks of TT.

The visit revealed something separate - that the ancient incoming service cable, belonging to the power company, has a conductive sheath that isn’t continuous back to the supply earth/neutral, nor bonded at the customer end.

To me, this is a fault for the reasons given above, post #38. Not grounds for immediate panic, but something that would form a case for service cable replacement. What say the EEs?

 

[Averagesupernova]

@sophiecentaur based on your last post then I see this as you simply want an updated system. Why the beating around the bush?
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As far as this goes:

There are many accepted practices in the past which are not acceptable now.

This is a straw man argument. It may be your opinion and of course you are entitled to it. If systems like this exist and are not on a list for updating then I'd say it is acceptable now.

 

[sophiecentaur]

This is a straw man argument. It may be your opinion and of course you are entitled to it. If systems like this exist and are not on a list for updating then I'd say it is acceptable now.

Hardly a straw man argument. I could ask you if you had the opportunity to change from a TN-S system to T-T would you change? What would be a good reason? T-T is a last resort when there is no low resistance company earth supplied? I notice you write from US which is often a different electrical environment from UK.

RCDs work pretty well but not as reliably as an earth connection which can blow a fuse and disconnect the supply from the faulty circuit.

If systems like this exist and are not on a list for updating then I'd say it is acceptable now.

Acceptable to whom? That demonstrates a strong faith in the politics and economics of your supply industry.

 

[Averagesupernova]

Acceptable to whom? That demonstrates a strong faith in the politics and economics of your supply industry.

In the USA no matter how old the service is there is nothing stopping me from updating to a 'safe' installation by adding three wire circuits or replacing the main panel, etc. Those costs are on me. There is no TT, TN-S, etc. All residences get two hots and a neutral. Very very old residences may get only one hot and a neutral. It is expected that at the main service panel the neutral will be attached to the chassis of the panel and from there a third wire will attach to the chassis of anything plugged in. Doesn't matter how old the wires are feeding the service. We don't have, nor ever had the variability that the UK seems to have. Yes, I feel safe and have faith in the system as it is intended.
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Yes, the third wire attached to chassis as I described is certainly safe. However, don't think a ground rod or two connecting to the neutral in your case will will cause a fuse to blow in the event that a hot wire touches a chassis. This is a common misconception. There are videos out there that demonstrate this. The actual earth is unable to provide a low resistance path in order to blow a fuse.
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I searched for a bit but couldn't find a video demonstrating this. Put out by Mike Holt.

 

[Guineafowl]

There are pros and cons to every earthing system.

 

[sophiecentaur]

Yes, the third wire attached to chassis as I described is certainly safe. However, don't think a ground rod or two connecting to the neutral in your case will will cause a fuse to blow in the event that a hot wire touches a chassis. This is a common misconception.

That statement includes the word "misconception" and I agree with that in some circumstances. Many faults are not just a simple 'short circuit' though. I'm used to fused plugs which prevent excessive current through an appliance. But even they will not blow when in series with a typical T-T earth resistance when the 'fault to case' is in series with an internal resistive path, leaving the case at a lethal voltage. True, any nearby conducting parts are likely to be bonded to the house earth so the pd may be very low but the internal fault could be nicely cooking the faulty appliance without blowing any fuse and still leave the appliance case live. A TN-S system will not allow that to happen because a fault current will always be enough to blow a fuse. I lived in a house in the 60's which had an Earth Leakage Trip which was a relay in series with the house earth and the rod in the ground. No biological protection there (no solid state products available at the time) and definitely not fail-safe. Strangely, they don't do it that way any more.

Of course, the RCDs should avoid disasters happening but, of all the millions of RCDs in homes around the world, how many are tested regularly? (That is recommended by the makers.) They're only tested when 'the man' calls to fix a fault or instal something else. Fail safe is a very comforting design. I don't think I should be criticised for wanting high levels of safety. The parallel with driving applies here; doing (the legally marked) 50mph in a country lane doesn't guarantee your safety all the way along the 50mph zone

 

[Averagesupernova]

So what is to stop you from joining the earth and neutral in your main panel? You would end up with TN-C-S right? I'm not suggesting you do this. It is truly a question. Post #34 touched on this a bit. I can't see how it would not be safer than leaving it as it is.

 

[Guineafowl]

So what is to stop you from joining the earth and neutral in your main panel? You would end up with TN-C-S right? I'm not suggesting you do this. It is truly a question. Post #34 touched on this a bit. I can't see how it would not be safer than leaving it as it is.

Various reasons:
- A TN-C-S earth is almost always PME (protective multiple earthing), meaning the neutral must be earthed at multiple points along the final distribution path, establishing a decent equipotential zone. So, it’s a network thing, not simple link in the home thing.

- You’d be making the earthing the DNO’s responsibility without their permission.

- As far as I know, the N-E link is never in the distribution board here, always at the service head. A glaringly obvious DIY bodge if anyone were to inspect.


Again, there’s nothing really wrong with a TT earth. I had one for 8 years. Many consider it safer than TN-C-S (or N-E link, or MEN, depending where you are)* and opt to use it even today. For example, a distant outbuilding (too far out of equipotential zone) or EV charger.

* Imagine you have TN-C-S/N-E link/MEN, and the supply neutral goes open circuit, for whatever reason. All the earthed metalwork in your property, including the charging EV, will be enlivened. A big drawback.

 

[Averagesupernova]

- A TN-C-S earth is almost always PME (protective multiple earthing), meaning the neutral must be earthed at multiple points along the final distribution path, establishing a decent equipotential zone.

Ok. I suspected as much. Here in the USA the transformer is seldom very far from the service panel. Several hundred feet at the most. There will be a ground rod at the transformer as well as one at the service panel for the residence. Two in a lot of cases because NEC says if the resistance is too high with one it needs to be two. This often translates into just put two in.
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In the UK I understand that distances are much farther between the transformer and the residence. It seems to me unless there are points along the run of cable feeding the residence that ground rods are installed, it wouldn't make a difference.
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The neutral is tied to the chassis of the meter box here in the USA as well as the main service panel. After the main service panel the ground (earth) are kept separate. Some places now require an emergency disconnect on the outside for fire department, etc. I'm not sure how that changes thing concerning where the earth and neutral split off.

 

[sophiecentaur]

Here in the USA

We are in different worlds and I need some advice which applies to a particular practical situation in UK.

I appreciate your input because of your high level technical knowledge but I need specific advice about what is approved over here in UK and how much I may have to pay. This whole issue is obviously a cloudy one and I say that because UKPN staff are reluctant to give me a definitive answer or opinion about T-T. Their answers would be loaded in terms of cost (to them in particular) so when they say something's acceptable it refers to economics and minimal safety considerations.

 

[Guineafowl]

Ok. I suspected as much. Here in the USA the transformer is seldom very far from the service panel. Several hundred feet at the most. There will be a ground rod at the transformer as well as one at the service panel for the residence. Two in a lot of cases because NEC says if the resistance is too high with one it needs to be two. This often translates into just put two in.
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In the UK I understand that distances are much farther between the transformer and the residence. It seems to me unless there are points along the run of cable feeding the residence that ground rods are installed, it wouldn't make a difference.
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The neutral is tied to the chassis of the meter box here in the USA as well as the main service panel. After the main service panel the ground (earth) are kept separate. Some places now require an emergency disconnect on the outside for fire department, etc. I'm not sure how that changes thing concerning where the earth and neutral split off.

Yes, I think we’re into ‘tomahto/tomayto” territory here. What you have is TN-C-S with extra earth rods at the installation end. You may not have N-E bonding along the power poles, but all those houses with rods would make it effectively a PME system.

Sometimes, TN-C-S supplies have “belt ‘n’ braces” auxiliary earth rods here, and metal service pipes (gas, water) would be bonded to supply earth/neutral, effectively the same as a rod.

What you must also have is the lurking danger of the broken PEN/neutral. Older EV chargers had to be installed in their own TT island if in a TN-C-S property, but I gather the newer ones have neutral loss detection. But of course, that’s placing safety in the hands of electronics.

 

[sophiecentaur]

Again, there’s nothing really wrong with a TT earth. I had one for 8 years. Many consider it safer than TN-C-S (or N-E link, or MEN, depending where you are)* and opt to use it even today. For example, a distant outbuilding (too far out of equipotential zone) or EV charger.

My last house had an overhead supply and T-T earthing. Nobody died so nothing was "really wrong".

There are obviously a range of opinions about this and I won't lose too much sleep over it, whatever the outcome. But it annoys / worries me that my house (and everyone around me) may be served by a truly ancient network or, even worse, by a network which was 'up-graded a bit' without taking a good earth connection to all the houses.

 

[Averagesupernova]

We are in different worlds and I need some advice which applies to a particular practical situation in UK.

This particular forum is not likely the place to get your needed advice. We've covered the technical end of what is safe and what is safest. We are pretty much in agreement. Even though we are on opposite sides of the Atlantic electricity behaves the same. At this point it is whether or not you are able to get your power company to do what you want or need. They may go to the trouble to update your service if it means selling you more power. It varies from one place to another and from one company to another here in the USA as to how cooperative they are about these things. Some places they'd be more than happy to increase the available current at a reasonable cost to the customer but their resources are finite and there's a lot of other work to be done farther up the list. So, be happy with what you have and wait. I can assume something similar where you are at.