Told I need to have Ze checked, an R1+R2 test, and possibly downrate a 40A fuse to 32A - any way to save money here?

[justanotherguy]

Recently our induction hob broke, so I bought a new one thinking it would be a drop in replacement. The circuit has a dedicated 40A breaker.

When I wired it up, it simply wouldn't switch on. We got a replacement for that and... same thing. So the third one we paid extra for John Lewis to install, but they refused saying the Ze return was 1.19 Ohms instead of the "maximum" 0.87 Ohms. They did mention they tested a plug socket and found the same issue. They did say it was only about safety, and wouldn't be the reason the first two didn't work.

We've now called an electrician and they are recommending an R1+R2 test (to see if it's just that circuit), a Ze test (to see if the connection into the house is the issue), and downgrading the MCB to a 32A. We can only find those second hand right now, presumably as it's so old (MK Sentry - LN5932s on eBay).

The wiring/consumer unit is probably 20 years old - and we're probably a year or two off finally doing a full refurb/rewire, so really want to spend the least possible to just get a working hob (safely).

The quotes I've been given are £100+VAT (so £120...) per hour - but noone seems to want to give me a fixed quote, or confirm how many hours (this is Zone 4 London).

I'm just wondering if that's the only option right now? Would the DNO just come and check for free before I go about fixing an issue that might not be internal? Is there anything I can check or narrow down myself?

 

[Adam_151]

Are you able to post a picture of your mains intake and consumer unit area, so we can establish what earthing system you have, and what you have in terms of RCD protection If you have an earth provided by the DNO they are obligated to maintain it. If you are getting high loops on sockets as well then we can probably rule out a poor joint on the cooker cirucit being an issue.

If the cirucit is on RCD then the loop being a little high for the 40A breaker is not so much of a drop everything and get it fixed before you can can cook dinner issue. Yes, high incomming loop does not need addressing, afterall it could deteriate further (which is why a 32A breaker is probably not the pancea others think it is, yes it tests out fine now, but what about in three months time?) but a 30mA RCD technically has a maximum allowed loop of 1666ohms (however if you are anywhere near this value, then its going to be to very unstable, but it does have the ability to act in a sticking plaster sort of way to give you fault protection until the DNO can dig up the road and fix the actual issue)

 

[justanotherguy]

Thanks for your help.

Have attached photos - it's a hideous mess and I can't wait for the day we replace it.

 

[ericmark]

For a non electrician to find loop impedance is hard, but an electrician will have an expensive loop impedance meter so he can either connect two leads or plug in press a button and read direct the loop impedance/prospective short circuit current.

So some thing seems wrong. Yes R1 + R2 is the standard method for a ring final, but a hob should not be on a ring final, it should be on a dedicated radial.

 

[Murdochcat]

The Ze is important, then the Zs and finally the R1 + R2 which is important as it’s a radial

Problem is that the “tester” they have is not to be trusted.

£100 + VAT sounds a lot but you are near London

I would be £60 - no VAT for the first hour

 

[Adam_151]

It looks like everything is RCD protected, so the risk of a high resistance suppliers earth is a lot less than if it wasn't. An RCD can actually be used as fault protection, but this is generally discouraged in TN systems, and definatly not really the done thing when you suspect there is a fault that ought to be fixed rather than ignored.

You have a TN-S earth (at least as far as its presented to you), SNE (separate neutral, earth) in DNO terms with the lead sheath of the cable providing the earth conductor, TNS supplies => 100A are allowed to have a Ze of upto 0.8ohms, but I'd suspect that yours is probably a little over that, plus having it that high in what I'm guessing is a tightely packed urban area is unusual without a fault. What often happens is the joint where your service cable is jointed to the street main breaks down over time (bearing in mind your incomming service is going to be at least 60 years old, likely even more) Or sometimes the supply is looped through a neighbouring property first, and the arrangement connecting the two sheathes is no longer as it should be (your earth connection does look to be using the wrong type of clamp, but doees not seem to look insecure - but dont try and fiddle with it)

Best think to do is get in touch with the DNO (Who I suspect is EDF as they look after the old LEB (London Electricity Board) area) and tell them you have been told you have a high Ze / and can they come and look at sorting it. The first thing the DNO will do is a loop impedance and then they'll tell you whether they think there is a problem or not, if you are on site when they are there, try and ask what they measured, and also note whether they turned you off, separtated their incomming cable from your earths and tested the incomming alone (as opposed to doing it with everything still connected - which can mask problems)

Its not really worth paying somoene to some and take a Ze, when the DNO will do that as the first step anyway after you get them out to investigate a high loop

 

[securespark]

That connection to the PILC incoming cable looks like it's via an earth clamp, which is a no-no.

Best replaced with a constant force spring which is a coiled strap which is wrapped around the lead sheath of the incoming cable. Only the DNO can work on the incoming supply.

The maximum allowable Ze for TN-S is 0.8 ohms, but hopefully the DNO will get a much lower Ze, allowing you to get a Zs comfortably within spec at the end of the hob circuit. Thought a 40A breaker max Zs was 0.92 ohms? Maybe my rusty brain misremembers?

That is for a type B. If it's a C the maximum Zs will be lower still.

 

[ericmark]

ZE the earth loop impedance at the incoming supply with a TN-C-S supply of 100 amp should be 0.35 ohm or less, however with lower supply amps, or a TN-S system it can be higher, and with TT it can be 200 ohms, above that it is considered unstable.

The line - neutral is normally measured as PSCC but ohms law 230 volt/either current or impedance gives the other. So the two are related. As well as the ability to trip an overload, it also relates to volt drop.

In the main it is the volt drop which means 0.35 ohms is the limit for a 100 amp supply. However this is line - neutral so only valid with a TN-C-S supply, this is the most common supply, but not sure what your supply is, so can't say what your limit is.

Looking at picture there seems to be an earth clamp around the supply cable, so could be TN-S or TN-C-S or even an unofficial link to DNO earth.

The DNO are duty bound to tell you max Ze and earth type. But you say an electrician attended, so back to first post, why did he not test it? It is so easy for a scheme member electrician who must have the test gear.

 

[JohnW2]

It looks like everything is RCD protected, so the risk of a high resistance suppliers earth is a lot less than if it wasn't.

I'm struggling to get my head around that statement The OP has TN-S, and the only other type of "supplier's earth" is TN-C_S, which surely would be (I would say 'inevitably') less likely to have a high Ze than does TN-S,wouldn't it ?

Can you clarify? What am I missing?

Kind Regards, John

 

[333rocky333]

I'm struggling to get my head around that statement The OP has TN-S, and the only other type of "supplier's earth" is TN-C_S, which surely would be (I would say 'inevitably') less likely to have a high Ze than does TN-S,wouldn't it ?

Can you clarify? What am I missing?

Kind Regards, John

Could he have meant the risk of injury by electricution to the client is less, due to having the rcd protection.
With a high Ze an rcd protected circuit is more likely to go off under fault than a mcb protected circuit, would it not

 

[justanotherguy]

Its not really worth paying somoene to some and take a Ze, when the DNO will do that as the first step anyway after you get them out to investigate a high loop

Thanks - I just checked and we're with UK Power Networks (great name!). Sadly it seems I do need to pay an electrician anyway - I've attached a list of checks they need before they attend.

 

[ericmark]

Thanks - I just checked and we're with UK Power Networks (great name!). Sadly it seems I do need to pay an electrician anyway - I've attached a list of checks they need before they attend.

How? Only way I know what size fuse I have is I was there when new meter fitted. They have seals on them. I have in the past used the enquiry methods to get the Ze, the DNO should have it on record, if not they have to send some one out to measure.

We may be on a ring, (not ring final) and that ring allows sections to be isolated while retaining the supply for work to be carried out, so the Ze can vary depending on if the ring is split or not when the measurement is made. So in theroy the domestic electrician can't work out normal Ze as he has no idea of how supplied when he measures.

So that makes no sense.

 

[JohnW2]

Could he have meant the risk of injury by electricution to the client is less, due to having the rcd protection.

Yes, I guess he may have meant that. As you will have realised, I read ...

It looks like everything is RCD protected, so the risk of a high resistance suppliers earth is a lot less than if it wasn't.

to mean ....

It looks like everything is RCD protected, so the risk of there being a high resistance suppliers earth is a lot less than if it wasn't.

... but, as you imply, maybe was meant to mean ...

It looks like everything is RCD protected, so the risk of electric shock if there is a high resistance suppliers earth is a lot less than if it wasn't.

... which would makes sense, since. as you goon to say,is true.

Kind Regards, John

 

[justanotherguy]

How? Only way I know what size fuse I have is I was there when new meter fitted. They have seals on them. I have in the past used the enquiry methods to get the Ze, the DNO should have it on record, if not they have to send some one out to measure.

We may be on a ring, (not ring final) and that ring allows sections to be isolated while retaining the supply for work to be carried out, so the Ze can vary depending on if the ring is split or not when the measurement is made. So in theroy the domestic electrician can't work out normal Ze as he has no idea of how supplied when he measures.

So that makes no sense.

I'm smiling thinking how it would go if I called UKPN and tried to argue with them

Not sure I have much choice? It's so annoying a broken hob as led down this expensive rabbit hole! But I guess if it's a safety issue, better to be poorer than dead.

 

[justanotherguy]

I called UKPN - went as well as I expected. They just redirected me to the form I already told them I'd seen. Computer says no.