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Electrical Earthing

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This topic contains 8 replies, has 7 voices, and was last updated by Les. 4 months, 2 weeks ago.

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  • #33858

    Drezha
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    @drezha

    Possibly someone might be able to give me some advice before I get myself an electrician in.

    Our oven died the other week, so we ordered one for delivery. We paid for installation and removal of the old one. However, on arrival, the guys took the old one, but when testing the circuit within the kitchen, found that the earthing (or impedence I think) was higher than they could sign off – there meter was reading 3.something Ohms, and they stated for a 13A socket, this should only be 1.something Ohms and therefore they couldn’t install it for us and we’d need to get an electrician out.

    Googling seems to suggest that this should have actually been Ze, not Ohms. I remember enough of A level and GCSE Physics that Ohms is resistance! The resistance of the earth loop makes sense, so I guess impedance is similar to this.

    How can the earthing be improved or can it? Are we looking at a rewire? The house is reasonably old (40’s build), but we ideally don’t want to be ripping out cables in full now etc (as I’ve got an interview in Dublin this month which may mean a move and house sale…)

    Should we just attempt to install the oven ourselves? (it’s a plug in one). From what I gather, this isn’t an issue unless an item goes live and needs to earth and it might be easier to earth through us.

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  • #33865

    Mark Turner
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    @turner74
    Forumite Points: 485

    Drezha

    Is it a full on electric cooker or a gas one with a plug for light and ignition?

    #33868

    Richard
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    @sawboman
    Forumite Points: 2,303

    The subject does appear quite challenging, here is one reference I found to the testing and standards requirements, http://www.seaward.co.uk/downloads/17th%20Edition%20Testing%20Guide.pdf

    I hope that will give you some assistance. If the installation is old then there is a possibility of many problems due both to wear and tear and the possible influence of dodgy modifications. Even if it has (very likely) been rewired, the quality of the work could be an issue. I was surprised that the oven came with a three pin plug termination, ours has its own direct feed on screw down terminations. I believe from memory it is a 30 amp supply. If your testing was done with other equipment plugged in, then according to the reference I quoted above, that could affect the issue.

    #33869

    Drezha
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    @drezha
    Forumite Points: 1,241

    Yeah, there aren’t many 13A plug in ovens left now we were told (and found) as it’s not strictly in compliance with the EU regs, but short of a rewire of the kitchen, that’s not a possibility. We might have the hard wire there hidden behind somewhere, but for now we wanted to replace like for like, especially as we might be moving (not only do I have an interview with Dublin FRS, I’m waiting to hear back from a role in Nottingham as well)

    It’s a pure electric oven, no gas involved.

    #33870

    Ed P
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    @edps
    Forumite Points: 6,508

    I cannot answer your question as it stands, but a good earth connection is an important part of your home’s protection.

    Normally electric cookers are on a stand-alone 30amp circuit, and I would not fiddle around with that as it is both illegal and will almost certainly void your house insurance.

    Impedance would not normally enter into the question unless you have a cooker with an induction hob. It is hard to guess why your mains would have unacceptable impedance figures unless you have some very large electric motors in your system.(unlikely).

    You can get away without a cooker in the short term (assuming you have a kettle) by buying an electric slow cooker or start down the road to full sous-vide cooking. (it results in amazing 5-star steaks!)

    Bottom line I’d call in an electrician as the tale is too tangled and the downsides too high. It would not involve rewiring your kitchen, instead he would extend your circuit board to add a 30 amp circuit and run one wire from your board to a new cooker outlet.

    #33872

    JayCeeDee
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    @jayceedee
    Forumite Points: 3,012

    Yeah, there aren’t many 13A plug in ovens left now we were told (and found) as it’s not strictly in compliance with the EU regs,

    HERE is a quick explanation about that.

    As Ed says, it might be quicker and easier to get a sparks in. It might just be the socket itself, but even if you swapped it out, you’d need a tester to see if that had solved the problem or not!!

    #33873

    Richard
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    @sawboman
    Forumite Points: 2,303

    I have to agree with ED on the need for a qualified electrician to inspect and advise. I have a very slight awareness that some modern power device connections can confuse protection devices and thus allow power when you might otherwise expect that protection should shut it off. I suspect that the impedance measurement has something to do with that aspect. Normally I would have expected a minimal resistance being the objective, but I am no expert.

    However whatever the level of ignorance I have expressed, ED is right. A question mark has been raised over the safety of your electrical installation and you could become liable in the event of a subsequent problem; insurance would only be one of several issues in the queue. A safety check now would give you peace of mind and be a valid sales point should you sell. If the wiring was faulty and caused an accident, if it had not been inspected; then your liability might become a serious issue. I cannot comment on the likely cost of adding a 30 amp direct feed, except that it would of necessity be a direct point to point link to Part P current standards. Only if the rest of the building’s ‘loom’ was in a failing state might a full rewire be necessary. In that event there would not really be an option anyway as the building would be unsaleable as well as uninsurable.

    Long story short; get it checked and be safe.

    #33884

    blacklion1725
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    @blacklion1725
    Forumite Points: 1,518

    Had to replace our oven about 18 months ago. MFI had originally fitted a 13A plug oven – inexplicably as there was a suitable spur in place for the oven it replaced (and they removed the spur!) Wonder why they went bust. Anyway managed to get an Indesit 13A plug model from Currys, but as linked above they are very few and far between.

    #33888

    Les.
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    @oldles
    Forumite Points: 648

    Going back to original point about resistance and impedance. The electrical supply people (and therefore all electricians) use the term impedance, but in reality they normally only measure the resistance between the neutral (with mains supply off) and the earth and then speak of the impedance. Some of the latest most sophisticated instrumentation may differ slightly (but may not) but a “practical electrician” armed with a meggar will get perfect answers. The high voltage test of the meggar is not used here, but there is a low resistance scale that is perfect for the job.

    Having said all that, there are at least three different sorts of supply (don’t ask for full details) and need to be assessed differently.

    My cottage receives a live and neutral via an overhead supply cable. I have an earth rod outside which provides my earth. I renewed this last year when I had a new meter fitted (NOT a “””smart””” one) and was told I needed various bits replacing. Since they were replacing my incoming supply (very old “Maconite” cables), I had a new RCD (earth leakage detector) and fitted a bigger cross section earth lead (16sq mm). My earthing RESISTANCE was around 30 ohms (I think it needed to be below 80 from memory).

    If you have a buried earth rod like mine, go outside after beer, pee on it, then call them back to recheck.

    Unfortunately, it seems you now need a “professional” to sign you off if you blow your nose.

    However, your supply type does matter, so take some advice, and if it is like mine, go for the beer then get the retest done.

    Les.

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