Earth resistance issue

[Lucasdemoley]

Hi all,
I had a new cooker delivered, but they wouldnt install it as my earth resistance was over 200mohms (like 275 or so it was showing).

Its likely that I've simply not connected the earthing rod correctly when we knocked it down for a concrete path (unfortunately connection was under the concrete so I couldn't check there and then).

I'm going to install a new earthing rod just to be on the safe side as I believe the old one was quite short.

Using a multi-meter, how do I check the resistance of the new rod installation?

Cheers.

 

[Harry Bloomfield]

You cannot, it requires an Earth Loop Impedance tester.

 

[Lucasdemoley]

You cannot, it requires an Earth Loop Impedance tester.

I was under the impression that you are simply testing the resistance? Most of the units I have seen to test simply plug in and give a resistance reading. I'm obviously totally wrong.

What equipment do I need to undertake the test?

Cheers

 

[Harry Bloomfield]

I was under the impression that you are simply testing the resistance? Most of the units I have seen to test simply plug in and give a resistance reading. I'm obviously totally wrong.

What equipment do I need to undertake the test?

Yes, they do simply plug in, but they use a low voltage high current to measure the resistance.

What equipment - Earth Loop Impedance tester, though I understand you can buy a much smaller plug sized gadget which is reputed to do a fair job of it - Never having used one, I couldn't possibly comment.

This might help explain - http://www.sparkyfacts.co.uk/Inspection-and-Testing-Earth-Loop-Impedance-Test.php

 

[Lucasdemoley]

Yes, they do simply plug in, but they use a low voltage high current to measure the resistance.

What equipment - Earth Loop Impedance tester, though I understand you can buy a much smaller plug sized gadget which is reputed to do a fair job of it - Never having used one, I couldn't possibly comment.

This might help explain - http://www.sparkyfacts.co.uk/Inspection-and-Testing-Earth-Loop-Impedance-Test.php

Ah right ok that makes sense. I'll take a look how much they cost vs just getting someone out to test.

Many thanks.

 

[EFLImpudence]

Just to note: You wrote 200 mohms. Presume you mean 200 Ohms - or 275 Ohms.


What did the Cooker fitter use for the test?

 

[endecotp]

Hi all,
I had a new cooker delivered, but they wouldnt install it

Based on many reports here. they never actually install them. There is always something that prevents them and lets them go home early.

 

[Harry Bloomfield]

What did the Cooker fitter use for the test?

I was about to ask that - I'm surprised they would send out a qualified guy out, with the kit to connect it.

 

[EFLImpudence]

Based on many reports here. they never actually install them. There is always something that prevents them and lets them go home early.

Yes, a customer reported they wouldn't fit her cooker because it did NOT have a 45A MCB on the circuit.

 

[EFLImpudence]

I was about to ask that - I'm surprised they would send out a qualified guy out, with the kit to connect it.

Yes, I assumed they might only have a plug-in tester but they don't show the actual measurement, do they? let alone one of 275Ω.

 

[Harry Bloomfield]

Yes, I assumed they might only have a plug-in tester but they don't show the actual measurement, do they? let alone one of 275Ω.

No, just LED's lights as a go/no go test.

 

[JohnW2]

Yes, I assumed they might only have a plug-in tester but they don't show the actual measurement, do they? let alone one of 275Ω.

The Martindale one would presumably allow one to determine that the loop impedance was between 200Ω and 500Ω (I presume the LEDS light up green up to 10Ω and red above that) ...

Kind Regards, John

 

[ericmark]

I remember testing earth rods, it needs access to the ground for around 30 meters from the rod, which in domestic your unlikely to have the space to test it. So the normal is to test combined DNO rod and your rod, DNO rod could be 15Ω, so it does not matter how good your earth pit, likely the result will not be low enough to not rely on a RCD to disconnect under fault conditions.

So maths, the voltage must not rise over 50 volt, and the RCD trips at 30 mA, so 50/0.03 = 1666Ω that is maximum reading, however it is considered as unstable if the reading is over 200Ω, and in the main we get readings of around 60Ω, from a single rod, so as an electrician I would be worried about a reading over 200Ω as being unstable.

However also one would need to check the RCD worked within the allowed parameters, and was a suitable type for appliances already installed. With most UK houses I could see this being a problem, with so many homes having type AC RCD's.

So would I want to test? It would open up a bag of worms. As well as saying it is not good enough, I would also be saying yes it is good enough, and as an appliance installer can I really in the time permitted test and inspect to a degree where I can say it's safe? OK with a TN supply, but with a TT not so cut and dried.

For a cooker we are not using a plug in tester, we have no socket to plug it into,

Just to note: You wrote 200 mohms. Presume you mean 200 Ohms - or 275 Ohms.


What did the Cooker fitter use for the test?

so a reading of 0.275Ω would be a pass, and 275MΩ would be an insulation measurement not earth loop impedance. At 275Ω I would have passed it, although would have made a comment. To test an earth rod it needs disconnecting from other bonded items, I have seen reports of earth rods with a 10Ω reading, which rings alarm bells, and one assumes whole installation was tested so it is the gas pipe, water pipe, earth rod all combined.

But what I would worry about if I came out of retirement would be RCD types, all well and good with a RCBO feeding just a cooker, one is not worried about DC so any RCD would do, but with a group of circuits on a single RCD one would need to look at what the other circuits supplied, which opens a can of worms, that to be frank I would not want to open.