Confirming continuity of earth

I think this is a bit too complicated.

The OP wants to determine that his lighting circuit earth is actually earthed.
That is - the wiring is correct and the other end of the cable is connected to an earth terminal.
While measuring between light circuit CPC and socket circuit CPC may not actually prove the whole installation is earthed it is an indication that the circuits are wired correctly.
He could then test between other sockets and, say, cooker switch CPC.

It is a possibility that the whole installation is not earthed but that is unlikely and a different matter.

If that were the concern, then he would not be asking about light switches.

I think this is a bit too complicated. ... The OP wants to determine that his lighting circuit earth is actually earthed. ... That is - the wiring is correct and the other end of the cable is connected to an earth terminal. ... While measuring between light circuit CPC and socket circuit CPC may not actually prove the whole installation is earthed it is an indication that the circuits are wired correctly.
Yes, I agree.

What I was really saying is that (probably contrary to what many/most might have said) what the OP proposed (to measure L-CPC voltage at the switch), probably secondary to an initial resistance/continuity measurement, actually goes further than resistance/continuity measurements alone, by giving a reasonable indication that not only is the light switch CPC connected to the MET but also that the installation is, to at lest some extent, earthed. In the absence of any earth (i.e. with 'floating' CPCs), I can't see that one would measure an L-CPC pd of anything approaching supply voltage.

Kind Regards, John

Thanks again all just to summarise, measuring voltage between phase and CPC at the switch will give me an indication that there is a route to earth - great, big tick - however resistance is an important consideration and I’d still get a reading of 230-240V whether it was 1ohm or 1000ohm, the latter making it likely the current will pass through me rather than draw enough current to trip the mcb.

In addition, testing the resistance of the CPC with the methods listed previously will give me the second part of the equation - at least as far as CPC to CU is concerned.

Saying all this, as the CU was replaced 2 years ago with the relevant Part P signoff it would be a reasonable assumption that the earth to CU CPC is sound and that I should be good!

Cheers all

Saying all this, as the CU was replaced 2 years ago with the relevant Part P signoff it would be a reasonable assumption that the earth to CU CPC is sound and that I should be good!
Unfortunately, not all electricians are as diligent as they should be.
As I indicated before, don’t trust or assume anything.

Thanks again all just to summarise, measuring voltage between phase and CPC at the switch will give me an indication that there is a route to earth - great, big tick - however resistance is an important consideration and I’d still get a reading of 230-240V whether it was 1ohm or 1000ohm, the latter making it likely the current will pass through me rather than draw enough current to trip the mcb.
All essentially true, although the chances of you having a 'high resistance' (but present) CPC connection within your installation is incredibly small. However, as you go on to say, if you undertake both a resistance measurement and a measurement of L-CPC voltage, you will have covered both of those bases and, I think, gone as far as you can with the test gear available to you.

As regards the adequacy of the connection to earth to facilitate operation of an MCB, an electrician would directly measure the entire 'earth fault loop' resistance, which includes the resistance of the earth connection as well as of the CPCs within the installation. However, that's not something you can measure.

Kind Regards, John

Thanks for the responses guys. Novice question here but would measuring the voltage between the L1 and CPC not give an indication that there was continuity of the CPC, I.e should show 240v approx. If the CPC was not connected it would show 0V?
Live working is not encouraged, more so as you say yourself a "Novice" to electrics

Live working is not encouraged, more so as you say yourself a "Novice" to electrics
Whilst that's obviously true, I assumed from the fact that the OP suggested it himself that he felt comfortable/competent to do it.

Kind Regards, John

Thank you for the concern Rocky, although I’m not qualified to discuss the intricacies of electrics, I’m mindful of my own safety and that off my family and would not attempt work that is not allowable under Part P or that I do not have the expertise to carry out. Saying this I know enough to safely isolate circuits and test for positive confirmation. With respect to live working I’ll ensure to take the appropriate precautions and will not be messing with the circuit apart from testing with the appropriate equipment.

I should note, asking a novice question is not the same as being a complete novice.

An old fashioned method of testing the integrity of an "earthing" system was to pass a current through the conductor and measure the volt drop along the "earthing system" being tested.

The crudest method uses a battery, torch bulb and a volt meter.

There are three plug in testers with a loop test Socket & See SOK 32, the Martindale EZ150, and the Kewtech loopcheck 107, the Martindale is
likely the best, but since the pass is 1.38Ω and even the Martindale shows OK at 1.5Ω non are really perfect, today loop testers pass a very small current around 6 mA so as not to trip the RCD, in the old days we would use 25A and the regulations use to say at least 200 mA has to be used to test an earth, however they are simple to use cost between £25 and £50 and a proper loop tester is around £200 mark so these plug in testers are relatively cheap.

Often old methods are best, a battery and bulb and long wire to know good earth with power off. My father-in-law said to me I would never forgive myself if my child was injured or killed with an electric shock, so he pushed me to fit RCD's in the house, although he never fitted them to his own, they will not stop you getting a shock, but will limit the time to 40 mS and offer reasonable protection.

As to if it works out cheaper to get an electrician to test or buy or make test equipment I don't know, my kids have grown up now, the boy I was keen to protect at 14 when he was learning and passing his exam to become a radio ham is now nearly 40, that means my house has had RCD protection for 25 years, but not having it did not kill my parents or me when I was a lad.

Cheers Eric, actually got an advanced socket tester. Went for the SOK36 which supposedly has surpassed the Martindale?

There are three plug in testers with a loop test Socket & See SOK 32, the Martindale EZ150, and the Kewtech
Whilst that is true, such testers are of no direct value in relation to testing the earth at a light switch. One could improvise some method of connecting it to the L and CPC at the switch, but one would have to find a neutral somewhere (which, as you know, is not commonly present at a light switch).

However, it could be used as part of a two-stage strategy. If such a tester shows a satisfactory earth connection at a nearby socket, and if ('dead') resistance testing shows a low resistance path between the earth pin of that socket and the CPC at the light switch, then that would essentially confirm that the CPC at the switch had an adequate connection to earth.

Kind Regards, John

Just a quick thank you for those who have helped in this topic.

I’ve now tested the resistance of my cpc from CU, R2, to all metal fixtures, maximum reading of 0.7 ohms so very happy. Obviously don’t have the equipment to test the impedances but I think that satisfies me

As far as earth bonding goes resistance = impedance in real terms, the proper tester must use 200 mA at least for the test, but I only use it because I have one, I would be satisfied with a multi meter for my own house.

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