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Posted By Topic: Neutral - Earth Continuity Loop

OctaneOutlaw
Oct 24 2019 21:17

When we are required to test earth continuity is testing between N-E an acceptable method of doing so provided that the value of both together is lower than the value the E is required to be on its own thus proving that if the N-E loop is low enough to pass then the E section must be even lower confirming a pass?

Also if using a trailing lead should the MEN link be removed? Because if it's not if there was a N-E transposition you'd still pick up continuity through the N back to the bar and through the MEN link giving a false positive?
   

DougP
Oct 24 2019 23:54

You can use whatever testing method you wish, as long as it can prove the desired objectives of the test.

You're talking about two different tests, earth continuity and part of the polarity or correct circuit connections tests.

Doing the tests with the N & E linked wouldn't necessarily save you any time, because it doesn't prove the earth is connected to the correct terminal as you know. Also, you would have to use a trailing lead for other earthed items anyway.

Usually you wouldn't remove the MEN link for any testing (other than some polarity tests).

For doing the earth continuity with a trailing lead and at the same time confirming that it is the earth connected and not the neutral, the usual method would be to disconnect the neutral of the circuit under test. Often, that only requires the RCD to be switched off, and a few neutrals be disconnected for non-RCD protected circuits.

Have you read through the testing methods in 3017?
   

OctaneOutlaw
Oct 25 2019 07:15

Thank you, maybe I've worded things wrong or maybe I have gotten mixed up myself but I am trying to solely refer to earth continuity

I have read 3017 and found it useful but was just thinking of the method I specified first as being a faster way without the use of the lead

Normally I'd run a lead from the E bar, MEN intact and test my earth points, confirm they are low enough, I think you've confirmed that it's a good idea to have the Ns of the circuit disconnected whilst doing so which is basically what I meant by the MEN disconnection, so that if there was a N-E transposition, you'd pick up on it then rather than if it wasn't disconnected in a state of transposition you'd still likely get a sufficient results and carry on test only having to start over and redo IR tests and anything else after you'd picked up on it later

My first point was simply if it's acceptable to test between N-E at let's say a socket outlet and if the results of that loop we're lower than the value needed to trip the protection device on the allowed time then just the E section must be acceptable, I just wanted to know if there was any flaws in using this method
   

DougP
Oct 25 2019 08:52

You need to try and keep it clear in your mind, that what you're talking about relates to 2 or 3 different mandatory tests.

Earth continuity is one test. Polarity and correct circuit connections are another test(s).

Now, you've also introduced the word "loop" and you have referred to "the value needed to trip the protection device". But as you know, the N-E resistance isn't part of the standard earth loop as it's not the fault current path. That would be A-E or A-N. So my advice is that it is pointless and confusing to test that "loop".

So I'll say it again. Testing with the trailing lead and the circuit neutral disconnected achieves two things. Firstly it confirms that the earth continuity is correct. Second it confirms that it is the earth connected to the earth terminal, and not the neutral.

And, your "N-E resistance test" isn't going to help you test the earth continuity on appliances like cooktops, ovens, hot water or class I light fittings etc.
   

AlecK
Oct 25 2019 08:57

As DougP said, it's achieving the purpose of the test that's important.
Testing to prove safety & compliance is different from testing to find a fault. The different purpose may require different range of tests.
Similarly testing a single circuit / circuit extension may be approached differently than testing an entire new installation.
The tests in 3017 are not just single-purpose tests; even though they are presented as if they are. In fact they work together as a whole testing regime. Which is why the Standard tells us that if we find a "fail" test result during any test, we have to consider whether that fault may have caused "false pass" results on other types of test that we did first.

We don't have to do the tests in any particular order, but there's a good reason why the order shown in "3017" was chosen. Basically it minimises the risk of having to go back and re-test stuff that looked OK the first time. If you don't do earth continuity before IR, then at least some of your IR test results have to be regarded as invalid.
Overall, our most important test tool is our own brain.

For earth continuity, yes your suggested method of measuring resistance of N + E can give a valid result. The total resistance must be more than the resistance of PEC alone, so if total below limit, then PEC is compliant.
Even if the total is a little higher than limit for PEC, then PEC is still OK - up to a certain point. Trouble is, we can't know where that point is. For a straight length of cable, we might rely on manufacturer data, so for cable with equal-sized N & E we could maybe assume the method is valid up to value double the PEC limit. But if there are any connections (or any manufacturing faults), that sort of assumption can't be made. And as soon as we have any doubt, then the test can't be considered reliable and we'll have to use another method.

So I might use it on an extension to an existing socket circuit, but not for an entire new circuit. More difficult to use it on a non-socket circuit; and of course impossible on a 3-phase circuit without N.
whereas using a trailing lead, with either MEN lifted or RCD open (assuming the RCD switches N - not all do) works every time, on any type of circuit, and without needing to dismantle equipment. Remember we have to test every point, not just the last point , on each circuit. Every light. Every fixed appliance. and every socket. Plus bonding.

And even for cases where the method can give a valid result; as you will have realised the result isn't confirmed until other tests, including polarity, provide confirmation.




   

OctaneOutlaw
Oct 25 2019 09:11

Right, I think that confirms what I needed to know, thank you both