115V P-e, N-e, 240V P-N

Post Reply
User avatar
BenAlchin
Posts: 19
Joined: Mon Apr 20, 2020 10:40 am
Answers: 0
Has thanked: 0
Been thanked: 0

115V P-e, N-e, 240V P-N

Post by BenAlchin »

Hi folks,

Hope you're all well! I'm not yet fully qualified, so forgive what's probably an obvious question!

I was wondering if anyone could venture an opinion or advice on a weird issue I found testing an outlet that was disconnected by a previous electrician for 'some earthing problem'. It's being used as a fault finding practice for me :P

Between phase and neutral there is 240V, but between Phase and earth or Neutral and earth there is about 115V.
Despite this, the RCD is not tripping.

My suspicion is that Phase and Neutral might be transposed somewhere.
Does that sound like a plausible explanation?

I believe I can use an independent earth test the prove it.

Thanks,

Ben
User avatar
DougP
Posts: 242
Joined: Sun Apr 12, 2020 10:11 pm
Answers: 3
Has thanked: 19 times
Been thanked: 31 times

Re: 115V P-e, N-e, 240V P-N

Post by DougP »

I would be checking if it has a MEN link.
User avatar
BenAlchin
Posts: 19
Joined: Mon Apr 20, 2020 10:40 am
Answers: 0
Has thanked: 0
Been thanked: 0

Re: 115V P-e, N-e, 240V P-N

Post by BenAlchin »

It does, and all the other circuits are behaving correctly
User avatar
DougP
Posts: 242
Joined: Sun Apr 12, 2020 10:11 pm
Answers: 3
Has thanked: 19 times
Been thanked: 31 times

Re: 115V P-e, N-e, 240V P-N

Post by DougP »

Sorry I missed that it was only one outlet. So that circuit probably doesn't have the earth connected correctly. You need to test the earth at the outlet with a trailing lead and a low-ohm meter (not a multimeter).
AlecK
Posts: 912
Joined: Thu Apr 16, 2020 11:24 am
Answers: 5
Has thanked: 2 times
Been thanked: 351 times

Re: 115V P-e, N-e, 240V P-N

Post by AlecK »

Those readings are typical of a supply from a 2-phase 180 degree supply, eg from an genset with "centre-tapped" winding.
Assuming this is on an MEN supply; that is probably coincidence.
But it shows that leaping to the "obvious" conclusion isn't a valid fault-finding technique.


So far you know the difference in potential between the three terminals at that point.
You don't know the "direction" of these differences.
Eg is the E terminal sitting approx halfway between A (230) & N (0); being 115 to actual earth?
Or some other combination (like the generator example above)?
You also don't know that N bar is actually at 0 V, or even that the earth bar is at 0 V.
We tend to assume they are, but it's not always true.
A voltage between independent earth and installation N could be live N & 0V earth; or it could be actually a live earth &b 0V N.

Agree that using a trailing lead from an independent earth for further voltage readings (to A, N, & E) will provide useful data.

The only other fact you know is that the RCD isn't tripping;
which suggests any current imbalance between A & N is insignificant.
A low range (mA) clamp test can confirm; but the results may well vary with load.
What this symptom does do is reduce the chance that the fault is a simple N-E reversal at some point on the circuit,
which would put load current on the PEC.

So step 1 is to confirm that supply is correct.
The fact that other points give the expected readings suggests, but doesn't prove, that supply polarity is correct.
Also suggests that at least part of the fault is within this subcircuit.
So proceed to step 2; but you may need to come back to this step 1, and do both polarity & EFLI testing of supply.

Step 2 is to check what loads are connected to this circuit.

Step 3; confirm integrity of subcircuit wiring
Isolate the circuit (A & N, eg at RCD); and do a full range of tests; including continuity & polarity with trailing lead
plus insulation resistance (A-N, A-E, N-E).
Remember that any connected loads can adversely influence test results.
User avatar
BenAlchin
Posts: 19
Joined: Mon Apr 20, 2020 10:40 am
Answers: 0
Has thanked: 0
Been thanked: 0

Re: 115V P-e, N-e, 240V P-N

Post by BenAlchin »

Thanks so much for all that advice folks.

It reassuring to know some of the tests you recommend are the ones I planned, too!

ELZ on other circuits is good, but on the one in question just fails saying <200V
User avatar
DougP
Posts: 242
Joined: Sun Apr 12, 2020 10:11 pm
Answers: 3
Has thanked: 19 times
Been thanked: 31 times

Re: 115V P-e, N-e, 240V P-N

Post by DougP »

You need to confirm an earth is connected before you can do a live loop test. Which is why there is a specific order of carrying out tests in section 8.
Do the trailing lead Re test first. Then also an IR test would be useful, but the results might be confusing if you don't know how to interpret them.
AlecK
Posts: 912
Joined: Thu Apr 16, 2020 11:24 am
Answers: 5
Has thanked: 2 times
Been thanked: 351 times

Re: 115V P-e, N-e, 240V P-N

Post by AlecK »

That fail is simply because - as you already established - the voltage A-E is only 115 V
As such; an EFLI tester designed for 230 V will not commence the test.
An EFLI tester works by applying a known resistor across a measured voltage for a limited time;
in order to then calculate the impedance of the circuit as a whole.
The resistor is such that test current is typically around 23 A, assuming the voltage is capable of sustaining that .

Fault-finding is a matter of identifying symptoms, considering possible causes, then eliminating them one by one.
The test sequence in Section 8 may help; but it's designed for a different purpose.

Right now you have only some symptoms; and no probable causes - which makes life difficult.
But you do have some fairly strong indications that the problem or at least part of it, is within this subcircuit.
So get rid of as many complicating factors as possible;
such as being live, and possibly having some load connected, and being connected to an installation.

Disconnect any loads.
Isolate by disconnecting at load side of RCD (A & N)
But don't trust this isolation completely - you know something's wrong, and it could be a crossed connection from another circuit.
So test to prove 'not live" at each point .

Having done that, you can get stuck into proving that each conductor - A, N, & PEC - is continuous.
You can leave PEC connected to E-bar ; and once you've proved that it's OK you can then use it as a reference.

Then that each is correctly isolated from the others - and from the rest of the installation.

Then that each is connected correctly at every point along the circuit.
Post Reply