Contact voltage on PE-conductor

[Bryn2017]

Hi, I'm trying to understand a fault displayed, and I'm hoping somebody might be able to help explain.

Recently purchased first home, and decided to use a socket tester to do a quick check of all sockets. I know there are limitations of socket testers (though don't claim to properly understand, or be a professional), and do intend to get the electrics fully tested and have some work done in the future.

Using a Schneider IMT23003 socket tester, the three lights were as they should be on all sockets on the circuit in question (1st N off, next two PE,L on). However, on touching the contact electrode, the warning symbol showed (indicating a dangerous contact voltage present on PE- conductor), but only for four of the sockets on the ring, it doesn't show on the other sockets in the same ring. This isn't a display pattern explained on their coding table.

I have since tried a second socket tester to try to understand the problem, a T.I.S 1005, which shows all E,N,L lights on indicating correct wiring, the loop check showing green for good (<1.8ohm resistance).

To be safe, I've turned he circuit off, and living off the circuit on an extension (which tests fine on both testers). I'm also getting a pro out to do an EICR and fault find, and will probably fit in some of the other work I would like if he has time.

In the mean time I've been trying to understand the cause, and why some sockets and not others. Why is the T.I.S. Tester not finding a problem? I'd be extremely grateful for thoughts or explanations.

If I'd have used the T.I.S tester first, I doubt I would have got hold of a second tester.

Thanks in advance,

 

[Iggifer]

Well the Schneider isn't necessarily saying there's dangerous voltage on the cpc, just that there's a voltage difference between you and the PE from the circuit. The spec sheet says the threshold for this test is <35vac dependant on coupling to earth.

This could mean, due to the all important < that the voltage is much less than 35V.

Not a bad idea to get it checked out - never a bad idea to have an EICR done anyway. But if you had or have access to a two pole tester you can quite easily work out the problem, if any depending on your level of skill.

Testing L-N, L-E and N-E should give you 230V, 230V and 0V respectively (allowing for variations in supply voltages of anywhere between 216.2V and 253v)

You can also test from the cpc of the suspect circuit to either the main earth terminal in the property and the affected circuit, or from a different circuit on the volt setting, the expected result would be 0v.

Loads applied to the circuit at the time of your limited testing could be skewing the results as some (quite a few) appliances leak voltage to earth. Computers and fridges can be major culprits. This will only be a small amount but could be enough for the tester to indicate a fault.

Because both testers are showing an voltage differential between L-E, and you're getting a (presumably) "good" loop check on the TIS this indicates these sockets do have an earth path. The reason the TIS is not showing a fault is because it only has one reference point for earth which is the cpc in the circuit.

 

[ban-all-sheds]

Well the Schneider isn't necessarily saying there's dangerous voltage on the cpc, just that there's a voltage difference between you and the PE from the circuit. The spec sheet says the threshold for this test is <35vac dependant on coupling to earth.

This could mean, due to the all important < that the voltage is much less than 35V.

I've not looked at the specs, but wouldn't that "threshold" mean that at < 35V it wouldn't give a warning?

 

[Iggifer]

I've not looked at the specs, but wouldn't that "threshold" mean that at < 35V it wouldn't give a warning?

You could be correct there. The spec sheet doesn't really go into more detail than I put in my post though so I can't expand on what I've written.

 

[EFLImpudence]

How can a threshold be "less than" whatever value?

 

[Iggifer]

*shrug*

 

[JohnW2]

How can a threshold be "less than" whatever value?

It's certainly rather odd, since one is used to 'fixed' thresholds for alarms such as this.

As BAS has said, the threshold is presumably the voltage above which it will give an alarm. However, what Iggifer has reported suggests that, for some reason, they cannot given an exact figure for that threshold, but are 'guaranteeing' that it will be less than 35V.

I suppose that, particularly because of their qualification "...dependant on coupling to earth" they could be talking about the threshold relative to true earth, but I can't really see how they could give an absolute figure for that, given that the extent of the coupling between the body of the user and true earth could be anything (theoretically including 'almost zero').

I think we need to scrutinise the documentation a bit more deeply, and see if any clarification is there to be had!

Kind Regards, John

 

[SFK]

Sorry, meant to put up this video...... look at about 7mins in

This video is less relevant, but was what I incorrectly posted the first time.

 

[ban-all-sheds]

How can a threshold be "less than" whatever value?

As in "The threshold for poverty is a household income of less than X"
"The threshold for hypothermia is a body core < 35.0 °C"

 

[ban-all-sheds]

*shrug*

Well yes - I don't think anybody doubted the accuracy of your reporting.

The question is what do they mean by the thing you reported....

 

[EFLImpudence]

As in "The threshold for poverty is a household income of less than X"
"The threshold for hypothermia is a body core < 35.0 °C"

Surely that by definition means the threshold is X and 35.0 °C.

As in "Poverty is a household income of less than X", "Hypothermia is a body core < 35.0 °C": i.e. less than the threshold.

 

[ban-all-sheds]

 

[ban-all-sheds]

Surely that by definition means the threshold is X and 35.0 °C.

Yes, the threshold is {whatever}.

And the condition exists when you are below the threshold.

It could just be a poor translation.

Sadly no user manual on the Schneider site.

 

[JohnW2]

Surely that by definition means the threshold is X and 35.0 °C. As in "Poverty is a household income of less than X", "Hypothermia is a body core < 35.0 °C": i.e. less than the threshold.

Indeed. BAS has stated the 'decision/classification criteria', not the 'thresholds' - the latter being the dividing lines between the binomial decisions/classifications.

Kind Regards, John

 

[EFLImpudence]

Yes, the speed limit in towns is not >30mph.