Socket Tester Neutral-Earth Voltage Meaning?

[vonsworld]

Hi

On a socket tester what is the correct measurement for N-E voltage? If the socket is wired properly should the Neutral to Earth voltage be zero?

If there is a small voltage present, why does that happen and what is an acceptable maximum please?

 

[EFLImpudence]

Hi

On a socket tester what is the correct measurement for N-E voltage?

Very low.

If the socket is wired properly should the Neutral to Earth voltage be zero?

Yes but it depends on the earthing method of the installation and the construction of the test meter.

If there is a small voltage present, why does that happen

Voltage drop on the Neutral.

and what is an acceptable maximum please?

Just a few real Volts ~2V maybe slightly more.


What Voltage have you encountered?

 

[ericmark]

The limit is 50 volt, although would not expect it to get anywhere near that.

I did not do much with the ELCB-v I know they must trip before 50 volt but can't remember the actual voltage.

It seems for EV charging points we are allowed 70 volt, not worked out why a higher valve is permitted.

We have been told for years earth should not be switched, but it seems that is exactly what is done with EV charging points, they measure the line - neutral volts with a TN-C-S supply and if not between 207 to 253 volts then first line and neutral are made open circuit then earth.

The big thing is the reference point, and method of earthing, TT would be very diffrence.

Within premises we have bonding, so unlikely to exceed 2 volt, as to socket testers, not a clue what earth - neutral voltage is required to light the indicator lamp.

 

[JohnW2]

The limit is 50 volt, although would not expect it to get anywhere near that.

Where do you get that 'limit' from?

Kind Regards, John

 

[ericmark]

There are many references to 50 V, like "Where doubt exists regarding the effectiveness of supplementary equipotential bonding, it shall be confirmed that the resistance R between simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts fulfils the following condition:
R ≤ 50 V/Ia in a.c. systems
R ≤ 120 V/Ia in d.c. systems"

However I would hope some thing would trip well before that point. I was told 25 volt is enough to kill a cow, when working on portable traffic lights, and using the earth wire as a neutral on 55-0-55 volt systems was not an option. This was in the 70's.

But a little maths, and to permit 207 to 253 volt before it triggers auto disconnection will allow the voltage to a broken PEN to raise by quite a large amount before disconnection.

Inside the Equipotential zone of the home, even with a loss of PEN there should not be a problem, except for the over and under voltage, the difference between neutral and earth with a TN system is only due to volt drop within the cables, so unlikely to exceed a couple of volt.

However to test a socket has a earth connection, only way without removing some protection like covers on the consumer unit is to measure the earth loop impedance, and the neutral loop impedance, which should not be the same. Even if very close to each other. And the socket tester with loop only tests the earth loop impedance, so can't detect if same wire used for earth and neutral, however if the RCD test works, then the socket clearly must have an earth, so testers like these

Do a reasonable job at testing the socket outlets, but one like this is of very limited use, can check if plug correct way around when using a caravan abroad, and if a wire is completely disconnected, but without the loop and RCD test it can't show that much. But we are looking at under £7 to more like £65 for the one with Loop and RCD test. But for a proper loop impedance and prospective short circuit current tester they start at around the £150 mark so the EZ365 is cheap when compared with them. Most of the proper loop testers are over £250 so one shown is rather cheap. Did find this one at under £120 but it only does loop and PSCC so would still need other test gear to do other tests. Insulation testers I have found down to £35 but loop and RCD testers not found any really cheap versions.

 

[JohnW2]

There are many references to 50 V, like ....

There are, but nearly always in relation to 'touch voltages' - I can't recall any explicit references of a '50V limit' in relation to N-E potential differences (which is what we are talking about), can you?

Forgetting 'limits' and just talking about significant/high N-E potential differences ....

... you talk about "broken PENs" (in TN-C-S installations) but that cannot possibly result in a significant N-E potential difference. At the origin of the installation, the N-E is obviously zero (even with a 'broken PEN'), since 'N' and 'E' are the same thing. Within the installation, there will be a small "N-E" (N-CPC" PD within the installation (in circuits which are loaded) because of VD in the neutral conductor, but that will be exactly the same N-CPC difference as would have been present in the absence of a 'broken PEN' (which is therefore 'a red herring' ).

Kind Regards, John

 

[ericmark]

At the origin of the installation, the N-E is obviously zero (even with a 'broken PEN'), since 'N' and 'E' are the same thing. Within the installation, there will be a small "N-E" (N-CPC" PD within the installation (in circuits which are loaded) because of VD in the neutral conductor, but that will be exactly the same N-CPC difference as would have been present in the absence of a 'broken PEN' (which is therefore 'a red herring' ).

Inside the Equipotential zone of the home, even with a loss of PEN there should not be a problem, except for the over and under voltage, the difference between neutral and earth with a TN system is only due to volt drop within the cables, so unlikely to exceed a couple of volt.

I did say within the equipotential zone it can't vary much, but how do we know the socket being tested is within the equipotential zone? I have seen may sockets fitted outside.

 

[JohnW2]

I did say within the equipotential zone it can't vary much, but how do we know the socket being tested is within the equipotential zone? I have seen may sockets fitted outside.

Even if it is outside of the equipotential zone, everything I wrote remains true of the PD between N and E of the socket (which is what this whole thread is about).

What you seem to be talking about the risk that, in the event of a 'broken PEN' (in a TN-C-S installation) the 'E' of the installation (i.e. CPCs and exposed-cps of accessories) may rise to considerably above true earth potential (which a person may be standing on whilst touching an exposed-c-p).

Whilst that is true, and a genuine concern, it has got absolutely nothing to do with 'N-E voltages' - which, as I wrote, will be exactly the same with a 'broken PEN' as with an intact one.

Kind Regards, John