Earthing - Voltage Reading!

[chipollini]

Just moved into a new house (built 1950's) and it would appear it's had a rewiring at some point.

Examining the lighting circuit and used a mains testing screwdriver just to check for the live cable. To my surprise the light came on when I touched the Earthing wire as well as the Live.

Got my multimeter out and checked the voltage difference between the Neutral and the Earth and it would appear it's picking up between 4 and 5 Volts?

Is this normal - I thought the Earth should read zero? Any ideas how to fault find if this is indeed something to be concerned about.

 

[Adam_151]

Its perfectly resonable that earth and neutral differ in potential, I'll assume TNS, both neutral and earth are connected at the star point of the local transformer, both cables will be at the same voltage at that point, and they both have resistance, the neutral will be carrying some load (the exact amount is variable depending on the balence of the 3 phase system in the street mains, also the service drop to your house will have the neutral carrying your full SP load current) the earth, carries nothing in a system thats not in a fault condition (assume leakage from SMPS units is negligable), ohms law will ensure that there is a potential between the ends of the neutral conductor due to volt drop, but not so in the earth conductor, and as the far ends are tied together, the near ends are at a different potential

Now moving onto the slightly more serious issue, a neon screwdriver cannot be relied on, a likely situation is the earth is floating and is capacitivly coupled to the phase conductor (capacitive coupling is a normal 'event') however if its allowed to float, it'll appear at the same potential as the phase conductor (however incapable of delivering much current) the danger is not so much the voltage on it, as much as the lack of effective earthing (the fact that a multimeter drags it down to 4v with reference to neutral demonstrates the high impediance of the coupling, I'd bet if you did the screwdriver test and the multimeter test at the same time, that it wouldn't light. Onto that lack of effective earthing conductor, you should put the multimeter between phase and earth and see if you get the nominal supply voltage, the better test is the standard R1+R2, you can do this with a multimeter if you know how to perform it, as the actuall value you obtain is of little significance at this stage, rather that you get something other than open circuit (which you might well get)

I'll look over this in the morning when I have had some sleep

 

[chipollini]

If I put the multimeter between phase and earth I get 234v (6pm last night) and 243v (11pm at the time of writing the original message) - I assume this is within range of the normal supply voltage.

Unfortunately I'm unable to test to see if the neon will fail to light if the multimeter is also testing at the same time, but I'll give it a go when I get in from work.

Would you be able to advise as to where I can find details of "the standard R1 + R2 test" and as to what it will demonstrate. Thanks for your assistance.

 

[Steve]

Just moved into a new house (built 1950's) and it would appear it's had a rewiring at some point.

I would hope so.

Examining the lighting circuit and used a mains testing screwdriver just to check for the live cable. To my surprise the light came on when I touched the Earthing wire as well as the Live.

Why did you need to test for the live wire? Wasn't it distinctively coloured red or brown? Were you replacing a light?

Got my multimeter out and checked the voltage difference between the Neutral and the Earth and it would appear it's picking up between 4 and 5 Volts?

Normal, especially on a TT earthing system with a local earth rod.

If I put the multimeter between phase and earth I get 234v (6pm last night) and 243v (11pm at the time of writing the original message) - I assume this is within range of the normal supply voltage.

Yes it is. Its due to volt drop i believe. At 6pm, lots of people are boiling kettles, cooking, washing clothes etc, and this drags the supply voltage down at the end of the line.

Unfortunately I'm unable to test to see if the neon will fail to light if the multimeter is also testing at the same time, but I'll give it a go when I get in from work.

What would be the point? What useful purpose do you think this would serve? They would give the same readings together as they would seperately. And are you the 4-handed monster? Because thats how many you will need!

A neon screwdriver is a dual purpose tool - it will undo screws, as well as tightening them too. Can be very useful. Its pretty too, with its little red decorative light that lights when you get in your car or walk past your consumer unit. Get my point? It is not a useful piece of test equipment

Furthermore, I dont think you realise that your wee multimeter cannot do every test under the sun. From my limited understanding, the insulation tests apply significant voltages (much more than 230V) to the circuit, and they require very expensive test equipment. If you wish to verify the integrity of your electrical installation, contact a local electrical contractor who will have all the nessecary tools and equipment, and issue a certificate.

 

[chipollini]

In response to craft1289.....

Why did you need to test for the live wire? Wasn't it distinctively coloured red or brown? Were you replacing a light?

At the time I was looking at installing a loft light (which is now up and working fine) and wished to check the existing switch running to the ceiling rose which I used to run to the new light. Although I believe you're making a valid point by stating the obvious, yes the live should be coloured red or brown, is there any harm in checking the other terminals to ensure they are not?.... am I breaking a cardinal sin by verifying the other cables are not live for some reason?... please remember all previous work was completed by the previous owner and I have no certification for the work.

Normal, especially on a TT earthing system with a local earth rod.

Thanks, that's what I needed to know.

What would be the point? What useful purpose do you think this would serve? They would give the same readings together as they would seperately. And are you the 4-handed monster? Because thats how many you will need!

I suggest you ask Adam_151 that one as it was a suggestion he made along with a wealth of other constructive information.

A neon screwdriver is a dual purpose tool - it will undo screws, as well as tightening them too. Can be very useful. Its pretty too, with its little red decorative light that lights when you get in your car or walk past your consumer unit. Get my point? It is not a useful piece of test equipment

Furthermore, I dont think you realise that your wee multimeter cannot do every test under the sun. From my limited understanding, the insulation tests apply significant voltages (much more than 230V) to the circuit, and they require very expensive test equipment. If you wish to verify the integrity of your electrical installation, contact a local electrical contractor who will have all the nessecary tools and equipment, and issue a certificate.

I understand a neon screwdriver is NOT an accurate tool and can be used as a guide only.

I also understand during installation tests a larger voltage than 230v is applied. Although my "wee multimeter" cannot complete every test under the sun, it is quite capable of reading the PD between the Neutral and Earth as measured in the first instance.

Although I appreciate constructive advice offered by most contributors on this website, I do not wish to be patronised with regards to wire colouration or how to use a screwdriver.

Crafty, thanks for confirming the 4v PD measured on the lighting circuit is normal - this answers my original post.

If there are others out there wishing to back up Crafty's constructive comments it would be appreciated. Many thanks.

 

[Qedelec]

I should like to add that the best place for a neon screwdriver is at the bottom of a very deep lake. The subject has been covered many times on this forum (and remember we only have your safety at heart), but only a fool would use their body as a conductor to test the presence of electricity.

Remember it only takes 50V to stop your heart!!

 

[Adam_151]

Just to clarify, the point about applying the screwdriver at the same time as the multimeter was a point to demostrate the high immediance nature of the coupling, a neon takes around 70v to strike, if you are getting 4v with a mulimeter the multimeter must be dragging it down, without the impediance of the instrument, we can't put a fiqure on the impediance of the coupling anyway (not that it really matters) oh and if you care, its more a less the way an ELFI tester works... apply a known load and measure the voltage sag.

if you are getting the nominal volatage between phase and earth, this would seem...indicate that you have earth continutity, so I'm still a bit confused about the tester lighting on earth.

The R1+R2 test is a test that done to check continuity of the earth conductor, and also polarity, it involves moving the phase wire from the fuseway into the earth block at the board and testing with a low ohm resistance meter, and gives you a value called R1+R2 (resistance of phase and earth wires) that when added to Ze gives you Z's (at least in theory... in actual fact, parrallel earth paths often conspire to give a lower Z's value)


Of course, poking around in a distrubution board, if you don't know what you are doing, is probably not a wise move...
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and I take it you wouldn't be a fan of these either, QED ?

 

[Qedelec]

Very pretty Adam just as long as we use your hand. Electricity deserves respect.