Earth testing

[Nemo2000]

Background:- the electrical supply to my house is TT with an earth rod at the premises. The consumer unit itself has rewireable fuses.

At one location on the 5A circuit of 1.0mm cable for the downstairs lights there was an old round bakelite-type JB to split the cable into two spurs - one to go to the front of the house and the other to he back. This was used as a straight-forward 3-way JB, no spurs to any switches or lights. At some stage the box has cracked, so while I had the floor up for some other work I removed it and replaced it with a Hager J804.

With the circuit fuse removed (before I started!) and testing with a multimeter afterwards there was about 0.3 Ohm resistance from E to the E on a different nearby circuit, so happy with the ECC continuity. There was NO reading between the L and E, so happy again. However when checking from N to E, although the meter gave no actual reading, the audio signal buzzed rather erratically as though there was some sort of intermittent connection.

Now, I don't recall that having happened before when testing N-E when (for example) replacing some old sockets or light fittings in the past. All that I've done on this occasion is swop out one JB for another JB and re-trimmed the cable ends. Having read other threads about N-E resistance, I'm not sure whether or not I should have any concern here and if so, why might this have happened so unexpectedly?

 

[jurassic sparkk]

N-E continuity absolutely normal with the main switch in the closed position.
Turn off main switch, then re check. If the continuity still remains there, you've got a problem.

 

[Taylortwocities]

Have a look at how electric gets to your house. At the substation the neutral is connected to earth, so you will get strange readings if you test with the main switch on.

 

[Nemo2000]

Thanks for the advice.

I tested again with the main switch off and got nothing at all this time, so I'm happy with that. Obviously one of those 'strange readings', it's just that I've not encountered it before and I wanted to check before I put the floor back down again.

 

[jurassic sparkk]

Obviously one of those 'strange readings',

It was not a strange reading. It's normal to see continuity. A TT (most likely) won't have a low resistance from n to e, but with parallel paths, it is possible, so there is no need to worry.

 

[ericmark]

To test the earth one needs to put probes in the ground, the centre probe is moved to show if distance is long enough, and in the main it can't be done with a domestic home as simply not enough distance to lay out the probes.

So in the main we measure the DNO earth and the homes earth together, the reading will be slightly high as a result, but the DNO earth is typically around 0.2 Ω so with a total of around 60 Ω the amount above true reading is not enough to worry about.

Continuity readings should have at least 200 mA flowing so can't do it with a multi-meter, and the insulation reading needs 500 volt so again multi-meter is no good, the impedance also needs a special meter, as does the earth rod resistance. So in the main not possible for the DIY person to measure. Same applies for the 40 mS time limit for a RCD to trip.

Full marks for trying, but one has to realise it can at best only give a rough result. And I see your results and wonder how you have managed to get them without tripping the RCD.

Years ago out water and gas came into the house in metallic pipes, these were bonded to the earth system, so although the earth rod was likely 60 Ω if fitted, in real terms the gas and water pipes resulted in the earth in real terms being around 0.4 Ω or less, however as the pipes were changed to non metallic the earth impedance started to rise to more like 60 Ω which resulted in it being no longer being able to rupture a fuse in the event of a fault.

The first idea was to use the earth rod as a voltage reference point, it the home earth raised to 50 volt above true earth the device would trip, called the ELCB-v soon it was realised all too easy for the home earth and true earth to be connected together, resulting in the device failing to work, so these were replaced with the ELCB-c using current instead of voltage, these came will a variety of tripping currents, up to 5 amp, but in the main 100 mA was used. They was called S type, and were designed so spikes would not trip them.

But then some one realised if we went to 30 mA then it would far less likely for the human body to be damaged, so we went to 30 mA at 40 mS, and then it was realised DC and high frequency could affect how the worked, so we got type AC, A, F, and B.

All this has resulted in more and more complex test equipment required to test it all works. Typically the test equipment cost over £500, some items can be got cheap, I paid £35 for a new insulation tester, but it was not calibrated so officially should not use it. But does the job. However not seen any cheap impedance meters or RCD testers.

 

[jurassic sparkk]

I paid £35 for a new insulation tester, but it was not calibrated so officially should not use it. But does the job. However not seen any cheap impedance meters or RCD testers.

I recently bought an old-fashioned vintage-style megger (loop tester with psc) for less than £30 (untested). It turns out it wasn't calibrated, but I got it done, and it functions fine.

 

[Murdochcat]

Dare I suggest that a TT system without rcd protection needs attention ?

 

[jurassic sparkk]

Dare I suggest that a TT system without rcd protection needs attention ?

It certainly does!
OP may also be misinterpreting it. No pictures provided whatsoever.

 

[Murdochcat]

Obviously one of those 'strange readings', it's just that I've not encountered it before and I wanted to check before I put the floor back down again.

Are you a spark or diyer?