ODD ELECTRIC RESULTS ON REWIRE

[dingbat]

5 day course sparkys I am guessings, they must have been cheap too. I stand to be corrected and for the OP's sake I hope I am.

I believe the OP has already corrected you - check out the first post again.

By the way, in my experience, the time-served sparks fare no better than new entrants into the industry when it comes to inspection testing. In fact some of them are worse because their heads are full of electrical folklore from their ill-remembered past!

 

[davelx]

To the OP:

Why not ask your electrician to come onto the forum and explain exactly what he's testing and what results he's getting?

There should be enough expertise here to help them with the diagnosis.

 

[holmslaw]

..

 

[houserenovator]

To the OP:

Why not ask your electrician to come onto the forum and explain exactly what he's testing and what results he's getting?

There should be enough expertise here to help them with the diagnosis.

Yes, that's a good idea. Thanks for all your ideas. I will try and get more precise information...

 

[houserenovator]

Oh dear!! What happens if it's tested on 1000v?

Can it ruin the wires? Am now worried about that too! What result figures should insulation resistance test show on meter?

What's the lowest that is acceptable to pass?

 

[ban-all-sheds]

Unlikely to damage the cables.

The minimum pass level is 1MΩ, but the guidance is that anything less than 2MΩ should be investigated.

However - as the place has been rewired, I'd expect much higher figures than that - an individual length of new cable should be 2-3 orders of magnitude higher.

If their tests show that each circuit is registering several '00MΩ, or into the GΩ range, which it should be doing for new circuits, and yet the installation as a whole is coming out incredibly low then something strange is going on.

Have they said which test is coming out low? L-N, L-E, N-E? Or are they testing (L&N) - E?

Have they tried adding circuits one by one and doing the whole installation test?

 

[ColJack]

frequent testing at higher than rated voltages ( most domestic cables are 600v.. ) will gradually break down the insulation.. IR testing is a destructive test if not done right..

 

[brig001]

houserenovator, I don't suppose you are close to a large transmitter? A friend was, and we could measure 5V on his radiators, but he was about a mile or so from Winter Hill. I expect that this would upset many digital testers, but might not upset an old wind-up type.

 

[houserenovator]

The N-E resistance test is coming out low, from what I understand, across several circuits, although the kitchen and garage are fine.

Also, when you use a Fluke meter, do you have to keep your finger on the button for a correct reading? It has been mentioned that perhaps the tester is getting low readings as he's not pressing it long enough?? Does that make any sense?!

 

[flameport]

Low readings between N-E could be dampness (from plastering etc), or damaged cables. However how low is 'low' ? Either way, if there is damage it is a simple matter to test individual sections until the defect is located.
Hopefully they were testing with the L&N wires disconnected from the consumer unit, otherwise silly readings between N&E are almost inevitable.

If they are testing individual circuits and getting good results, then connecting them all together and being amazed when the insulation resistance measured is lower, then clearly they have no idea how resistance actually works.

If the meter is one of the Fluke 165X series, then yes, you do have to hold the button down for insulation resistance and continuity tests. All explained in the manual.
Only pressing the button for a short time (such as 1 second or less) when measuring insulation resistance could result in low readings.

A few volts on a disconnected circuit when measured with a high impedance meter is very common. Usually caused by cables being adjacent to other energised circuits, or the house is located under powerlines etc. Easily clarified by testing using a meter with a lower impedance setting, or connecting a small load and then testing for voltage.
All of this is pretty basic stuff, and suggests that those doing the testing don't really understand what they are doing or why.

 

[SparkyNinja]

Can it be verified that along the run of the circuits, there are no connections with supplementary bonding anywhere?

 

[ColJack]

N-E is coming out low when everything is connected up, does this include the connections at the CU?

what breakers / rcd's / rcbo combination do you have?

some rcbo's have an earth flylead for monitoring earth voltages, you might be reading through that if they are connected up..

 

[LoveRocket]

Hopefully they were testing with the L&N wires disconnected from the consumer unit, otherwise silly readings between N&E are almost inevitable.

If they are testing individual circuits and getting good results, then connecting them all together and being amazed when the insulation resistance measured is lower, then clearly they have no idea how resistance actually works.

I think you've hit the nail on the head there, Flamester. They're checking each circuit individually, no problem, then connecting everything to the CU and then basically doing a somewhat unorthodox N - E 'looptest' back through the supply!

Tell them to test the whole installation with the main earth conductor disconnected.

 

[houserenovator]

Thankyou for all your ideas and suggestions - I will pass them onto our electrician, who is experienced and time served.
Hopefully, he can then clarify the situation better than I ever could (not being at all electrically-minded I've probably got a few details mixed up!). So thanks all for trying to shed some light!

 

[securespark]

Right. The way I see it is quite simple. The time-served qualified guy is baffled. He has tried everything.

What he should do now is graciously admit defeat and pay for another spark to do a PIR.

Any defects noted in that PIR should then be rectified at his cost. Then and only then, should you pay his bill. Less, of course, the cost of the PIR and any rectification work.