Basic question

[davphi338]

Hi,

Can someone tell me what the resistance should be between earth and live and earth neutral when measured with a meter, obviously measured with the supply off on a 32A ring main.

 

[dingbat]

Very high indeed.

Minimum (currently) required is 500,000 Ohms, but expect figures in excess of 200Megohms

 

[Lectrician]

But if N is still connected at the CU then that could alter things considerably! Assuming you are only isolating the ring final circuit by using a SP MCB.

 

[davphi338]

Will the figure be affected by appliances on the ringmain. The worrying thing is the ringmain is working perfectly well and not tripping the RCD and yet the resistance between earth and live is virtually zero and about 40ohm between Neutral and Earth. I can't easily remove all the appliances on the ring to check because there is a heated towel rail on the ring and that is wired on a spur

 

[Steve]

I can't easily remove all the appliances on the ring to check because there is a heated towel rail on the ring and that is wired on a spur

Turn it off?

 

[dingbat]

Nobody should be carrying out IR measurements with the main isolator closed.

I suspect, however, that the OP is not sure exactly what he is measuring, or why he is measuring it.

 

[davphi338]

I haven't actually isolated anything. Just turned the mains off via RCD. I had a problem with a heated towel rail element which I'm 99% certain was the cause of my RCD tripping. When this was removed everything operated fine but I was doing a quick check by measuring resistance between earth and live and neutral live at the FCU where the towel element was originally connected and got the reading of minimal ohms between Live-Earth and Neutral-Earth. I thought the reading should be really high but then thought other factors may come into play thus my original post.

 

[RF Lighting]

What sort of tester are you using?

 

[davphi338]

Just a normal meter for measuring, Volts, Amps and Resistance.

 

[bernardgreen]

""Normal"" resistance measurements can be with a couple of volts or less applied to the circuit or with a working voltage applied of 230 or 500 volts.

Testing a mains system with a couple of volts will indicate any "hard" short circuits ( wire to wire ) but may not show faults in cable insulation that will fail when 230 volts is applied.

With a couple of volts applied non linear insulation may show very high resistance as the insulator has not broken down. Applying a few hundred volts to the same material may break down the insulation and show a very low resistance.

New ish PVC has non linear charactoristics when the voltage is around 1000volts and above. Old, damp or chemical affected PVC can become significantly non linear at much lower voltages

Extreme example on non linear insulation is two wires separated by air. Using low voltage testing there is no current flow. Air is a perfect insulator.

Test with a couple of thousand volts and there may be an arc when the air insulation breaks down to near zero resistance.

 

[Lectrician]

If it is a DC sence RCD, you will also be getting some odd values from that - Turn off all MCB's and remove neutrals from the neutral bar of the CU to do the tests.

 

[davphi338]

All getting a bit too complicated for me. I've now replaced the faulty towel heater element and everything is working OK, well the RCD isn't tripping and I don't get a shock when I touch the towel rail, if that means anything. I was just a little concerned when I had a short between live and eath on the ring main but maybe thats normal when there are appliances plugged into the ring and evrything is still connected at the CU.