Live/earth fault ?

Hi again,

I'd been doing some work on the ring main and before I reconnected the socket I'd been working on I thought I'd check the continuity between L&L and N&N wires to check there wasn't a break anywhere else in the system. This was fine. However I also checked the resistance between the L&E conductors. This revealed a resistance of 19 ohms, somewhat less than I was expecting.

Going around the house unplugging things appeared to indicate that a small freezer was the culprit. On unplugging it the L-E reistance when open circuit ish (unsteady readings kohm range).

(everything else unplugged/switched off by this stage)

I unplugged the freezer and measured the resistances accross the plug terminals. L-E was open circuit, L-N was 19 ohms. N-E was open circuit.

This made me think that there was maybe a fault with that socket so I plugged a lamp into it expecting to see a L-E resitance of70 ohms but instead was still open circuit.

Plugged freezer into different socket L-E open circuit.

Now confused! I appear to have a fault that only occurs with one socket/appliance combination.

Ran out of time to investigate further (complaints about lack of TV etc)

When I re-connected the ring and put the fuse back in I got a reading of about 0.5V between L&E.

Am I missing something. Presumably I wouldn’t expect 19 ohms between L&E.

Any suggestions as to what maybe wrong or what to try next. I was thinking of measuring the current in the earth conductor by connecting meter in series at the cu just to confirm if I really have a problem.

Jim

Make sure everything is unplugged (even fuse spurs), shouldn't be getting any readings between phase/earth & phase/neutral (should be open), unless something is plugged in or there is a fault...sounds like your using a low ohm reading meter.....to make sure everthing is clear try using an insulation resistance tester (megger) phase-phase should read 0.00 / phase-neutral should read >999 etc.

The appliances plugged in can cause false readings (ie, fridges with motors / neons, cookers with elements etc etc)

Hope this helps.

il78 said:

Make sure everything is unplugged (even fuse spurs), shouldn't be getting any readings between phase/earth & phase/neutral (should be open), unless something is plugged in or there is a fault...sounds like your using a low ohm reading meter.....to make sure everthing is clear try using an insulation resistance tester (megger) phase-phase should read 0.00 / phase-neutral should read >999 etc.

The appliances plugged in can cause false readings (ie, fridges with motors / neons, cookers with elements etc etc)

Hope this helps.

Click to expand...

With everything unplugged it checks out OK. Why should plugging the appliance (freezer) in gives me a false reading (could my meter be charging an inductor or something?). I could understand it if it was a couple of kohms but 19ohms is nothing. My meter should read up to 40Mohm but I'll try and get hold of a megger.

Can an appliance cause a low resistance reading between phase and E and not be faulty (either the circuit or itself?).

Jim

If everything checks out OK without anything plugged in, then your wiring is OK...so it must be something you have plugged in..

When I'm testing an installation I make sure everything is unplugged :

A) I dont want anything giving me a dodgy reading
B) I dont want to blow some geezers sound system up

Appliances will give some wierd readings depending on what they are...

I would like to Echo il78's point regarding a megger. an "ordainairy" multimeter is not really ideal for testing phase to earth, the other point is were you touching your probes when testing? as this can also give a false reading, you certinly wouldn't be holding the probes with a megger test @ 500v it does make you use the colourful side of the Queens English

breezer said:

I would like to Echo il78's point regarding a megger. an "ordainairy" multimeter is not really ideal for testing phase to earth, the other point is were you touching your probes when testing? as this can also give a false reading, you certinly wouldn't be holding the probes with a megger test @ 500v it does make you use the colourful side of the Queens English

Click to expand...

Unfortunately I don't have a megger (and they look relatively expensive new)

I was trying not to touch but I only tested the circuit as an afterthought before I turned the power back on. I'll probably have a proper look at it this weekend when SWMBO's not wanting to watch telly (maybe use some chock block to clamp the probes). I was rather hoping someone might have a credible explanation of how I could get a low resistance between phase and earth without something being at fault but I guess that's not possible?

Out of interest what "testing" would normally be carried out after electrical work (other than previously discussed).

Many Thanks

Jim

Anything within the fridge or any appliance can cause the reading, noise suppressors, capicitors, motors etc etc if it was a major fault I think you would know about it.....

other tests would include :

Continuity of protective conductors
Continuity of ring final circuits
Insulation resistance
Polarity
Earth electrode resistance (TT systems etc)
Earth fault loop impedance
Prospective fault current
Functional testing (RCD’s etc)

Best grab yourself a test kit......

About halfway down here:

http://www.iee.org/Publish/WireRegs/forms_2001.pdf

are the IEE forms that the tester completes - gives you an idea of what tests are run.