Residual Current v Earth Leakage

[Timotei]

I think I must be missing the blindingly obvious, but.....

TT system......100ma time delayed as main switch, then some final circuits (which will require 30ma RCD as includes sockets etc)...then 30ma RCD for other final circuits...

Prior to changing CU at customer's request to include 100ma time delayed and 2 x 30ma RCD I have tested for earth leakage using clamp meter....(amongst other pertinent tests inc IR which is overscale on all circuits)

clamping LN tails gives 20ma as "regular" reading....lots of tv's / computers / fridges etc, so not entirely surprised by cumulative leakage...

but if boiling kettle, LN reading rises to 45+ma....so tested kettle using "stripped" extension lead technique, again clamping LN, to show 2-9ma max reading, but mostly 2-3ma.....and measuring on CPC only mirrors this result...

so why is increased residual current at tails so much higher than that measured "at source"?

have to admit to having tough time personally at the moment, so mind not as fresh as it should be.......hence opening statement about missing the blindingly obvious....

Thanks in advance,

Timotei

 

[PEG]

Hi,is this TT deriving it's earth from a rod? and have you an Ra for that?

 

[Timotei]

Hi PEG, earthing is via rod with RA of 143ohms, not perfect but have seen worse. Copper gas and water services also correctly bonded.

 

[stevethesparks]

In my experience using an earth leakage clamp meter at source, I would say the readings should only be taken as a rough guide, not as a accurate measurement.

 

[Wilko]

Hi Timotei, sorry to hear about your rough time. I hope things get better soon - including this job!
Agree with Steve. I've also seen strange numbers from time to time doing similar things, which I've put down to my el-cheapo meter, other nearby cables and me. If the kettle really did have 25mA leakage it would be about 10K to earth (perhaps a quick check). Perhaps also confirm RCD tests and add a ramp test for peace of mind .

 

[Midwest]

I got a cheapo earth clamp meter (£80ish), readings varies with the time of day, but useful as a guide. Nice fresh battery might help.

My meter suggests my house has a leakage across the board of 19mA. We don't have lots of pc's etc.

 

[PEG]

Hi,if we accept that the measuring equipment,is giving accurate readings,i would be tempted to repeat the test,with another load of similar size,connected where the kettle was,and see if things altered.

There are several reasons for why this issue could occur,including anomalies,on that particular circuit.

 

[The Ghost]

Do you put the clamp on the tails or the main earth cable?

 

[Timotei]

Thanks to all for very useful replies.

I have primarily tested with LN tails in the clamp, this gives the "high" reading I have mentioned (c. 19mA standard and >40mA with kettle on).

If I test the earthing / bonding cables I get 0.7mA on main earthing conductor, 0.7mA on gas bonding and around 14mA on water bonding.

I've not had the clamp long, thought it would be a useful assist in fault finding jobs. It's definitely a cheapo, £80 as opposed to£100's!

HandyMAN TEK775 Mini AC Earth Leakage Clamp Meter - https://www.test-meter.co.uk/handyman-tek775-mini-ac-earth-leakage-clamp-meter/

I think it's readings on LN via extension lead seem accurate (by what I would expect to find), but the tails measurements seem well out by the same standard.

Could this be because the meter is not true RMS? When measuring at the tails am I getting readings affected by sinusoidal / non-sinusoidal waveforms etc?

Boiled kettle on different socket circuit, same results within a couple of mA.

Ramp tested RCD's, 100mA tripped @ 70mA, 30mA @ 30mA, both well within max disconnection times.

Thanks again for advice / support.

Timotei

 

[Midwest]

OP said 'clamping LN tails' in his opening post.

 

[Midwest]

OP said 'clamping LN tails' in his opening post.

Apologies, never saw OP's response before I posted.

 

[Midwest]

Thanks to all for very useful replies.

I have primarily tested with LN tails in the clamp, this gives the "high" reading I have mentioned (c. 19mA standard and >40mA with kettle on).

If I test the earthing / bonding cables I get 0.7mA on main earthing conductor, 0.7mA on gas bonding and around 14mA on water bonding.

I've not had the clamp long, thought it would be a useful assist in fault finding jobs. It's definitely a cheapo, £80 as opposed to£100's!

HandyMAN TEK775 Mini AC Earth Leakage Clamp Meter - https://www.test-meter.co.uk/handyman-tek775-mini-ac-earth-leakage-clamp-meter/

I think it's readings on LN via extension lead seem accurate (by what I would expect to find), but the tails measurements seem well out by the same standard.

Could this be because the meter is not true RMS? When measuring at the tails am I getting readings affected by sinusoidal / non-sinusoidal waveforms etc?

Boiled kettle on different socket circuit, same results within a couple of mA.

Ramp tested RCD's, 100mA tripped @ 70mA, 30mA @ 30mA, both well within max disconnection times.

Thanks again for advice / support.

Timotei

I have similar tester. I just use it for a rough guide. Sure one of the forum fathers may be able to explain your readings. In any case, your install is not causing you any issues, what are your concerns?

 

[PEG]

Hi,your water pipe isn't lead,by any chance?

 

[Timotei]

Midwest, no real concerns, just want to understand the readings so that I am better prepared to interpret them when I do have an issue. Thanks for your comments / advice. I think that,like you, I shall use the clamp meter as guidance only.

PEG, not sure about the pipe outside the house, but all visible internal is copper.

 

[Timotei]

PEG, you got me thinking now.....client is elderly customer who I have to admit developing a soft spot for over the past couple of years....obviously lead pipe would not be good.....may I ask what made you think it might be lead and is there any easy way of telling "what lies beyond" the house other than starting to dig?