High earth fault loop readings

[sparkybird]

Hi there

I recently invested in an all in one tester and a friend offered to let me try it out on his house. However, the earth fault loop readings for upstairs and downstairs sockets are outside the regs. Ze is OK, but this is what I got:

Downstairs sockets - Zs = 1.39 max, also R1 = 0.52, R2 = 0.80
Upstairs sockets - Zs = 1.3 max, and R1 = 0.45, R2 = 0.59

Now I thought that if 2.5 mm T and E was used, you could estimate R2 as being 1.67 x R1. The values I got were not (would have been 0.93 and 0.75 respectively).

So what do you think the problem is? The installation is only a year or so old. I'm thinking along the lines of:
1 Poor connection(s) in sockets?
2 Poorly laid cables giving higher resistance?

What should be the next step?

SB

 

[SparkyTris]

How does the meter connect to the circuit?

 

[sparkybird]

It has a plug in cable ie direct to the sockets

 

[pdcelec]

Are you removing all paralell paths?

Is yor Ze about 1.05? If so what type of supply do you have

 

[sparkybird]

Thanks for getting back.

It's a TN-S supply and although I don't have an Ze value, I think it was OK coz the kitchen sockets tested out OK (about 0.5 Ohms for loop). I disconnected the main bonding (gas and water) before I did the test. I would have thought though that parallel paths would have reduced the loop value??

Any ideas gratefully received!

SB

 

[pdcelec]

If you can get your Zs values, you can also get your Ze. If you tell us what it is then I may be able help

 

[pdcelec]

I disconnected the main bonding (gas and water) before I did the test.

Never do this as it could be dangerous!

 

[sparkybird]

pdcelec

I'll try and get over tomorrow to measure Ze.

Sorry, got a bit confused there about disconnecting the main bonding. (not the main earth - which would be very dangerous!). Just to clarify that I know my onions - you do not have to do this for earth loop measurement, but you do have to do it for measurement of Ze - to avoid parallel paths. Obviously reconnect straight after the test.

BTW surely we can work out Ze as Zs= Ze+R1+R2?
From the figures earlier:
For upstairs sockets this would be 0.03 ohms and for downstairs 0.26 ohms. Shouldn't they be the same (or at least very similar?) Me thinks therefore that there must be problems within the installation....

 

[pdcelec]

BTW surely we can work out Ze as Zs= Ze+R1+R2?

For a ring circuit:

Zs = Ze + (0.25r1 + 0.25r2)

which gives approx 1.05 for the Ze

 

[sparkybird]

Oh, I see. But how come I got a reading of 0.5 ohms for Loop on the kitchen sockets? Surely all loop readings would have to be a minimum of 1.05 if this is the value of Ze?? Arrg - this is doing my head in!

Also, I notice that this Ze i(1.05) is outside the regs for the type of supply... Will confirm by measurement tomorrow

SB

 

[dingbat]

The only sure way is to start from scratch and confirm the Ze reading with installation isolated, main earth attached and bonding disconnected. If this is satisfactory then confirm that the equipotential bonding is actually continuous, reconnect it and test Zs again on each circuit. Your upstairs and downstairs circuit continuity readings seem okay, so if, as suspected, you have a high Ze you need to contact your DNO.