High zs

[jimsparks]

Evening all,
Got a strange one that I could
Do with a little advice.

I have extended a radial circuit for an extra socket. R1r2 0.41 IR 84.5mohm and rcd trips in time.
When doing zs I got a reading of 6.83 when I was expecting about 0.63 ish when ze was 0.21 on tns supply .
Started to investigate disconnected that circuit from board and found that on the disconnected CPc I was getting continuity if I tested between that and earth bar. Took all the earths out of the Earth bar but the continuity disappeared except for testing between incoming Earth and the original one that was disconnected from the first circuit. I then tested zs on cooker circuit to see if that gave a proper reading but got 6.91 and then after testing the other circuits got similar readings ! A new split load board was fitted a yr ago and got the certs and obviously no issue.
If anyone has got any suggestions I would be very grateful.

 

[Murdoch]

What is the Ze?
What is the earthing arrangement?

 

[Fleeting]

What is the Ze?
What is the earthing arrangement?

TN-S.

 

[Fleeting]

Did you recheck your Ze I have has TN-S lose it's external impedance half way through testing.

 

[jimsparks]

yes, kept rechecking it. Since I posted I have started to break the circuits down I found broken earth on one of the ring circuits but not in socket or jb but the cable itself which I will replace. Another strange thing which I found just before I stopped for the day, set my tester to continuity and put one lead on neutral load side of rcd2 and then the other lead on neutral bar for rcd1 and I was getting continuity reading.

 

[Murdoch]

Are you using a plug in adaptor

How close is the cu to the service head?

 

[jimsparks]

I have done both adaptor and croc clips on terminals taken out of socket. Service ghead is about a meter/half from cu

 

[Murdoch]

I have done both adaptor and croc clips on terminals taken out of socket. Service ghead is about a meter/half from cu

I would try to do the Ze , when the CPC is connected to the service head

Is the 0.21 Ze your reading or off the previous cert ?

 

[kerching]

Meterzeroed, calibrated, batteries ok, leads ok?

 

[jimsparks]

0.21 is my reading with earth taken out of board. Prev cert was 0.70. Leads all checked and new fully charged batteries

 

[binky]

Weird stuff tends to be neutral issues, and cpc can be damp in walls - spent half a day once chasing CPC continuity before realising it was fresh plaster still being damp.

 

[Murdoch]

What make of RCDs are they ?

I would do Zs tests on the incoming and out going sides of the RCDs

 

[jimsparks]

They are Bg thanks I will try that Monday

 

[Murdoch]

They are Bg thanks I will try that Monday

Any updates

 

[7029 Dave]

Mmm If all the circuits are high Zs. IMO it must be an external issue. Is the main earth lead nice and tight at the MET.

 

[jimsparks]

Well after pretty much going through each circuit and finding three faults. 2 seperate loose Cpcs on ring final circuit and a neutral to earth fault where the screw of a socket was pinching a lighting circuit cable that had been run through a dual box to get to switch,However I still was left with high zs reading . I then linked out rcd and tested on loop test with out rcd trip function. The readings all came out as expected or what they should be .So I now think something is causing interference. I have also just found out they have dug up the rd recently to provide services to two new houses opposite. I have asked power networks to check ze and borrowing another tester for my return Friday to see if it is my metrel ?

 

[Murdoch]

Did you do incoming and outgoing sides CPC the RCD

What make of CU is it ?