Increasing Zs readings on a ring as I progress through the house.

[HappyHippyDad]

I have just fitted a new consumer unit into a house which previously just had a fuse box BS3036 with no RCD protection.

Whilst doing the testing the Zs readings were not as expected. They are around 1Ω throughout the house but at one socket they are 3.7Ω. The customer states this was the socket that was used to extend the ring into the new conservatory. The Zs readings rise further in the conservatory until they reach 4.4Ω at the far end. There are 4 sockets in the conservatory.

I shall be returning next week to investigate but as this is not something I have come across before I thought I would offer my suggestions and see if I had covered everything.

1. Check socket that tests as 3.7Ω and see if they have indeed extended the ring or just made a radial from it. Even if they had made a radial this doesn't explain the high figure at this socket but it would explain the increasing results as you go further into the conservatory.

2. Check same socket for loose connections, screw going into insulation etc which may explain higher Zs at this socket?

Any other ideas so I am prepared?

 

[TJ Anderson]

what was the IR results that what have told you some thing .

Not for this fault mate. You could have perfect IR for L -N, L-L & N-E and it won't affect the high Zs.

 

[MFS Electrical]

Just as a follow up..

I have just had an email from the elderly customer saying that they will 'think' about getting me back to fault find. Before the CU change I had made it clear verbally and in the quote that I may need to carry out fault finding and extra work if there are faults found/complications.

I always carry out a prework visit for a CU change where I do some standard tests at the CU, Ze, global IR etc but I dont do a full EICR.

I'm not going to leave this elderly couple without sockets so all is live, just as it has been for the last 36 years they have been there, apart from now they have RCD protection.

What would you do with regards the certificate and notification if they choose not to get the work carried out to remedy the high Zs values on the sockets?

With initial verification you should withhold the cert until the defects are remedied

 

[Deleted member 26818]

The defects are nothing to do with the work conducted.
As such issue the EIC with a note regarding the high Zs under comments on the existing installation.

 

[MFS Electrical]

The defects are nothing to do with the work conducted.
As such issue the EIC with a note regarding the high Zs under comments on the existing installation.

They may not have been connected but as the designer of all of the circuits connected to the CU, because let’s face it you’ve just changed every circuits parameters you shouldn’t be certifying it unless it complies. Although if your sticking RCBOs in the Max Zs would be 1667ohms so it does comply.

 

[HappyHippyDad]

Thread conclusion!

The customer at last agreed to the fault finding on the high Zs results.

I returned today, the customer told me his son had opened up the conservatory sockets and may have fixed the problem!! I tested the Zs, it was now 12Ω rather than 4Ω

I opened the socket that I thought was were they had extended the ring and found 3 earths loosely connected together and one earth not connected at all. Connected it all up properly and the Zs is now 1.06Ω. (Still close to the 1.1Ω max Zs for a 32A Type B MCB, those max figures are coming down aren't they!)

A happy customer as he thought I was going to be there much longer, as did I!

 

[buzzlightyear]

those max figures are coming down aren't they!)

you can sing to this when you get good results and not football ones .

 

[Liamsparks]

Thread conclusion!

The customer at last agreed to the fault finding on the high Zs results.

I returned today, the customer told me his son had opened up the conservatory sockets and may have fixed the problem!! I tested the Zs, it was now 12Ω rather than 4Ω

I opened the socket that I thought was were they had extended the ring and found 3 earths loosely connected together and one earth not connected at all. Connected it all up properly and the Zs is now 1.06Ω. (Still close to the 1.1Ω max Zs for a 32A Type B MCB, those max figures are coming down aren't they!)

A happy customer as he thought I was going to be there much longer, as did I!

Good work there HHD, but as stated above - as you've installed a 30mA RCD to every circuit as your additional protection you have in turn increased your max Zs reading to 1667ohms for every circuit.

 

[HappyHippyDad]

Good work there HHD, but as stated above - as you've installed a 30mA RCD to every circuit as your additional protection you have in turn increased your max Zs reading to 1667ohms for every circuit.

Surely that's not right Liam?
I fully understand the 1667 figure but the RCD is 'additional' protection and we should still focus on ADS and therefore meet the max Zs figures.
I can't see an inspector saying 12ohms is ok. Plus with a particularly high Zs on a tns or tncs it may well be a fault (as it was in this case) as with a Zs of 4ohms that would have to be a mighty long run.

 

[Liamsparks]

If you have the 30ma rcd it would be perfectly fine to put the 1667ohms in max zs option.

However that said, it's not to say with the knowledge you have as a sparks not to further investigate an abnormal reading. It's all in the regs book.

 

[Baddegg]

Technically fine yes but still poor design......