Very high direct zs and pefc

[sidekick]

After adding a SFCU as a spur off a kitchen ring circuit I took a direct Zs reading which was 18.4 ohms / 13A. When I then checked from main incomming tails I took a reading of 17.89 ohms / 13A. Rcd trip tests work fine and within required times etc.
As this is a very high Zs, what should I do ref issue of minor works for the work I have done. i.e note on cert of problem or is there something I should do to rectify first.

 

[Des 56]

What is the earthing system ?

 

[ackbarthestar]

Perfectly acceptable if the the installation's earthing arrangement is TT, but not so perfect if it is TN.

So what is the earthing arrangement here?

 

[sidekick]

Tn - s

 

[ackbarthestar]

Poor / cut earth connection somewhere, this could be at the installation or out in the road

1/ Check all your connections in the installation
2/ Contact the DNO regarding any works they may have undertaken and compare your results with theirs.

3/ You may have to TT the installation....

 

[One_of_them]

so you didnt think of checking the Ze before starting the work?

Give the DNO a call and let them know. They will decide your next move

 

[sidekick]

Poor / cut / loose earth connection somewhere, this could be at the installation or out in the road

My thoughts were to note the problem on the minor works cert and advise to contact supplier to check and rectify the problem.

 

[sidekick]

so you didnt think of checking the Ze before starting the work?

Give the DNO a call and let them know. They will decide your next move

Maybe should have done. But at the end of the day it was just a simple spur for an extractor fan and the work would not have made it any worse or caused the fault.

DNO call my next job. Thanks

 

[Octopus]

Maybe should have done. But at the end of the day it was just a simple spur for an extractor fan and the work would not have made it any worse or caused the fault.

DNO call my next job. Thanks

If I were you I would be adding a note on the invoice and cert stating the issue and the fact it needs to investigated too.

 

[telectrix]

the reading could be high due to internal resistance of MCB or RCD. you need to get a reading of the Ze before being able to tell if it's 1 thing or the other.

 

[Dunc]

the reading could be high due to internal resistance of MCB or RCD. you need to get a reading of the Ze before being able to tell if it's 1 thing or the other.

Agreed, I've had high resistances across an RCD before which put the Zs readings way above what was expected. If you get higher then expected Zs readings then carry out a proper Ze test (make sure the install is isolated completely and you're testing the main earth only which you disconnect at the DB). If the Ze is high and beyond what the electricity board quote for a TNS system (0.8ohms I think) then I believe it is the suppliers responsibility to sort the issue FOC as they have to maintain a good earth to the property if one is already supplied. I'm sure I read this somewhere and the argument is based around the Electricity at Work Regulations as work carried by the board must ensure that a safe supply is still maintained to a property.

 

[Sav]

I once had a similar situation.
Hi Zs on socket outlets, but ZE was 0.13 on a TN-S system.

Turns out that the RCD was faulty, even though it was tripping at correct times.
Once I replaced the RCD, the Zs was normal, I think 0.21ohms or something.
I hope this helps, or if someone can shed some light on this, if this has ever happened to them ??

Have a great day everyone,
Sav

 

[telectrix]

have once had a RCD give a Zs of 349 ohms on a Ze of 0.2.

 

[plugsandsparks]

As its high on the incoming tails, have you checked the earth sheath clamp on the incoming cable, it may have broken away or be loose.

 

[tony mc]

IMO with your ZS test so high and as you say the Earthing is TNS your right to question the result.

What was your R1 + R2 reading for the additional FCU ?

ZE would have to be the next test to confirm that you do have a earth via the sheath and the reading is acceptable.

If it is OK (ZE that is) try testing the circuit on Hi current loop bypassing the RCD and see what reading you get!