Hi Ze at distribution board, how would you approach?

[Gringoking88]

Hi all,

Looking to exploit (please) those older and wiser lol. I was asked to extend / relocate a cooker circuit (6mm t&e) and did so no problems. I then started doing the minor works, R1+R2 came in at 0.05 ohms so was perfect. However, I then went to do do an Ze at the board and get very high readings e.g. around the 6 ohms Mark, this obviously resulted in my Zs being outside acceptable values.

The first learn I guess is always do a Zs / Ze before you take a job (I know, a rookie mistake)! But the question is how would you guys approach this now e.g. what would you do with the minor works, would you advise the customer to contact dno then retest or just calculate Zs from the given Ze and complete the minor works when the Ze is rectified, or would you do a minor works now, note the customer needs to rectify Ze then reissue another minor works?

Many thanks in advance,

Martin.

 

[Pete999]

Was the main earth dissed?, had you nulled the MFT leads? what type and size was the OCPD? and it's max Zs? What is the supply system?

 

[Octopus]

Customer needs to contact supplier.

Is the circuit RCD protected?

 

[westward10]

You now have a situation where you have altered a circuit possibly leaving it without fault protection even if there is an rcd in place. What is the "apparent" earthing system.

 

[The Ghost]

Is it TN or TT system. Is there an RCD on the circuit? 1667 ohms?

 

[DJ Ovengloves]

...don’t forget the bonding too!

 

[Gringoking88]

Was the main earth dissed?, had you nulled the MFT leads? what type and size was the OCPD? and it's max Zs? What is the supply system?

Pete,

Yes the main earth was disconnected, yes leads mulled, 16mm main earth, it is a tn-s earthing system Max Zs should be 1.36.

Thanks,

Martin.

 

[Gringoking88]

Customer needs to contact supplier.

Is the circuit RCD protected?

Hi Murdoch,

Yes there is rcd protection and fortunately disconnects in required time. .

Cheers,

Martin.

 

[Pete999]

...don’t forget the bonding too!

Pete,

Yes the main earth was disconnected, yes leads mulled, 16mm main earth, it is a tn-s earthing system Max Zs should be 1.36.

Thanks,

Martin.

Try testing the Ze again Martin could have been a bad connection on your test leads, if it's round about the same then as suggested your customer needs to contact the DNO.

 

[Jim90]

I had this once on a phase at a police station, arranged UKPN to attend, within a day they dug out the road and located a loose connection. They had a meter that could locate the fault in approx. Metres! Nice bit of kit.

 

[westward10]

I would not energize that circuit until it is resolved, you should not be relying on that rcd for fault protection if your external earth is breaking down.

 

[Wilko]

TNS with Ze of 6 is clearly a fault for DNO to fix. You could call DNO, or have Customer call them with you standing there.
You are right that Ze should be checked before starting work, that way you could have this chat up front and get it fixed first. Now I would say "cert will be done when work is completed, and that's when the means of earthing has been restored".
Agree @westward10 not to energise a circuit with a known fault.

 

[Jamie']

I have a question, I'm still in training and my college tutor said if a circuit fails on a Zs to stick it on an rcbo or RCD. obviously with this it's the Ze so it's the DNOs responsibility. Now is my tutor right on this? Where do you draw the line? And also what do you put on the test sheet?

 

[Strima]

I have a question, I'm still in training and my college tutor said if a circuit fails on a Zs to stick it on an rcbo or RCD. obviously with this it's the Ze so it's the DNOs responsibility. Now is my tutor right on this? Where do you draw the line? And also what do you put on the test sheet?

Tutor is being lazy. Think about why we need a low Ze/Zs and what effect it has upon the installation.

 

[westward10]

I have a question, I'm still in training and my college tutor said if a circuit fails on a Zs to stick it on an rcbo or RCD. obviously with this it's the Ze so it's the DNOs responsibility. Now is my tutor right on this? Where do you draw the line? And also what do you put on the test sheet?

When he says fails on Zs is this for an EICR or a new circuit. For a TN system you should not be relying on an rcd for fault protection. So if this is an EICR you can remedy the situation by changing the protective device for a different type or even a lower rating and as a last resort an rcd/rcbo. For a new circuit this should never happen if correctly designed. A 30ma rcd on a TN system is for additional protection, e.g. socket outlets and cables without earth containment in walls etc.

 

[Ian1981]

You’d only need a 100mA rcd/rcbo to deal with the fault protection issue on a high zs tho depending if it’s protecting socket outlets or cables buried in walls or additional protection is needed for other reasons, then 30mA will be required.
As said a new circuit on a TN system should be designed that the mcb provides satisfactory fault protection for said circuit.

 

[Gringoking88]

Thanks all for the reply's!!! I also spoke to NICEIC technical helpline and they advised to exactly as Wilko said, A+ Wilko lol! I have advised customer to call DNO and given them the details of the fault etc. Said I would retest when its rectified at no cost. They seemed very happy that I actually cared and didnt just make up the certificate lol

 

[Wilko]

Oh dear, I must've put the radio on NIC FM

 

[Gringoking88]

Oh dear, I must've put the radio on NIC FM

LOL, What I am confused about, or perhaps, I should say, there is confusion about is; if the Ze is high, some have suggested the circuit should not be energised, but this fault would affect the entire property so would you therefor not turn off the entire installation??? Not sure how the property owner would feel about that lol.

 

[Toneyz]

Oh dear, I must've put the radio on NIC FM

Replacement of Tony Cable job vacancy filled.

 

[Wilko]

Yes the fault affects the whole property, but you've modified one circuit. My view is the circuit is "out of normal service" during your work (including the live tests) and by testing you know it does not meet the intended design. So the circuit should not be returned to "normal service" by you.

 

[westward10]

LOL, What I am confused about, or perhaps, I should say, there is confusion about is; if the Ze is high, some have suggested the circuit should not be energised, but this fault would affect the entire property so would you therefor not turn off the entire installation??? Not sure how the property owner would feel about that lol.

Yes judge I was fully aware of the possible outcome of making the circuit live knowing the earth was not satisfactory.

 

[lurch]

Agree with all those that state advise dno to rectify but hypothetically or technically what is the difference of a tt System with zs readings sub 200 ohms?
Edit to say c/w 30ma rcd

 

[Wilko]

To me the difference is - if it was TT it would be working as per its design - if it's a TN then it has a fault, which could get worse ...