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?

 

[Risteard]

Notwithstanding the fact that clearly something is wrong there, the earth fault loop impedance won't be the same throughout a ring final circuit. That only applies to (R1+R2) readings and (R1+Rn) readings due to the cross-connection.

 

[Wilko]

Hi - it does read (from afar) like that is a long radial off the ring with a cabling fault. I've been to a few of those older installations where one CPC of the ring has been snapped off just behind the backbox, probably dating from installation...
Happy hunting HHD .

 

[Strima]

Spur off a spur off a spur off a spur etc etc etc

 

[buzzlightyear]

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

 

[Harry_P]

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?

You haven't mentioned what the main earth reading is,have you checked the main earth and also the connections?

 

[HappyHippyDad]

Thanks all for replying do far.

You haven't mentioned what the main earth reading is,have you checked the main earth and also the connections?

Harry, do you mean the Ze? If so, Ze was o.35Ω TNS. All other Zs's for other circuits were fine.

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

IR results were surprisingly good for an older property. Can't remember exactly but 50 or 60MΩ.

 

[Harry_P]

Thanks all for replying do far.



Harry, do you mean the Ze? If so, Ze was o.35Ω TNS. All other Zs's for other circuits were fine.



IR results were surprisingly good for an older property. Can't remember exactly but 50 or 60MΩ.

Thanks all for replying do far.



Harry, do you mean the Ze? If so, Ze was o.35Ω TNS. All other Zs's for other circuits were fine.



IR results were surprisingly good for an older property. Can't remember exactly but 50 or 60MΩ.

First I would disconnect the link from the socket to the conservatory and check if the reading is still high.if its still high would disconnect all the neutrals from the neutral bar and check the reading again. Then i would trace the cables route from the consumer unit to find where it rises to 1 ohm from .35

 

[HappyHippyDad]

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?

 

[telectrix]

What would you do with regards the certificate and notification?

i'd hang fire till they decide whether to have further investigation and remedials. give them a month, and if no further just complete as normal with a not on the cert re.the high Zs and that you have recommended further work to rectify.

 

[buzzlightyear]

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?

I have just done a EICR,before I do a c/u change I was unsatisfactory ,but the customer is getting me back to do the remedials,where the IR was that low needs investigation . so in order for me to change the c/u that must be carried out .

 

[Lucien Nunes]

1. Agree fully.
2. Yes, sounds like a poor connection, but not a

...screw going into insulation etc which may explain higher Zs at this socket?

As I can't see the relationship between the two.

What's the remedy if it's a radial-off-a-ring? Split at the junction into 2x 20A radial leaving the conservatory on one half, or fuse the conservatory down to 13A with an FCU? This is an old problem to which I don't like either of the solutions unless the ring is very lightly loaded.

 

[Nebu1a]

As long as the RFC doesnt serve the kitchen or utility I would temporarily fit a 20amp MCB until the problem is recitified. If the conservatory is all spurred then a local 13amp FCU could be fitted.
Sometimes on older socket outlets the pins become worn and give high Zs readings although 4.4 ohms is very high. I think it is likely to be loose connections. What size was the cpc in the RFC, 1.0mm or 1.5mm?

 

[HappyHippyDad]

As long as the RFC doesnt serve the kitchen or utility I would temporarily fit a 20amp MCB until the problem is recitified. If the conservatory is all spurred then a local 13amp FCU could be fitted.
Sometimes on older socket outlets the pins become worn and give high Zs readings although 4.4 ohms is very high. I think it is likely to be loose connections. What size was the cpc in the RFC, 1.0mm or 1.5mm?

1mm

 

[Nebu1a]

Installation to the 14th edition then.
Were the CPCs twisted together in one piece of green sleeving?
I did a EICR in a flat last year with high Zs and I found that the CPC had a layer of oxidised copper. After reterminating and fitting new socket fronts it dropped down to acceptable levels.

 

[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......