readings?

[SIIVION]

Good day.
I have recently been involved in an EICR on a property with 15 DB's.

The average PFC appears as 90A and the average Ze is 2.9
The system is TN-S

I suspect there is an earth fault somewhere which would explain the Ze but why the PFC reading?

Each DB is isolated by a BS88 80A or 100A at distances ranging from 5m to 30m

Any ideas?

 

[SIIVION]

I hope there isn't any main bonding conductors being taken to this and the any other DB's that are being supplied with 2 core SWA cables, as the armouring will not have a sufficient CSA for compliance??

Apart from that, something is very wrong here with the values you are stating for the Zdb's of these DB's....

I was aware of the necessity to establish the armouring Cu equivalency and have assumed from the age of the installation and other factors it is SWA rather than XPLE. 2 core 10mm SWA armour is 17.7mm whereas XPLE is 8.36mm.

Yes the readings are a concern, as previously mentioned it is probably a DNO problem, will ket you guys know soon

 

[Engineer54]

I was aware of the necessity to establish the armouring Cu equivalency and have assumed from the age of the installation and other factors it is SWA rather than XPLE. 2 core 10mm SWA armour is 17.7mm whereas XPLE is 8.36mm.

Yes the readings are a concern, as previously mentioned it is probably a DNO problem, will ket you guys know soon

Neither will comply as far as main bonding is concerned, the CSA of both PVC and XLPE SWA cables, the steel wire armouring will only comply as far as the circuits CPC!!

I wouldn't be waiting too long to confirm or otherwise that it's a DNO cable sheath problem....

 

[SIIVION]

DNO test today... Ze 1.48ohms , PFC 146A. The system is 'sort of' TT but also TN-S, all a bit odd really.
The DNO are going to do works in the new year to improve the supply.

 

[SIIVION]

Neither will comply as far as main bonding is concerned, the CSA of both PVC and XLPE SWA cables, the steel wire armouring will only comply as far as the circuits CPC!!

I wouldn't be waiting too long to confirm or otherwise that it's a DNO cable sheath problem....

Could you tell me why the 2 core 10mm armoured sheath will not be suitable as an earth please?
I have read it is equivalent to 17.7mm.

 

[Geoffsd]

DNO test today... Ze 1.48ohms , PFC 146A.

That' the PEFC.

The PSCC will be higher so that will be the PFC.

 

[davesparks]

Could you tell me why the 2 core 10mm armoured sheath will not be suitable as an earth please?
I have read it is equivalent to 17.7mm.

It will most likely be fine for a cpc but not for a bond.

 

[Engineer54]

Deleted

 

[Engineer54]

Could you tell me why the 2 core 10mm armoured sheath will not be suitable as an earth please?
I have read it is equivalent to 17.7mm.

A PVC 10mm 2 core SWA cable has a steel wire CSA OF 41mm, to comply as a circuit s CPC would require 22.6mm, so complies as a CPC. But for bonding purposes, it requires the equivalent of 10mm of copper, unfortunately there is an 8 X difference between steel and coppers conductivity...

 

[Engineer54]

DNO test today... Ze 1.48ohms , PFC 146A. The system is 'sort of' TT but also TN-S, all a bit odd really.
The DNO are going to do works in the new year to improve the supply.

A TN-S supply with an additional earth rod connection to the MET is not a problem at all and was once a very common occurrence, it is still very much classed as a TN-S supply.

Really, i would have a word with your client and get them to contact the DNO Area Engineer with a follow up email placing full responsibility on the DNO's shoulders for any and all consequences on it's failure to correct a known problem with it's supplied equipment.

As it stands at the moment, the OCPD within those 15 DB's will be unable to operate on an earth fault, to the required times, given the Zs values you have stated in your earlier posts

 

[SIIVION]

A PVC 10mm 2 core SWA cable has a steel wire CSA OF 41mm, to comply as a circuit s CPC would require 22.6mm, so complies as a CPC. But for bonding purposes, it requires the equivalent of 10mm of copper, unfortunately there is an 8 X difference between steel and coppers conductivity...

Thank you, I misunderstood the equivalent values. Your help is much appreciated.

 

[SIIVION]

Yes absolutely. The DNO engineer came out to do the tests the other day, I have spoken to the DNO who are going to rectify the problem early in the new year. The building will not be occupied over the Christmas holiday.
The system had a large plate installed rather than a spike for the earthing and I had been informed it was more a TT but will trust your information it is TN-S.
Obviously TT would have allowed me to use the 1600 allowance relative to all RCD main switches.