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?

 

[telectrix]

are you talking a bout Ze ( at the origin) or Zdb at each board? pefc will be possibly higher than expected due to parallel paths.

 

[ipf]

Not too much in it, though...if you divide 240 by 90....

 

[davesparks]

Average Ze and PFC?

What values are you calculating this average from?

 

[Andy78]

Average Ze ? are you on about Zdb ? Your PEFC sounds about right for an EFLI of 2.9 ohms. You can work it out with sums and that. Talking of sums, do your Zdb readings match with what you calculated from your continuity readings of the sub feeds ?

 

[ipf]

Average Ze and PFC?

What values are you calculating this average from?

I was wondering that! Only one Ze how do you get an average?

 

[SIIVION]

sorry not enough information for you...
The average was taken from the readings of all 15 boards.
Also, yes you are right all Ze should be Zdb.
Range of PEFC is from 88 to 90.
Zdb range is 2.8 to 3.0

Thank you for your information, I should have thought about it a bit more rather than just jumping straight on here. So used to getting far lower Ze and much high PFC on single board tests.

 

[davesparks]

Why are you calculating an average Zdb and PFC from multiple boards ?
What did you hope to get from doing this?

What is the Ze of the installation?

 

[SIIVION]

Average Ze ? are you on about Zdb ? Your PEFC sounds about right for an EFLI of 2.9 ohms. You can work it out with sums and that. Talking of sums, do your Zdb readings match with what you calculated from your continuity readings of the sub feeds ?

Thankyou. All the Zdb values do seem very high. As an example one Zdb reading is 2.7 where the calculation for a 30m run of 10mm 2 core SWA (using armour as earth) is 0.2 .

 

[telectrix]

but you've not yet told us the Ze at source. it could be a DNO problem.

 

[davesparks]

Thankyou. All the Zdb values do seem very high. As an example one Zdb reading is 2.7 where the calculation for a 30m run of 10mm 2 core SWA (using armour as earth) is 0.2 .

Something is a bit wrong then methinks.

 

[Andy78]

Agree with the above comments.... a measured Ze for this install is needed.

 

[SIIVION]

Why are you calculating an average Zdb and PFC from multiple boards ?
What did you hope to get from doing this?

What is the Ze of the installation?

I am only giving the average to save listing all the readings.
I will find out the installation Ze tomorrow, it has not been physically possible for me to measure.

 

[davesparks]

I am only giving the average to save listing all the readings.
I will find out the installation Ze tomorrow, it has not been physically possible for me to measure.

How did you come up with the predicted Zdb in post 9 without a Ze to make that prediction with?
Do you have a Zdb at the intake position?

 

[Engineer54]

Thankyou. All the Zdb values do seem very high. As an example one Zdb reading is 2.7 where the calculation for a 30m run of 10mm 2 core SWA (using armour as earth) is 0.2 .

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....

 

[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.