DIY Advice Needed!

Mr.Simple · Dec 6, 2013

Hi, firstly I wanted to publish this in the DIY Forum as I am not an electrician, however the DIY forum appears locked and new members cant post there.

So, my problem is trying to interpret an EIC for a neighbour. I will say again I am not en electrician however I do know about electrics, I have a degree in electronic engineering and my neighbour wanted some technical help.

He has just had an extension and this was completed correctly by a NICEIC Domestic Installer and the system tested. ON page 2ofthe NICEIC Domestic installation Certificate, under "Supply Characteristics" is a value for External Earth Fault Loop impedance Ze of 0.35 Ohms (this is a TNCS system, and I think the ma permissible Ze is 035 Ohms). My understanding f Ze is that it measures the impedance of the circuit from the customers from Consumer unit main earth (disconnected from the board) through the suppliers transformer and back along the live to the live side of the main breaker (hope that this is right). If this is correct then it appears to be on the limit but is in tolerance, so everything is ok.

However in the sexton "Particulars of the installation at the origin' there is another box labelled Measured Ze,and this is 0.14 Ohms. Basically I simply don't understand the difference between the two. Furthermore also in the "Supply Characteristics" area is a box called Prospective Fault Current Ipf recorded as 1.8kA Now using Ohm law and assuming the supply is actually 250v (yes I know that Uo is 230v, but isn't that a sop to the EU) 250 divided by 1800 comes out at 0.138 Ohms, which is near enough measured 0.14 Ohms. So where does the vale 0.35 Ohms come from in the Supply Characteristic area?

I also have an earlier Periodic Inspection Report dating from 2004, before the extension and new wiring, which talks about the Ze as being "measured as 0.32 which is close to the supply limit o 0.35, caution advised" (what caution did they expect my neighbour to take?). This is recorded in the "Supply characteristics" of the Periodic Inspection Report with the comment in observations Section.

Can someone please clarify why there are two entries for Ze on the NICEIC Domestic Electrical Installation Certificate and why they should be different.

Many thanks

telectrix · Dec 7, 2013

it might just be an error in that he's put the max. recommended value of 0.35 in the box. 0.14 does appear to tie up with the PFC.

Kate · Dec 7, 2013

On the niceic forms i believe the only time you record two Ze values is for a TT system. So sounds to be a mistake.

Can you take a photo of the page and upload?

Mr.Simple · Dec 7, 2013

I would tend to agree that the installer in this case could have made a mistake, but for the similar Ze reading on the old report for the original installation. I have attached a copy of the new Cert and old report. The conflicting Ze's are on the certificate about 3/4 across the page in both supply characteristics and Particulars at the origin.
.

Also as the new cert is for a consumer unit fed from a breaker in another (old) consumer unit is the Ze measured at the old CU or the new one?

telectrix · Dec 7, 2013

Ze is always measured at source. on a 2nd DB ( CU) the term is Zdb.

Deleted member 9648 · Dec 7, 2013

Mr.Simple said:
Hi, firstly I wanted to publish this in the DIY Forum as I am not an electrician, however the DIY forum appears locked and new members cant post there.

So, my problem is trying to interpret an EIC for a neighbour. I will say again I am not en electrician however I do know about electrics, I have a degree in electronic engineering and my neighbour wanted some technical help.

He has just had an extension and this was completed correctly by a NICEIC Domestic Installer and the system tested. ON page 2ofthe NICEIC Domestic installation Certificate, under "Supply Characteristics" is a value for External Earth Fault Loop impedance Ze of 0.35 Ohms (this is a TNCS system, and I think the ma permissible Ze is 035 Ohms). My understanding f Ze is that it measures the impedance of the circuit from the customers from Consumer unit main earth (disconnected from the board) through the suppliers transformer and back along the live to the live side of the main breaker (hope that this is right). If this is correct then it appears to be on the limit but is in tolerance, so everything is ok.

However in the sexton "Particulars of the installation at the origin' there is another box labelled Measured Ze,and this is 0.14 Ohms. Basically I simply don't understand the difference between the two.The first box relates to Ze by enquiry,ie...the DNO stated Ze.....the second box is the actual measured Ze. It would be normal practice on an existing install to only fill in the measured Ze,as it is obviously not necessary to ascertain Ze by enquiry. Furthermore also in the "Supply Characteristics" area is a box called Prospective Fault Current Ipf recorded as 1.8kA Now using Ohm law and assuming the supply is actually 250v (yes I know that Uo is 230v, but isn't that a sop to the EU) 250 divided by 1800 comes out at 0.138 Ohms, which is near enough measured 0.14 Ohms. So where does the vale 0.35 Ohms come from in the Supply Characteristic area?

I also have an earlier Periodic Inspection Report dating from 2004, before the extension and new wiring, which talks about the Ze as being "measured as 0.32 which is close to the supply limit o 0.35, caution advised" (what caution did they expect my neighbour to take?). This is recorded in the "Supply characteristics" of the Periodic Inspection Report with the comment in observations Section.

Can someone please clarify why there are two entries for Ze on the NICEIC Domestic Electrical Installation Certificate and why they should be different.

Many thanks