Anyone know how to get ZE from Scottish Power ???

Mike - many many thanks the info is excellent

Chris Pearson: 
 

OK, time for a wee fib. Call them and say that you have measured Ze above the maximum permitted. When I did this (almost 1 ohm on TN-S) they were round like a shot.

lol…. love it

Chris Pearson: 
 

OK, time for a wee fib. Call them and say that you have measured Ze above the maximum permitted. When I did this (almost 1 ohm on TN-S) they were round like a shot.

I know of someone who did this to UKPN and they got a letter a bit like this one, telling them to convert to TT or pay lots of money for a supply cable upgrade.

OK, I would have attached the file as it is somewhere on the IET server, but not possible in the new editor.  

Mike,

I had a pleasant surprise the other week on my last job. Entering the location post code on the particular DNO website produced a record of available design details, plans, schematics which contributed to successfully determining supply characteristics/particular details of the origin of supply.

tonydon: 
 

Chris Pearson: 
OK, time for a wee fib. Call them and say that you have measured Ze above the maximum permitted. When I did this (almost 1 ohm on TN-S) they were round like a shot.

lol…. love it

I should make it clear that in my case, the excessive Ze was genuine and the DNO fixed it by “PMEing” my supply.

There is another way pull the main earth cable off then fone them tell them its come off then they will turn up to reconnect it  and measure the loop whilst they are at it. My main earth  to the supply cable sheath came off because I knocked it whist getting something out the cupboard a very helpful man came and did the tests and re connected it simples

Kelly Marie Angel: 
 

There is another way pull the main earth cable off then fone them tell them its come off then they will turn up to reconnect it  and measure the loop whilst they are at it. My main earth  to the supply cable sheath came off because I knocked it whist getting something out the cupboard a very helpful man came and did the tests and re connected it simples

lol…

On a site pulling maybe 600A per phase…. perhaps not lol

OK point taken i assumed you were in a house or small shop  or whatever didn't realise was such a big supply

Tony, your site is pulling about 400kVA from your figure. I presume you have a local Transformer, which is close to you. The supply type doesn't matter the Ze will be very low whether TN-S or TNC-S, although most likely TN-S. It will certainly be low enough to open your main breaker of whatever type (Fuses, ACB, etc) for an Earth fault at the main switchboard. It will be much less than the 0.35 Ohm figure above, this is for supplies to 100A that are TNC-S. What size are your incoming cables, this will give you something to estimate its likely value from the length and size, remembering that the N-E bond will probably be at the TX? If it is a private supply the bond may be at your switchboard which will give Ze of essentially zero. You can check this by the metering position, the primary or secondary of the TX, if primary it only supplies you.

This is where BS 7671 has its limitations!

I am guessing that a 600 A fuse would blow in under 5 s with a fault current of around 3 - 5 kA, which would be possible if Ze is 0.08 - 0.05 ohms.

The DNO must know the size of its fuses and their blow times (could be more than 600 A, but unlikely to be less) and therefore the maximum possible Ze. I feel sure that they would not just say 0.35 ohms because that would be impossible.

As a matter of interest, how precisely can Ze be measured and with what? I am not thinking of my MFT here. ?