Old chestnut

[grumpyjohn01]

Hi there, Have recently undertaken a job for a client who has openly 'flouted' the Part P requirements and completely rewired his own house as part of several different planning processes. Have been in touch with the building control officer in charge and he is keen to get the whole thing 'signed off' and has told me an EICR followed by notified remedials will suffice. All good. Actually the guy has done pretty well and there isnt much more to do other than 'tidy up' the earthing & bonding (he slightly had the wrong end of the stick) and install a CU. DNO have been out at some point and fitted new TNC-S service with unknown fuse size (as yet!) problem is Ze is at 0.4ohm...obv 7671 requirement is 0.35, however if fuse turns out to be 80A or lower will that suffice given that it would meet disconnection times? or do I have to begin the process of convincing them there is an issue (DNO that is) and then getting them to address it? I have been down that road several times before and found them hard to deal with given their regs are different than ours. Opinions please! cheers John

 

[telectrix]

the 0.35 is a guide, not set in stone. you won't get very far with DNO unless there is a fault causing the 0.4 value.

 

[Simonslimline]

Did you take the Ze using a high loop test?

 

[Plonker 3]

As ^^^^^^, 0.4 isn't in the BGB it's a guideline that the DNOs use so don't worry about it as long as your Zs values all meet BS7671 values.

 

[grumpyjohn01]

the 0.35 is a guide, not set in stone. you won't get very far with DNO unless there is a fault causing the 0.4 value.

So what would you do?
1. TT the install.
2 Keep the network earth, if 80A all good, if 100A make note on cert
3 Hassle DNO?

 

[grumpyjohn01]

As ^^^^^^, 0.4 isn't in the BGB it's a guideline that the DNOs use so don't worry about it as long as your Zs values all meet BS7671 values.

All max Zs's are met, So would you accept any figure of Ze so long as that condition is met?

 

[grumpyjohn01]

Did you take the Ze using a high loop test?

Yes used high current test

 

[telectrix]

your concern is with your Zs values. as long as they are within BS7671 values that's it. as dilb said. the Ze is not a problem.

 

[Plonker 3]

All max Zs's are met, So would you accept any figure of Ze so long as that condition is met?

Depends on what Ze I was getting, if it was higher than the guidelines and my Zs values were affected then I would contact the DNO, as yours are fine there is no problem IMO

 

[Plonker 3]

So what would you do?
1. TT the install.
2 Keep the network earth, if 80A all good, if 100A make note on cert
3 Hassle DNO?

1. Why?
2. No need for anything to be done as it's not your problem
3. Why?

 

[telectrix]

^^^^ eggsackly. if it ain't broke, why fix it?

 

[Engineer54]

So what would you do?
1. TT the install.
2 Keep the network earth, if 80A all good, if 100A make note on cert
3 Hassle DNO?

1. Surely you're not being serious here?? You would actually dump a TN earth in favour of a TT earth?? ....Jesus!!!
2. DNO Service fuses are based on their own disconnection times. So long as your Zs comply i wouldn't be worrying too much about a Ze that is 0.05 ohm over it's guidance figure!!
3. You won't get very far by trying to hassle them, they tend to have far more time and money to waste than you...

 

[grumpyjohn01]

1. Surely you're not being serious here?? You would actually dump a TN earth in favour of a TT earth?? ....Jesus!!!

Not at all, would overwhelmingly prefer not to do that! just looking for opinions and I know some would say do that. Now that you are here o respected one...would you say a note on cert is appropriate if the cut out is 100A fused?

 

[Knobhead]

So what would you do?
1. TT the install.
2 Keep the network earth, if 80A all good, if 100A make note on cert
3 Hassle DNO?

Out of interest John, what figure would you accept for a TT system?

Please don’t say ≤200Ω, I’ve not taken my medication yet.

 

[nickblake]

Actually you may well find that the .4 reading you took could well be 0.05 or more resistance lost in the test kit or resistance of the leads , or even the cabling, bet if the test was carried out across the main fuse it would be way lower , please do do that, it was a comment !!