Max ze values

[martinh89]

This may sound like a daft question but in bs 7671 does it state anywhere in it about max ze values
ie

tncs--0.35
tns--0.80
tt--21

I was jut wondering,I can find max zs but not ze ?????

 

[Octopus]

Where did you get the TT figure from?

 

[martinh89]

from my tutor at college , but since then ive connected generators which have needed readings of 200-250 ohms to be correct for the rcd used , but to my question is it anywhere in the bs 7671 about zE values ? because I were talking to a contractor on site a few days ago and he were quoting regs to me (ive done and passed my 17th edition) but looking through I can not for the life of me find any values for ze? is this anything to do with esqcr regs ?

 

[Silly Sausage]

Look at the bottom of page 11, part iv in the OSG.

 

[martinh89]

im aware of it being in the osg , but cheers . Is it in the bs 7671 at all ?

 

[Clive P]

Hi,

As Murdoch says you need to re-visit your TT figure.

Regards.

 

[martinh89]

ok cheers clive will do , unless any of you would like to point me in the direction of where im going wrong ?

 

[Geoffsd]

The supply, hence Ze, does not come under BS7671.

 

[7029 dave]

If you want ze supply figures, ring them they will have that data for all their installations.

 

[telectrix]

If you want ze supply figures, ring them they will have that data for all their installations.

in your dreams. most of the time they don't even know what , if any, earthing is present. & @ OP. the 21 ohms you mentioned is nothing to do with TT. it's a figure for DNO earthing upstream of your installation

 

[7029 dave]

Tel, if they are responsible for the earth?, they are duty bound to supply such info.

 

[Doomed]

If you want ze supply figures, ring them they will have that data for all their installations.

Ah ha ha ha ha - oh heck - you were being SERIOUS?

 

[telectrix]

took them 5 months to answer my query as to whether or not PME was available at a location. and that only happened because i spoke to them on another matter.

 

[SirKit Breaker]

No they are not in the BGB, but they are in the OSG.

 

[Pjd]

You will also find max ze in GN3.
The max Ze for TT is 21 when DNO supplies earth through PEN conductor.
If it is a client owned TT, anything over 200 Mohms is unstable.

 

[trev]

If it is a client owned TT, anything over 200 Mohms is unstable.

So anything under 200 meg is perfectly fine for Ra?

 

[Pjd]

So anything under 200 meg is perfectly fine for Ra?

200ohms, my mistake. Should pay better attention to what I am typing.

 

[trev]

200ohms, my mistake. Should pay better attention to what I am typing.

Any excuse to take the mick and I'm gonna take it mate. Sorry

 

[davesparks]

I believe 21ohms is the figure that a DNO wil supposedly quote for their end of a TT system.

 

[Rockingit]

As far as Ze is concerned, the fabled values are guidelines only, the DNO's are under no obligation to honour them. You get's what you're given and you put up with it!