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.