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!

 

[Engineer54]

but since then ive connected generators which have needed readings of 200-250 ohms to be correct for the rcd used ,

Exactly where are you getting this load of drivel from???

Oh, the 21 ohm figure is that of DNO fenced substation and distribution centres. In reality the actual values found, will be very much lower, usually low single figure values...

 

[Knobhead]

Only one ohm out. 20Ω for a LV nest at >8m from the MV nest. <8M then <1Ω max and they can be linked.

It’s the difference between a “hot” and “cold” site.

 

[DNO]

We do not guarantee any "data" for external earth loop impedance. the expected max for PME
networks in the UKPN area for single phase in 0.38. For cable sheath it is much higher, normally
up to 1 ohm. The 21 mentioned is the figure with no earth provided.

It could be much higher in any of the above examples.

 

[Engineer54]

Only one ohm out. 20Ω for a LV nest at >8m from the MV nest. <8M then <1Ω max and they can be linked.

It’s the difference between a “hot” and “cold” site.

It always used to be 21 ohms!! lol!!

To be honest, we have always achieved sub 1 ohm on our earthing systems and have generally linked MV-LV earthing via a removable link bus bar arrangements. An 8 metre separation can at times be almost impossible, to achieve, especially when trying to also comply with other related factors...

 

[Engineer54]

We do not guarantee any "data" for external earth loop impedance. the expected max for PME
networks in the UKPN area for single phase in 0.38. For cable sheath it is much higher, normally
up to 1 ohm. The 21 mentioned is the figure with no earth provided.

It could be much higher in any of the above examples.

You''ll have to explain that one for me?? So where does this Phantom 21 ohms come from then??

 

[Knobhead]

We do not guarantee any "data" for external earth loop impedance. the expected max for PME
networks in the UKPN area for single phase in 0.38. For cable sheath it is much higher, normally
up to 1 ohm. The 21 mentioned is the figure with no earth provided.

It could be much higher in any of the above examples.

I’d also like DNO to explain something.

“We do not guarantee any "data" for external earth loop impedance.”

Do you speak for a DNO or UKPN?

If you do, you’ve had a meteoric rise since 21/01/12 when you were bemoaning being offered £28 per EICR.

 

[Knobhead]

It always used to be 21 ohms!! lol!!

To be honest, we have always achieved sub 1 ohm on our earthing systems and have generally linked MV-LV earthing via a removable link bus bar arrangements. An 8 metre separation can at times be almost impossible, to achieve, especially when trying to also comply with other related factors...

I’ll dig the UKPN document out for you if you’re feeling desperately in need of something to read. Personally I’d rather drown in cold porridge.

The best bit was after I was sent the design document and asked about the MV→LV link I was told “forget that bit.”
I wasn’t worried about it at all, as per usual for me the transformer was in a steel framed building nearly ¼ mile long. Who needs an earth nest?

 

[DRG]

I was told by niceic inspector to input this 21 ohm figure for earthing arrangement Ze for TT systems for the DNO earth instead of the 200 ohm i used to input into that box to cover your installation ZE.

Slightly confusing in the sense you're supplying your own earth, which must meet the 200 ohm (obviously you want it as low as possible) criteria and you have no control over the DNO supply earth which doesnt even enter the property. Doesn't make 100% sense to me but I'll follow orders and do as I'm told

 

[Plonker 3]

I was told by niceic inspector to input this 21 ohm figure for earthing arrangement Ze for TT systems for the DNO earth instead of the 200 ohm i used to input into that box to cover your installation ZE.

Slightly confusing in the sense you're supplying your own earth, which must meet the 200 ohm (obviously you want it as low as possible) criteria and you have no control over the DNO supply earth which doesnt even enter the property. Doesn't make 100% sense to me but I'll follow orders and do as I'm told

Where do you think the NIC will be if you are ever questioned in a court over one of your certificates?

 

[Engineer54]

I was told by niceic inspector to input this 21 ohm figure for earthing arrangement Ze for TT systems for the DNO earth instead of the 200 ohm i used to input into that box to cover your installation ZE.

Slightly confusing in the sense you're supplying your own earth, which must meet the 200 ohm (obviously you want it as low as possible) criteria and you have no control over the DNO supply earth which doesnt even enter the property. Doesn't make 100% sense to me but I'll follow orders and do as I'm told

Don't suppose you even questioned him about WHY you would reduce the measured Ra value of 200 ohms by almost a factor of 10!!! By the way what do you mean by '' instead of the 200 ohm i used to input into that box to cover your installation ZE.'' ?? You don't input any value on a Test Certificate unless it's been measured by you testing it, or by calculation...

More fool you if you're going to blindly follow instructions from a bloody idiot!!

 

[Silly Sausage]

I can't understand that post.
The acronym beginning with N immediately raised my suspicions, but if you shelled out over £500 for their over whelming authority in the electrical industry, I suppose you best take their word for it.


If you really have to sign up, do it with BSI.
Shut the scum, sorry, scams, sorry schemes down!

 

[Engineer54]

I can't understand that post.
The acronym beginning with N immediately raised my suspicions, but if you shelled out over £500 for their over whelming authority in the electrical industry, I suppose you best take their word for it.


If you really have to sign up, do it with BSI.
Shut the scum, sorry, scams, sorry schemes down!

Would you really, even though you KNOW he's telling you a load of garbage??

 

[Silly Sausage]

Would you really, even though you KNOW he's telling you a load of garbage??

Of course I would, I've paid good money for that wisdom! :tounge_smile:

 

[Engineer54]

By the way Archy, how much do the BSI charge to be registered as a competent domestic Part Pee operative??

 

[nickblake]

Of course I would, I've paid good money for that wisdom! :tounge_smile:

Get it down on paper first !!!

 

[Rockingit]

By the way Archy, how much do the BSI charge to be registered as a competent domestic Part Pee operative??

I'm BSI Kitemark mate, and its not quite like that. You are inspected as a Kitemark quality supplier for anything that you want to include - domestic, commercial, alarms etc - so it's an overall marque of quality, not a Part P scheme. Yes, you get access to the Part P notification system as a by-product, it's not the aim of the end result.

And my annual registration is about £750ish from memory.

 

[Knobhead]

You’ll find the “official” maximum Ze readings in here. Network Design Manual Have fun!