Max ze values

[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!!