Ze of 0.50 ohms on a TN-C-S system

[Mikeatkinson]

Hi. I’ve just tested a friends DB and it recorded a Ze of 0.50 ohms on a TN-C-S with an 80amp fuse is this ok. As the DNO state that the 0.35 ohms is for 100 amp..

 

[Deleted account]

No it's not ok. Contact DNO.

They will probably tell you it is ok, but you've done your duty.

It doesn't make any difference if it's a 80A or 100A.

 

[Baddegg]

Out of interest if the dno does say it’s ok what’s his next move? Obviously he can’t work on the installation and record a Ze within what’s acceptable,still learning so these things keep me awake at night......

 

[Deleted account]

Out of interest if the dno does say it’s ok what’s his next move? Obviously he can’t work on the installation and recited a Ze within what’s acceptable,still learning so these things keep me awake at night......

If all his Zs reads are within parameters he potentially can. Let's be honest in domestic the R1 +R2 is so low the Zs is normally way under the permissible.
However I would make a note on cert DNO informed with name of person who ok'ed it.

 

[westward10]

For a bit of a read on such things have a look at Engineering Recommendation P23/2:2018.

 

[Strima]

For a bit of a read on such things have a look at Engineering Recommendation P23/2:2018.

I know what I’m reading tonight, thanks.

 

[Baddegg]

Thanks westward,

 

[Octopus]

What was the Zl? (L-N)?

 

[Adam W]

They'll probably get you to take the Ze at the cutout to eliminate the possibility of any consumer's equipment (eg tails) causing the problem.
If they OK it get a reference number and quote that on the cert.

 

[Nigel]

Perfectly fine.

 

[bigspark17]

I wouldnt loose any sleep over it. If it was that high that is pushed your final circuits outside max zs parameters then id consider acting on it, highly unlikely in a domestic situation.

 

[davesparks]

Out of interest if the dno does say it’s ok what’s his next move? Obviously he can’t work on the installation and record a Ze within what’s acceptable,still learning so these things keep me awake at night......

Regardless of what the DNO say he records the measured Ze on the certificate, and if everything else complies then no problem.
The figure of 0.35 is a guideline typical value, not a fixed maximum permissible value.
As long as the Ze of 0.50 is stable and all circuits comply for Zs then it is fine.

 

[Baddegg]

Thanks Dave all advice appreciated,read through the link westward put up,explains a lot to a novice,

 

[Deleted member 26818]

Regardless of what the DNO say he records the measured Ze on the certificate, and if everything else complies then no problem.
The figure of 0.35 is a guideline typical value, not a fixed maximum permissible value.
As long as the Ze of 0.50 is stable and all circuits comply for Zs then it is fine.

Apparently 0.35 ohms is the typical maximum.

 

[westward10]

It is the typical figure for a 100A supply, other supplies will differ.