Problematic Ze........

[GBDamo]

I'm at a bit of a loss as to how to proceed.

Had a Ze measured at 0.55 Ohms on a TNCS, contacted DNO who gave a asserted value of 0.52 Ohms. DNO came out and confirmed the result then checked over the earth path to the transformer, worked some magic and came back and measured at 0.44 ohms, they also de-rated the main fuse to 80 Amps from 100 Amps.

All good then? well no, tested today and it's back to 0.55 Ohms. So really wondering how to proceed with EIC.

1, record the result the DNO tested - 0.44 Ohms
2, record my measured result but make a note on the EIC that the DNO is aware of the high Ze.

This is not a test instrument issue as my, another independent spark and the DNO test instrument all tested to within 0.01 Ohms of each other.

This is my second visit, 300 mile round trip, to try to resolve an issue with the DNOs equipment, where do my responsibilities with this end?

The DNO have been contacted again, not sure of what they'll do but if they decide to de-rate the main fuse again we're going to have some serious load issues.

Any thoughts??

 

[westward10]

Just connect both leads to neutral and the other to line.

 

[Ian1981]

Yeah I had a rural installation where the ze stipulated on tncs from DNO was 0.5 ohms.
Thankful it wasn't problematic as my zs values where still within tolerances.
It was all rcd protected via 30mA anyway.

 

[7029 dave]

IMO, it is their issue, call them up again. You can not change the Ze measurement. Good luck.

 

[GBDamo]

Just connect both leads to neutral and the other to line.

Will try, just PM me your address to mail the charred remains to........

 

[westward10]

Will try, just PM me your address to mail the charred remains to........

Never going to happen, black and green to neutral and red to line I do it all the time.

 

[Ian1981]

Will try, just PM me your address to mail the charred remains to........

What mft is it?

 

[westward10]

The tester will not be able to recognise if the leads are both at neutral as opposed to neutral and earth.

 

[Wilko]

0.35 is a typical value for a 100A fuse at the head. I suspect the reason they reduced the fuse to 80A is to allow for a higher Ze of 0.50 but I can't recall the exact figure.

Correctamundo - 0.38 for 100A and 0.51 for 80A (Table 41.4 BS88-3 with 5sec)

 

[Ian1981]

Correctamundo - 0.38 for 100A and 0.51 for 80A (Table 41.4 BS88-3 with 5sec)

For a bs 1361 type 2 80amp is 0.48
100amp is 0.34
This is at operating temperature not the 0.8 rule of thumb.

 

[Ian1981]

Correctamundo - 0.38 for 100A and 0.51 for 80A (Table 41.4 BS88-3 with 5sec)

Is that bs 88-3?
Never mind

 

[Dave OCD]

All this is academic really if all circuits within the installation are 30ma RCD protected, it would have been more relevant in the past before such widespread use of RCDs was regulatory.

 

[Ian1981]

All this is academic really if all circuits within the installation are 30ma RCD protected, it would have been more relevant in the past before such widespread use of RCDs was regulatory.

It becomes academic if the protective device in the cut out takes longer than 5 seconds to disconnect due to a high ze/zs at the origin.

 

[Dave OCD]

Yes of course but any circuit fault with RCD and MCB protection will disconnect well before the main fuse gets involved.

 

[Ian1981]

Yes of course but any circuit fault with RCD and MCB protection will disconnect well before the main fuse gets involved.

But if the ze is high when measured at the incoming main switch and above what is required to disconnect the fuse in 5 seconds at the cut out then that's ultimately the issue.

 

[Andy-1960]

Wait for us to have a really good wet spell, then visit site again!