Problematic Ze........

[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!

 

[Dave OCD]

Yes but what of the thousands and thousands of TT systems with much higher Ze , they all have the same problem but much worse than your sub 1 ohm reading.

 

[Ian1981]

Yes but what of the thousands and thousands of TT systems with much higher Ze , they all have the same problem but much worse than your sub 1 ohm reading.

By that reasoning what's the point of the DNO stipulating max ze figures on tn-c-s and tn-s systems.
They are supplying an earth for the system and their figures should be upheld In my opinion

 

[Dave OCD]

I'm not being argumentative Ian, just wondering if your perceived issue is really worth worrying about with the Ze that you have on this particular job. I just think a note stating DNO informed on your EIC would cover you fine.

 

[Andy-1960]

The only way around the problem if the DNO cannot improve the Ze is to fit a main incoming RCD at the point of the incoming supply, that way the DNO fuses will take care of overcurrent and the RCD will take care of earth faults.

 

[Ian1981]

I'm not being argumentative Ian, just wondering if your perceived issue is really worth worrying about with the Ze that you have on this particular job. I just think a note stating DNO informed on your EIC would cover you fine.

Apologies dave I don't mean to seem to be arguing, I do see what your saying though about the tt system scenario.
I've never really thought about it in that light.
Food for thought mate.

 

[Dave OCD]

No worries mate, basically just trying to help put your mind at ease.

 

[westward10]

With an all insulated head a TT system is not relevant, the DNO are not providing an earth so any fault requirement issues are not their problem.
With the OPs issue I am certain the fuse was reduced to 80A to allow an increased Ze.

 

[hightower]

Surely if your Zs readings are still under the limit you'll be alright? All you gotta do is meet disconnection times

 

[Lee42]

Hi
Did you power down the installation and remove the main cpc when taking the ze reading?

 

[Adie moore]

are you using a high current test as opposed to the non trip test as this will offer give lower readings.
edit, I mean the high current will give the lower reading.