High Zs on submains and high Ze

[Nigel]

Just what I can reasonably do practically, tightened all connections, contact cleaner, supplementary bonding.

But there is nothing that can be done to get a Zs lower then the Ze so it was always a design issue.

 

[ChrisElectrical88]

Can you not put the Main Tails into Henley blocks and install a 60A Fused Switch as previously stated.

I understand some schools this will not be possible due to the cables coming from an outside substation into a panel board. However with your Ze of 0.35 I would assume the school is not large enough for its own transformer and sub.

 

[Ian1981]

S type rcd to protect the sub mains perhaps?
Assuming that you still will achieve short circuit protection with the C type mcb

 

[Wilko]

Hi - with B63 the max goes up from 0.35 to 0.69 and job's done (?).

 

[Nigel]

Hi - with B63 the max goes up from 0.35 to 0.69 and job's done (?).

I don't think a B type 60898 will be suitable for a distribution circuit.

 

[Wilko]

You are probably right. I just don't see how C63 can work here. It's got to be your fuses then (?).

 

[Strima]

Have you measured Zn to ensure that your OCPD will disconnect in short circuit conditions?

If good then I would add an RCD as previously mentioned, I'm not a fan of using RCDs for fault protection however you've been left with a poorly designed installation that you need to rectify at limited cost.

Also I would opt for fuses rather than MCB to give you a little more scope for supply variances etc.

 

[johnduffell]

Have you measured Zn to ensure that your OCPD will disconnect in short circuit conditions?

That was my thought, if it's tncs then if the pscc is higher than the pefc at the cutout then there is an issue for the dno to resolve, as you'd be measuring the same cable through two different terminals.
However in its favour the fact that the cable is armoured could make short circuit less likely without an earth fault, so overload protection is more important

 

[Andy-1960]

How about supplementing the DNO earth with an earth rod or mat?

 

[Toneyz]

Whatever is the solution time is running out they are back to school in a week! Have you got no access to the O&M manual/test certificates? as if it complied when installed.

 

[stevels4]

I’m not being patronising here but with such low value measurements you did remember to zero your MFT first...it is easily forgotten to do so.

 

[Toneyz]

I’m not being patronising here but with such low value measurements you did remember to zero your MFT first...it is easily forgotten to do so.

I have never had to zero out to do a loop test.

 

[Teddy Lop]

Holy thread bump batman etc.

How did you resolve this? Any new ideas?

Had a CU on a submain reading 0.29 - made compliant by using an 80a fuse at the origin, but not wholly satisfied as this could be swapped out, and while unlikely to ever give issue probably doesn't meet the max demand (silly as it is.)

Origin was 0.28 (TNS) and submain was a few meters of 25mm SWA

Looking for inspiration as the DNO in these parts is fairly hostile towards enquiries on safety or maintenance of their largely interwar network, and given its crumbling state it's a problem I expect to encounter again.

Single RCD protecting submain and therefore whole house is not ideal.

Two submains/CUs on 60amp? Seems lunacy but it works.

However BS7671 states its zs figures are worst case scenarios and that manufactures may offer devices compliant with higher values... Anyone researched who/what?

And finally.... How is it that my submain reading .29 would not comply but the 100 amp fuse at the origin is quite alright for anything up to 0.8 then?

 

[pc1966]

Single RCD protecting submain and therefore whole house is not ideal.

If it is 100mA/300mA delay RCD feeding the sub-main and the house CU is all on RCD (or neutral-switching RCBOs like compact Wylex/Fusebox) then you keep final circuit selectivity and avoid marginal Zs limits.

Not a substitute for anything very suspect in the way of supply Ze, but if the system is otherwise sound but just not up to meeting 5s disconnect on the sub-main fuse then it is a valid approach and ticks all of the regulations.

 

[pc1966]

And finally.... How is it that my submain reading .29 would not comply but the 100 amp fuse at the origin is quite alright for anything up to 0.8 then?

The DNO fuse is for overload protection of their network, it is not required to meet the BS7671 regulations on ADS for an installation.