High Ze causing headache !

[sythai]

Hi Guys

Any advice/ pointers would be appreciated

Have ‘pretty big’ new build house we’re wiring from scratch.

New supply in from DNO is PME, but we have Ze of 0.51

Are DNO obliged to keep this under 0.35?

Wouldn’t be that much of an issue normally if regular size house and meter was attached to house.

Before we came along on the scene Client had separate garage block built, meter permanently sited in here.

We’ve run a SWA sub-main across to house DB 25metres away, but struggling on suitable overcurrent protection. Even at 80amps BS 88…. Max EFLI 0.40!

Don’t really want to be putting new build on TT system, if it comes to it.

Thank you
Sy

 

[7029 dave]

Are you taking the Ze from source (at the new supply)

 

[Gurnatronics]

Listened to a podcast from the E5 group earlier today and DNO guidance states PME ZEs can actually be as high as 0.8. I will try to dig out the pdf

 

[buzzlightyear]

New supply in from DNO is PME, but we have Ze of 0.51

Have you spoken to the DNO about it.
They might sort it out.

 

[Julie.]

DNO is not obliged to supply Ze lower than 0.35 ohm; although all DNO strive to achieve it.

It appears to be a common myth!

Edit:

And why on earth (sic) would you consider changing it to TT?

 

[SJD]

Two 60A / 63A sub-mains, to two separate DBs in the house, circuits suitably divided between them!

 

[Aaron b]

Is it the Zs of the sub main that is high or the Ze in the garage block? What size is the SWA?

 

[Deleted account]

if disconnection times cannot be achieved by the Breakers then residual current devices can be used.

no need to TT. a PME with a slightly high Ze gives a far better EFLI than any TT could achieve.



although a combination of earth rod and pme may be in order in case of dropped neutrals, but that’s a whole different thread on an abandoned proposed reg change.

 

[sythai]

Are you taking the Ze from source (at the new supply)

reading at cut out

 

[sythai]

Listened to a podcast from the E5 group earlier today and DNO guidance states PME ZEs can actually be as high as 0.8. I will try to dig out the pdf

I did hear that a while back.... that they wouldn't be concerned unless above 0.8! But always hoped they wouldn't go by that

 

[sythai]

Have you spoken to the DNO about it.
They might sort it out.

Did raise my concerns with Client before Xmas.... DNO apparently came out straight away (I wasn't on site) said no problem all fine !

Re-checked last week still 0.51

I'm going to raise it myself again directly with them, making sure I'm onsite when (if) they re-visit.

 

[sythai]

Is it the Zs of the sub main that is high or the Ze in the garage block? What size is the SWA?

High Ze at garage block.... 25mm SWA

 

[sythai]

Two 60A / 63A sub-mains, to two separate DBs in the house, circuits suitably divided between them!

Like that one ! I'll see what Client thinks

 

[sythai]

And why on earth (sic) would you consider changing it to TT?

Open to any other suggestions, any advice would be appreciated .... definitely not my preferred method by far

 

[sythai]

if disconnection times cannot be achieved by the Breakers then residual current devices can be used.

no need to TT. a PME with a slightly high Ze gives a far better EFLI than any TT could achieve.

Thanks Pete

How does that work with my 80amp sub-main supply though max EFLI is 0.40 for BS-88 ?

My thinking for TT as an option, was if so could then put all on an RCD (time delayed for sub / house db rcbo's) and use 1667 for max ZS values.

Maybe I'm missing something - has been long week!

 

[mattg4321]

100mA S Type RCD on sub main if it comes to it. Then make sure all RCD protection disconnects both line and neutral downstream

 

[Julie.]

Thanks Pete

How does that work with my 80amp sub-main supply though max EFLI is 0.40 for BS-88 ?

My thinking for TT as an option, was if so could then put all on an RCD (time delayed for sub / house db rcbo's) and use 1667 for max ZS values.

Maybe I'm missing something - has been long week!

Yes, time delayed rcd on distribution circuits, standard rcd on final circuits - rcds as fault protection.

No need to convert to TT

leave it as TN-C-S and you can still use 0.4s / 5s but add rcd as fault protection where required.

So if the cable from the meter location to cu fails to operate in 5s due to fault, add a time delayed rcd.

If any final circuit fails to operate in 0.4s due to fault add a standard rcd.

If a circuit will operate within these times, then a rcd is not required for fault protection, but may be required for additional protection.

 

[TJ Anderson]

You also need to consider the protection for L-N faults which won't be covered by adding RCD's. This often gets missed.

If you have a circuit that has cpc size equal or greater than the neutral and you have inadequate fault current to operate ocpd fast enough with regard to thermal constraints, then you will have same issue with L-N faults. You can add RCD to cover the L-E fault protection, but L-N will not be covered.

 

[Julie.]

You also need to consider the protection for L-N faults which won't be covered by adding RCD's. This often gets missed.

Yes, you still need overcurrent protection for fault protection, and where necessary overload protection, but there is no requirement to achieve the same trip time.

You also have to achieve voltage drop limits etc, etc - basically all the other requirements remain in place.

Using rcd for fault protection is merely a technique to achieve ads

 

[Pretty Mouth]

You also need to consider the protection for L-N faults which won't be covered by adding RCD's. This often gets missed.

The disconnection times only need to be met for L-E faults, as far as the regs are concerned anyway. Though interestingly there is something in the OSG that leads you to believe they do need to be met for L-N faults. I'll find a page number...