Fluctuating voltage!

[sparks72]

Hi

I hope this makes sense.

Went to look at a job a few days ago, did a quick test on the incomer of a 3 phase supply TNC-S system (seperate from the main consumer board) got a Ze of 0.19, all good, went to test the seperate consumer board & got a Ze of 0.33 & a Zs at a few RCBO protected sockets, the max being 1.80!
Went back a few days later & did the same test, Ze at incomer was 0.19 but Ze at property had shot up to 3.88!! did an IR test on mains cable & all good.
I added a new 25 mm earth conductor to the main incomer & the Ze at the property lowered to 0.22 & Ze at the RCBO sockets was a max of 0.68!
went back today & the Ze at the property board had gone up again to 0.31 & the Zs at the RCBO protected sockets had risen to a max of 1.77!
Had the LEB look into their side of the supply & got the all clear.
The cable from the main incomer to the property is a 25mm 2 core SWA using the steel wire as the earthing, the run from incomer to property is around 50 metres.
I think their is a split in the SWA sheathing. probable damage when installed 10 years ago (no certs)
has anyone else come across this & if so what was the problem.

Thanks.

 

[johnboy6083]

i agree with you on the split sheath/ corroded armour. this is the reason that i never use the armour as CPC when cable is run underground.
I worked on a job in wolverhampton city centre, and when i carried out site survey to size submains to our panels the ZE was 0.08 (winter). When i tested the job at the commisioning stage (summer) the ZE was 0.56. the LEB weren't interested. they said that fluctuation due to water levels is normal. this was a TNC-S system

 

[sparks72]

i agree with you on the split sheath/ corroded armour. this is the reason that i never use the armour as CPC when cable is run underground.
I worked on a job in wolverhampton city centre, and when i carried out site survey to size submains to our panels the ZE was 0.08 (winter). When i tested the job at the commisioning stage (summer) the ZE was 0.56. the LEB weren't interested. they said that fluctuation due to water levels is normal. this was a TNC-S system

Cheers johnboy.
The company that did the initial installation is well known for "cutting corners".
I'm scratching my head as to what to do really, told the customer what might have happened but they
don't want any excavation done.
Could turn it into a TT System but same thing again, customer dosn't want all the hassle as they've recently moved in & had to endure a barrage of bad workmanship that has cost them a small fortune.
I've warned them of the danger without the woman bursting into tears (again)
Now the big question, on a periodic......satisfactory with a code 2 or un-satisfactory ?
I would say unsatisfactory personally but I can't endure more tears from the customer!!

 

[pennywise]

It would go down as an unstable main earth surely. TT it

 

[johnboy6083]

could you not bring down the ZS of the ring final circuit with supplementary bonding? i would also run a seperate earth next to the submain. If its 16mm2 or over, it can be buried without protection. This should bring your readings down.

 

[sparks72]

could you not bring down the ZS of the ring final circuit with supplementary bonding? i would also run a seperate earth next to the submain. If its 16mm2 or over, it can be buried without protection. This should bring your readings down.

Their are 3 ringmains in the house, I have no access anywhere really as the house has water fed underfloor heating throughout (chipboard on top of pipes)
I would run a seperate earth next to the submain but they have a stone courtyard that the cable runs underneath & they don't want it digging up, to dig it up (excavation quote) would cost at least £5k!

 

[sparks72]

It would go down as an unstable main earth surely. TT it

Was thinking that, already done some minor work in the property (added socket to ring) when the Ze & Zs was acceptable, sods law strikes again!!

 

[pennywise]

Typical it's like an MOT pass one day fail the next

 

[sparks72]

Typical it's like an MOT pass one day fail the next

Will put unstable earth down on the sheet, at least my van passed it's MOT

 

[sparks72]

Just slapped myself!

Apparently I can install an earth electrode on a TNS system ( PME) for additional protection, the IEE recommend it.