earth fault.

[telectrix]

hi, folks, went out to a job today. Central Heating contract, guy has refused to do any work on the boiler due to " unacceptable Zs reading at boiler spur, >200ohms. checked with MFT and got a ZSreading of 347ohms. hmmm. goes to CU and it's 16th with 30mA RCD covering all circuits. bonding to gas and water connected and look OK. as a quick check, took a Ze reading ( 3 lead with all earths/bonding still connected, ) read 0.16ohms. then after shifting several boxes etc. to access main fuse, not TNC-S and not TN-S, incoming cable is plastic sheath, not lead, as i had originally supposed from the age of the property. OK, thinks, must be TT, so where's the rod? also, Zs on every circuit is around 350. earth cable disappears down through floor to cellar, now inaccessible, so before identifying and disconnecting bonding ( as there are 3 cables in earth bar, all 10mm, so can't tell which is main earth, i've come to the conclusion that it is TT and somehow this has been disconnected. i've quoted for fitting a rod ( accessible) with the proviso that further work may be necessary . question is why has the boiler guy refused to do a ssevice as on a TT system with RCD, any reading below 1667 is acceptable?

 

[SMB]

Maybe give the DNO a call Tel ?

 

[telectrix]

want to do the job this side of xmas though. called em on a job 3 months ago, got a letter saying they'd be out within 5 working days. still waiting.

 

[SMB]

Yes I understand.

I just thought that they might be able to offer advice about the earthing system in the property.

 

[telectrix]

right, i've slept on this one and have come to a plausible answer. one of 2.

1. there is no earth, and the Zs value of 347ohms is due to parallel paths, gas and water. and the 0.16 Ze was a false reading.

2. when the house was split into 2 dwellings, the new supply waqs looped from thee original supply, and either TN-S or TNC-S at the origin, and Sintra's idea that the RCD has a high internal resistance is the answer.

if i get to go back, i'll measure the pssc and pefc. if they read the same, then i can assume it's TNC-S.

maybe worth a look in next door's cellar to see if it's looped across and earthed there.

 

[nickblake]

What it could be mate if you have a neutral to earth fault in one of the circuits say lights you could get that reading ,and i know you have said the readings were high on all circuits which could be because there is a neutral to earth fault but the cpc could be open circuit after the fault point , if you see what i mean and the readings are as you have said parallel earth paths if you understand what im saying it is sunday and im about to go out and make some new racking for the van

 

[telectrix]

not a fault in the circuits, as i get a Zs reading in the CU, 3 lead/no trip, on all circuits ( leads on neutral bar, earth bar and each MCB) of 345ohms. this only tallies with either of the scenarios in my last post. favorite is high internal resistance of RCD. it's MEM, the one with the lever switch.