earth fault.

[IQ Electrical]

Never rely on one test result, always conduct Ze and Zs tests more than once and take the average. That way ''fluke'' readings can be ignored. You can also try using the 3 leads with the neutral and earth leads connected to earth, but normally the 3 lead loop test, is more accurate than a 2 lead test.

Anyway, ....I think you have a lot more investigating and testing to do, before arriving at a conclusion and solution to this present problem mate ...lol!!

Not on the Megger MFT's !!

 

[Engineer54]

I'll take your word for that then ...lol!!!

 

[telectrix]

i've quoted to fit a rod, with the proviso that , if that doesn't cure the problem, extra work will be charged for once i have done that. i think that's all it can be, but problem is no access to earth cable once it's in the cellar, so i,m going to fit rod outside in front garden, then see what i,ve got. i wasn't going to do any stripping down as the visit was only to quote. if i get the job. i,ll keep you all posted.

 

[Sintra]

What it could be Tel is a RCD unit that has a high impedance across it. You could try and do a loop impedance on both supply and load side of RCD unit with the meter on no trip setting and see what you get.

 

[telectrix]

hmmm, but that would indicate an internal resistance in the RCD of 347ohms!!! and the 0.16 cannot be right as it's TT.

 

[Sintra]

Have seen it before mate in excess of 400 ohms accross an RCD. Are you 100% sure it is TT?

 

[Marvo]

just can't figure out the 0.16 initial Ze reading.

0.16 is a bit too good to be true.
Usually if I've had a weird test result that I can't duplicate later I just restore the whole system to what it was before I arrived and start the complete test procedure again from scratch and just ignore completely any previous results.

 

[telectrix]

i.m assuming TT as the incoming cable is 16mm concentric, plastic sheath, and there is no earth teminal on the cut-out. problem is , cables go down in to cellar which has been boarded up, no access. so i am assuming there is a rod down there.

 

[Marvo]

I don't know what your rule book says but if I couldn't visually check an earth rod I would have to treat the system as if there wasn't one present and quote for a new one.

 

[telectrix]

what has mabe me wonder now is,,, suppose a new supply has been put in and the earth is to the lead sheath of the original supply cable, making it TN-S? would love to get in that cellar, but the house was originally one big house, split into two. the cellar access is from next door and they have boarded the divide up so i can't get in.

 

[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.