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?

 

[telectrix]

megger 1552. and reason didn't dis earth from CU was that there are 3 10mm cables in nearth bar,and all disappear so no way of telling which is which, without using wander lead to gas and water, and disconnecting. bear in mind that so far i've only got to the stage of quoting to put the situation right. it's even possibly a very high earth rod resistance. just can't figure out the 0.16 initial Ze reading.

 

[Engineer54]

A Ze reading of 0.16 ohms points towards a PME incoming earth connection, but you say definitely not!!! One things for sure, you'll not see 0.16 ohms from a Rod!!!! Can't see either or both of the bonding connections giving you that low a reading. ....Interesting to say the least!!! ...As IQ suggests, can you safely by-pass the existing RCD and conduct a full current loop test?? Have you tested each of the 3 earth cables individually??

Somehow, your losing that 0.16 ohms between the cu and the outlets?? ....Velly Strangey, ...as the Chinese would say ...lol!!!

 

[tony mc]

Did you try a 2 lead test for ZE with the 1552 as I have known the 3 lead test to be mixed up and give you the resistance of the line and neutral conductor only.

 

[telectrix]

this is along the lines of what i am thinking. decide to ignore the 0.16 reading as error of some sort. all sockets have cpc's connected. it's got to be at source, was not going to take any more time though, as customer had told me was another electrician coming to quote, so i thought 1 hour was enough, quoted for fitting a rod. end of kneeling down with head in low cupboard.

 

[Engineer54]

yeah, i thought something weird. OK , Ze was read. main switch off, L/N/E.0.16... all sockets read around 347. then just to make it more confusing, 3 lead test in CU. top of each MCB/N/E. all readings around 347. meter calibrated 2 weeks ago. reckon the 0.16 must have been a fluke reading somehow.

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!!

 

[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.