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?

 

[R1CH-G]

Sounds very much like the boiler dude wasn't aware of the RCD/RCBO max permitted Zs under those conditions. He's probably looked at his max permitted Zs for the 60898 or 3871 type breaker.

 

[ezzzekiel]

because as usual they think they know more than they do

 

[johnboy6083]

at least it has generated some work for tel. If the boilerfellas are taking EFLI readings, then it will identify any problems with the earthing of that installation. No hot water tends to make people sit up and listen. Im all for it if it generates work, and improves safety.

 

[chiplard]

To be fair to the plumber, at least he's checking.

How many times have you seen them just spur off the nearest outlet without checking. Seen many boiler fcu's spurred from a spur.

There should be more like him IMO, erring on the side of safety rather than doing what he has to do then leaving.

 

[Dayo]

if the ze at the fuseboard is 0.16 ohms how the hell is it over 300 ohms at the spur??

 

[R1CH-G]

if the ze at the fuseboard is 0.16 ohms how the hell is it over 300 ohms at the spur??

Possible parallel paths at the DB bringing the readings lower?

 

[Dayo]

no, def something else. zs is the ze reading plus r1 r2 therefore 0.16 plus ????? 300 ohms on r1 and r2 dont think so

 

[Dayo]

check your meter on your electical supply at home to make sure its ok. did you connect the earth lead to earth bar, live and neutral to the tails? you should have had really high reading at the fuseboard if the rod was disconnnected

 

[IQ Electrical]

Something's certainly weird here, a DB Zs (all bonding still connected) of 0.16 Ohms can't possibly translate into domestic final circuit Zs's of 350 Ohms.

I wonder if this has something to do with the 'no trip' Zs readings? Maybe 'noise' on the supply?

I just have a feeling that were the RCD to be (if safe) removed from the testing and a full current earth loop impedance test performed then the results might be in line with what would be expected.

I'm very interested in this one, Telectrix!

 

[poheZ]

the best thing to do here is an continuity test on the circuit with the spur to ascertain whther the circuit is earthed or what resistance it is measuring. If this is fines then it can only be a test or user error

 

[telectrix]

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.

 

[poheZ]

in this case the zs wont tell you anything, you need to do some dead testing as said before to make sure the circuit os sound first

 

[Dayo]

only thing that makes feel its a faulty meter or human error is if the water is a copper pipe into the ground and gas is a metal pipe into the ground these would normally be as good as any rod

 

[Dayo]

just as a matter of interest what meter are you using?

 

[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.

 

[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.