Live earths in a TT system

[South Spark]

Went to put in a new circuit (for a solar inverter) on a property with a TT system (the first time I've encountered one). The existing system is a dog's breakfast with three splits at the after the meter at a Henley block. There's some kind of 60 amp breaker labelled BS 842 in the mix. This may be an early earth leakage device but I don't know.

So I split off from the Henley block again and connected a mini DB with a 30ma/40 amp RCBO. All tests were fine - including a very low ZE (for a TT system) of 0.35, until the RCD test - the thing just wouldn't trip.

Taking the cover off to investigate I brushed my hand on the earth bar - which was live!

After investigation all the earths were live - testing with my MFT and a straight voltage tester between live and earth showed 240v.

I disconnected my entire new circuit and tested again - same result.*

My question is : Have the fuses not blown because there is in fact no path to earth (a long lead test to the earth rod gave a reading of 55 ohms) or am I doing something really dumb?

Chances are it's been like this for a while - the system works merrily along but presumably is very dangerous.

 

[Deleted member 9648]

Went to put in a new circuit (for a solar inverter) on a property with a TT system (the first time I've encountered one). The existing system is a dog's breakfast with three splits at the after the meter at a Henley block. There's some kind of 60 amp breaker labelled BS 842 in the mix. This may be an early earth leakage device but I don't know.

So I split off from the Henley block again and connected a mini DB with a 30ma/40 amp RCBO. All tests were fine - including a very low ZE (for a TT system) of 0.35, until the RCD test - the thing just wouldn't trip.

Taking the cover off to investigate I brushed my hand on the earth bar - which was live!

After investigation all the earths were live - testing with my MFT and a straight voltage tester between live and earth showed 240v.

I disconnected my entire new circuit and tested again - same result.*

My question is : Have the fuses not blown because there is in fact no path to earth (a long lead test to the earth rod gave a reading of 55 ohms) or am I doing something really dumb?

Chances are it's been like this for a while - the system works merrily along but presumably is very dangerous.

You would expect a reading of 230v between live and earth.....and if you had a Ze of 0.35 ohms it cant be live!....I'm confused and/or missing something.

 

[SimpleSimon]

I agree with wirepuller. Your results show fine, maybe you where picking up a static shock when touching earth bar and the RCD is faulty? two unrelated incidents? Best check between N-E if you think it's live.

 

[davelerave]

if it's TT -sounds like a high impedance or non-existent fault path


don't know how you're getting 0.35 here

 

[Des 56]

if it's TT -sounds like a high impedance or non-existent fault path


don't know how you're getting 0.35 here

If he had a neutral earth fault on the existing system and the system earth was non existent,he may be reading the neutral path to get the 0.44

The system earth could be at neutral potential and the 240 volt between live-system earth/neutral

Lots of questions
Have you had a satisfactory existing installation insulation resistance test ?
Is there a voltage reading between the system earth and the electrode ?

The long lead test could have just measured the system earth to the connection at the electrode where there was no connection to the actual true earth

The old earth trips had seperate connections for the electrode cable and the system earth,was a Ze done at the correct side ?
Did you perhaps obtain your earth from the electrode earth or the frame side of the old trip ?

 

[South Spark]

Ok - back to look at the job and maybe install new consumer unit in a fortnight's time.

Thanks for all your advice - My best guess at the moment is the system is in fact not earthed (mistake made with MFT on Zs reading?) and there is inadequate insulation resistance somewhere.

A later phone conversation with the residents they said that they frequently have nuisance tripping during thunderstorms. My guess is that only when the earth rod is soaking wet does it earth properly which provides an adequate fault path for the BS 842 (which is apparently a 1965 earth leakage detector) to trip.

So first step is an insulation resistance test on the whole set-up

Next is to dig up the earth electrode and inspect it and then replace it.

Depending on the results - put in a price for modernising the set up.

 

[malcolmsanford]

If it is the old V-ELCB which normally as a yellow button and the earth conductor going through it you should change this as a matter of course as unless you have a voltagfe transformer type meter you can not test it. Tripping out durung electrical storms was thier spciality.

You may find that the earthing conductor as over the years either been disconnected or perished, which is why it may be prudent to ask your customer to give the DNO a call and see if it is possible to upgrade to a PME system. Be a couple of hundred pounds normally but IMO a wise investment.

 

[giio]

where are the earth rods installed are they under a driveway or flagging and has new drainage been put in since they were installed,a bit of detective work with the owner might throw up a few ideas, the once moist soil could now be a dry area because of subsequent flagging or drain laying.

 

[nickblake]

Ok - back to look at the job and maybe install new consumer unit in a fortnight's time.

Thanks for all your advice - My best guess at the moment is the system is in fact not earthed (mistake made with MFT on Zs reading?) and there is inadequate insulation resistance somewhere.

A later phone conversation with the residents they said that they frequently have nuisance tripping during thunderstorms. My guess is that only when the earth rod is soaking wet does it earth properly which provides an adequate fault path for the BS 842 (which is apparently a 1965 earth leakage detector) to trip.

So first step is an insulation resistance test on the whole set-up

Next is to dig up the earth electrode and inspect it and then replace it.

Depending on the results - put in a price for modernising the set up.

The BS 842 is an RCD installed in the mid to late 60's up till the mid 80's when an amendment in the regs stopped there use , they were used on TT systems , i agree with Malc here you need to locate the earth rod and connect up as a matter of urgency ,leaving a house with no earth ,is not realy a good idea , remember as sparks we have a duty of care , and you need to cover your butt too ,what i think you should have done was install a rod even if its tempuary and a main switch RCD , not having a go mate just making a point

 

[South Spark]

As it stands the owner won't let me back in for a fortnight!

I was being subbed out from my usual firm on a one off day-rate just to install one new circuit - I've told the customer that I consider their set-up to be potentially dangerous and advised them to have it examined professionally a.s.a.p.

 

[EddieB]

please enlighten me to these two mystery 3LA's (three letter acroynms), what is a DNO and an IMO?

 

[Engineer54]

Think it's ..... IMO=In My Opinion and DNO = Distribution Network Operator....

 

[mayfair]

new one WWGD .......... What would gibbs do.......................ie ncis

 

[nickblake]

As it stands the owner won't let me back in for a fortnight!

I was being subbed out from my usual firm on a one off day-rate just to install one new circuit - I've told the customer that I consider their set-up to be potentially dangerous and advised them to have it examined professionally a.s.a.p.

Fare comment like i said i wasnt having ago mate , were in a funny world where you have to cover your back all the time , could be an idea to get yourself a book of DN's all realy for is to cover your back on paper