Stange fault on new installation

[suffolkspark]

today i have a fault that is driving me crazy. new installation - barn conversion
end of overhead lines, transformer in garden serving 3 properties, this one has a new underground cable from pole to new cutout and has
been left on TT by dno (originally a temp supply)


i have a 2 way ccu with 2x rcbos, each serving a submain on 16mm t/e +10mmearth to a main switch / mcb board at each end of the barn (its huge!)


one end behaves perfectly
the other has a strange fault.
with everything on EXCEPT the hob and cooker isolators the installation has been running fine for weeks.
turn either of the above on and out goes the rcbo.. easy yes ?? dodgy new appliance??? NO
as with the rest of the installation off at each mcb, they run fine, flat out .then connect back in a lighting circuit that was otherwise fine (running for
3 weeks) and out goes rcbo :*( same if i connect in heating system, or insert a plug in socket tester!


rcbos test out fine
IR of entire installation is spot on, mostly clear, worst reading was 12mohm on said lighting circuit which rises to clear after a few secs
L-PEN at incomer is 0.18ohm (i put pme link in made no difference) current Ze as still TT is only 2ohm as it wet and transformer in the garden is only 10m away


whats going on!!!!!!

 

[Deleted member 26818]

Could it be that the cumulative earth leakage exceeds 30mA?
Conduct a ramp test on the RCBO without any circuits energised, then again as you energise each circuit.

 

[suffolkspark]

Thanks for reply, the property is still incomplete so there is very little current using equipment present > 3x mira digital shower pumps, 1 neff induction hob, 2 neff ovens, 2 neff warming drawers (unplugged) most light fittings are running sylvania gu10 led bulbs (about 80 in total) theres a few sets of selv leds and tapes too.

 

[Midwest]

Clamp it as well, see what earth leakage you have.

 

[UKMeterman]

Ok,
1. Induction hob == Low Frequency Radio Transmitter, it will have a filter capacitor L to E which will be quite leaky
2. New oven, also very leaky.

Do a standiing leakage test on the installation and regardless of the result put the above on RCBOs. If the standing leakage of the installation is anywere near 15mA then split the load over RCBOs.

Also check the circits with L+N joined and check the combined resistaince against the circuit E at 500V

 

[suffolkspark]

The installation is already split approx 60/40 on two rcbo's at scource, unfortunately the submains to each consumer unit are wired in 16mm t/e so i have to leave my 30ma protection where it is, i shall cirtainly carry out some leakage testing next week and will come back to you all!

 

[Deleted member 26818]

Why does the 16mm T&E require RCD protection?

 

[suffolkspark]

There are some areas of its run that i cannot gurantee it is more than 50mm in the wall (runs in under screed trunking but rises to each consumer unit in stud wall with no earthed mechanical protection)

 

[Octopus]

Dare I suggest you remove the RCBO from the main board and put 1 on each of the sub board to see if you can spot any consistancy?

This is a bit like fault finding a cu with a single up front rcd!

have you clamped the sub db tails and switched off the mcb's one by one?

 

[suffolkspark]

I know, rather than putting a dual rcd board at scource though i put a ccu at each end of the property to reduce my cable runs so its still on 2 rcds effectively like a dual rcd board, just half of the board is at the other end of the house if you like in hindsight i could have swa'd from scource to a dual rcd board at each end but it just seemed overkill at the time. Would have been lovely now though! I havnt done any clamping yet so thats for Tuesday and yes if i get no joy there i will try to temp remove the main rcbo and fit some rcbos in the board to search for the cause

 

[richy3333]

I know, rather than putting a dual rcd board at scource though i put a ccu at each end of the property to reduce my cable runs so its still on 2 rcds effectively like a dual rcd board, just half of the board is at the other end of the house if you like in hindsight i could have swa'd from scource to a dual rcd board at each end but it just seemed overkill at the time. Would have been lovely now though! I havnt done any clamping yet so thats for Tuesday and yes if i get no joy there i will try to temp remove the main rcbo and fit some rcbos in the board to search for the cause

Just my opinion but crikey, whoever designed that installation should think again. How is SWA an overkill? More like a better idea to me. The whole install on 2 RCBO's, no wonder they are tripping out. I can't ever see that install not being problematic

 

[tony mc]

IMO when you clamp the tailsyou will find the RCBO is only doing its job and I would put money on this being cumulative leakage.

It is a double pole RCBO at the front end?

 

[suffolkspark]

>richy3333 - It's no different to a dual rcd protected property, as i say i was trying to reduce my final curcuit runs

>tony mc - The incoming ccu houses a wilex 100A main switch and 2x wilex 50A 30mA single pole RCBO's.

 

[richy3333]

Apart from the fact that they have to walk a distance to reset the RCBO? Still an appalling design IMHO. And using your split load board argument, at least they'd be double pole RCD's

Single pole RCBO's really?

Wylex - hope they dont catch fire

 

[suffolkspark]

Jeesus any need for that? As stated in post #1 the installation is TT so i would need some sort of RCD at the scource anyway even with SWA I'd need type S 100mA. Yes there would be discrimination but still could go out loosing the whole house.

IF it does turn out to be cumulative earth leakage i will have to expose the short vertical run in the stud wall and install some earthed mechanical protection, and fit a pair of rcd's into the consumer units.