Thanks all. If interested then here goes on the next episode.
2 Dr DIYer and the TT turned to TNCS.
Whilst at the home of my former GP,an avid DIYer ( Have you noticed a lot of "proffessionals", Drs, Dentists and even Vets, fancy themselves as Electricians?, sorry Patch none intended)
wiring out his massive new conservatory, (well on 100 grand a year they must have to spend some of it), I just did a quick Zs on an existing socket and got 1.2 ohms. Thats good I thought especially as this is a TT installation and I new last time I tested the Ra it was 169 ohms!. As a matter of interest I also did an RCD test and NO tripp @ 30 mA none @ 5x and none even on a full 25 Amp Loop test.
So what have we got here? A TT install looking more like a TNCS and No RCD protection.
Check RCD test button and Click Off it goes.
Now having been around the block a few times I recall something like this happening in the past, and remember N-E faults can disable RCD s on a TT.
So remove cover off CU (Double decker Duel RCD job) test Other Rcd and all tests ok.
Stick Meter across between N and E on continuity and see 1 ohm, remove Nuetrals one at a time and discover fault on downstairs lighting circuit.
Leave disconnected and now do a Zdb test, get 64.1 ohms, thats more like TT and even better Test the RCD again and yup all OK. (26.7mS @1x and 16.5mS @5x)
So just to analise things this N-E fault had disabled the RCD and turned the install into a TNCS, only trouble being instead of a nice fat 16mm Earth lead, any fault current would be taking a scenic route around the house on a 1mm (i.e. a L-E fault on the cooker for instance would have travelled back from the cooker on its cpc to the earth bar in the CU, down the faulty lighting circuit 1mm cpc to the N-E fault and back up the 1mm N to the C Unit and off in search of home via the N supply conductor to centre point on the supply transformer)
A previous example of just this scenario that I have seen had resulted in a Burnt out 1mm Twin and earth from Cu right thru the house!
Well the Dr was in the house and very interested in all this, passing by now and again with nail gun in hand, (Dont ask, but think he had a floor panel to fix down in the loft)
So as he passed by on route to rig me a scaffol platform to refit the Landing light fitting "While your here", (4 bits of 3x2 and a 6"strip of chipboard) I ask "has he fitted any new lights downstairs lately? " not recently, was the reply, no clues there then, so set about seaching for the fault.
Split the lighting circuit at a convenient point in the middle and fault disappears, good its further on then, all wall lights from here wired in singles, N looped in and out of all fittings, L looped in and out of all switches with switch wires to fittings.
Split N at wall light in hall, fault further on, into lounge ( 3 wall lights, split at one, fault further on, go to next one in room, fault back between the two. Now what could be between the two wall lights? search high and low, nothing in the room, getting late in the day now, so leave 2 walllights dis and prepare Dr for return visit and possible floors up above, NOPE laminated (dont you just love the stuff), well holes in ceiling then!
In between all this I dice with death on the landing scaffolding and re fit the landing light,(Dr had put his foot thru the ceiling previously and builders had been summoned by the wife to repair the damage which ment them taking the light down)
So back a few days later to finish the job.
Dust sheets at the ready Mrs Dr is all prepared, but having slept on it, I wonder what route the N took between the 2 wall lights, and take a look at a wall light in the Dining room which may have been on route for convenience, and yup there it is N short to metal back plate. Great Mrs Dr is very pleased, we celebrate with a coffee.
And yes Hubby had fitted same Wall light a few years ago. They had been very fortunate not to had any serious faults to earth in the meantime.
All in a days work, role on legislation to prevent such like events in the future, oh forgot we have it already Part something or other......
PS
For all you budding sparky's out there (NO LLs PLEASE), would you like to have ago at explaining what would be the normal result of N-CPC short on a TN install with an RCD and why the RCD was disabled with it on this TT Installation.
