Callout Today on a TT Install tripping RCD

Vitesse · Jan 11, 2016

I got called out this evening to a tripping RCD on a TT install after a lightening strike in the area.

Its quite an old CU with mcb's and a standalone 100ma RCD on the tails between the supply and the 100A DP main switch on the board. The RCD would switch on but as soon as the CU main switch is switched on it trips the RCD.

Tested the Ra and it was 155Ω, so not brilliant but within spec for TT.

I did an IR between the CPC bar and neutral bar and found a dead short, so I disconnected all the earths from the bar and reconnected them up one by one until I found the offending circuit causing the tripping which was the ring final for the lounge. Unplugged everything, still short between Neutral and CPC on the circuit.

I split the ring and found which leg is causing the problem but it was getting late and I didn't get a chance to go any further as they had a young family they needed to be put to bed. I explained that I had to leave the offending circuit isolated on the board and locked it off so they couldn't switch it back on. I am going back tomorrow morning to split the ring further and hopefully find the problem.

I live not far away and there a massive lightening strike in the area and according to the owner it was very close to them. My only conclusion at the moment is that a voltage surge from the lightening has travelled up the earth rod, as there is no surge protection, it has somehow managed to breakdown an existing fault between neutral and cpc and has made the fault worse to the point that it is now causing the RCD to trip?

Does anyone have any more logical explanations as to what has caused this problem to appear all of a sudden?

davesparks · Jan 11, 2016

More likely coincidence than being related to the lightning strike.

Vitesse · Jan 11, 2016

davesparks said:
More likely coincidence than being related to the lightning strike.

Funny you should mention that Dave. I was going to amend my original post with that thought as well after I had posted it.

Risteard · Jan 11, 2016

Vitesse said:
I disconnected all the earths from the bar and reconnected them up one by one until I found the offending circuit causing the tripping

I would definitely have preferred the neutrals to have been disconnected and then tested to earth with cpcs connected to the means of earthing.

Vitesse · Jan 11, 2016

Risteard said:
I would definitely have preferred the neutrals to have been disconnected and then tested to earth with cpcs connected to the means of earthing.

So would I but the board was so tight and poorly installed it was virtually impossible to get near the neutral rail without disconnecting all the circuits, it was a mess to say the least. The neutral rail wasn't one of those installed at the top of the board it was halfway down behind all the wiring and the circuits were like guitar strings over the top of the rail.

Gavin A · Jan 11, 2016

is it an overhead line coming in? If so then it's probably an induced surge rather than direct strike.

Vitesse · Jan 11, 2016

Gavin A said:
is it an overhead line coming in? If so then it's probably an induced surge rather than direct strike.

Yes overhead lines. I was about two miles away at the time and it was very close as the lightening and thunder were literally a second apart.

plantpot · Jan 11, 2016

Many years ago we got called to fault after lightning strike.tracked it down to a length of 1.0mm txe under a floor that had literally melted into a solid block .

UKMeterman · Jan 11, 2016

If you want to get a little more money on this job, suggest to the customer that they might need whole house surge protection. ABB sell the kit, have a chat with their technical line, you can buy the stuff from RS.

malcolmsanford · Jan 11, 2016

[ElectriciansForums.net] Callout Today on a TT Install tripping RCD

Wasn't one of these old babies.............the infamous VOELCB?

These were notorious for tripping in electric storms

NDG Elecs · Jan 11, 2016

There are no outside lights spurred off the RFC are there. Very slim chance that an exterior light got fried perhaps.

We had a lightning strike a couple of years back that knocked out around 200 homes telephones. My missus was at home at the time and she actually went and hid under the stairs with the kids for 20 mins as she said it was terrifying and the air was strange! Ionisation I guess. And my missus isn't a timid thing!

The strike hit the BT pole in our back lane I was told, and thankfully not our TV aerial!

Vitesse · Jan 11, 2016

Problem fixed. Split the ring down, probably about 12 double sockets and some spurs . Found the short on the last leg of the ring between two sockets. Opened them both up and the last socket on the ring was the only one with no CPC sleeving. On closer inspection there was the tiniest of nicks out of the neutral and the unsheathed CPC was resting against it, so possibly caused by heat between the two wires? Tested IR again and got >2000 on all combinations. The owner is adamant that the problem occurred at the time of the lightening, I don't know if it is possible that a voltage surge caused the CPC to heat up and just push the insulation on the neutral over the edge?

Got a CU change out of it as well, as I explained to the home owner that the current situation with the standalone RCD protecting all circuits is not the best solution plus the current CU is a mess and really cramped and some other bits and bobs, so turned out to be a nice little earner for a Sunday callout.

I will take a look at the surge protection as suggested. So thanks for all your helpful suggestions and a satisfactory conclusion.

NDG Elecs · Jan 11, 2016

Yeah could be heat between the neutral and CPC. Once the insulation breaks down a little, some electricity could leak to the CPC, and this then gradually worsens over time. The lightning strike may well have been a red herring and just coincidental. Always nice to rectify a fault. Can be a pita at the time!

bigspark17 · Jan 11, 2016

Are you planning on removing standalone 100ma when doing board upgrade.? Was it type s.?

Vitesse · Jan 11, 2016

TheGayOwl said:
Yeah could be heat between the neutral and CPC. Once the insulation breaks down a little, some electricity could leak to the CPC, and this then gradually worsens over time. The lightning strike may well have been a red herring and just coincidental. Always nice to rectify a fault. Can be a pita at the time!

As you say it can be a pita at the time, there is always some little gem, a socket hidden away out of sight. This offending item was tucked away down the side of a radiator in the hallway, so not even in the same room as the rest of them. I must admit fault finding and getting a result at the end of it is probably the part of the job I enjoy the most.