Discuss Callout Today on a TT Install tripping RCD in the Electrical Forum area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

V

Vitesse

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?
 
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.
 
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.
 
is it an overhead line coming in? If so then it's probably an induced surge rather than direct strike.
 
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.
 
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!
 
Last edited:
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.
 
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!
 
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.
 
When there is a lightning strike there can be a huge voltage difference between the local rod earth and the voltage at the earth of the local transformer. Also the overhead cable is a huge aerial with the N earthed at the local transformer, these effects combine to make for a huge N to E spike that could jump a small gap...
 
Are you planning on removing standalone 100ma when doing board upgrade.? Was it type s.?

It is a 100ma DP type S, so it will stay and a couple of 30ma RCD's on the new board or maybe even RCBO's. I haven't really thought too much about the board change at the moment as I am snowed under.
 
When there is a lightning strike there can be a huge voltage difference between the local rod earth and the voltage at the earth of the local transformer. Also the overhead cable is a huge aerial with the N earthed at the local transformer, these effects combine to make for a huge N to E spike that could jump a small gap...

Thanks I figured that it was something like this I was just looking for clarification that I was thinking along the right lines. There was obvious signs on heat on the insulation of the neutral. I took a picture which I will try to post, so that people can see how small the hole in the insulation is.

Callout Today on a TT Install tripping RCD image - EletriciansForums.net
 
Problem fixed.
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.
Handy stuff that lightning. If you'd go round there to quote for some minor work, bet they would never of gone for a new CU etc. Its opened their eyes.
 
Handy stuff that lightning. If you'd go round there to quote for some minor work, bet they would never of gone for a new CU etc. Its opened their eyes.

Yeah it's turned into a nice little earner and it's nice to get good feedback. They made a comment about me being really thorough compared to other electricians they have had in the past.
 
Last edited by a moderator:

Reply to Callout Today on a TT Install tripping RCD in the Electrical Forum area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc

Electrical Forum

Welcome to the Electrical Forum at ElectriciansForums.net. The friendliest electrical forum online. General electrical questions and answers can be found in the electrical forum.
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock