Lighting Strike - Ground Potential Rise - Cable Damage

[SpannerSpark]

Hi all
Called to a neighbour whod had almost direct hit lighting strike, I know this because I witnessed it and the same strike turned my desktop PC and bband modem into charcoal briquettes/paperweights. It also took out her telephone and bband permanently.
All her electrics had tripped but after reset the upstairs lighting and kitchen ring trips wouldnt stay in.
After removing all appliances & lamps and a bit of divide and conquer the only answer left was that one particular cable run in each circuit having a L-cpc short circuit. The lighting was easiest to get at a replace the run and sure enough when the offending cable had its cores stripped out it looks like the cpc had melted at one point melting the live ptfe sleeve, fusing the two copper cores together. I would post an image but dont have the permissions.
Did MW test on the lighting and all ok now. Assume this is the same fault case for the kitchen ring, but the fault is in the fabric of the building and is a floor-up job, so temporarily split it to two 16a radials.
Assume this is ground potential rise, instantaneously melting the cpcs due to fast overcurrent. Assume also that the cpcs went at a weak point, perhaps a nick in the core or manufacturing error.
Firstly has anyone come across this? Secondly I will be recommending a periodic as there is no telling if any other runs/cpcs have been damaged, possibly now not continuous. And thirdly would anyone recommend a new cu, it seems fine (all trips still function and RCD trip times ok), but wonder how stressed it has been.
BTW her system is @15 years old, PME, wiring generally looks in good nick, MCBs in CU with RCD LHS for sockets etc, lighting etc RHS no RCD.
Any advice gratefully received.

 

[Pete999]

Don't mean to be cheeky but what's an MW test or am I missing something

 

[telectrix]

Megger Whacker. ain't you learnt anything?:smilewinkgrin:

 

[jimmymac]

Massive W....r?

 

[SpannerSpark]

Sorry did the tests as per a Minor Works. e.g. L-PE continuity, insul resistance, external earth loop Z, RCDs times etc.

 

[Pete999]

wtf has a minor works test got to do with the issue, more to the point what is a minor works test?

 

[telectrix]

he said tests as per MW. i.e. testing of a circuit as you would after a minor works. poor phraseology, but who cares?

 

[SpannerSpark]

After replacing the faulty cable I did all the tests as you would normally do after a minor works (addition or alteration to existing circuit). Seemed to me to be a sensible thing to do to check the lighting circuit was now ok. Perhaps others with more experience would do something different. In any case if anyone has any comments on the ground potential rise damage rather than the sequence of tests performed after fixing the fault that would be appreciated.

 

[telectrix]

your theory may be correct. another possibility is the lightning down the TV aerial into the TV or hit the phone line, and then on to other parts of the installation.

 

[SpannerSpark]

Don't think it was the aerial as her TV still works, assuming if its powerful enough to melt 1mm cable then the electronics in her telly would be fried. The phoneline is a possibility as mine is knackered too and I have heard that at least 3 other neighbours have damaged phone/bband equipment. Having said that why would that trip all the electrics. And I would have expected phone cable would melt before 1mm twin and earth.

 

[telectrix]

you would expect that, but it's also possible that the strike found a weak point in the insulation of the 1.0mm T/E

 

[shanky887614]

have you checked your bband cable within you property?

it could be damage at the exchange outside and that would account for multiple properties having this fault

 

[Gavin A]

induced voltage into the earth circuits of the t&e?

induction being relative to the area between within the circuit, which will be pretty massive in a house with t&e buried all over the walls and ceilings, so a close by lightning strike could induce a fairly significant charge into the circuit.

There are some formulas around that would allow someone to calculate this if they could be arsed, but that's why solar PV singles cables should be run together / under the panels.

Not entirely sure about earth cables in t&e but assume the same principle would apply.

 

[Darkwood]

If the lightning hit close then prior to the strike their would have been several faint positive streamers and it sounds like one was off your house, it may piggy back the wiring system namely the earth as its usually interlinked and connected throughout the building - the voltages are very large and can do alot of damage even though the positive streamer is not the one to make contact with the actual bolt..... check the link out ...towards the end you can see footage of several streamers a split second before the bolt find its target.

https://www.youtube.com/watch?v=RLWIBrweSU8

 

[scammell man]

hope she is well insured.

potential earth rise to several thousand volts likely. In this sort of situation all sorts of appliances are likely to get fried along with the fixed wiring. Even if Megga OK the wiring may have been damaged at places other than where you found complete failure. As a minmum sample other wiring, checking for internal signs of overheating. MCB may well have experinced currents well in excess of their rated breaking current. If in doubt rewire the lot, like I said, hope well insured