Nightmare....!

[sythai]

Just need to share this and feel free to offer any advice suggestions...

I'll hold my hands up I've messed up and am slightly out of my comfort zone !

Scenario : 1970's ground floor flat concrete all round (walls/ floors/ ceiling) wired in imperial MI which has been skimmed over. Recent CU upgrade, RCBOs for each circuit.

Carried out EICR as requested by customer few weeks back prior to further works. No major problems, some low IR readings but passable.

Started kitchen refurb this week , part of which was to install 3 meter track light.

Right where we were to install down the middle there snaked along a length of MI, so spent the next hour chipping away exposing it so we could get fixings in the right place.

At one point just tapped the edge of MI slightly, no visible damage but RCBO tripped, reset it and stayed in so thought nothing more of it (just a blip maybe?)

Fitted track light all good. Where the original ceiling rose was sat on top of the terminal box just needed to put a box lid on this on the next visit, so for time being cables exposed but in their original terminal blocks (3 x MI's in here)

Lights been working fine for a few days.

Plasterer has been in today and now the lights have tripped!

He hasnt filled in the terminal box, which I thought he may have done at first, but kitchen has been fully skimmed and just worried damp may have somehow got into the MI as he would have skimmed near to the terminal box?

To be honest havent done any MI since at college 12 years ago and that was about it.

I completely forgot about damp problems related to MI until another sparky friend mentioned it to me today.

So wondering wether it is a damp issue or something to with when I first tapped the MI and it tripped.

Over there first thing tomorrow to split it all down and megger out.

Haven't figured out yet how to go about repairing/ replacing or explaining what is going on to the customer as of yet.

One thing in my favour is the whole flat is being decorated and this hasn't started yet.

Thanks for your time if you've managed to read this far.

Have a good weekend

Sy

 

[Wiresandthings]

I think id start at where you think you damaged it, as for repairing any potential damage its hard to say without seeing it, im assuming the sheath is your cpc? So it will need cutting back and terminating into a metal box (another conduit box chopped in??) shouldn't be a major issue with the right amount of MI exposed and the right tools etc, doesn't do much for the plastering though, good luck

 

[nickblake]

that doesn't sound good mate my only advise would be to remove all the fittings and join the cables together so that can do the insulation test with out any fittings being connected this way you will be able to see if its a problem with the cabling it could be just a simple short to neutral which is effected by temperature changes hence the reason its not all the time could just be a defective pot , cant see it being where you caught it seen this stuff heated till it glowed red and belted with a hammer and the insulation tests were fine one of the biggest problems is its imperial and very hard to get the pots etc

 

[Jay Sparks]

Sounds like damp from the new plaster. I've had this a few times but with twin. Did first and second fix, plasterer gos in and skims the chases, rcd trips. Done the ir test and the results were low but rising. Waited a few hours till plaster had dried a bit, tested again and readings were higher. Rcd stayed on after a few hours.



You will only really know properly, when you test it again but I would say damp from the plaster.

 

[Darkwood]

MI is the most resilient tough cable out there IMHO that you'll come across unless the damage is on the blind side of the cable i.e. a sharp piece of concrete has punctured it then i suspect the termination pot to be the culprit ... these are only as good as the guy who terminated it and if it was a poor job then they are prone to been an issue whenever the joint is disturbed or moved, doing insulation tests while gently moving the termination cores about will identify a poor pot but be careful to discharge the capacitive voltage you megger leaves or you'll get a nice surprise .... if it is the pot then dig out remake all the joint off with new pot etc and test again.

 

[tazz]

Even the potting compound can play havic with insulation tests and rcds, sometimes just needs time to dry out. In days gone by, you would see a spark walking up and down with a blow torch trying to get rid of moisture. Quite common to get readings of 1 meg when fitted, 2 meg the next day, and full 999+ after 4 days or so

 

[itselectric]

If the power tripped as you hit the cable then I would assume you've damaged it and the wet plaster patch has caused a short.
Try a bit of heat where damage is and if readings change then you've got a bit of a job on. i.e. cable replacement

 

[Guitarist]

I'd agree with above. If it tripped when you caught it, then that sounds like the problem area. If you can see all around the cable, check for puncture marks, however small.

 

[flyingsausage]

The old trick when you are doing an IR test on pyro and getting a dampness reading is to leave the megger on the pyro for a while which will dry it out a bit and make the reading go higher,500 volts does this nicely.

 

[Des 56]

If this is an empty property you could remove the rcbo and rely temporary on the mcb to see if the leakage is reduced enough to refit the rcbo