MCB not resetting on MICC lighting circuit

[JohnnyG]

I had a look at someone's external lighting circuit today but didn't go too far on it as it's in MICC and I've never touched that before so didn't really want to start messing with the integrity of it.
There's about 2kw of lighting & a 1kw photocell (well designed) and the mcb has tripped and won't reset.
I tried bypassing all the lights and the photocell but it still won't stay on, if the mains power is cut it will though so I *think* the mcb is ok.
I took the neutral out of the bar in the DB and it still wouldn't stay on, does that sound like it might be a L-E fault?

 

[Spark123]

Carry out an insulation resistance test
Probably got moisture in it.

 

[JohnnyG]

I thought about that but figured if it failed an IR test I wouldn't have been able to do anything about it anyway
One of the JB's I opened (well removed the last remaining screw of the four) looked like it had some sort of gunk coming out of the gland, is that likely to be the insulation coming out?

 

[Spark123]

Not sure without seeing, the ends are sealed using a compound which is then crimped in using a special tool, think there is a method in the wiki for potting these.
If the compound is at fault and moisture gets in it makes the mineral insulation (magnesium oxide??) damp and hence a low insulation resistance.

Found it here:
//www.diynot.com/wiki/electrics:installation_techniques:micc

 

[ELZ4742]

ahhh pyro...that takes me back to the good ole days of zapping my fellow college pals with the trusty ole mega

 

[Spark123]

 

[1john]

if the potting compound has come out of the pot then that is the likely place for the insulation to have taken in moisture. The cable is packed with magnesium oxide which is extremely hygroscopic if left to the elements. You need to split up all the lengths of MI cable at junction boxes and accessories and insulation test each length. find the one(s) that are 'down' and the ends will need stripping back until you get a good reading and then re-pot the end. When you pack the pot out you need to dress the cables so they are nice and straight and offer the sealing disc up to the pot to keep them on place. leave about an inch gap. then fill the pot with the putty, make sure you do this from one side by scraping it in from one side so as the pot fills completely and you don't get any air trapped in there then once it is full you press the disk into the pot and then crimp it in place. you will need some specialist tools for this. You may end up having to replace the cable if you end up stripping it back so far that is is unfeasible to keep going.

 

[JohnnyG]

Thanks for the input, I won't be playing with it myself, I was just interested in the physics of it all