clearing fault on micc

[wattsup]

Fault on micc, one leg of two-core down to earth.

Anyhow big probs cos fixed / glanded into conduit box in concrete ceiling...supposed to be phased and tested before, the fault magically appeared after!

Ideas what to do and get out of hole before boss finds out

We settled on one guys idea who had done it before and worked...

wang 240v down it on 30A rewirable it may clear...

.....yep blew the pot out of the ceiling!

Just talked to him on phone, have to laugh about it now

So if any of you guys are faced with the same prob, try a 5A first -;

 

[Knobhead]

Try a blowlamp first!

Did it clear the fault BTW?

 

[oxocube]

There's more than one way to skin a cat, but what damage has been done to the shorted core? so what's the next move?

 

[malcolmsanford]

There's down to Earth and there is low reading!!!

Low reading is a sign of a little dampness and yes it as been known for "sparks" to liven up a MICC to dry it out. This is a quick fix and to be honest not a particularly good one as often as not dampness will get into the pot again and breakdown the sealant and do exactly what happened to your pot.

The liven up the circuit really was a "good enough for the country" job, and let someone a few months down the line worry about exploding pots.

This though would not work on a fully down to earth

 

[wattsup]

sure did tony, except nothing left to connect anything to.

The gaffer wern't too pleased but said run another leg surface, there's a suspended ceiling going in here....nobheads

 

[wattsup]

this was in the days of wind up meggers, clear, low, dead short.
We was dead short

 

[wattsup]

This though would not work on a fully down to earth

I would argue it could and has worked before, a hairline fault would blow before the 30A rewirable fuse

 

[Engineer54]

Fault on micc, one leg of two-core down to earth.

Anyhow big probs cos fixed / glanded into conduit box in concrete ceiling...supposed to be phased and tested before, the fault magically appeared after!

Ideas what to do and get out of hole before boss finds out

We settled on one guys idea who had done it before and worked...

wang 240v down it on 30A rewirable it may clear...

.....yep blew the pot out of the ceiling!

Just talked to him on phone, have to laugh about it now

So if any of you guys are faced with the same prob, try a 5A first -;

The times i've heard this gem of a solution to clear a MICC pot earth fault!! All it will do at best is leave a carbon deposit from the pot to the conductor by the fault flash.

I'm afraid the only way to correct, is to chop out the concrete from around the Besa box to release it, and around 6'' along the route of the cable to allow you to remake the pot, then repair the area when all back in place. A real pain of a job, and why you ALWAYS check and double check any made off lengths of MICC cable, BEFORE it's embedded in the fabric of the building.

 

[malcolmsanford]

Fault on micc, one leg of two-core down to earth.

Anyhow big probs cos fixed / glanded into conduit box in concrete ceiling...supposed to be phased and tested before, the fault magically appeared after!

Ideas what to do and get out of hole before boss finds out

We settled on one guys idea who had done it before and worked...

wang 240v down it on 30A rewirable it may clear...

.....yep blew the pot out of the ceiling!

Just talked to him on phone, have to laugh about it now

So if any of you guys are faced with the same prob, try a 5A first -;

As I said in the post if it was dampness then you may have cleared it.

But no matter what you do with a full earth fault it will not clear, it will either blow the pot or the fuse

I thought you were trying to clear it as you stated in your OP, you will never clear a full earth fault.

 

[wattsup]

And by the sheer nature of an explosion, carbon flash points may or may not leave direct conductive paths.

I have been assured it can work, by a trusted and yes alright eccentric sparks

 

[wattsup]

malc, i did state we were attempting to clear it, it didn't work on this occaision. We were aware of damp, this was a dead short.
I ought to have stated, sorry

 

[Engineer54]

And by the sheer nature of an explosion, carbon flash points may or may not leave direct conductive paths.

I have been assured it can work, by a trusted and yes alright eccentric sparks

Rather you than me, this would be the very last thing i would attempt. The chances of that fault flash leaving a conductive path between the conductor/ pot / sheath is a dam sight higher than it not!! Even if the conductive path is minimal, it will create a permanent weakness in this lighting circuit. One that i certainly wouldn't be happy about walking away from!!!

 

[wattsup]

For qualified only:
Assuming rewirable fuses, you can easily test the theory behind
Split a length of .75mm flex, take out one strand (I have no idea what size this would be, but would guess arround 0.075mm)
Turn off power
Take off socket front and connect strand between line and earth. If you want you could surround the connections with compound to replicate an micc termination.
(If tested this would show a dead short)

Turn on power
The strand will blow (assuming earthing is up to spec.)
The rewirable fuse may also blow, replace rewirable fusewire, and reconnect to supply.
You will see no fault, although you may or may not see a low reading
Of course not to be attempted by diy

 

[wattsup]

Why? If a fault it will trip, if not it won't?

 

[Engineer54]

For qualified only:
Assuming rewirable fuses, you can easily test the theory behind
Split a length of .75mm flex, take out one strand (I have no idea what size this would be, but would guess arround 0.075mm)
Turn off power
Take off socket front and connect strand between line and earth. If you want you could surround the connections with compound to replicate an micc termination.
(If tested this would show a dead short)

Turn on power
The strand will blow (assuming earthing is up to spec.)
The rewirable fuse may also blow, replace rewirable fusewire, and reconnect to supply.
You will see no fault, although you may or may not see a low reading
Of course not to be attempted by diy


Are you actually being serious here??

Why? If a fault it will trip, if not it won't?


It could well do, but at a latter date, it then may never re-set, so leaving the original fault with this circuit, for some other poor bugger to fix!! Brilliant!!!

So why not correct this MICC fault using a proven conventional means, and not trying to bodge your way through it??

 

[wattsup]

yes absolutely serious engineer, btw please explain why the experiment is flawed?

 

[wattsup]

Bodge? If its clear, its clear...or maybe you know down the line all clear always clear?

Are you paul daniels?

 

[Engineer54]

yes absolutely serious engineer, btw please explain why the experiment is flawed?

I'm not even going to grace this with an answer!!

 

[Engineer54]

Bodge? If its clear, its clear...or maybe you know down the line all clear always clear?

Are you paul daniels?

I've been in this game long enough to know that little statement can come back and bite you in the arse, especially when talking about MICC pot faults. Blowing an earth fault apart in a MICC pot that you can't inspect is NOT a good idea ...Full Stop!!!

And you don't have to be a Paul Daniels or a clairvoyant to know that!!

 

[mechelec]

I have in the distant past removed pots to check. IR test still showing phase to earth fault. I was shown a trick that has worked on several occasions especially if the Pot has been twisted slightly when gland was tightened, give the conductors a pull with a pair of pliers directly inline with the MICC to pull them straight IR test again and bingo fault gone (sometimes) Fit new pot fill with compound and retest.