Uh oh...shorted MICC. How scr*wed am I?

johnnyuk · May 8, 2024

I've just been fettling some 50 year old 2L2.5 pyro (MICC) to relocate a plug socket. I probably bent the cable a little more than I should have (although I didn't feel it was particularly tight when I did it) when I was doing the fettling, but now I've pulled the cable straightish again, I seem to have a short across the live, neutral and earth on the cable in question.

This is my first time working on pryo - assuming I've managed to cause a short within the cable somehow by bending it, is this something that happens a lot and is there anything I can do to fix the short in the cable? Or is it a case of stripping the MICC back until I find the short? Plan B will be to replace the damaged leg if I can.

Or am I being incredibly dim somehow?

johnnyuk · May 9, 2024

UNG said:
If the pyro has not been terminated you could be seeing damp in the end of the cable

The short was evident immediately after I cut the cable so I don't think it is damp. It was in a live circuit up until I cut it. Unless you're thinking that the cable could have been damp before and all i'm seeing is the "short" that was already in the cable before I started messing with it?

littlespark · May 9, 2024

You said early on it had orange sheath…. Did imperial MICC come with a pvc sheath, or did that come in after the change to metric?

johnnyuk · May 9, 2024

westward10 said:
1.8mm diameter is 2.5. If you have the pot there is normally writing stamped on it even the imperial ones.

I can't get at the pots - they're inside a pot clamp that's been buried in the blockwork wall behind some concrete. I could probably hack it out but not doing it for the moment. There are remnants of a slightly torn label on one of the conductors which looks like it read "Pyrotenax 734-02" and "A.f5". A google hasn't thrown anything up yet but I'll see if I can find a better label elsewhere on the circuit.

[ElectriciansForums.net] Uh oh...shorted MICC. How scr*wed am I?

davesparks · May 9, 2024

johnnyuk said:
Thanks Dave - i snipped the pot off

Ooops, that potentially made the repair a whole lot more difficult!

johnnyuk said:
(it was in a pot clamp behind concrete) and made a clean cut of the cable end with a dremel, then stripped off the first cm of copper sheath just to be sure it wasn't shorted at the end, but still get the "continuity beep" on my multimeter between the cores and sheath

Forget the multimeter, isolate the cable and IR test it.

johnnyuk · May 9, 2024

littlespark said:
You said early on it had orange sheath…. Did imperial MICC come with a pvc sheath, or did that come in after the change to metric?

pass. i was born in the 80s! Seems to be orange sheath in very good nick so don't think it's rubber.

johnnyuk · May 9, 2024

davesparks said:
Ooops, that potentially made the repair a whole lot more difficult!

Forget the multimeter, isolate the cable and IR test it.

thanks dave - when you say isolate, do you mean "turn the circuit off" or do you mean "find the other end of the cable and disconnect it"?

re. sniping the pot off, i'm trying to lower the socket in question so i needed to shorten the cable (fed from below).

UNG · May 9, 2024

johnnyuk said:
ok thanks for this. part of the problem is that I can't be 100% sure of the routing of the circuit without pulling everything to pieces as it's all under a concrete slab. I can do an IR test between cores/earth on the open end of the problematic cable. Forgive me a basic question but my confidence has been a bit knocked on this one - do I need to find and disconnect the other end of the "damaged" cable before I test IR? The circuit isn't live but obviously the ring is broken where i cut the cable.

You really need to find the other end before doing an IR test even using a multimeter as you have been without knowing what is attached to the other end of the cable any readings are a bit meaningless
Even if you terminate the open end you will still need to access the other end of the cable to "bell out" the cores to identify which is the live core
Whereabouts in the UK are you it is possible someone experienced with Pyro may be close by and may be able to help you out or offer advice

nicebutdim · May 9, 2024

johnnyuk said:
thanks dave - when you say isolate, do you mean "turn the circuit off" or do you mean "find the other end of the cable and disconnect it"?

This circuit isn't disconnected?

freddo · May 9, 2024

littlespark said:
You said early on it had orange sheath…. Did imperial MICC come with a pvc sheath, or did that come in after the change to metric?

Imperial MICC is listed in all the books I have, available bare or with a PVC sheath coloured orange as standard, also available to order in black.

johnnyuk · May 9, 2024

UNG said:
You really need to find the other end before doing an IR test even using a multimeter as you have been without knowing what is attached to the other end of the cable any readings are a bit meaningless
Even if you terminate the open end you will still need to access the other end of the cable to "bell out" the cores to identify which is the live core
Whereabouts in the UK are you it is possible someone experienced with Pyro may be close by and may be able to help you out or offer advice

southwest

johnnyuk · May 10, 2024

nicebutdim said:
This circuit isn't disconnected?

yes. I was just clarifying what Dave meant by "isolate"

davesparks · May 12, 2024

johnnyuk said:
thanks dave - when you say isolate, do you mean "turn the circuit off" or do you mean "find the other end of the cable and disconnect it"?

I mean open an isolator if available or otherwise disconnect the cable, switching the circuit off does not isolate the cable.
By isolate I mean disconnect all live conductors.

johnnyuk said:
re. sniping the pot off, i'm trying to lower the socket in question so i needed to shorten the cable (fed from below).

I still think it is unwise to cut it until you are ready to reterminate it.

johnnyuk · May 13, 2024

ok that's what I thought - thanks Dave.
Thanks to everyone for the help. I'll update when I have more news.

7029 dave · May 13, 2024

You need to use an IR tester at 500v

johnnyuk · Oct 8, 2024

A little update for everyone that helped me out before. Feel free to laugh at me:

- It turned out the short on the MICC was actually somewhere else entirely on the circuit. The end I had been testing at on was fine. The short must have already been there before I started testing and disappeared once I traced and disconnected the other end of the leg I was looking at. I feel there's a lesson learned in there somewhere!

Anyway after all that, having belled out the circuit and established exactly what would be required to recommission the MICC for this circuit, I've decided to abandon in and just get a new circuit installed in pvc.

So I was wondering if anyone could advise - what is best practice (apart from pulling out all out) to terminate the abandoned MICC before it gets buried in the walls? It won't be visible in any of the new back boxes as I'm having sockets relocated.

Thanks in advance for any advice.