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

[johnnyuk]

MI cable is annealed to soften the sheath to aid installation but too much bending soon hardens it back. You can normally give MI all sorts of abuse with no ill effects and due to it's construction it is hard to destroy it's insulation properties unless you split the sheath. You call it 2L2.5 so assume it is metric.

i measured the diameter of the conductors inside and they measured 1.7mm, which seems nearer 2.5mm than the nearest imperial equivalent. It's got a 32a mcb on the circuit (probably not the original one) and I'm fairly confident it was designed as a ring circuit. Built around 1973 so could be imperial - not 100% sure, but leaning towards 2.5mm

 

[johnnyuk]

This is pretty unlikely, far more likely you've disturbed it inside the pot at the termination.
My guess without further information would be that you've twisted the pot on the cable when undoing the gland and it has brought the conductors too close together.

Thanks Dave - i snipped the pot off (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

 

[westward10]

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

 

[johnnyuk]

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

No matter how good the quality of the multimeter it is not the correct test equipment, if you have damp in the pyro doing a few IR tests will generally give an indication if it is damp in the pyro

With old pyro that has work hardened bending it can sometimes cause the sheath to rupture / split that is why it is preferable to anneal the pyro sheath before reworking it

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.

 

[johnnyuk]

I had to google the word fettling, I hadn't heard it before. The closest Afrikaans word I know would be vrootel (pronounced fruitle) which means fiddle with. Thanks for the new word.

Haha. No worries.

 

[johnnyuk]

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]

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]

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.

 

[davesparks]

Thanks Dave - i snipped the pot off

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

(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]

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.