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

[johnnyuk]

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?

 

[Marvo]

Visually check the cable for cracks where you bent it. You can bend Pyro fairly tight but it doesn't like being straightened again, it loses it's flexibility after the first bend and it often cracks if you bend it back again. What was the resistance between the L, N and E? If you've got a low impedance short circuit between them you're probably going to end up replacing the entire leg.

 

[johnnyuk]

Visually check the cable for cracks where you bent it. You can bend Pyro fairly tight but it doesn't like being straightened again, it loses it's flexibility after the first bend and it often cracks if you bend it back again. What was the resistance between the L, N and E? If you've got a low impedance short circuit between them you're probably going to end up replacing the entire leg.

i did manage to nick the orange plastic sheathing in one spot when I was fettling but don't think i did any damage to the copper cladding or the two cores.

looks like 24 ohms between cores, 24 ohms between line and cpc copper cladding, and 0.4 ohms between neutral and cpc copper cladding.

Would it be worth trying to hammer it straight, or to try cutting the cable back until i lose the short? Or just cut my losses? Sod's law, the cable runs under a concrete slab so a pig to trace. I have a wander lead though so doable.

 

[littlespark]

Have you ruled out a bad potted end? Or have you not potted yet?

Might just have to take it back to before the bend, and lesson learned about trying to re straighten it.

 

[UNG]

Reworking old Pyro requires a lot of skill and patience and the work hardening during installation and age will create problems that need to be addressed. The cables will more often than not need to be annealled before being moved to make it easier to rework and reduce the risk of fracturing, going in gung ho will more often than not end in disaster although with metric cables it can be fixed as the terminations are still available

looks like 24 ohms between cores, 24 ohms between line and cpc copper cladding, and 0.4 ohms between neutral and cpc copper cladding.

What test equipment are you using

A job I have coming up that needs some imperial sized Pyro's that are possibly 80+ years old moving so the heat gun will be getting some use to anneal the copper prior to even thinking about trying to move them and even then it may be a leave well alone outcome

 

[johnnyuk]

not potted yet. resistance readings were just done with a good quality multimeter. I have a kewtech tester but haven't used it yet as the multimeter has identified the short.

just to check my understanding then - is it likely (ie it is a known thing that happens with old pyro) that just by bending the cable, I've created a short somewhere at one of the bends?

 

[westward10]

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.

 

[Marvo]

24 ohms is far from a short circuit, even 0.4 ohms is very high for a dead short. If the conductors were contacting each other or the copper sheath I'd expect it to be a sub-ohm impedance reading unless the end of the cable where you're testing is very far away from your bending or fettling. Try warming the cable with a heat gun for a few minutes, get it pretty hot and see if the reading between conductors improves.

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.

 

[davesparks]

assuming I've managed to cause a short within the cable somehow by bending it,

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.

 

[UNG]

not potted yet.

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

resistance readings were just done with a good quality multimeter. I have a kewtech tester but haven't used it yet as the multimeter has identified the short.

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

just to check my understanding then - is it likely (ie it is a known thing that happens with old pyro) that just by bending the cable, I've created a short somewhere at one of the bends?

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

 

[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.

 

[johnnyuk]

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]

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]

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]

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]

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]

This circuit isn't disconnected?

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

 

[davesparks]

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.

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]

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

 

[7029 dave]

You need to use an IR tester at 500v