Repairing imperial mineral cable

[D Skelton]

Last week I did a job for forum member joramtor. Link to the original post here: http://www.electriciansforums.co.uk...s-your-area/87967-repair-micc-pyro-cable.html

He had an underground mineral cable that had been severed by the groundworkers so off we went to sort it out for him. The length of cable that was broken was about 50m long and ran between two lamp posts in his driveway. I thought I might put up some photos of the work along with an explanation so that those less experienced with mineral might learn something. Feel free to add to it if you want (I'm sure much flaw picking and critisism will ensue also ).

First off the customer snipped a piece off and posted it to me so that I could determine whether it was imperial or metric and also so that I could size the pots and glands needed if it was imperial (the job was over an hour away so couldn't just pop round). Upon receiving it it was immediately apparent that it was imperial so I used my calipers to size the pots and glands needed. The pots needed were 2L2.5, the glands needed were 4L1.5 and the seals needed were 4L1. The cable in question was 4 core with the conductors being equivalent to 0.75mm-1mm.

When I arrived at the job this is what I found, it had been lying here like this, exposed, for about four months:




First thing's first, isolate and lock off (circuit was already off at the breaker). Then to start... I chopped the ends away with an angle grinder for a neat and square application of the stripper (a square end always makes starting off the stripper easier) and stripped back a good three quaters of a meter of the sheath with a ring and pull. Next thing to do is to get the blowtorch out and give the cable a good blasting. Short, slow even strokes towards the cut end of the cable from about 650mm back to get rid of any moisture ingress. Remember the magnesium oxide that makes up the insulating mineral compound is hygroscopic, it wicks up moisture like nobody's business! So we have to get that out.




After ten or so minutes of doing this I let the cable cool and after it was, I slid on the glands and then started stripping it back with the joistripper. This was tougher than usual as the cable was very slightly too fat to fit in the 2L2.5 hole and a bit loose in the 4L1.5 hole. I went through two blades with this bit! In hindsight I should really have used a heavy duty stripper but hey ho. When the cables were stripped back the pots were screwed on and potting compound was applied from one side only to eliminate air pockets within the pot. Notice the caps are half slid down, this helps to keep the conductors in place through the potting compound filling process. The caps were then put in place and crimped using a mineral crimping tool.




When this was done it was time to IR the conductors and copper sheath. The lowest result was roughly 20Mohms live-live. Not perfect but okay. After this I belled out the ends between the break and each lamp post with my long lead to determine which core was which and also to check earth continuity on the sheath which was being used as the cpc. Once this was done the insulation was applied and the cables identified. Two cores were not used and were disconnected at the lamp posts. At the supply end I ensured that these were earthed as they previously weren't.





Once this was done I terminated the gland at one of two Pratley underground junction boxes making sure to clean the copper on the exposed mineral sheath to ensure zero resistance between this and the olive within the compression gland. I then prepared a short length of 4 core 1mm SWA to bridge the gap and terminated this into the other side of the Pratley.




This process was then repeated on the second Prately on the other end of mineral cable so that the short length of SWA was the bridge in the gap of the break. Once this was done it was time to seal the gland terminations. I tightly wrapped some insulation tape at the join between the glands and the cable to ease the transition and then wrapped tightly and carefully with a couple of layers of self amalgamating tape. This will seal the joins and prevent corrosion of any previously exposed metal.




Once sealed up the cables were joined with wagos and the circuit tested, dead tests then live. Total lowest IR had now reduced to 15Mohms (a product of the increase in length [2x25m = 50m]), again, not perfect but okay. Pratleys have a built in method of preserving cpc continuity so no need for fly leads. All tested out ok so MCB on and a functional test of the circuit carried out. All ok. MCB off, JBs sealed up with the lids and put in place, circuit energised and away we go!




The customer was told by more than one person that it couldn't be done, and here it is! Never say never!

 

[ipf]

If there isn't enough cable to repair, take the switch up 6 inches, new box with knockouts, or as existing with earth clamps fitted, strip back and re terminate.
Need a decent, experienced spark, though.
High switch, but what's the cost?

 

[7029 dave]

How the hell did that 3rd core snap off

 

[7029 dave]

If there isn't enough cable to repair, take the switch up 6 inches, new box with knockouts, or as existing with earth clamps fitted, strip back and re terminate.
Need a decent, experienced spark, though.
High switch, but what's the cost?

With Gland couplers and bush would make it a tad lower

 

[nicebutdim]

I haven’t made pyro off since my AM2 circ 1999 but I’m sure I still could with little issue.

I know this is not in the exam anymore, but is it still taught at college?

Indeed it is, although few will ever get much experience of using it.

 

[Boonsy641]

Yes I can see it now, that is definately a challenge to repair but I see no reason why it would not be possible to repair that with relatively little further damage to the wall.


I very much doubt that the cable has become brittle, without outside influences causing it copper doesn't just degrade and become brittle.

OK, that makes a bit more sense now.


I think you have 2 possible options here.

1 repair the existing cable

2 repurpose the switched live (marked yellow) as a permanent live and then fit a wireless switch with a reviver at the light to replace the switched live. The wireless switch may have to go on a surface box mounted over the existing switch position.

Dave. Can you recommend a wireless switch. If repurposing the yellow cable to permanent live then that would provide power to bathroom and bedroom that is currently in darkness. Is that right? Forgive my naïve Rss but how would the witless switch be powered? And you mean mounting witless switch over the top of normal switch? Is this something an electrician would find challenging or easy? Many thanks. Really appreciate your help on this I truly do

 

[Boonsy641]

How the hell did that 3rd core snap off

I have my doubts but initial electrician came round due to flickering light and tightened cables but before he did so he opened a few of the light switches and on this one when he opened it he said the cable easily came away.

 

[Boonsy641]

If there isn't enough cable to repair, take the switch up 6 inches, new box with knockouts, or as existing with earth clamps fitted, strip back and re terminate.
Need a decent, experienced spark, though.
High switch, but what's the cost?

My issue it seems is getting someone who could do this. It’s totally beyond the electricians I have had around. (So far 3). They have suggested a rewrite and then trunking or lowering ceilings!! I am absolutely dumbstruck that a single core of a cable is broken and I have been told it could abe a rewire running into the thousands. I know I don’t know electrics but I can’t understand why this cannot be fixed. Are any of you guys available to fix this please. Or recommend someone who can? Last electrician came around and I used all your comments and he went quiet. I feel he expected me just to accept a rewire. Thanks everyone. You are the light in a dark bathroom and 2nd bedroom!!

 

[westward10]

As is #90 this can be done. Carefully cutting the stub back with a razor blade or very fine file then snap it away and solder, someone who has patience and a deft hand.

 

[davesparks]

I know I don’t know electrics but I can’t understand why this cannot be fixed.

This type of cable has fallen out of common use, it can be time consuming to work with and the skills needed to fix it are rarely found within the domestic sector. So most electrician's you find doing domestic work either don't want to work with it, don't know how to work with it or some won't even recognise what it is.

Are any of you guys available to fix this please.

Depends how long you can wait! My other half is in Stevenage so I'm often in the area but I am fully booked now for a few weeks.

 

[Boonsy641]

This type of cable has fallen out of common use, it can be time consuming to work with and the skills needed to fix it are rarely found within the domestic sector. So most electrician's you find doing domestic work either don't want to work with it, don't know how to work with it or some won't even recognise what it is.



Depends how long you can wait! My other half is in Stevenage so I'm often in the area but I am fully booked now for a few weeks.

All the good one are fully booked is the rule. It just says to me that those fully booked are worthwaiting for. Sorry to ask and I am humble asking. How confident are you that you can fix it ?(sorry Dave. Have to ask) hope you don’t mind. Ime in Hitchin by the way

 

[westward10]

If anyone can fix it then @davesparks is someone you should trust to use his skills to do so.

 

[Boonsy641]

If anyone can fix it then @davesparks is someone you should trust to use his skills to do so.

Yes. I truly get that. I suppose I have just been hammered last week or so and just feeling a little lacking in confidence. Not like me at all. I do hope I cause no offence as you guys are the light at the end of this. It would be great if I can get Dave to do this.

 

[westward10]

No one has been offended.

 

[Boonsy641]

This type of cable has fallen out of common use, it can be time consuming to work with and the skills needed to fix it are rarely found within the domestic sector. So most electrician's you find doing domestic work either don't want to work with it, don't know how to work with it or some won't even recognise what it is.



Depends how long you can wait! My other half is in Stevenage so I'm often in the area but I am fully booked now for a few weeks.

Happy to wait Dave. I’ll get some lamps in flat. Many thanks

 

[Boonsy641]

This type of cable has fallen out of common use, it can be time consuming to work with and the skills needed to fix it are rarely found within the domestic sector. So most electrician's you find doing domestic work either don't want to work with it, don't know how to work with it or some won't even recognise what it is.



Depends how long you can wait! My other half is in Stevenage so I'm often in the area but I am fully booked now for a few weeks.

Wife had a guy come around and said he would put some trunking up and new cable to the main circuit box as he would not want to channel into concrete as is the ceiling. He wouldn’t touch MICC. He wanted 700 pounds! That sounds a lot for putting up trunking. am so glad I found you guys.

Dave. Believe me I can wait for you and obviously very happy to pay what you need to put this right.

Thanks to all of you. You’ve been a great find.