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!

 

[7029 dave]

I keep looking at the picture and it looks like 2L1, even with the wiring configuration N's connected together in back box ? Perm Live in Common S/L out ? But hey am not there if you say 3core okay.
Silly question I take it the light/ lights dont work?
Could be a 3L1 disc but have only potted 2core

 

[littlespark]

Is it just my eyes looking at a phone screen, but I count only 4 cores between the 2 cables. 2 into a connector and 2 into the switch.

If there is even a short stump of the broken core, can a small 5A connector be screwed on and extend the wire?

 

[LeeH]

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?

 

[davesparks]

I keep looking at the picture and it looks like 2L1, even with the wiring configuration N's connected together in back box ? Perm Live in Common S/L out ? But hey am not there if you say 3core okay.
Silly question I take it the light/ lights dont work?
Could be a 3L1 disc but have only potted 2core

They look like wedge pots rather than the usual screw on pots, were they ever used on metric cable or were they older than that?

 

[7029 dave]

They look like wedge pots rather than the usual screw on pots, were they ever used on metric cable or were they older than that?

No idea , I do remember Pyrotenax's heatshrink pot/ seals but never used them.

 

[davesparks]

No idea , I do remember Pyrotenax's heatshrink pot/ seals but never used them.

I keep a few of the heatshrink seals for emergency, 1 size fits most, type repairs.

I got one of the wedge pot tools in a box of second hand tools a few years ago but have never used it or even found new wedge pots.

 

[7029 dave]

I keep a few of the heatshrink seals for emergency, 1 size fits most, type repairs.

I got one of the wedge pot tools in a box of second hand tools a few years ago but have never used it or even found new wedge pots.

Can you still buy them, struggled to find them got a pic to show other members if they haven't seen them before.

 

[davesparks]

Can you still buy them, struggled to find them got a pic to show other members if they haven't seen them before.

They aren't manufactured anymore.
I keep an eye out for new old stock appearing on ebay.

I've collected a stock of cable and fittings from various people clearing out their stores.

 

[Boonsy641]

Good, this clamp makes the earth connection to the copper cable and the circuit must not be energised without it fitted.


I can't make out the 3rd core for either of them.
Has anyone said what that thrid core is being used for?


It is very hard to make a repair if it is snapped off at the pot, but not always impossible.

Cutting back 2 feet is unnecessary, it will only need to be cut back enough to expose enough of the snapped off core to be able to join onto it.
Has anyone carried out any testing yet to see if the cable is in useable condition or not?


The only real risk is that the cable may have corroded due to chemicals in the plaster. But it looks like it is plastic coated MICC which has just had the last inch or two removed, you can see the orange outer sheath on one of them.
The sheath will have protected the buried cable from corrosion.

Hey Dave. Trying to attach further photo

They look like wedge pots rather than the usual screw on pots, were they ever used on metric cable or were they older than that?

hi dave struggling to get low resolution images attached due to size. Defo 3 core each cable physically counted hopefully attached will show. Please see right cable. If you zoom in you can see ehats left of small cable. Electrician that came around said that if was to reterminate then the cables may be brittle in which case may not be able to reterminate at all in which case it defo would mean rewiring. At the moment he says the cable that broke is not live but is the common core that goes to next light in the loop. It connects to light switch and feeds of the live that is already there. He says best bet is to run a new twin and earth from the circuit breaker to the light that isn’t working but that would mean conduit across ceiling as in these flats the ceiling is concrete. I really would like to reterminate the pyro but worried about it being brittle as he says and making it worse

 

[Boonsy641]

They look like wedge pots rather than the usual screw on pots, were they ever used on metric cable or were they older than that?

Dave please see pictures

 

[littlespark]

For the photos, what I do when I take a photo with my phone is to screenshot it, then crop the screenshot down to just the original photo.
On my iPhone, that brings the image size just low enough for the forum to accept.

Do you have a photo looking up into the seal side of the pot, showing where the core is broken?

I still think you might get enough of a stump of copper for a very small connector to fit on…. Even the Ideal push fit type, rather than a block connector.

 

[Boonsy641]

For the photos, what I do when I take a photo with my phone is to screenshot it, then crop the screenshot down to just the original photo.
On my iPhone, that brings the image size just low enough for the forum to accept.

Do you have a photo looking up into the seal side of the pot, showing where the core is broken?

I still think you might get enough of a stump of copper for a very small connector to fit on…. Even the Ideal push fit type, rather than a block connector.

Thanks. That works. Please see. Just see the stump

 

[Boonsy641]

For the photos, what I do when I take a photo with my phone is to screenshot it, then crop the screenshot down to just the original photo.
On my iPhone, that brings the image size just low enough for the forum to accept.

Do you have a photo looking up into the seal side of the pot, showing where the core is broken?

I still think you might get enough of a stump of copper for a very small connector to fit on…. Even the Ideal push fit type, rather than a block connector.

Can you recommend a connector? I’ll try it myself I think. Many thanks

 

[davesparks]

Defo 3 core each cable physically counted hopefully attached will show. Please see right cable. If you zoom in you can see ehats left of small cable.

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.

Electrician that came around said that if was to reterminate then the cables may be brittle in which case may not be able to reterminate at all in which case it defo would mean rewiring.

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

At the moment he says the cable that broke is not live but is the common core that goes to next light in the loop.

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.

 

[davesparks]

Can you recommend a connector? I’ll try it myself I think. Many thanks

The only connection you could make onto that is by, with great care, cutting the plastic stub back and soldering then heat-shrinking.

There's no mechanical connector I can think of that will make a good connection to that tiny stub