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!

 

[tazz]

I think it proves the point, that there is a very big difference between the two cables, in the real world....LUL have always had their own specifications on the underground, as well as the own specifications on the fire alarm system...With Advanced fire alarm systems being the only system they will now use, being manufacture especially for LUL

 

[UNG]

just to keep the theme of old times Does anybody remember or indeed used, the crappy little modification that was brought out to try and reduce the potting procedure
It was a resin filled spacer come shroud type thing that had to be heated with a purpose made torch come oven
It never became popular or replaced the traditional method,

Ahhh the shrink on seals they were a good idea but the Pyro heatgun / oven / blowtorch thing they produced as the tooling needed was absolutely useless

When they first came out in about 1980 one of the new build Pilkington sites in St Helens was used as a test site and all the fire alarm pyro's were terminated with them. They also had an earth tail seal that was a shrink on job. Never heard of any problems and I assume they are still in place although the site was extended some years ago

They were very good for repairing old imperial pyros quickly as they would fit a number of cable sizes, when they were discontinued the pyro rep gave the company I was with at the time all the remaining stock from BICC in Prescot ( a briefcase full ) and they were slowly used up over a few years

but I would give them 1 out of ten for effort

I think a pyro terminated with a shrink seal is still a lot better than an FP cable, they could have done something better as in the early days they had a few issues with the blowtorches due to being cheap and nasty (loads were returned to BICC) and getting people to accept change (nothing new there) they were never going to be a big seller with people so used to the quality a pot termination gives. I liked them for certain things and used both pots and the shrink seals while they were available


D. Skelton good to see someone repairing a pyro rather than rip it out and a good write up too a bit of a trip down memory lane for me

 

[Geordie Spark]

nice one.

Oh for God's sake ....... don't make his head any bigger - he's struggling to get his gun turret lid fastened down as it is!!

Seriously though Rommel, thanks for taking the trouble to post that - I enjoyed it.

 

[Lucien Nunes]

If you get the job to do the upgrade, it would be interesting to find out what the IR is then. If the old seals on the other ends are tight that residual moisture might have dispersed far enough to give a nice high reading.

I always used to doubt that bit at first, where they said that even quite a low reading will come good over time as the moisture spreads out rather than being all in a ring around the end. But you only have to heat an old sheathed heating element to see how true this is. Could be megs when cold but while it's warming up the insulation drops to tens of K as all the moisture from along the length condenses around the cold ends.

Nice explanatory post there, hope it will persuade people to brush up on their MI skills.

 

[Boonsy641]

Last week I did a job for forum member joramtor. Link to the original post here: Electrical Blog - Electrical Advice | Free Electricians Advice Forum - http://www.electriciansforums.co.uk/electricians-unite-jobs-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:

View attachment 23494View attachment 23495


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.

View attachment 23496View attachment 23497


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.

View attachment 23498View attachment 23499


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.

View attachment 23500View attachment 23501



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.

View attachment 23502View attachment 23503


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.

View attachment 23504View attachment 23505


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!

View attachment 23506View attachment 23507


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

Hi are you still working with imperial micc. I have electricians saying to me just because of one strand of the micc cable has been cut that I need a new rearing job done. They say parts not available to reterminate imperial mic. This is for connection to light switch. You defo look like you know what you are doing. Where are you and can we chat please. Ime in Hertfordshire by the way. Thanks Andy.

 

[davesparks]

Hi are you still working with imperial micc. I have electricians saying to me just because of one strand of the micc cable has been cut that I need a new rearing job done. They say parts not available to reterminate imperial mic. This is for connection to light switch. You defo look like you know what you are doing. Where are you and can we chat please. Ime in Hertfordshire by the way. Thanks Andy.

Imperial MICC can be reterminated using metric parts, there is a conversion chart which shows which pots and seals are required.

You might have to shop around a bit to find an electrician who can do the job, but there are still a lot of us around.

 

[7029 dave]

Imperial MICC can be reterminated using metric parts, there is a conversion chart which shows which pots and seals are required.

You might have to shop around a bit to find an electrician who can do the job, but there are still a lot of us around.

MICC is classed as a specialised job these days, unbelievable it was once an everyday task for any Electrician back in the day.

 

[oscar21]

I've still got some pyro stuff in the shed, haven't touched it in years though.

 

[davesparks]

MICC is classed as a specialised job these days, unbelievable it was once an everyday task for any Electrician back in the day.

Unfortunately the price of it makes it hard to justify on a job these days, a 100m drum of 2l1.5 will set you back nearly £900 and a pack of 10 glands is easily £40.

I still work with it occasionally, usually repairs or small alterations/extensions to existing MICC installs.

 

[nicebutdim]

Unfortunately the price of it makes it hard to justify on a job these days, a 100m drum of 2l1.5 will set you back nearly £900 and a pack of 10 glands is easily £40.

I still work with it occasionally, usually repairs or small alterations/extensions to existing MICC installs.

Certain brands of pvc coated steel flexible conduit aren't much cheaper.

 

[davesparks]

Certain brands of pvc coated steel flexible conduit aren't much cheaper.

Wow, I rarely use any floppy conduit, which brand?

 

[nicebutdim]

Wow, I rarely use any floppy conduit, which brand?

Can't name brands, but prices vary wildly from wholesalers. Some stuff is reasonably cheap, but £45-£50/10m isn't uncommon.

 

[pc1966]

Wow, I rarely use any floppy conduit, which brand?

We have used this, I think we got it a bit cheaper elsewhere:

(1) x 64453710 & (10) x 55501340 | Lapp Flexible Contractor Pack Conduit, 27mm Nominal Diameter, Steel Spring Wire, Grey | RS

uk.rs-online.com

Works out at just over £30/m ouch!

At this point I'm sure folks are thinking "WTF why use that?" but we found it lasts outdoors when flexing, while most others we tried would split after a year or so, etc.

 

[nicebutdim]

We have used this, I think we got it a bit cheaper elsewhere:

(1) x 64453710 & (10) x 55501340 | Lapp Flexible Contractor Pack Conduit, 27mm Nominal Diameter, Steel Spring Wire, Grey | RS

uk.rs-online.com

Works out at just over £30/m ouch!

I use quite a bit of similar stuff, but at a much better price. The steel in that is more for support than protection.

 

[pc1966]

Unfortunately the price of it makes it hard to justify on a job these days

Yes, I would struggle to think of cases where MICC is going to be your go-to choice as the traditional fire alarm case is now almost always FP200 or similar. Off the top of my head:

• Churches and similar historic buildings where the bare copper would look the part, even if in reality the building pre-dated electricity by several centuries.
• Blast furnace and similar where operating at seriously elevated temperatures is needed.
• Nuclear sites (not just power stations, but some research or medical) where radiation tolerance is needed.
• Military situations where NEMP protection is needed.

Any other suggestions?