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]

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?

Petrol stations

 

[pc1966]

Petrol stations

Good call, that and refineries / chemical works might also come in the the list.

In fact any case where really high reliability is needed.

 

[ipf]

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

Right or wrong, that is because of the electrical trade in general and it's views regarding other cheaper and inferior methods of wiring being produced and introduced as replacements.
For the last 40 years we've been subject to so many alternatives. Ease for labour and pocket, being the main reasons why. Still nothing to rival it.
If nobody had ever seen or heard of the stuff and it suddenly came out on the market today, it would probably warrant it's own apprenticeship.

 

[ipf]

Paint mix areas, boiler house systems, glass fibre production areas, fire stations.

 

[Boonsy641]

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.

You anywhere near Hertfordshire? I really don’t want to rewire the flat because of single strand....failing that where or how do I find the measurements of existing imperial pyro such that I can convert and get new termination kit... many thanks

 

[Boonsy641]

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.

Sorry about my posts, I sound like a pining teenager.. having come across all you guys I am confident I can get micc imperial repaired.. i have an electrician coming around today but over the phone he says it’s a rewire job.. and not knowing I would have taken his word... mainly him suggesting because you cannot get termination kit for imperial these days. His words.. I don’t doubt his ability at all, but I do think he has perhaps used my lack of knowledge and worry over this to make it a larger job.. I just can’t believe that because 1 single strand of the micc cable has broken near the pot that it means rewiring the flats lights... I will take some photos of the MICC connection if ok and post them, if you all don’t mind I will then ask your advice on size of pots I needs etc (imperial to metric conversion).. I’ll get it all, and then I just need someone to do it.... thanks.. glad I found this thread

 

[littlespark]

There could be ways to solve your switch problem without working on the MICC.

Do you know which core is broken? Live? Switchwire?
Could you do away with the switch altogether and use a “quinetic” switch instead?


You don’t want an inexperienced electrician messibg with MICC. If this one thinks it can’t be done, he could make a hash of it.

Sorry to say, it’s definitely not a diy job

 

[Boonsy641]

There could be ways to solve your switch problem without working on the MICC.

Do you know which core is broken? Live? Switchwire?
Could you do away with the switch altogether and use a “quinetic” switch instead?


You don’t want an inexperienced electrician messibg with MICC. If this one thinks it can’t be done, he could make a hash of it.

Sorry to say, it’s definitely not a diy job

Oh totally agree. I have researched enough to see that I need someone who knows what they are doing that’s for sure. I see him in an hour so I’ll sound him out. The Quinetic switch sounds interesting I don’t know what that is but will research. I think the wire that broken is but the wire for the immediate light it operates. I think it is for the two lights that follow in bathroom and bedroom as they are not working because of it. As I say I think I can source a gland and pot and tool for that matter. Just need someone to do the job. I hate not knowing and really am in hands of professionals but don’t know if the advice they are offering is to benefit them or me. Hence I am pleased finding this forum and you guys. If you can offer any suggestions I would be very grateful. Having redecorated just and spent a fair amount getting the flat ready for sister. There’s not a lot of cash to pay for a rewire that’s for sure

 

[davesparks]

Could you do away with the switch altogether and use a “quinetic” switch instead?

Most flush MICC installs of that era I've worked on used shallow boxes which had the built in clamp to hold the pot. If that's the case here it would be quite a bit of work to get a deeper box in there to take anything other than a standard plateswitch.

 

[davesparks]

. I just can’t believe that because 1 single strand of the micc cable has broken near the pot that it means rewiring the flats lights..

It is always possible that this could be a symptom of a bigger issue and rewiring is necessary. But if it is a simple case of someone has done something to damage this one cable then it should be repairable.

. I will take some photos of the MICC connection if ok and post them, if you all don’t mind I will then ask your advice on size of pots I needs etc (imperial to metric conversion).

Yes, post up some clear pictures so we can see exactly what has happened and get an idea of what repair is required.

Is it the MICC cable which has been damaged or is it one individual conductor within the switch box that has been damaged?

Wherabouts are you?

 

[davesparks]

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?

Licence conditions on some West end theatres still require it, or required it up until recently, which is why I'm still working with it semi-regularly.
Although with some of them it's more a case of in-house staff hanging on to old rules and refusing to keep up to date, some of them still have a fit if you dare to bring 2 phases within 6 feet of each other.

The natural history museum also specify it for all of the areas with exposed stonework. You can see some fairly recently installed MICC in there which has been done to a very low standard, nowhere near straight and no thought to aesthetics.

 

[Boonsy641]

Most flush MICC installs of that era I've worked on used shallow boxes which had the built in clamp to hold the pot. If that's the case here it would be quite a bit of work to get a deeper box in there to take anything other than a standard plateswitch.

Some photos of the switch and connection

 

[davesparks]

Some photos of the switch and connection

Do you have the front half of the pot clamp? I assume this circuit is currently isolated if the clamp is missing?

Wherabouts is the damage, it doesn't look like anything is broken as far as I can see.

 

[Boonsy641]

Do you have the front half of the pot clamp? I assume this circuit is currently isolated if the clamp is missing?

Wherabouts is the damage, it doesn't look like anything is broken as far as I can see.

If you mean the cover that goes over these two cables yes I do. I unscrewed the cover and this was what lay beneath. They are both 3 core, the right hand one you can only see two cores and the third basically snapped off so you only see two. I had an electrician around who seemed to know his stuff, I think. He said not enough left of R/H 3rd core to do anything with as too short. Said that pots etc can get online also said the cable would need to be stripped back about two foot as to ensure any moisture is taken account of. He said copper becomes brittle so no guarantee when he digs out two foot of pyro that he will be able to terminate as it may be too brittle to do so but that is a risk that I must consider. He said that it would be terminated if not brittle to a sealed junction box and then twin and earth to the light switch. The big concern was if the pyro was brittle and it could not be terminated it would definitely need a rewire. The core cable that came away was for feed to other lights in the circuit I.e. bathroom and 2nd bedroom. He said that pyro was encased by concrete and that would be a big job to trace. I guess the concern was the cable being potentially too brittle to terminate if cut back

 

[davesparks]

If you mean the cover that goes over these two cables yes I do.

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

I unscrewed the cover and this was what lay beneath. They are both 3 core, the right hand one you can only see two cores and the third basically snapped off so you only see two.

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

I had an electrician around who seemed to know his stuff, I think. He said not enough left of R/H 3rd core to do anything with as too short.

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

Said that pots etc can get online also said the cable would need to be stripped back about two foot as to ensure any moisture is taken account of.

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?

He said copper becomes brittle so no guarantee when he digs out two foot of pyro that he will be able to terminate as it may be too brittle to do so but that is a risk that I must consider.

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.