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rubber insulated cloth covered tinned copper submain cable in galv. conduit.

Legh Richardson
Had a look at an installation which has a submain running approximately 25m to supply a kitchen.

We thought umm this looks interesting, cut a slice off at the output of the switched fuse and re-terminated. The cloth, and rubber appears to be in reasonable condition for what appears to be nearly 70 years old. An insulation test at 500V gave a reasonable result (>500M) and a leakage test gave the same, 0.41mA.

The cable is out of date but appears to be still serviceable. It is not heavily used and the ambient temperature is generally quite low. I think I might advise that the cable is replaced by a newer edition or at least regularly checked every year. Has anybody got any other ideas?


Legh
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    If the insulation at both ends is still flexible and in intact, and it all tests out OK I'd stuggle to find a reason to condemn it. If it's survived 70-off years and conditions remain about the same I don't see it suddenly failing catestrophically any time soon. I'd certainly make a note about it (and your findings), maybe suggest a slightly shorter interval between inspection (maybe 5 years instead of 10?) but I wouldn't worry unduely.


      -  Andy.
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    Legh Richardson:

    The cloth, and rubber appears to be in reasonable condition for what appears to be nearly 70 years old. An insulation test at 500V gave a reasonable result (>500M) and a leakage test gave the same, 0.41mA.

    The cable is out of date but appears to be still serviceable. It is not heavily used and the ambient temperature is generally quite low. I think I might advise that the cable is replaced by a newer edition or at least regularly checked every year. Has anybody got any other ideas?

     



    Legh, the enemy of this cable includes heat, causing brittleness of the insulation. This cable insulation seems to be still fit for purpose ......... at present; but if the user increases the loading? I would recommend that It is replaced asap. 


    Jaymack
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    Old rubber insulated cable, including cotton covered or rubber insulated in a lead sheath, lasts for many years. This cable in a conduit is similar in that it exists in its own protective "micro climate". It is normally at the ends that changes of temperature and humidity cause rubber deterioration. I have seen just the ends of such cable oversleeved with P.V.C. sleeving after just the ends deteriorated. I inspected a house in Sheringham a few years ago, and the main runs of the old rubber insulated cables were in a remarkably good condition considering that they were probably installed in the 1040s/50s. They were still flexible and the insulation resistance was very high. The rubber insulation was remarkably think. The old boy owner would not have a rewire, all I could do was to improve the earthing for safety.


    Z.
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    Ok, thanks chaps, as I suspected it is a boarderline case, the cable appears serviceable but is regarded as not suitable by todays standards. I will now take a view.


    Legh
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    I was asked to investigate kitchen sockets that had had been causing the fuses to blow. Unscrewed a socket, pulled it out as you do. The VRI insulation crumbled leaving bare wires - gingerly screwed the socket back in and .................Ran away!. (Some mistakenly state VIR). 


    Jaymack
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    Jaymack:

    I was asked to investigate kitchen sockets that had had been causing the fuses to blow. Unscrewed a socket, pulled it out as you do. The VRI insulation crumbled leaving bare wires - gingerly screwed the socket back in and .................Ran away!. (Some mistakenly state VIR). 


    Jaymack




    "Run away" you say?  - oh dear, lol. The quality of both the insulation and cloth covering at and around the terminals is still flexible as there is quite a bit of spare left in a rather large galvanized switch fused enclosure. That surplus allowed me to take a biopsy! .the cable available to be viewed appears to be healthy for its age; I can't say what the quality is within the conduit but as Zoom has stated it has its own protected micro climate.


    Legh

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    Legh Richardson:




    The quality of both the insulation and cloth covering at and around the terminals is still flexible as there is quite a bit of spare left in a rather large galvanized switch fused enclosure. That surplus allowed me to take a biopsy! .the cable available to be viewed appears to be healthy for its age; I can't say what the quality is within the conduit but as Zoom has stated it has its own protected micro climate.

    Legh




     




    Legh, the enemy of this cable (and any others) is heat, caused by loose connections or the cyclic nature of the load, causing expansion and contraction at screw terminals, the connections at sockets etc. are then vulnerable. If not checked at routine intervals and tightened when loosened, you get an escalating resistance effect. VRI cable insulation is particularly vulnerable to embrittlement, as you know. It is probably OK inside conduit, provided it is not subject to a heat causing, chimney effect. 


    Jaymack  

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    I noted the same with the TRS cable in our house. The ends were perished to all hades, and held together with sleeving, tape, and paper in a couple of instances... the main runs through the attic were fine. Rubber can be remarkably resilient. The fact it is in metal conduit also adds to the safety. Ensure the conduit is well bonded (given the age, i'm assuming it forms the CPC anyway?), take some high current Zs readings, to make sure corrosion isn't messing up the continuity...Any breakdown in service will simply cause ADS to occur as expected. In short, it's fine.
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    MHRestorations:

    I noted the same with the TRS cable in our house. sharpened tile bits for tiles



    TRS is Tough Rubber Sheathed cable and not the same as Vulcanised Rubber Insulated cable.


    Jaymack
  • 0
    One would have thought it was easy to find a 'responsible person' to be the maitenance duty holder of a village hall. I was told by a person who is on the committee that the person ordering the work is the committee and all correspondance should be by email only. There is no post box facility at the village hall anyway but the person  would niether provide her name or a postal address. This one's a first for me as I usually deal with people face to face. I can also appreciate that there maybe issues associated with privacy. I suppose it will be up to the insurance company to decide whether the 'committee' is a suitable person ordering the work.


    Legh

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