Terminating Pyro cable

I am being asked to re-terminate some pyro cable because of a broken cable and replacing a diy joint made with electrical tape, not something I have done before. Watched some videos and the process doesn't look too difficult, will get some materials to practice with first.

Now some questions  

The cable is 50 or 60 years old, therefore presumably an imperial size with cores close to 1mm, I can get a micrometre on them if the job goes ahead. Can I use a metric size gland close to the imperial size. Alternatively the gland is marked L238, I can find RGM238 glands, cable pots etc  on ebay, presumably more or less the same thing. This is for use in a church, so not hazardous but damp.

I am looking at also buying suitable tools on ebay, kits seem to go for £50 to £90 for used, looking to save money and finding a new imperial pot crimper would be challenging.

Appreciate any advice or tips that can be provided.

  • Usually the three-figure numbers on old MICC imperial sized glands were the OD of the cable in thousandths of an inch - so 250 would be quarter inch, and 238 is 5.84mm or 15/64ths of an inch. I'd still check before cutting anything, but that is a starting point. The metric olive will not really fit imperial tube properly, but the rest of the bits in a 6mm metric  kit might.

    Makers datasheets, https://www.pyrosales.com.au/media/blfa_files/IMPERIAL_CABLE_DATA.pdf  suggests you can solder rather than crimp a metric pot to an imperial cable in some cases, presumably to make up the tenth of a mm of slack.

    Soldering saves on the pot crimping generally but things do need to be shiny clean and snug, and the angles need to be right with just a little flux so the solder wets the joint without flooding it.- not a commonly used option.

    Also if it has been open to air for a bit, unless it is super dry, then  increase the chance of passing an IR test by using a hot air gun or gas torch to stroke any absorbed moisture back out of the open end prior to sealing it.



  • Thanks for the insight Mike. I am competent at soldering from plumbing etc but I think that using the proper glands, which are available would be the best option. They cost more but a fairly small incremental increase on the project cost.

    Do I have much chance of getting old glands off, or do I assume I will need to cut the cable.

    Just looked at the data sheet you suggested again. By the look of it as I have 2 core cable, if I emery down the outer a bit I can use a combination of metric parts. Not sure how I feel about using emery paper and potential contamination of the insulation with copper.

  • What follows is my very limited experience, and others who have done more of this may well want to disagree or chip in with other hints and tips.

    Getting old glands off can be very hit and miss.  If it has been gorilla tightened then the olive may have been undercut into the outer, but if the recommended torques were used, it should slip out with a bit of friggling.

    Removing old pots is usually destructive - the filling compounds have varied over the years from something  like blue tac that never sets and can be dug out, to a rock like  epoxy that never moves again. That stuff is in dremel country sadly. In any case the thee point crimp, unless very badly done, should be a one-time event,.

    If you need to re-gland and end up just a bit short, but can rescue a connection to the cores, then a common dodge is to extend the gland away from the box with a short length of conduit plus the right sex of threaded nipple or barrel as required.  This leaves a length of free tails in the conduit so they need to be sleeved, but done well this is not an issue. Some installations were like that originally as it gives room for some slack for remakes.

    The emery thing is interesting - I've not tried it, but I might prefer to ease open the pot, rather than thin the cable, as it is easier to control, but I'd share your worry about the wisdom of thinning the outer and contamination with metal dust after it has been cut to length, perhaps do it before peeling the last little bit of the outer.

    I raised my eyebrows at the solder thing too, but a colleague later told me it was actually preferred at some of the early nuclear sites as giving an all-round gas tight seal.


  • Soldering of Pyro glands works fine provided that there is no vibration or movement. Soldering changes the "temper" of the metal and can result in fractures after vibration or movement.