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Tony S

Sheathed transformer tails

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Tony S

Sheathed transformer tails:


Each is a single core cable with an earthed outer sheath.


PILC (Paper Insulated Lead Covered)

PIAS (Paper Insulated Aluminium Sheaved)

AWA (PVC Aluminium Wire Armoured)



What ever the type the principal is the same, only one end is collectively earthed and by convention it is at the transformer end. The fun and games start when terminating the cables at the switchgear end, PILC and PIAS have to be plumbed to a wiping cone. The papers are then sealed by taping over half lap with resin impregnated silk* followed by filling the cones some form of compound.



This would be for a LV switchboad.







I suspect these are for 11kV 20 MVA 500mm². You can just make out the insulation at the top of the cone.


Whatever they are they are a work of art and a credit to the cable jointer.







The inside of a 11kV oil filled terminal box. Note the stress relief where the lead has been terminated.







AWA needs the armours terminating in to a gland of some form to prevent damage to the inner serving. Again each gland must be isolated from earth and each other.


The reason for insulating one end of the tails is to stop circulating currents in each cable sheath which may be equal to the current carried by the single core. The solidly earthed terminations at the transformer provide the fault path to earth. An advantage of the transformer terminations being earthed is the REF protection relay covers both the transformer windings and the tails.








For LV cables a Paxolin gland plate may suffice but given the forces involved during a fault it may not be sufficient to restrain the cables.


For PILC and PIAS terminations the separate brass wiping cones are insulated from the iron or steel gland plate and each other by a Tufnell or Paxolin pad. The retaining bolts, washers and nuts also have insulating sleeves and washers so the termination is entirely clear of earth.







There is the cheats method of plumbing the PILC and PIAS cable to the brass cone and it is the recommended method for Ellison switchgear.

  • Pack the bottom of the cone with wire wool or asbestos string. A small mirror is handy to see what is happening,

There’s now two options.

  • Chop up some lumps of grade D plumbing metal and drop them with some tallow in to the cone. Heat the cone until the metal turns liquid, remove the heat.
  • Melt the grade D plumbing metal in a metal pot and ladle it in to the cone.





*As I was writing about the resined silk I could remember the smell and feel of it, it’s soft, sooth, squidgy and fragrant and in a way very sensuous.


OK it’s time for a cold bath!




© Tony S

Photographs of the 11kV terminal box and the transformer earthing arrangement © RoB

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