Question - The cable inside a light switch is covered with a green-blue �gluey� substance. A similar liquid can be found in the junction boxes in the roof space. It looks like the substance is on the cable insulation but under the sheathing. There have been no problems with the electricity supply to date and the cable, which is PVC, does not seem to have otherwise deteriorated.

Answer - This is a problem that affects some brands of cables used in the late 1960�s. The outer sheathing is normal PVC, but the red and black insulation around the conductors was made from a type of plastic called capathene, which has since been found to react with the copper wire to produce a green runny liquid. It is likely to have affected all the cables in the house, but is most often noticed on the cables connected to the light switches, as these drop down from the ceiling and the liquid runs out due to gravity. It is not necessarily hazardous, but you should have your wiring checked by a qualified electrician.

Article taken from the Sunday Telegraph 18 March 2007