1970s PVC insulated cable which has degraded.
This is an example of flat twin and earth cable manufactured around 1970.
PVC cable insulation consists of the PVC itself and a plasticiser. The PVC alone would be a fairly hard, inflexible material which is used for conduit, pipes, window frames and similar building applications. The addition of plasticiser changes this into the flexible material commonly used for cable insulation.
With excessive prolonged heating of the cable, the plasticiser may migrate out of the cable insulation, forming a sticky residue. Normally, cables will not be overheated, so this problem will not occur.
However from the mid 1960s to early 1970s, an antioxidant was included in the cable insulation, and this had the unwanted effect of causing the plasticiser to leach out of the cable even at ambient temperatures.
The plasticiser itself is colourless, but a chemical reaction with the copper wire creates a green colour.
The example shown in the video is cable installed in 1970. The original socket outlet was replaced in 2011 so the amount of gunge visible is only what has collected during the last three years.
The green slime can be cleaned away easily, however this only postpones the inevitable - there is no way to fix the problem other than replacing the cable. For installations such as the one in the video, this will involve rewiring the entire property.
The green slime is unsightly, and in extreme cases will cause cosmetic damage to sockets, switches or even the wall to which they are attached. However the performance of the cable and insulation is not usually affected, unless the problem is so severe that is has made the cable insulation brittle.
It is sensible to wear gloves when removing the slime or affected cables, as the slime may cause skin irritation. The slime will also stain fabrics and other materials, possibly permanently.