Hi David,
Welcome to ECN.
The problem you describe sounds like migration of the plasticiser in the pvc insulation.
This occurs as a result of thermal cycling of the conductor due to changes in load and conductor temperature drawing the thermodynamically unstable insulation plasticiser to the copper conductor.
The sticky blue/green liquid is not corrosive and can be cleaned with methylated spirits.
There is no cure for plasticiser migration, other than replacement of the cable.
This migration only affects older PVC cable (pre mid 70's), as newer cables use refomulated pvc compounds.
I would strongly advise you to have a qualified electrician inspect & test your wiring to ensure that it is still in safe condition.
I have seen this problem several times, but usually the sticky liquid stays inside the mounting box.
Hope this helps.
