Very interesting.I would add that ,since plastic is technically a non-conductor of electricity, would it be more correct to describe these Crabtree makes as "non-plastic"? They must have contained a component or additive that allowed for conduction.
There are many different plastics with very different characteristics, but electrical accessories like traditional UK light switches made of hard white 'plastic' aren't really plastic at all, but thermosetting resins with fillers (thermosets.) There are many different permutations of resins and fillers, for example one might use a phenolic resin with wood flour filler for a non-technical application, but prefer an inorganic filler for an electrical component. This makes a difference to the moisture absorption properties, e.g. some thermosets lose much of their insulation resistance when exposed to damp heat, because they are microscopically porous and absorb a measurable amount of water. Others are more resistant and as soon as the surface is dried off the insulation is good again. FWIW, 50 years ago in East Germany, there was a coding system where the type of resin and fillers used in the mix had to be identified on a product and were moulded into the approval mark. You would see these on plugs, sockets, switches etc.
Electrical components, accessories and importantly PCBs are (or should) be designed with minimum creepage (over the surface) and clearance (through air) spacing between parts that are supposed to be insulated from one another. The combination of material surface resistivity, creepage distance, environmental pollution and humidity determine the insulation resistance achieved in practice, and all should be considered in the design stage. Unfortunately sometimes things still get overlooked; a capillary such as Brian describes where a tiny amount of free liquid can be drawn across a long creepage path, or a material with an unsuitable surface that occludes moisture and dirt, carbonises and tracks across, even metal migration.
I have some connectors with paxolin insulators (same material as the brown part of the back of this switch) which is resin-bonded calendered paper, which have trees of metallic leakage paths across the surface where silver has migrated from one pin to another under the electric field. Remember when 'NON-TRACK' used to be moulded into some fittings like junction boxes? That's what it was referring to.
As for bulk resistivity, we've seen on this forum some thermoplastics with unsatisfactory performance. I think one was an IEC power cordset where people were getting shocks off the dry external surface of the connector. It was probably a substandard product with fake approvals, where the moulder had used an incorrectly graded recycled PVC feedstock containing carbon black.
