[Lucien Nunes]

I'm not sure whether it was I who mentioned bootlace ferrules, but they are preferred when connecting fine-stranded (flexible) cables to some types of terminals that are intended for solid or 7-stranded installation cables. Those in certain brands of cooker switch and outlet box, for example, tend to shear off some of the strands or allow them to escape past the screw, when terminating fine stranded flex. Cage-clamp terminals and stud terminals with cable-retaining cup washers are usually OK for either stranded or flexible without rings or ferrules.

Insulation of sheathed heating elements has always been a rather variable quanitity. Some deteriorate with age, disuse and exposure to moisture without any tendency to improve with use. Others recover to nearly their original value if they are regularly run up to full temperature, although during warm-up the insulation may plummet to a few tens of kilohms as the accumulated moisture is driven from the warm middle to condense at the cold ends. Then once the element is fully hot it begins to redistribute again, slowly escaping into the atmosphere. It's the warmup process that is most likely to trip an RCD, which of course prevents further drying of the element to the point where the insulation is high again.

There is no universal requirement for a fixed appliance to be RCD protected, provided certain criteria are met within the installation. This will include a suitably low earth-loop impedance (not usually achievable on TT / rod earths) and cabling installed in such a way as to be unlikely to sustain hidden damage (e.g. not buried less than 50mm deep in a wall). Any point leaking more than 10mA should have a high-integrity earth connection.