RCD and SPD do not protect against electrical fires.

RCDs are pretty good at spotting insulation breakdown that can lead to fires (both with and without arcing) especially where there's an earthed part in close proximity to the basic insulation - and that's reasonably common - not just in steel conduit and metal clad accessories, but even the humble T&E cable with an uninsulated c.p.c. is pretty likely to produce a detectable earth fault if damaged to the point where fire is likely. For a long time now the regs have recognised that ≤300mA RCDs provide some useful protection (e.g. 422.3.9, 705.422.7).

.I suppose that SPDs may also provide some protection against the initial insulation breakdown that could lead to the parallel arc situation that AFDDs might be able to detect. So also providing some protection, if in a avoidance rather than detection form.

   - Andy.

????????

How common is insulation breakdown in PVC? It happens with old rubber insulated cables but it's rare to find any still in use nowadays.

There is the possibility of PVC insulation burning as a result of arcing, leading to a low L-E insulation resistance, where a sufficient current flows to trip a RCD before the MCB.

I suspect insulation break down in well installed PVC or polyethene is almost non-existent, unless there is some other factor.

I'm thinking of things like cables exposed to bright sunshine lose their flexibility, and will perhaps crack if flexed - a problem for T and E on catenery perhaps,  but given the UK weather, only in on a timescale of the odd decade, and maybe never if on the shady side of the building.

Damage by heating for wires stuffed in beside the pipes to the radiators is something I have certainly seen more than once, but neither of these are examples of 'well installed'.  The insulation test may or may not find either of these examples - a crack in a cable or a bit of browned off plastic, may be in a pre-cursor to trouble, but does not reliably result in a low resistance between L and E.

Mike.

I suspect insulation break down in well installed PVC or polyethene is almost non-existent, unless there is some other factor.

I'm thinking of things like cables exposed to bright sunshine lose their flexibility, and will perhaps crack if flexed - a problem for T and E on catenery perhaps,  but given the UK weather, only in on a timescale of the odd decade, and maybe never if on the shady side of the building.

Damage by heating for wires stuffed in beside the pipes to the radiators is something I have certainly seen more than once, but neither of these are examples of 'well installed'.  The insulation test may or may not find either of these examples - a crack in a cable or a bit of browned off plastic, may be in a pre-cursor to trouble, but does not reliably result in a low resistance between L and E.

Mike.