whjohnson said:

Given that on Xmas Eve was called out to yet another example of an electric shower isolator burning off it's neutral at the terminal, is it time to cease fitting isolators for the likes of ovens and showers?

To answer that question, we need to get to the bottom of what's happening in that particular case(s).

And I think the answer could be, at least in some cases, that the Neutral conductor gets slightly hotter than the line conductor for shorter lengths of run from shower to isolator. Copper is a good conductor of heat as well as electricity, and, depending on installation method, if there's not a lot of opportunity for some heat to be lost through the sheath, if the cable is close, or over, its 70 deg C operating temp for its current carrying capacity, the isolator switch (and possibly shower terminations) are overheating.

Why more often Neutral and not Line under these conditions? On a number of electric showers, the Neutral comes straight off the element via a very short wire to the fixed wiring terminal, whereas there's a loop through the thermostat/over-temp cutout, and switching, internal to the shower.

So, it's possible in some cases it's cables near, or above their conductor operating temp for one reason or another, rather than a particular problem with isolators?

Other cases ... well, cable access vs enclosure size etc. has often been stated.

But I think more evidence/statistics of the real causes might be needed to determine what ought to be advised?

gkenyon said:

But I think more evidence/statistics of the real causes might be needed to determine what ought to be advised?

I will have a go at getting that, at least in terms of how common it is.

I took the electrical team of one of the biggest social landlords in the U.K. through the 2391. They told me that one of the top maintenance issues found during inspections was what they called “neutral burn-back” at the shower pullcord. 

I will seek to get further info from the section manager. 

gkenyon said:

But I think more evidence/statistics of the real causes might be needed to determine what ought to be advised?

I will have a go at getting that, at least in terms of how common it is.

I took the electrical team of one of the biggest social landlords in the U.K. through the 2391. They told me that one of the top maintenance issues found during inspections was what they called “neutral burn-back” at the shower pullcord. 

I will seek to get further info from the section manager.