Given that shower isolators are prevalent and a handy means of switching at the bathroom; there are probably many that give satisfactory service, I suggest that the problem is with limp wristed installers or the switch location being covered up with excess, thermal insulation. The ones that simply switch pressure pumps are probably most troublefree, having a lower current rating.

Jaymack   

Given that shower isolators are prevalent and a handy means of switching at the bathroom; there are probably many that give satisfactory service, I suggest that the problem is with limp wristed installers or the switch location being covered up with excess, thermal insulation. The ones that simply switch pressure pumps are probably most troublefree, having a lower current rating.

Jaymack   

Well, I went to CEF (only buggers open this week) and picked up a Click isolator - no 2 screw option though, only a single screw for each terminal. I fitted the thing but am forecasting that it won't be long before I have to go back and fit another. Shower is a Mira jobbie but I couldn't find the KW rating label on it anywhere. All I know is that it was fitted roughly 9 months ago.

A resistance measurement of the elements might have given a fair indication. There does seem to be some corelation on the few I`ve tested, enough to hint at an estimation anyway.

In fact I did the measurements of a couple of 3 element types and noted some kind of corelation as being "within the parish" sort o` speak. The reason I was doing it was to calculate a reconfig to put elements in series to get an idea of the likely resulting KW for a lower power in line water heater jobby.

Jaymack said:

I suggest that the problem is with limp wristed installers or the switch location being covered up with excess, thermal insulation.

The general observation being it's "usually the Neutral" that shows signs of overheating, sort of leads us down the path that the problem is not only due to "limp wristed installers" as you put it (although overtightening could also be a problem ... having said that, now more manufacturers are specifying torque settings with most of their range, that ought to be addressed over time).

Just wondering, though, whether installers check the shower end of the cable too - to check whether it's overheating at that end as well? I've certainly seen Neutrals at the pullcord and shower overheat at the same time, with cables arguably right at the limit to comply with BS 7671, but not under-rated, if you ignore a very short - < 300 mm -  through thermal insulation.

I am erring towards the N not checked on testing scenario as being the leading factor. I think the only case where L might be longer than N is in lighting switch circuits but not so much the reverse and therefore N being longer. Yes I`d be rather inclined to suspect the N not checked as much for being the main deciding factor if most other things are equal.

I checker the shower-end and all was well. The cable looks to be 10.00mm but I couldn't find a KW rating label on the shower unit itself. The old isolator was a Contactium Inspire range one. I don't see many problems with Contactum gear as a rule, but can no longer get it.

We must also take into account the gradual dilution of product quality which has taken place over decades due to value engineering exercises mandated by bean counters. Smaller steel cage terminals with poorly plated steel screws which have a different thermal coefficient of expansion/contraction to that of copper which doesn't  help.

MK, the former Rolls Royce of electrical accessories is a prime example of this. I won't fit MK any longer as a consequence. Even their Logic range of light switches seem to fail with monotonous regularity.

whjohnson said:

We must also take into account the gradual dilution of product quality which has taken place over decades due to value engineering exercises mandated by bean counters. Smaller steel cage terminals with poorly plated steel screws which have a different thermal coefficient of expansion/contraction to that of copper which doesn't  help.

Whilst I don't want to discount this observation, I'm interested in how "dilution of product quality" explains why "it's the neutral that burns" ... well at least most of the time, if the general observation (without statistics) on this observed phenomena is true?

If product quality were the issue, then surely we'd expect to see as many Lines burning as Neutrals?

gkenyon said:

If product quality were the issue, then surely we'd expect to see as many Lines burning as Neutrals?

I suspect that there may be some confirmation bias going on.

And of course - like the washing machine repair man, firmly convinced that all modern machines are rubbish as he only sees the broken ones, we do not count all the ones we go past that are working just fine. Especially for pull switches that get damp from below and cold air from the loft coming in with the cables from behind, the conditions are good for a condensation problem, and that may well finish otherwise perfect contacts.

I am not sure if any of the switches where whcih pole is "N" has been  defined break neutral after live to allow the neutral contacts to be undersized - it is not true in DP sockets, where the L and N rockers are the same,  but in 3 phase switch gear having an odd-legs neutral is not uncommon. NExt time someone is staring at a hot neutral shower switch, if they could take a hammer to it, and compare the weight of the metal work on the N and L sides it would be very interesting.
Mike

I would suspect that the dissimilar metals thing is indeed a contributory factor, especially when we move further away from copper to steel. All-brass construction didn't seem to be as big or common a problem back in the day. If it burned, then usually it could be put down to a loose connection. What I have found is that the steel screws are still tight when it comes to changing out the faulty item - not burnt/welded tight due to molten material clogging up the threads, but mechanically tight. So suggestion is that the cause was a high resistance connection inside the unit itself.