My contact in the social housing provider's maintenance team has reported back that there is indeed a sizeable number of shower pullcord switches with thermal damage with a disproportionate number where the neutral is burned back. He tells me that often bubbling is evident on the surface face of the pullcord.

I am sure that if someone in IET wanted to establish more precise facts, he would be willing to engage.

I have replaced a few of these in my own home (around one every couple of years), such is the paranoia I actually keep a new one in stock in my garage. When I change them the cores are not found loose. When the new one goes in I take time in manipulating the cores in the box for the least stress, push the switch into the box then lower it just enough to get a screwdriver on to recheck the terminal screw tightness. I have taken old switches apart and generally the switch contact faces are blackened, mostly on the neutral.

Interestingly if you consider the operation of the shower; current breaking at the ceiling switch should not be a problem because everyone turns the shower off at the shower then at the ceiling switch. Logically at the time the ceiling switch is operated you are only switching off the neon lamp within the shower that indicates the power is on.

Not sure if anyone else has taken an old shower apart for fun, sadly I have. When you switch it on a solenoid valve operates which admits the water to the heater and this has a pressure switch on it that switches the heater elements in. This is done for safety so that the heaters cannot be on unless the water flow will be adequate to prevent overheating.

From memory inside the shower unit only the live is switched on the heater and solenoid valve and the wiring in there is not particularly large, certainly smaller than the 10mm^2 supply cable. I am wondering if there is something else going on, perhaps due to the inductance/capacitance of the heater elements, is the damage to the neutral contacts occurring when the switch is opened?

If the neutrals are being disproportionately affected, why? The current is the same. The terminals are not obviously smaller. The conductors are the same. If any current were being diverted down the CPC the RCD would trip, but if not fitted, the current in the neutral would be reduced compared with the line. So let's have an explanation please!

Another variable to chuck into the arena is this - some folk religiously switch on and off their shower isolators after each use whilst others are content to just leave the switch in the ON position until something fails, yet we still see problems regardless of frequency of use or lack of.

The one I did recently cannot have been touched in the 9 months since it was installed - it being sited behind a heavy double wardrobe, yet it still burnt out. Same with cooker switches unless they incorporate a socket and are situated on the wall above worktop height - most people seem to leave them permanently ON, and those buried inside kitchen units never get touched at all.

It is a fair point that some of these switches are not used but still exhibit the same issue, my comments were based on how I use my shower.

It has always amazed me the amount of effort put into choosing cable when it comes to loading and heat rejection in comparison to the actual contact area within switches. Perhaps the heat is generated there and conducts to the cable terminal. Not sure why this would only affect the neutral though. Does anyone fancy changing their house to 230V two phase power to see what happens to their shower switch?

It is a fair point that some of these switches are not used but still exhibit the same issue, my comments were based on how I use my shower.

It has always amazed me the amount of effort put into choosing cable when it comes to loading and heat rejection in comparison to the actual contact area within switches. Perhaps the heat is generated there and conducts to the cable terminal. Not sure why this would only affect the neutral though. Does anyone fancy changing their house to 230V two phase power to see what happens to their shower switch?