Broken shower pull switch

After using the shower last week I tried to turn the power off using the ceiling mounted pull cord switch but found the switch didn’t click like usual and seemed stuck. For a few days the switch had been quite stiff to pull, but still seemed to work OK. However, as you can see from the image above, something went very wrong with it. The last time I pulled the cord the power turned off, but the switch got stuck and was quite hot to the touch. I also noticed the crack on the casing and the slight brown mark too.

Today the electrician came to replace the switch and after he’d left I took some photos. Click on the links below to see them

Burnt out shower pull switch showing a crack in the plastic casing and a small brown mark.
Internal shot of the burnt out shower pull switch, showing extensive burning of the plastic.
Closeup of the terminals inside the burnt shower pull switch.

After taking the switch apart it became more clear why the switch had failed. The switch contacts hadn’t shorted together and most of the burnt plastic was localised around the screw terminal for the live wire coming from the fusebox. The electrician also told me that because the shower wires are quite thick it’s difficult to make them all fit into the terminal holes and tighten the retaining screw enough. So rather than the switch failing, which would have welded the contacts together, tripping the circuit breaker, the terminal must have been loose making the electricity arc inside the switch.

According to the British Gas energy meter I own, the shower draws around 10KW of energy when in use (it’s winter, we have the heat setting turned up quite high). While the switches are made from very thick thermosetting plastic and large pieces of copper I feel there is a design fault because this is the second switch I’ve known to fail in this way, and the electrician said it was fairly common for this to happen also.

Next time the electrician visits, I’ll have to ask him what the strange switch at the top of the stairs is for.