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[lenny23]

my shower pullcord keeps blowing. i have a 10mm cable connected from a consumer unit through to a 8.5kw shower intersected by a 45amp pullcord.i fit a new pullcord unit and it works for a few months then blows any help please.

[ericmark]

Normally a bad connection. Any poor connection will heat up. Once heated up it tends to coat parts with oxide which results in poor connection so heats up again.

[PaulWhite]

A friend of mine had this but turned out to be because they were leaving the shower pull cord on (not ever having a power shower before they didn't know)!
Are the wires melted/burnt when you replace the unit?
As ericmark says it's probably a cycle of heating that is eventually too much and it blows.

[ericmark]

I see no problem leaving an isolator switched on. The idea of a local isolator is to switch off when working on item and it should be where the person working on the item can see it.

A power shower takes very little power they run on the stored hot water and pump it. A normal electric shower is not a power shower.

For any switch to fail we look at a number of problems.
1) Used very often especially under load. An isolator would normally not be touched from one year to next so that is unlikely to be a problem.
2) Over loaded since rated within the showers power this should not be a problem.
3) Lose connections. This would seem most likely. I have found that heavy cables as they heat and cool can move just slightly and settle. I have found the main meter cables become slack a few times. Also poor machining can mean screws become tight without gripping cable. Or flex cable can allow a strand to get into threads. Welding cable normally comes with a brass or copper strip to wind around the cables to stop it happening.

Likely best option is after fitting the switch to re-test the tightness after some time. Normally one slowly increases the time between checks. If slack re-check after short time if tight then leave for longer time.

4) Cables or mounting straining on switch. With heavy cables or uneven wall the switch can be twisted.

5) Children who think it's funny to switch off the power while Mummy's in the shower and hear her scream as water goes cold can also reduce the life of the switch.