Shower Switch burning.

[hugh.donnelly2]

Hi Group,

My friend asked me to look at a problem concerning a shower switch with a burning chemical smell.

I opened the switch. It was a 45A DP pull cord ceiling mounted switch that had been installed several years ago. On examination both of the neutral conductors were blackened to 1" away from the terminals. One of the terminals fell off when I touched the wire. The live terminals were OK but some of the burning had spread from one of the N terminals to one of the L terminals.

The installation was a dedicated circuit protected by an RCD in the Consumer Unit.

The burning smell had been present for a number of weeks so it must have been a gradual process. There had been sporadic trippings of the main breaker but no trippings of the shower RCD.

I have replaced the switch but I am at a loss to explain what caused the fault.

Please does anyone have any ideas?

Thanks, Hugh

[Rob Morley]

Poor connection caused heating, heating caused thermal movement, thermal movement made the connection worse to the point that it started to arc. The RCD wouldn't trip because there wasn't any earth leakage, but once it started to arc the inrush current on the intermittent connection upset the main breaker.

[hugh.donnelly2]

Thanks Rob. I see this now. A connection not quite tight probably loosened further by the pull switch actions over a time resulting in what you have described. Regards, Hugh

[Paul]

we had an identical problem recently, could smell a fishy smell in the bathroom but could not trace the source. eventually you could not switch the shower on with the pull cord, i took the switch apart and the wires were very burnt. i was surprised at how loose the screws on the lugs were, i am sure i did them tighter than that when i fitted it. replaced the switch and made sure the screws were TIGHT. will check it in a few months to make sure its still ok.

[Dave Liquorice]

Slightly high contact resistance on the netral side. At 30 odd amps it doesn't take much resistance to dissipate a lot of power as heat...

Just 1 ohm will generate 30 odd watts and as the switch isn't designed to dissipate heat it all gets very hot rather quickly. Think how hot a

40w light bulb gets in free air with plenty of circulation and surface area... I'd say 0.1 ohm is enough to fry a switch and wires over a period of time.

[mike ring]

i was surprised at how loose the screws on

It prolly wasn't as bad when it was fitted; the way Rob explained it (beautifully!) you can see it's a self accelerating process, the burning causes oxidation, so you've got less metal there than when the connection was made. , so the lugs are looser than when you left them

mike

[BIG NIGE]

Similar thing happened to me, the cable was not loose in the connector but one of the contacts in the switch had burnt, (possibly started by dust getting into contact)

[Ian Stirling]

[Dave Liquorice]

Sorry I have a cold. Quite correct P = I^2R, instant fire...

Means that 0.01 ohm contact resistance dissipates 9W...

[Tony Williams]

Which is why those stupidly tiny microswitches in Triton electric showers are almost guaranteed to burn out. :(

[Chip]

On Sun, 25 Sep 2005 11:54:28 +0100,it is alleged that Tony Williams spake thusly in uk.d-i-y:

After 367 days of course. Much simpler than the complex hidden 'malfunction as soon as the warranty runs out' timers they used to fit to VCRs

To the OP, rather than merely replacing the switch, I would suggest checking the terminals at _all_ points along the circuit, the tops of MCBs and the terminals in the shower could be loosening also.

[Dave Plowman (News)]

The connections weren't tightened properly. On many things this doesn't seem to matter, since they're not carrying heavy currents. But in this case they are. I've lost count of how many 'pro' installations where faults have been caused by this. You need to use a screwdriver with the same width blade as the screw and tighten it firmly. Those little 'electricians' mains tester screwdrivers aren't up to the job. Fine for ceiling roses - but not high current connections.

[Stuart]

Don't Triton give 2 years warranty .Thats what I got .?? Stuart