I'm wondering if a stray strand of fine stranded wire somehow got inside during installation then some time later happened to short it out (and then vaporise).
Yup, that could do it. There's a very good demonstration here of that. Although they've used 480V, the spacing is much, much larger than in that switch.
I think halogens will be classed as resistive?
Dimmer packs are not that nice though, they clip the sine wave and have inductors in them so might be a tad of an issue, but wouldn't have thought that they would cause that - my betapacks don't fire up for a few seconds after powering on.
Mabe just the cold loading causing the arc.
I think halogens will be classed as resistive?
Dimmer packs are not that nice though, they clip the sine wave and have inductors in them so might be a tad of an issue, but wouldn't have thought that they would cause that - my betapacks don't fire up for a few seconds after powering on.
Mabe just the cold loading causing the arc.
Cold halogen is a resistive load, but it behaves like an inductive load with a large inrush current, and as such switchgear needs to be over rated, and C type or sometimes even D type circuit breakers used.
The switch doesn't switch the dimmers, just the control room sockets, but someone had made some adaptors and plugged the tails of the patch straight into the control room sockets.
There obvioulsy won't be an inrush current until the contacts close so, unless the contacts then bounce open again, I wouldn't have thought there would be an arc.
Carbon soot can build up inside of the isolator if it is operated under load due to arcing when the contacts opens and closes, some of the carbon soot may have provided a path between the L and N to allow arcing during the inrush of current when switching on?
The damage may be due to the switch being operated slowly. The construction is not snap action so the speed the contacts close depends on the speed the rocker is operated.
When switching ON a circuit with a load applied the very small arc at first touch of the contacts will push them apart enough to break the contact and this On-Off action can repeat several times before the rocker has been moved enough to ensure the contacts remain in contact with each other.
One reason why this type of slow acting switch mechanism is not recommended for switching when under load.
When switching ON a circuit with a load applied the very small arc at first touch of the contacts will push them apart enough to break the contact and this On-Off action can repeat several times before the rocker has been moved enough to ensure the contacts remain in contact with each other.
As I said in my last post, I can certainly see that as a possibility if there is some (mechanical) 'bounce' (bouncing on and off) of the contacts during closure. In the absence of any such (mechanical) bouncing, there shouldn't be any "small arc at first such" which you described as initiating a series of (arc-mediated) bounces, should there?
With a high load current passing through the first very small area of contact there is very rapid heating of the surface of the contact which can result in near instant vapourising of a micron or two thickness. This micro explosion is not itself a true arc but does create the disconnection that becomes an arc. This can repeat several times.
Much of the vapourised metal condenses on the still cold area of the contacts but some will condense elsewhere in the switch creating conductive coatings on insulating parts.
FWIW, I as taught the same and, furthermore, that isolators are not necessarily rated to be able to switch the circuit's full current 'on load'.
For example, I wouldn't be at all comfortable using the little isolator fitted into some domestic meters to 'switch off' the whole installation when it was heavily loaded - and doubt that it is rated to do that.
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