What caused this?

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I don't think it was caused by a stray strand of wire. There was no sign of any damage to the actual wiring.

The switch supplies 6 twin sockets in a theatre control room, which someone had plugged some of the theatre lights in to rather than using the dimmers :rolleyes:

It would have been switching a load of about 5kW of cold halogen which behaves like an inductuive load, so it could be something to do with that then.

I have been reading the technical specs from MK, and there is no inductive rating for these switches, just a small note in the specs

Note: These switches are not recommended for switching large banks of PCs

I had hoped that using MK and over-rating the switch should have led to a long trouble free life. :rolleyes:
 
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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.
 
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i think the main problem these days is the mouldings dont tend to insulate between the poles like they used to

Reminds me of the big old flashguards on the mem isolators
big chunky thick things yet the poles were about 25 mm apart
 
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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.
 
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It doesn't act like an inductive load with respect to collapsing emf and arcing ;)
 
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Yeah true. It does have a high inrush current though which I guess could cause the initial arc?
 
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Yeah true. It does have a high inrush current though which I guess could cause the initial arc?
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.

Kind Regards, John
 
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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?

Just a thought :)
 
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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.
 
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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?

Kind Regards, John
 
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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.
 
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I noticed RF referes to it as an isolator :)

I was taught
An isolator, isolates and is not for general on/off use
A switch switches and is for general on/off use
 
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I was taught ... An isolator, isolates and is not for general on/off use ... A switch switches and is for general on/off use
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.

Kind Regards, John
 

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