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I have a double light switch in my house (1 inside, 1 outside light) that will trip the main switch in the circuit breaker when both switches are turned off at the same time. Turning either one off individually works fine, and turning them both on at the same time also works, but if both lights are on and I hit both switches, it'll trip the main switch and I lose power in the whole house.

Has anyone seen this before, or can someone suggest how I can start to diagnose this? I don't understand why this could be happening.

Thanks in advance!

Edit: I'm in the UK - completely forgot about that!

Just to be specific here, the unit in question is something like this: image. The left hand switch is responsible for the outside light (and is the only switch for these lights), the right hand one is responsible for the inside lights. There is another switch for the inside lights, so this is a 3 way switch.

This is not a new installation, has never worked properly that I'm aware of, and the switch has never been replaced.

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    Where are you in the world. Are there any other loads other than lights? – Ed Beal Sep 5 '17 at 22:03
  • Yes, where are you on this planet? – ThreePhaseEel Sep 5 '17 at 22:22
  • Do you mean the breaker trips when you operate both switches together, or the breaker trips when you turn one switch off, wait a few seconds, and turn the other switch off? – A. I. Breveleri Sep 5 '17 at 22:50
  • What happens if you leave both switches off and reset the main switch? – A. I. Breveleri Sep 5 '17 at 22:50
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    So, slight turn of events now. Little did I know, my dad had replaced this switch a few days ago and the problem is now fixed. He didn't change the wiring, just the switch. I'm still intrigued why this would happen though? @Harper, yes we have an RCCB. – tom982 Sep 6 '17 at 8:43
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Tom - we need to know where on the Planet you are as Electrical Codes vary and so does the wiring.

In the USA .. simplistic description: normally your electric panel would be wired Power Line, to Main Panel ( you might have a main breaker in the panel or on the outside of the house ) your lights would be wired through a separate circuit breaker after the main breaker in that panel. Lets be clear this is NOT a Main Breaker.

So if you have a MAIN breaker that is tripping - Your circuit protection for those lights (the light circuit) is not working or it is Sized completely Wrong - hint it is probably not working - welded in the on position.

What you refer to as a double light switch is what is known as a 3-way switch arrangement. You do not specify if this is a new installation, has ever worked or if a switch has been recently replaced.

If a switch was recently replaced (my guess is yes):

I suspect the Switch; 
someone purchased and installed a normal light switch instead of the 3-way switch.
  • Ken, thanks for your reply. I've added more detail to my initial post which should help clarify things. When I refer to a double switch, I meant something like this: image. The left switch is for an outdoor light and is the only switch for this light. The right hand switch controls the indoor light, for which there is also another switch on the other side of the room, so this one is a 3-way switch. – tom982 Sep 5 '17 at 23:49
  • @tom982 Image is B&Q Kingfisher logo .... – Ken Sep 5 '17 at 23:56
  • Strange, try this one instead: link – tom982 Sep 6 '17 at 8:32
  • @tom982 Never worked to your recollection, not a new switch - probably the switch - verify that is a 3 way in each location. Also try the 3rd switch - part of the 3-way. If it is in the off position turn it on and then run your tests of turning the double set of switches off / on. Edit your question with what happens.. – Ken Sep 6 '17 at 8:40
  • Ken, just coping my comment on the question here to make sure you see it: "So, slight turn of events now. Little did I know, my dad had replaced this switch a few days ago and the problem is now fixed. He didn't change the wiring, just the switch. I'm still intrigued why this would happen though?" – tom982 Sep 6 '17 at 8:44
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You have switces wired badly (you have the live on one 'inverted' contact). Just rewire the switches this way:

central should be Phase on one switch and 'lamp' from the other. Other two (deviated) contacts should be connected each other.

This is a schema about how 3-way should be wired right 3-way

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