2 way switch tripping rcd..

Hi guys. Recently out of my time so go easy on me.. today is the second time in a week i have come across this problem and im a bit stumped to what the issue is. Im currently re wiring a house with a mate, basic rewire with a 2 way 2 gang light switch in the corridor as you come in through the front door and a 2 way way directly above on the landing. We've wired it in a 3 plate system, switch wire going down to each switch from both rose's and then 2 strappers linking the switches as you would normally. Brown from each strapper in common on each side of switch (im using black as L1 grey as L2) and then brown from the  t+e in the rose into L1 with black,  and then blue sleeved in with the grey into L2. We have made sure both strappers are not crossed as we have belled them out and they are in with the correct pair. 

All seems straight foward so far, however when flicking one of the switches on the two way it takes out the rcd? (Its a split way dual rcd board) Now i have meggered every leg of the lighting circuit ive disconnected each leg and methodically checked from point to point and everything is clear.. now the interesting thing is if i do away with both 2 ways and replace them for 1 ways switches all is fine, rcd holds and every light both up and down switches as it should.. now is this pointing to an issue with the strappers but i just cant understand what it can be as ive meggered them through and im getting perfectly clear readings so that rules out any shorts between neutral and earth etc? 

sorry for the long winded first post its just been baffling me, ive come from a more industrial background and im somewhat new to the domestic game! 

Any replies would be welcome cheers!

would normally answer a borrowed neutral, but in this case I would suspect a screw touching one of the strappers, doesn't necessarily show when testing, but when energised causes a problem.

I would test all 4 cores of the 2 way switch...... to check for damage

You either have a  shared  (borrowed )  neutral  or 2 different circuits .       

I presume your Lts up  and Lts down are on two different RCDs .  One of your neutrals could be on the wrong Neutral bar.

The way your post reads ...you have  a 2g  sw in hall ....one is  2 wayed to landing   & one is 2 wayed  down the corridor ? 

Do you have two different circuits in this system ?

Thanks for the replies. We can deffo rule out a borrowed neutral as its a brand new install... both up and down lights are on separate RCD's, first thing I suspected was that one of the neutrals was on a the wrong bar but double and triple checked that and they are deffinetly on the corresponding bars.

its just a smiple case of the downstairs corridor and upstairs landings are 2 wayed. 2 strappers running in between and thats it nothing more nothing less.

Evans electric when you say 'two different circuits' what do you mean by that exactly? The up and down are seperate yes but they are technically linked by these strappers as obviously you will have voltage passing through the up and down light circuits when the switchs are in different position's thus giving you your 2 way capability.. just seems mental ive drawn it out followed it through in my head and the wiring cant be wrong. Madness.

Kieren991 said:

 The up and down are seperate yes but they are technically linked by these strappers as obviously you will have voltage passing through the up and down light circuits when the switchs are in different position's thus giving you your 2 way capability.. just seems mental ive drawn it out followed it through in my head and the wiring cant be wrong. Madness.

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Mixed up strappers?

When you say Strappers,

Do you mean traditional, as in a twin and e, or a 3c and e,?

The circuits should not be interconnected at any point, no matter what position of the switches.

Kieren991 said:

its just a smiple case of the downstairs corridor and upstairs landings are 2 wayed. 2 strappers running in between and thats it nothing more nothing less.

Evans electric when you say 'two different circuits' what do you mean by that exactly? The up and down are seperate yes but they are technically linked by these strappers as obviously you will have voltage passing through the up and down light circuits when the switches are in different position's thus giving you your 2 way capability.. just seems mental ive drawn it out followed it through in my head and the wiring cant be wrong. Madness.

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Your switch downstairs and upstairs, for the landing light (my example) MUST be on the same circuit.... so the feed and neutral only come from 1 circuit.

If you are 100% sure nothing is mixed up, then cable damage is the next suspect.

Does this happen with both gangs of both switches?

steptoe said:

When you say Strappers,

Do you mean traditional, as in a twin and e, or a 3c and e,?

The circuits should not be interconnected at any point, no matter what position of the switches.

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Just 3c and e mate..

Murdoch said:

Your switch downstairs and upstairs, for the landing light (my example) MUST be on the same circuit.... so the feed and neutral only come from 1 circuit.

If you are 100% sure nothing is mixed up, then cable damage is the next suspect.

Does this happen with both gangs of both switches?

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Hmmm aslong as i have the right strappers on the right side of the switch with the right switch wire all should be okay shouldnt it? There must be a cross somewhere and ive mixed crossing tbe strappers thus im switching between up and down circuits maybe? 

Ye, if its 3c+e deffo sounds like you have crossed a cable/wire somewhere.

Kieren991 said:

Hmmm aslong as i have the right strappers on the right side of the switch with the right switch wire all should be okay shouldnt it? There must be a cross somewhere and ive mixed crossing tbe strappers thus im switching between up and down circuits maybe? 

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Seems likely. Maybe the switch is incorrectly marked? Had that before.

Try wiring the 2 ways on 2 1 gang switches rather than the 2 gang. If it still does it you must have the wires crossed. Easily done, I have done it on more than one occasion, even when pulling it apart and checking again, they can look like they are all in the same cable but they aren't!

BTW, how have you connected the switch wire from the light, ? At both the light and the switch,?

Lurch said:

Seems likely. Maybe the switch is incorrectly marked? Had that before.

Try wiring the 2 ways on 2 1 gang switches rather than the 2 gang. If it still does it you must have the wires crossed. Easily done, I have done it on more than one occasion, even when pulling it apart and checking again, they can look like they are all in the same cable but they aren't!

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Thats the mad thing mate ive put them on two 1 gang switches and boom its fine. Must be a mix up in strappers cant be anything else can it.

steptoe said:

BTW, how have you connected the switch wire from the light, ? At both the light and the switch,?

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The switch wire that comes from the light is just a a T+e L N and cpc ive obviously used the blue as a switch wire and brown as permanent.Wired as a Standard 3 plate at therose. Then down at the switch that same switch cable is wired into one side of the 2 gang with one of the strapers. In L1 of the switch is the permanent brown of the T+e and black of the strapper, in L2 is the blue sleeved switch wire of the t+e and grey of the strapper. 

put them both on the same circuit / RCD. if it doesnt trip you have mixed the circuits. if it still trips then there is a fault

Kieren991 said:

Thats the mad thing mate ive put them on two 1 gang switches and boom its fine. Must be a mix up in strappers cant be anything else can it.

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Well, if it works on separate plates then it is not the strappers. You must have them connected up wrong on the 2G switch.

Andy™ said:

put them both on the same circuit / RCD. if it doesnt trip you have mixed the circuits. if it still trips then there is a fault

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Seems like the best option mate not tryed this as yet. As i said me and my mate first fixed and second fixed this, id be shocked if we have mixed/crossed the up and down lights but it is a possibility and would make sense, if they are crossed effectively all im doing when im

switching the two ways is crossing over the two circuits and throwing the imbalance hence tripping the rcd. 

Kieren991 said:

Seems like the best option mate not tryed this as yet. As i said me and my mate first fixed and second fixed this, id be shocked if we have mixed/crossed the up and down lights but it is a possibility and would make sense, if they are crossed effectively all im doing when im

switching the two ways is crossing over the two circuits and throwing the imbalance hence tripping the rcd. 

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That still doesn't make sense if you say it works fine when doing it on 2 1 gangs instead of the 2 gang.

I think you 'may' have split one of the twin switch wires across one of the switches.

or, as said, you have mixed up one of the cores of the 3c