Help needed with adding a 2 way switch to a switched fused spur


Postby DAZ3269 » Sat Apr 23, 2016 12:44 pm

I currently have a light which is fed from a switched fused spur.
In the spur i have load (live neutral and earth) and supply (live neutral and earth)
I want to run a second switch from the spur and i understand that the connections between switches requires 3 core cable i am confused as all diagrams i have seen show a "com" on the back of the spur (a place for the 3rd cable) but mine does not have one does this mean i cant do this or i need another spur with a "com" connector on it?
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Postby ericmark » Sat Apr 23, 2016 9:57 pm

To retain the fuse and have a two way switch means using a grid switch. You need a deep back box but that way you can put any switch you like with the fuse.

However for an outside light we like to be able to switch the neutral as well as line. The idea is should water get in you can simply switch off the light and every thing else in the house will still work.

So better to have a separate switch for the two way system and leave the FCU always switched on except if there is a fault. To the new switch you only need two wires and earth.

So remove the load line wire and put it in a chock block. Then two core and earth brown to load line on FCU and blue to chock block plus earth.

Take this wire to first two way switch and brown and blue go to L1 and L2 (assuming there is a com, if no com then L2 and L3) now used some three core and earth and wire switches like for like so if grey is com with one switch it is also com with second switch.

Note without second switch first switch will do nothing.

Without the two pole switching of the FCU water can cause the neutral to connect to earth, and this can cause the RCD to trip, this is why we give the option to switch both live wires (neutral is considered as live and well as the line wire) but in doors it is unlikely to ever have a neutral earth problem so we only switch the line wire.

I still some times call the line live in error. 40 odd years ago when I started it was called live. If you do not have RCD protection then swapping the FCU to a RCD FCU is easy.
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Postby DAZ3269 » Sun Apr 24, 2016 7:08 pm

Thanks ericmark

Such a detailed explanation i will make sure i totally understand your explanation (my lack of knowledge) then give this a go next weekend if it is beyond my capability i will of course get an expert in.

Once again thanks for the step by step explanation.
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