Difficulties adding intermediate switch

[Whitling2k]

Hello! I wonder if anyone could help me - whilst I'm building new walls and ceilings, I am moving and updating light switch locations - mostly adding intermediate switches to existing 2-way setups. I've done 3 or 4 now with no hiccups, using this method. When I went to update the last one - it's thrown me - I can't see any circuit diagrams that look like this set-up, and when I did what I thought would work - I popped the MCB when closing the main circuit breaker.

There is a diagram at the bottom - it's quite hard to explain using text!

There are two separate light bulbs in this circuit (which I want to keep), and both bulb cables are wired into different junction boxes.

Also, I currently have two 2-way light switches, which are connected via a junction box - from which another web of junction boxes are connected... from which the two lighting cables come. (this network of junction boxes also seems to link up to the other lights on the ground floor).

My first thought was that the cable going to the switches was the circuit cable which I could just wire according to the diagram I showed above. To test the theory I wired it up to a one way switch, turned the mains back on, but then when I closed the 1-way switch, it popped the MCB for the ground floor lighting circuit.

I have put a diagram of what I mean at the bottom of the post.
- does anyone recognise this type of circuit?
- what do I need to do to add my intermediate switch?
- is it normal/safe to have this many junction boxes?

Ideally - I would like to de-complicate this particular switch set-up it and make it like the diagram in the Wiki - but the network of junction boxes is just too complex for me to safely detangle. It's difficult to map out fully as quite a few cables disappear through holes in joists and I've not yet lifted those floors yet.

Untitled

• Whitling2k
• 10 Jul 2014
• 1

 

[flyingsparks]

Where the Red and yellow are coming into the junctino box from the switch on the left, your intermidiate switch needs to go there. So red and yellow into intermitiate. Out of intermidiate and reconnect to the brown and black of the cable going to the switch on the right.

 

[flyingsparks]

- is it normal/safe to have this many junction boxes?

Its very common to find junction boxes. Now if they are hidden under floors etc they should be Maintenance Free type, (yours are not). It could certainly do with a tidy up, you should be able to get rid of all of those boxes and have just one larger box with wago's in. Thats what I would do if you couldnt rewire it so as to have all joints at switches or ceiling roses.

 

[AndyPRK]

and when I did what I thought would work - I popped the MCB when closing the main circuit breaker.


My first thought was that the cable going to the switches was the circuit cable which I could just wire according to the diagram I showed above. To test the theory I wired it up to a one way switch, turned the mains back on, but then when I closed the 1-way switch, it popped the MCB for the ground floor lighting circuit.

Your understanding of 2 way & Intermediate switching is fine.

However to be causing the MCB to trip somewhere in the circuit you have got a neutral connected to a live. (or live to earth).

It could be anywhere on the downstairs lighting circuit. Think back to what you have done since the the circuit powered up ok and the mcb didn't trip.

or you have misidentified a cable, which would not be a surprise in that rats nest.

You could disconnect that top cable from the junction box and power the circuit up again. If the mcb still trips, its nothing to do with this rooms lights/switches.

You need to disconnect the downstairs lights Live and Neutral & E at the CU, and with a meter on resistance measure these cables to see the short. or near 0R.

Then at approx the half point of the lighting circuit, disconnect half the lights. See if the short on the meter has gone. This will help you trace where the fault is.

Fault could be a bulb or light fitting if you have fitted one recently.

Also make sure you haven't got a damaged cable insulation somewhere causing a short to earth or metal

 

[Whitling2k]

Where the Red and yellow are coming into the junctino box from the switch on the left, your intermidiate switch needs to go there. So red and yellow into intermitiate. Out of intermidiate and reconnect to the brown and black of the cable going to the switch on the right.

I like simplicity - so I'll give this a try when I get home from work.

 

[Whitling2k]

However to be causing the MCB to trip somewhere in the circuit you have got a neutral connected to a live. (or live to earth).

That's what I thought - I was a little surprised when it tripped to be honest - I thought the one switch would be offering the same function as the junction boxed two. If there are only two conductors going into the junction box (the cable from the top - the one I called the circuit cable). I think what I'm trying to say is how come the two switches act differently to the one switch?

It could be anywhere on the downstairs lighting circuit. Think back to what you have done since the the circuit powered up ok and the mcb didn't trip.

or you have misidentified a cable, which would not be a surprise in that rats nest.

You could disconnect that top cable from the junction box and power the circuit up again. If the mcb still trips, its nothing to do with this rooms lights/switches.

Luckilly I have not made any other changes for a number of months - and everything else has been running fine. I the MCB doesn't trip when the black and red wires are removed - only when the one-way switch is closed - i.e. the red and black are shorted. (which is my understanding of what the 2x2-way switches must be configured to do - again there are only two conductors so what else can they do?

You need to disconnect the downstairs lights Live and Neutral & E at the CU, and with a meter on resistance measure these cables to see the short. or near 0R.

Then at approx the half point of the lighting circuit, disconnect half the lights. See if the short on the meter has gone. This will help you trace where the fault is.

I'll probably leave it if this is what's needed - I would live without the third switch :-/

Fault could be a bulb or light fitting if you have fitted one recently.


Also make sure you haven't got a damaged cable insulation somewhere causing a short to earth or metal

I think this is unlikely because everything else is working fine - and has been checked when I had some other work done (between moving/adding the other light switches and starting this task)


What's confusing me the most is how the red and black from the top are being switched by two switches joined through a junction box. when I remove the junction box and switch the red and black directly - it trips, when in my understanding of the set-up it's just performing the same function - because there are only two conductors, they must only be able to be short (switch closed) or not (switch open).

Could it be an earth-something fault that manifests when disconnect the black and red, but leave the earth circuit connected the existing way, that would not cause it to trip when all connected via the two switches/junction box combo?

 

[AndyPRK]

I'm confused. If the MCB is tripping now then you need to do something.
You say its tripping with 1 switch and 2 switches, so you need to do something to correct this?

Are you sure the red & black are the switch wires? Where did they used to go?

 

[Whitling2k]

Hi Andy,

Sorry for confusing things! I'll try reiterate my key points with the rest of my ramblings stripped out!

The MCB isn't tripping now - i.e. in the config shown in the diagram I posted.

When I temporarily replaced the entire bottom junction box with a 1-way switch the MCB tripped when the 1-way switch was closed.

I then rewired it as shown in the diagram, and all worked again with two switches.

I thought this would work because I assumed the red, black and earth cable was the circuit cable

My logic behind the 1-way switch being that the bottom junction box has a red, black and earth cable going into it and two switches coming out of it both linked with 3 & earth cables, nothing else.

So I don't understand how replacing the two 2-way switches & junction box with a 1-way switch will make any difference other than isolating the two 2-way switches - unless something is going on that I hadn't accounted for.

My overall aim is to get three switches in the circuit - 2x 2-way and 1x intermediate.

I'm afraid I don't know what the red, black and earth are if they are not a standard circuit cable - they go into the mess of junction boxes that I don't understand - I'd rather not start messing with the junction boxes as I can't fully trace where they go without taking floors up - which I'll do eventually, but not now. If I can unjumble little bits at a time as and when I need to - there will be less mess to clean up when I do lift the floors on my 1st floor.

Does that clear things up a bit?

 

[AndyPRK]

Ok. I think your logic is sound.

I can only assume the cables don't actually go where you think they do.

The easiest option is to run a 3C+E cable from one of the existing light switches to your new light switch position.

This will allow you to add a 3rd switch when you want.

You would need to replace the existing switch with an intermediate (2 gang)

 

[AndyPRK]

When I temporarily replaced the entire bottom junction box with a 1-way switch the MCB tripped when the 1-way switch was closed.

How did you wire the red, black and earth to

the 1 way switch ?

 

[Whitling2k]

there were only two holes in the 1-way switch - so one to each.

I think I see where you are coming from with the intermediate and 3C+E.

I'll have a look at the practicalities of that tonight - but think that's the solution if it works - don't know why I didn't think of that in the first place!

[code:1]

Going to this:

|
___O___
| |
+ X====+

from this

|
___O___
| |
+ +

with:
+ being 2-way
X being intermediate
O being the existing junction box
= being new 3C and E[/code:1]

 

[flyingsparks]

Going to this:
|
___O___
| |
+ X====+

from this
|
___O___
| |
+ +

with:
+ being 2-way
X being intermediate
O being the existing junction box
= being new 3C and E

Now you have lost me!!

 

[AndyPRK]

I managed to crack the code


+ X====+


2G
===3C+E==== SWITCH
SWITCH


THey are usually both intermediates if you order a 2G Intermediate
but you can use an intermediate for a 2 way

 

[Whitling2k]

Haha yes! It "helped" by removing the spaces from my diagram - it looked OK when i typed them in!

 

[Spark123]

Editing the spacing:
[code:1]
Going to this:
|
___O___
| |
+ X====+

from this
|
___O___
| |
+ +

with:
+ being 2-way
X being intermediate
O being the existing junction box
= being new 3C and E[/code:1]