Hi, I'm rewiring at the moment and have come across this system of wiring up two lights, the first with one-way switching and the second with two-way switching:
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you can make out how it works from the circuit diagram!
Essentially the one-way light is fed from three core and earth, with the third core used as the switched live for the second (two-way) light. The Live for both lights is shared at the twin switch, by means of a "jumper cable" whic I hope is clear in the picture. I *think* it was done this way to save disturbing decoration as it would have impossible to squeeze 2 twin and earths and 1 three core and earth down to the twin switch, in the usual fashion. As you can see, the system illustrated relies on just 2 three core and earths to the twin switch instead.
My causes for concern are: A: One three core and earth cable is acting as the switched live for two DIFFERENT luminaires. B: The switched live for Light A is joined together with a crimp connector within the ceiling rose for Light B.
Is this setup perfectly acceptable? If so, I will simply ignore, otherwise I will cut out a new chase to accommodate the more standard switching practice.
That's a very odd "standard method", given that the more orthodox method requires no more wires. Why the heck wasn't the new 3-core wire wired as follows instead?
Use two of the cores to connect the two strappers of the two switches. This is the same as above.
Connect the third core to the common of the remote switch. Then use the local end of that third core, and the common of the local switch, as the replacement for the original one way switch.
I.e. either connect the local end of the third core (which is the common of the remote switch) to the live feed, and then connect the common of the local switch to the live end of the light bulb, or the other way round.
Apart from anything else, the circuit is easier to understand.
Though I do see the inherent simplicity of wiring the thing up by using the three cores to parallel up all three terminals on both switches.
So here's a wee exercise for you. You have two lamps up a pole. Call them lamp A and lamp B. You never want both of them on at once, i.e. you want either both off, or A on with B off, or A off with B on.
There is only a 2 core cable going up the pole, but that's OK because they are on a DC supply and there are a couple of diodes up there, wired so that the polarity of the supply determines which bulb comes on.
Question 1 (easy): You are given a two pole changeover switch with a centre off position. How do you wire the battery supply, the switch's 6 terminals, and the cable to the pole to get what you need?
Question 2 (more difficult): You don't like the changeover switch and decide you would rather have two separate switches, one for lamp A and one for lamp B. What types of switch do you need and how do you wire them? The obvious solution involving two double pole one way switches will short-circuit the battery if you are stupid enough to turn both switches on at once. So you need a safer solution which avoids that.
Well the question did not make sense. The obvious solution using two double pole switches works fine depending on how RR wants to place the switches, cables and diodes and more importantly, he never gave a rule on switch positions and which lights should be on depending upon the switch positions.
If it is
battery =={2 core cable}==switches== [2 core cable] == [2 LEDS wired in parallel in opposite directions] then it is easy.
prefer to avoid it unless I have can't, as having five wires coming out of the switchplate backbox can confuse future maintainers, even more than a loop-in ceiling rose can.
It is not a standard method that the OP referred to and gave a diagram for. His concern was not the 3 core and earth cable used for the two way switching but the three core and earth cable used for the switched drop that used one core for permanent live and two different switched lives on the other cores.
I thought it did, but perhaps I unwittingly left out some detail.
Perhaps it would help if I give some background. See below.
No it doesn't, I'll explain below why not.
I meant the cable which runs up the pole is 2 core, the two lamps and diodes are up on top of the pole. The battery and switches are at the bottom of the pole where you can do anything you like.
The "pole" is actually a mast on a sailing boat and the two lamps are navigation lights. Lamp A is an anchor light which shines white all round (in a 360 degree arc). It is used to warn other boats of your presence when you are at anchor. Think of it as a "parking light". When sailing, boats are expected to exhibit sidelights (red to port (the left), green to starboard (the right), and white astern (backwards)). Sailing boats below a certain size are permitted to combine these lights into a single tricolour (which uses a single light bulb inside a multi coloured housing) at the top of the mast. This is lamp B.
Imagine a boat which was originally fitted with only an anchor light (and which previously used ordinary side and stern lights at deck level), or alternatively was fitted with only a mast top tricolour, with other arrangements for anchor light being made at deck level. Therefore a two core cable would have been used in the original installation.
Now the boat is upgraded to use one of these combined tricolour/anchor lights which are normally fed by 3-core cable because it contains two bulbs. But the cable is threaded up the inside of the mast and it would be a major operation to rip out the 2-core and thread a 3-core in its place. Hence the trick with the diodes, mounted inside the mast top unit, so that if you label the two cable ends at the bottom X and Y, if you connect XY to +- you light lamp A, and if you connect XY to -+ you light lamp B.
The wiring question simply concerns how you wire up a switch or two, to make XY either +- or -+ or something else (++ or -- or 00) when you want both lamps to be off.
Problem 1 with a two pole changeover switch:
The switch has three positions, namely a centre off, and two "on"s. The idea is that in one "on" position it lights the anchor light, in the other it lights the tricolour.
The switch has six terminals: two commons A and B, which it connects either to A1 and B1, or to A2 and B2, or to neither in the centre off position.
The obvious and correct solution is to cross-wire A1 to B2 and B1 to A2. You then either wire B2 to X and A2 to Y, and A to + and B to -; or else you wire A to X and B to Y, and A1 to + and B1 to -.
Problem 2 is that you want one switch to turn on the anchor light, and a separate switch to turn on the tricolour. If you use double pole one way switches for this, then the obvious way to wire them up is as follows. Switch 1 has poles A and B which it either connects to A1 and B1 or not; switch 2 likewise connects C and D to C1 and D1 or not.
Wire + to A and C, and - to B and D. Wire A1 to D1 to X, and B1 to C1 to Y.
Then if switch 1 (with A and B) is on, XY is +-. If switch 2 (with C and D) is on, XY is -+. But if you switch both switches on, then you short + to -, because A1 is connected to D1 and also because B1 is connected to C1.
To prevent this, you could of course deploy an extra four diodes, but you will then get 3 diode voltage drops in the line, which on a 12V system will dim the lamps rathe more than a single one will, which is already bad enough.
However, there is a solution involving two double pole changeover switches.
When both switches are on, you don't want a short, nor do you want both lamps on (which they can't be anyway), so the safe option is to have both lamps off in this case.
Surely you can do it with two single-pole changeover (SPDT) switches. Using your terminal naming, so SW1 has 'fixed' contacts A1 & A2 and common or moving contact A (and SW2 similarly has B1, B2 and B). Now connect
- A1 and B1 to -ve supply
- A2 and B2 to +ve supply
- A and B to the outgoing wires X and Y.
If both switches are 'up' or 'down' X & Y are connected to the same supply pole, so both lights are off. If one is up and the other down there will be an output to X & Y, with the polarity dependent on which switch is up.
There is no prize other than your own satisfaction, but if there were a prize, you would be disqualified on grounds of being too expert. I was interested in whether harry could do it. :-)
Presuming that the "g" in your "2g 2way" stands for "gang", i.e. that the switch has two poles, each of which can be switched 2 ways, in what is commonly referred to as a DPDT switch, then yes, that's the answer to Q1. Indeed I have already given wiring solution for that one.
But Q2 explicitly specifies that you must use two switches, one to operate each lamp. I find I need two DPDT switches for that (I don't know if there is a simpler solution), and the puzzle lies in working out how to wire them up so that when switch A is on and B is off, then the two cores X and Y of the cable are connected to + and -, respectively, whereas when switch B is on and A is off, you want X and Y to be - and +, respectively, instead.
When you turn both switches on, you don't want to short out the battery, or blow your fuse if there is one, as you would if your solution were based on two 2g 1way (DPST) switches.
Turning both switches on simultaneously is an operator error condition, since you can't turn both lamps on together. What gets connected to X and Y in those circumstances may be undefined if you wish, but preferably it should be that X and Y are both disconnected from the battery, same as when both switches are off.
What I had was two DPDT switches. Using the same notation, one switch has moving contacts A and B which it switches either to A1 and B1 or to A2 and B2, with the other switch using the letters C and D correspondingly.
I connect A1 and C1 to +, and B1 and D1 to -. I connect A to C, and B to D. I connect X to A2 and D2, and Y to B2 and C2.
Call each switch "off" when it is in the "1" position, i.e. A=A1, etc, and "on" when in the "2" position, i.e. A=A2, etc.
When both switches are "off", X and Y are not connected to anything. When both switches are "on", + and - are not connected to anything, but X and Y are conncted harmlessly to each other. When at least one switch is "off", A and C are connected to +, and B and D are connected to -. When at least one switch is "on", X is connected either to A and hence + or else to D and hence -, depending on whether switch AB or CD is on. Correspondingly Y would be connected to B and hence - or to C and hence +.
I know what SPDT etc means, it's the "2g" part that was unfamiliar. It's also misleading if you use a term containing the word "switch" in the singular to denote what is in fact two independent switches. That's what led me to the wrong conclusion above about what you meant.
I didn't mean to make it harder.
That's fair enough, I only said "preferably". That X and Y are at the same potential is good enough.
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