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breezer
why not just look out the window?
Intermediate neon switch
- Thread starter regsmyth
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Plugwash and 333rocky333 are correct. You can do it with four cores (two lengths of flat twin and earth perhaps) if you use the old-fashioned method of multi-way wiring: live feed to COM at one end and switched live from COM at the other end. 
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Spark123 said:
Agreed. Apart from the fact that two earths are better than one, anybody stumbling onto those cables in future will expect their earth wires to be earthed! Your floating earth wires will, by virtue of capacitive coupling, sit at the supply voltage when the lights are on (half when they're off).
That same capacitive coupling can be enough to make low energy lamps flicker when switched off. An earthed wire between live and switched live will prevent this.
4-core Flexishield might be a good choice.
I thought I'd got this sorted, but now you're saying I have to connect an earthed wire between live and switched live?
I don't understand, can you expand on this a little please? I don't think he means for you to join together the live, switched live and earth, that's just silly...., just that having an earthed wire laying in between the live and switched live (as when using twin and earth or three core and earth for lighting) will quell the inductive effect that he's on about!
Sorry for the confusion. You don't CONNECT the earthed wire between live and switched live. That would be a really bad idea! The presence of an earthed wire LYING BETWEEN live and switched live in the cables will block the capacitive leakage current.
I'll elaborate a bit. You have two wires lying side by side, close together, over a long distance. Together they form a capacitor which isn't on your circuit diagram. (A ball park figure for the capacitance is about 100 picofarad per metre.) This hidden capacitor allows a small leakage current to cross the gap between them. It's enough to light a neon driver, enough to give you a tingle if you touch it and, possibly, enough to make a low energy lamp flicker.
Now lets put a third wire midway between the first two. Because of the geometry inside the cable you now have two capacitors: one each from the centre wire to the other two. The capacitor between the two outer wires has all but vanished. If you leave the centre wire floating, the leakage current will flow from the live wire to the floating wire and on to switched live just as if the centre wire wasn't there. But if you earth the centre wire, the leakage current will be shunted away to earth and virtually none will reach switched live.
Did that make sense?
The presence of an earthed wire LYING BETWEEN live and switched live in the cables will block the capacitive leakage current.I'll elaborate a bit. You have two wires lying side by side, close together, over a long distance. Together they form a capacitor which isn't on your circuit diagram. (A ball park figure for the capacitance is about 100 picofarad per metre.) This hidden capacitor allows a small leakage current to cross the gap between them. It's enough to light a neon driver, enough to give you a tingle if you touch it and, possibly, enough to make a low energy lamp flicker.
Now lets put a third wire midway between the first two. Because of the geometry inside the cable you now have two capacitors: one each from the centre wire to the other two. The capacitor between the two outer wires has all but vanished. If you leave the centre wire floating, the leakage current will flow from the live wire to the floating wire and on to switched live just as if the centre wire wasn't there.
But if you earth the centre wire, the leakage current will be shunted away to earth and virtually none will reach switched live. Did that make sense?
Umm, no.
Are you saying that if I cut the earth back as per drawing, I could get the neons and the outside lights (if low energy) flickering even when they're off?
And that if I keep the earth connected on both cables, this is avoided?
Hope so.
But I can't get my head around how the capacitance would build up and between which wires. You mention between L and swL, why not between L and N? or would that happen also, just not show itself?
But the L and the swL are in different cables, aren't they... or are they..you don't mean adding another single core earth and sandwiching it between the two cables do you?
And as for just getting a tingle if you touch L and swL.....
Getting out of my depth here...
Are you saying that if I cut the earth back as per drawing, I could get the neons and the outside lights (if low energy) flickering even when they're off?
And that if I keep the earth connected on both cables, this is avoided?
Hope so.
But I can't get my head around how the capacitance would build up and between which wires. You mention between L and swL, why not between L and N? or would that happen also, just not show itself?
But the L and the swL are in different cables, aren't they... or are they..you don't mean adding another single core earth and sandwiching it between the two cables do you?
And as for just getting a tingle if you touch L and swL.....
Getting out of my depth here...
if you put 2 wires close together and put current down one, it produces a magnetic field..
the other wire picks this field up and turns it back into current.. ( this is the way transformers work.. )
if the other wire is not connected to anything at both ends then the current can't flow so it appears as a potential voltage.
if one or both ends are connected to earth then the small current induced runs to earth and the potential voltage dissapears.
the other wire picks this field up and turns it back into current.. ( this is the way transformers work.. )
if the other wire is not connected to anything at both ends then the current can't flow so it appears as a potential voltage.
if one or both ends are connected to earth then the small current induced runs to earth and the potential voltage dissapears.
The drawing shown , is good and will work ,but could cause issues with voltdrop to the last fitting, due to it running via all the switches , back through the indicators , then through all the lights.
As you now have a switch line and neutral at each switch/indicator position , you could go from any switch to the nearest light, if it is more convenient, for wiring, the downside being you would have 3 wires in the terminals.
If you was wiring it in two core and earth, you could wire from indicator to light to indicator, in any combination ,which may be easier and shorten the cable length.
If the earth wire is in the cable then connect it.
Can you quesstimate the lengths involved
As you now have a switch line and neutral at each switch/indicator position , you could go from any switch to the nearest light, if it is more convenient, for wiring, the downside being you would have 3 wires in the terminals.
If you was wiring it in two core and earth, you could wire from indicator to light to indicator, in any combination ,which may be easier and shorten the cable length.
If the earth wire is in the cable then connect it.
Can you quesstimate the lengths involved
In your drawing yes
One cable has live only , which switches between the two cores.
second cable has switched live and neutral, which i think may cancel out any capacitance,due to the direction of current flow.
Imo
I think the capacitance problem is more with live and switchline in same cable.
This is possible if you used four core, or wired it different, but will be reduced due to the earth wire being inside the cable, providing it is terminated
Thanks for all your thoughts and help. You people really are the business!
I think I see what you mean about supplying the lights from the switches, as the four lights more or less correspond with the five switches, though I haven't tried to work it out yet. It might make things a lot more rational. (Or maybe it'll drive me round the bend)
As far as lengths go...
CU to Switch 1 = 6m
Switch 1-2 = 12m
Switch 2-3 = 6m
Switch 3-4 = 6m
Switch 4-5 = 14m.
Lighting cable similar lengths.
The outside lights will be on their own circuit, I was planning to use all 1mm, as the max load won't exceed 500W, or will the volt drop issue dictate 1.5mm?
I think I see what you mean about supplying the lights from the switches, as the four lights more or less correspond with the five switches, though I haven't tried to work it out yet. It might make things a lot more rational. (Or maybe it'll drive me round the bend)
As far as lengths go...
CU to Switch 1 = 6m
Switch 1-2 = 12m
Switch 2-3 = 6m
Switch 3-4 = 6m
Switch 4-5 = 14m.
Lighting cable similar lengths.
The outside lights will be on their own circuit, I was planning to use all 1mm, as the max load won't exceed 500W, or will the volt drop issue dictate 1.5mm?
regsmyth said:
Sorry; my fault.
The important cores here are the ones running through the intermediate switches - in the cables whose earths you were going to cut back. These cores run side by side all the way from one two-way switch to the other, a total length of 38 metres in your case. Together they form a capacitor. One side of this hidden capacitor will be live. The other side feeds the lights and it is this one that I - misleadingly - called switched live.A capacitor of that size could bleed a quarter milli-amp of leakage current into your lights. That's enough to light a neon. I doubt that you would notice if you grabbed the non-live wire but you might feel it on the back of your hand.
But it's more complex than that.
You have, as you say, another capacitor in the other cable; not between switched live and neutral but between switched live and earth. This will carry the leakage current to earth and drop the voltage to less than half - but that's still enough to light a neon. Now let's put some lamps in. A cold filament lamp has a very low resistance; so low that the leakage current will put only millivolts across it. No lit neons and no tingles. A low energy lamp is different. The leakage can slowly build up voltage in its internal electronics until it gives a faint flicker. You can - and should - eliminate all the possible effects of that hidden capacitor by connecting the earths in the switch cables. Leakage will then go directly from live to earth. No lit neons, no flickering fluorescents and no tingles.
ColJack said:
Sorry but that's not quite right. A current carrying conductor does have a magnetic field and an AC field does induce voltage into adjacent wires but the induced voltage cannot exceed the voltage drop in the first wire. (In practice it will be much less because at 50 Hz the voltage drop is mostly resistive.) Moreover, interposing an earth wire will not block magnetic coupling. The dominant coupling in mains wiring is capacitive.
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