Can a neon indicator light work across L and N?

[Penners]

I have an electric towel rail, rated at 600W. It is supplied through a fused connector which is in turn supplied from a pullswitch (switched live only).

At the moment there is no way of telling whether the rail is switched on or not, unless you can feel its warmth. So I want to add an indicator light to the connection box to show that it's on.

I have bought an MK Neon Indicator for light switches. It's a clear plastic "frame" that fits between the faceplate and the back-box of a light switch, and has two little neon lights that make the edge of the frame glow when the light is off (so that you can find it in the dark). My intention is simply to connect the neons across the switched live and the neutral of the connection box, so that they glow when the towel rail is on.

However, before I do this, I'm just looking for reassurance that the neons will work OK like this. The device was primarily designed to work without a neutral, so is it OK to connect it to live and neutral? Or will it blow?

Many thanks for any advice.

 

[Lime]

Something like this?-

http://www.screwfix.com/p/mk-1-gang-neon-locator-plate-switch/73782


That should be fine.

 

[Penners]

Yes, that's the exact thing, Lime.

Spurred on by your approval, I have connected it and switched on - success! Thanks very much for your advice.

Just to improve my education, would the neon also work if it were connected in parallel across the live supply only - in other words, live into it and live out of it?

 

[Flyfisher]

I don't think a neon indicator would light up when connected in that way. They typically require 90 volts across their terminals in order to ionise the neon gas and light up, though it depends on the size of the 'bulb'.

Actually, thinking about it, it might well light up when the switch is 'off', because the 'live in' would be at mains voltage but the 'live out' would be at (or near) zero volts, thus giving mains voltage across the neon, so it would light up.

When the switch is 'on' the voltage on the 'mains in' and 'mains out' connections would be the same and therefore there would be no voltage between them or across the neon, so it would go out.

Handy if you wanted a "Not On" indicator though. :wink:

 

[plasticpigeon]

I'd have thought connecting lives to each side of the neon in the circuit with the towel rail would be connecting it in series with the towel rail. Maybe there would be enough potential difference across it to get the gas to glow I really don't know (it depends on the relative resistance of both), but I wouldn't think that the bulb would allow the necessary current through it for the towel rail to heat up! I'm no expert though!

 

[Owain]

Something like this?-
http://www.screwfix.com/p/mk-1-gang-neon-locator-plate-switch/73782
That should be fine.

AIUI those are intended for use in series with a filament bulb i.e. wired across the Supply Live and Load Live terminals of a lightswitch. They rely on a small current flowing through the filament bulb to illuminate the neon (and on the filament bulb to provide a series resistance for the neon). The neon is shorted when the switch is on and the room light illuminated.

It would be better to replace the fused connection box with one with a neon in which will be connected across Load Live and Load Neutral and incorporate the required series resistance. eg MK K377

Note however that a fixed heating appliance should be supplied through a double pole switch for isolation and this must include a mechanical indication of on/off status eg either a toggle/rocker switch or a pull switch with a mechanical flag. The neon cannot be relied upon as a visual indication of isolated status as it can fail to give a false safe indication. EG

 

[Lime]

Something like this?-
http://www.screwfix.com/p/mk-1-gang-neon-locator-plate-switch/73782
That should be fine.

AIUI those are intended for use in series with a filament bulb i.e. wired across the Supply Live and Load Live terminals of a lightswitch. They rely on a small current flowing through the filament bulb to illuminate the neon (and on the filament bulb to provide a series resistance for the neon). The neon is shorted when the switch is on and the room light illuminated.

It would be better to replace the fused connection box with one with a neon in which will be connected across Load Live and Load Neutral and incorporate the required series resistance. eg MK K377

Note however that a fixed heating appliance should be supplied through a double pole switch for isolation and this must include a mechanical indication of on/off status eg either a toggle/rocker switch or a pull switch with a mechanical flag. The neon cannot be relied upon as a visual indication of isolated status as it can fail to give a false safe indication. EG

No.
The MK 1-Gang Neon Locator is rated at 200-250V / 50Hz.
They are able to operate directly on the mains.
There is a series resistor with each neon.
The resistance of any filament lamp is far too low to operate a neon without its own series resistor.

 

[Flyfisher]

Neon bulbs become highly conductive when alight (because the neon gas is ionised) and without something to limit the current flow they would very quickly burn out. Because they typically require only a tiny current to operate (around one milliamp), the series resistor must be a fairly high value (usually measured in thousands of ohms) and certainly much greater than the resistance of a filament bulb.

 

[Penners]

I knew that... ahem... :?

 

[Zebra]

Dammit. Arrived late to the party. Yes, Penners, that would work. Oh, you already know that. Oh, OK.

 

[Zebra]

Actually, thinking about it, it might well light up when the switch is 'off', because the 'live in' would be at mains voltage but the 'live out' would be at (or near) zero volts, thus giving mains voltage across the neon, so it would light up.

At the risk of starting up a lengthy electrical argument, I don't think this is correct. Switched live is not at (or near) zero volts. It doesn't have a voltage as such, because it's not connected to anything. You don't get 230volts between live and switched live on a normal house wiring circuit, if you did it would go bang every time you switched the light on.....

 

[Flyfisher]

If the switched live was not connected to anything then nothing would happen when it was switched on would it?

Imagine a simple circuit of a switch and a lightbulb. Live connects to one side of the switch ('Live in'). The other side of the switch ('switched live') connects to one side of the lightbulb. The other side of the lightbulb connects to Neutral.

Live---------(SwitchIn)/(SwitchedLive)---------(Lightbulb)--------Neutral

When the switch is off, the 'switched live' side is connected to Neutral through the lightbulb. If you measured the voltage across the two switch terminals it would be 230v (ignoring the tiny volts drop because of the lightbulb resistance).

If you now connected a neon across the switch terminals it would light up (but the lightbulb would not).

When the switch is turned on, it won't go bang because the lightbulb limits the current flow (and lights up of course). If you now measure the voltage across the two switch terminals, it will be zero and the neon would not light.

 

[Lime]

If the switched live was not connected to anything then nothing would happen when it was switched on would it?

Imagine a simple circuit of a switch and a lightbulb. Live connects to one side of the switch ('Live in'). The other side of the switch ('switched live') connects to one side of the lightbulb. The other side of the lightbulb connects to Neutral.

Live---------(SwitchIn)/(SwitchedLive)---------(Lightbulb)--------Neutral

When the switch is off, the 'switched live' side is connected to Neutral through the lightbulb. If you measured the voltage across the two switch terminals it would be 230v (ignoring the tiny volts drop because of the lightbulb resistance).

If you now connected a neon across the switch terminals it would light up (but the lightbulb would not).

When the switch is turned on, it won't go bang because the lightbulb limits the current flow (and lights up of course). If you now measure the voltage across the two switch terminals, it will be zero and the neon would not light.

I agree.

 

[skier-hughes]

Oh well, at least I got through two kit kats trying to ingest that info.............. I'll stick to plastering, no good at it either, but at least I understand it

 

[Penners]

When the switch is turned on, it won't go bang because the lightbulb limits the current flow (and lights up of course). If you now measure the voltage across the two switch terminals, it will be zero and the neon would not light.

You see... this is where an ignoramus like me gets into trouble.

If the in-line switch is OFF and you measure voltage across its two terminals, I understand that you are, in effect making the circuit through your meter - so you read 230V. But if the switch is ON, and you put your meter across the terminals, surely you would again read 230V, wouldn't you?

I know this is bound to be wrong, but I don't understand why. In the simplest possible terms, the circuit (AIUI) consists of a loop - from the generator up the live wire, through the (closed) switch, through the light bulb, and down the neutral back to the generator. If we regard the closed switch as non-existent, then - if I understand you correctly - connecting a meter across the live and neutral will show a reading of 230V, but connecting a meter across two points on the live wire will show zero. Why? Isn't there 230V everywhere in the circuit?

My 'ead 'urts...

 

[LadyArowana]

When the switch is turned on, it won't go bang because the lightbulb limits the current flow (and lights up of course). If you now measure the voltage across the two switch terminals, it will be zero and the neon would not light.

You see... this is where an ignoramus like me gets into trouble.

If the in-line switch is OFF and you measure voltage across its two terminals, I understand that you are, in effect making the circuit through your meter - so you read 230V. But if the switch is ON, and you put your meter across the terminals, surely you would again read 230V, wouldn't you?

I know this is bound to be wrong, but I don't understand why. In the simplest possible terms, the circuit (AIUI) consists of a loop - from the generator up the live wire, through the (closed) switch, through the light bulb, and down the neutral back to the generator. If we regard the closed switch as non-existent, then - if I understand you correctly - connecting a meter across the live and neutral will show a reading of 230V, but connecting a meter across two points on the live wire will show zero. Why? Isn't there 230V everywhere in the circuit?

My 'ead 'urts...

Please sir, is the answer the magic pixies again?

 

[Penners]

When it comes to electrickery, it's not the pixies you want to be worrying about, it's the Waveries.

http://jennre.wordpress.com/2012/07/02/the-waverlies-fredric-brown-1945/

 

[philpjuk100]

This electricity can be confusing,many years ago I tried explaining to my wife how to wire up a three pin plug,The earth wire was easy (safety) and then I told her the electricity came up the live wire and returned on the neutral (I know this is not the case but it seemed a good way of explaining it),she then asked "if they get it back why do we have to pay for it?".I considered explaining watts but didn`t.

 

[Zebra]

If the switched live was not connected to anything then nothing would happen when it was switched on would it?
When the switch is off, the 'switched live' side is connected to Neutral through the lightbulb.

Sorry, you're right, I was thinking without the load connected, ie without the lamp in.

If we regard the closed switch as non-existent, then - if I understand you correctly - connecting a meter across the live and neutral will show a reading of 230V, but connecting a meter across two points on the live wire will show zero. Why? Isn't there 230V everywhere in the circuit?

Connecting a meter across two points on the live wire you have 230v at one end of your meter and 230v at the other, so no voltage between them. You only get a voltage reading on your meter when you connect one end to 230v and the other end to either neutral or earth. Can be a problem sometimes when testing a lighting circuit with no earth - you can't tell if it's live at the switch because there's nothing at zero v to test to, you end up stretching to get the other end of your tester onto a radiator.

 

[Zebra]

Please sir, is the answer the magic pixies again?

Yes, there are magic pixies in the cables, and if tempted with sugary sweeties, they can be coerced into lighting things up, powering your vacuum, etc, but only if they want to. If something has stopped working, and you think you have fixed it, then before you switch it on you must ask the pixies nicely. Works for me.
They do steal your tools though.