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Discuss LED light in bathroom flashes when extractor fan TP pole switch is closed. in the Electrical Forum area at ElectriciansForums.net

Pretty Mouth

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If you're really brave you could fit a 2-way light switch, put the switched live on the 'common' terminal, the live on one of the other terminals and an earth on the last terminal. The earth is functional as well as protective after all...
I suppose if the N was brought to the last terminal instead of the earth, that would equalise any potential across the lamp when switched off? Not sure it's a good idea/compliant though, something to do with switching neutrals
 
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pc1966

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I suppose if the N was brought to the last terminal instead of the earth, that would equalise any potential across the lamp when switched off? Not sure it's a good idea/compliant though, something to do with switching neutrals
Using a change-over switch to short out the lamp is perfectly OK as you are not switching the neutral, you are switching the live feed to the lamp (i.e. at no point would both lamp terminals be live due to an open on the neutral path).

But...I would worry about the next one changing the switch getting a nasty surprise as it is so common/normal to have unsleeved blue (or black) as the switched live to a light (not really neutral)!
 

Marvo

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I've just been reading one of your past posts from a few years back Marvo, which said exactly the same thing with regards the earth. You got a minor telling off from @Lucien Nunes :)
Yeah, quite possibly and quite rightly so, hence the smileys. As mentioned by Pretty Mouth it would be a considerably more legitimate remedy if you used the neutral rather than the earth. Creating a path between neutral and earth, even a very resistance path, within an installation isn't a good idea for several reasons and it's forbidden in my local regs so I'm guessing it's also forbidden in the UK regs.
 

Lucien Nunes

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There's another aspect to using a changeover (2-way) switch to short the lamp which is that L and N are present on N/O and N/C contacts (L1 and L2). In a functional switch that is not designed as an isolator, and made for AC only, the contact separation when open is very small, only a millimetre or two; too small to guarantee that the separation alone will extinguish any arc that forms when a load is switched off. This is deliberate, because the arc will extinguish itself at the next current-zero in the AC cycle and the smaller it is in the meantime, the less heat it will dissipate.

Consider first what will happen with a normal switching configuration, either 1- or 2-way, the effect is the same. When the current-carrying contacts open, an arc forms that behaves like a conductor, continuing to pass current through to the lamp across the contact gap. It doesn't matter whether the common contact then makes contact with another other fixed one, perhaps bringing another strapper into circuit. The current flows through the arc and lamp until, not more than 1/100 of a second later, it drops to zero in the AC cycle, the arc extinguishes and the lamp goes out.

Now consider this lamp-shorting arrangement when you switch it off. The common contact separates from the line at L1, drawing an arc with it that keeps feeding current to the lamp. But then the common makes contact with L2 which is connected to neutral, and the arc that is conducting from L1 to common now forms a short-circuit between line and neutral. It will almost certainly extinguish before the MCB trips, but not before chewing a chunk out of the contact.

Back in the bad old days there was a 2-way switching scheme that had line and neutral across each 2-way (called Carter 2-way, look it up if not familiar). It had a serious disadvantage that one of the off combinations made both terminals of the lamp live, but it worked. However, in those days, the contact separation of switches designed for DC was more like half an inch so that the separation alone was sufficient to extinguish any arc, i.e. the switch design avoided drawing an arc from L to N when used in the Carter configuration. I would not trust a modern micro-break AC-only light switch to achieve this, so would never put L and N on opposite contacts of the same pole.

You could however use the 2-way switch to connect the snubber / dummy load, so that it is not always in circuit when the lamp is on. Save a few pence per year!
 

happyhippydad

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There's another aspect to using a changeover (2-way) switch to short the lamp which is that L and N are present on N/O and N/C contacts (L1 and L2). In a functional switch that is not designed as an isolator, and made for AC only, the contact separation when open is very small, only a millimetre or two; too small to guarantee that the separation alone will extinguish any arc that forms when a load is switched off. This is deliberate, because the arc will extinguish itself at the next current-zero in the AC cycle and the smaller it is in the meantime, the less heat it will dissipate.

Consider first what will happen with a normal switching configuration, either 1- or 2-way, the effect is the same. When the current-carrying contacts open, an arc forms that behaves like a conductor, continuing to pass current through to the lamp across the contact gap. It doesn't matter whether the common contact then makes contact with another other fixed one, perhaps bringing another strapper into circuit. The current flows through the arc and lamp until, not more than 1/100 of a second later, it drops to zero in the AC cycle, the arc extinguishes and the lamp goes out.

Now consider this lamp-shorting arrangement when you switch it off. The common contact separates from the line at L1, drawing an arc with it that keeps feeding current to the lamp. But then the common makes contact with L2 which is connected to neutral, and the arc that is conducting from L1 to common now forms a short-circuit between line and neutral. It will almost certainly extinguish before the MCB trips, but not before chewing a chunk out of the contact.

Back in the bad old days there was a 2-way switching scheme that had line and neutral across each 2-way (called Carter 2-way, look it up if not familiar). It had a serious disadvantage that one of the off combinations made both terminals of the lamp live, but it worked. However, in those days, the contact separation of switches designed for DC was more like half an inch so that the separation alone was sufficient to extinguish any arc, i.e. the switch design avoided drawing an arc from L to N when used in the Carter configuration. I would not trust a modern micro-break AC-only light switch to achieve this, so would never put L and N on opposite contacts of the same pole.

You could however use the 2-way switch to connect the snubber / dummy load, so that it is not always in circuit when the lamp is on. Save a few pence per year!
Could you explain how to use the snubber at the switch without the neutral Lucien?
 

Lucien Nunes

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But the snubber load will probably work OK though. You could fit it in the isolator, lamp or humidistat, since all have access to SL and N.

One reason we use a snubber (RC network) is that for a given ability to dispose of leakage current, it dissipates less heat when the SL is energised. The capacitor stands most of the 230V, passing a current 230/Xc where Xc is its capacitive reactance. The resistor limits peak currents at sudden changes in voltage e.g. to stop it welding switch contacts. As per the OP, it is possible to use pure resistance instead, but to achieve the same leakage-sinking ability there will be more heat in your back box when live.
 

pc1966

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There's another aspect to using a changeover (2-way) switch to short the lamp which is that L and N are present on N/O and N/C contacts (L1 and L2). In a functional switch that is not designed as an isolator, and made for AC only, the contact separation when open is very small, only a millimetre or two; too small to guarantee that the separation alone will extinguish any arc that forms when a load is switched off. This is deliberate, because the arc will extinguish itself at the next current-zero in the AC cycle and the smaller it is in the meantime, the less heat it will dissipate.
But said switches are rates at 10A, so there could well be a load on the other pole that would sustain an arc of at least 10A when cold, possibly more. So while it is a risk with a historic precident I would be most surprised if any switch rated at 230V/10A was so designed that it could not safely switch from pole to pole without an arc jumping them.
 

Lucien Nunes

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In the scenario you describe, if the switch only handles the line, the arc will always be in series with one or other of the loads, limiting its current to within the switch rating. OTOH if L & N are both present on one pole, any arc that forms can transfer to directly between them, bypassing the load resistance, therefore could rise to the short-circuit current.

I agree that very little arc is likely to form in this low-load scenario but if it does, there is nothing to stop it escalating.
 

Lucien Nunes

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It's not altogether unlike crossfire in a controlled bridge. Doesn't often happen but can cause immediate destruction if it does. Crossfire can happen in Mercury Arcs too under both controlled and passive rectification. They usually had surge protection across the anodes to minimise the risk of a transient setting it off.
 

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