Using earth for dry contact switch

[Keithmac]

If you watch bigclive on YouTube it's absolutely shocking what's available for sale in the UK safety wise.

 

[plugwash]

I think I might have sussed out another way to make this work using momentary switches. You can set the sonoff at software level to recognise a momentary switch as an 'ON' signal. The following schematic will allow me to connect both switches with the pre-existing twin and earth. I'll just have to replace my standard switches with momentary switches:

Sounds like a plan.

BTW manufacturers of electrical accessories use the term "retractive switch" for momentry switches.

To make sure I've understood correctly... he shouldn't use the unshielded earth because the mains voltage could jump from the live terminal to S1 and S2. Is this correct?

If i've understood the design correctly it's not a case of "could jump", it's a case of those terminals being live with a substantial pulsating DC voltage in normal operation.

 

[JohnW2]

BTW manufacturers of electrical accessories use the term "retractive switch" for momentry switches.

They do, and I've often wondered why, since the switches in question only seem to have a slightly passing relationship with the dictionary definition of the word. Does anyone know how this use of the word came about?

Kind Regards, John

 

[CrazyFool]

Thanks for all the help guys.
Much appreciated.
CF

 

[user_id_for_q]

In case it helps anyone, I've just had a near-miss with one of these Sonoff Mini R2 devices. I too thought the switch connections were at a low safe voltage and so attempted a quick job without disconnecting the mains! All I was doing was swapping one switch for another so I unscrewed the 2-core bell wire from the existing switch, leaving the other end connected to the switch terminals of the Sonoff. Unfortunately, the bare end then slid away from me and managed to touch (and stay touching) the back of my dishwasher (which is earthed), causing the Sonoff to start arcing inside! Fortunately my fuse box did it's thing and spotted the current going to earth and turned off the circuit. But there was still enough time for the Sonoff to turn black inside. Interestingly it all still works except for the switch input, which looks to have a resistor burnt out near by.

Being a bit surprised at how much current must have been flowing to nearly start a fire, I did some measuring on a replacement. Sure enough, there is only 3v between one switch terminal and the other, but 110v between each terminal and earth! Interestingly, this is not enough to sense with my hand since I touched the connection a few times while attaching the switch and felt nothing. I wasn't touching earth at the same time though so not sure what would have happened if I had. If the wire hadn't touched earth I would be none the wiser.

Anyway, I know it doesn't sound like it, but I do have a reasonable understanding of electricity and I'm normally very careful when I know I'm dealing with mains voltage i.e. I connect a meter, verify voltage is present, turn power off at the fuse box, verify voltage is zero, turn fuse box on again and verify voltage returns (this checks that voltage went to zero because of fuse box and not some other coincidental reason), and finally turn circuit off again and verify zero voltage before starting work. I got caught out this time because I trusted the Sonoff documentation (and discussions on the Internet). I also completely forgot the idea that voltage is actually potential difference so 3v can just be the intuitive 3v above 0v, but it can also be 113v above 110v, which seems to be the case in the Sonoff.

 

[oldbutnotdead]

Thanks guys
I think I might have sussed out another way to make this work using momentary switches. You can set the sonoff at software level to recognise a momentary switch as an 'ON' signal. The following schematic will allow me to connect both switches with the pre-existing twin and earth. I'll just have to replace my standard switches with momentary switches:
View attachment 232599

Going back to my brother-in-law. I'll try to feed this back to him.
To make sure I've understood correctly... he shouldn't use the unshielded earth because the mains voltage could jump from the live terminal to S1 and S2. Is this correct?

Thanks again

He must not use the bare copper CPC in T & E cable for anything other than CPC. If you had 3 core flex you can use the green/yellow core for other purposes in some circumstances if you sleeve it with something not green and yellow.

 

[bernardgreen]

since the switches in question only seem to have a slightly passing relationship

The expression " switching from daylight to lamp light " may be the origin from days long gone ( gas lights on the street )

 

[JohnW2]

The expression " switching from daylight to lamp light " may be the origin from days long gone ( gas lights on the street )

Albeit you're referring to something I wrote about 9 months ago, in what sense does that explain how momentary-action switches came to be known as "retractive" (which is what my comment/question was about) ?

Kind Regards, John

 

[EFLImpudence]

They do, and I've often wondered why, since the switches in question only seem to have a slightly passing relationship with the dictionary definition of the word.

What is your objection? I think it far less dubious than other terms we have to endure.

Does anyone know how this use of the word came about?

A minion at CEF or similar ???

 

[JohnW2]

What is your objection? I think it far less dubious than other terms we have to endure.

No objection at all. I was just wondering (~9 months ago) whether anyone knew how this word "retractive" came to be used in the context of switches!

I've always thought of, and described, them as "momentary-action switches", and undoubtedly will continue so to do, although I obviously also have to 'understand' the language which some others use

Kind Regards, John

 

[bernardgreen]

For they are momentary-action switches,

The first time I heard "retractive" applied to momentary-action switches was when talking about pull cord switches in the warden call system of a sheltered home community. The cord will "retract" when the person lets go of it.

 

[JohnW2]

... The first time I heard "retractive" applied to momentary-action switches was when talking about pull cord switches in the warden call system of a sheltered home community. The cord will "retract" when the person lets go of it.

That's probably the nearest to a credible 'explanation' I've heard.

However, the cord of a NON-retractive pull-switch also "retracts" when one lets go of it, doesn't it?

Kind Regards, John

 

[EFLImpudence]

No objection at all.

Oh, sorry. I thought you were implying it was inappropriate.

 

[Jackrae]

More than likely a term dreamt up by someone with minimal knowledge and experience of electrical or control terminology. "Fleeting" or "Momentary" are far better terms.

 

[JohnW2]

Oh, sorry. I thought you were implying it was inappropriate.

As I said, not at all - I' was merely curious/intrigued!

As I said, it's probably not a term that I would ever use myself, unless it seemed that it were the only way of communicating what I was talking about!

Kind Regards, John