Using earth for dry contact switch

Discussion in 'Electrics UK' started by CrazyFool, 4 May 2021.

  1. CrazyFool

    CrazyFool

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    Hi guys

    My brother in law has bought me a bunch of sonoff mini smart switch modules and invited me round to his house to show me how he has wired them up.

    Please note the following diagram from the manufacturer to wire a 2 way switch to the smart relay:
    https://community-assets.home-assis.../d04531ec59ede1c19890a702dd8e0ad668d1e0d7.png

    Basically it requires 3 wires between the second switch and the first switch/relay.

    My brother in law only has twin and earth between both switches so he's used the earth cable as the third wire, despite the fact it is unshielded and there is only the pvc jacket protecting it.

    I challenged him about this but he said there is no mains voltage running through the wire. Its just a dry contact switch.
    There are other cables coming into his back plate so the backbox is appropriately earthed.

    I still felt uneasy about this so went home and had a look at my wiring.
    Sods law states there is only twin and earth between my 2 way switches too.

    I've had a quick Google search and he's right about it being a dry switch but I'm not sure if he should be using unshielded cable for this purpose.

    What do you guys think?
    How concerned should I be about my brother in laws wiring?
    I'm not sure how far I should push him about this. He knows a little bit more about electrics than I do so I'm mindful about not wanting to be teaching him how to suck on eggs.

    Thanks in advance

    CF
     
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  3. EFLImpudence

    EFLImpudence

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    Strictly speaking, no. Flexible cable could be used because the "earth wire" is also insulated.

    Why not just buy and use the correct cable?
     
  4. SUNRAY

    SUNRAY

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    Please explain what you (EDIT: or for that matter your BIL) understand by:
     
    Last edited: 4 May 2021
  5. plugwash

    plugwash

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    I don't see anything in the manual about what if-any isolation there is between the switch connections and the mains. So IMO you need to treat them as potentially connected to mains voltages and wire accordingly.
     
  6. CrazyFool

    CrazyFool

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    Sorry, no to which question?
    Unfortunately the twin and earth is already plastered into the wall between both switches so there is no chance of running new cable. The earth wire is only protected by the pvc jacket.

    Now your testing me lol.
    Dry contact switch as in no substantial voltage running through the wire.
    Activation of a switch just completes a relay circuit. The relay activates a separate circuit where all the mains voltage is handled.

    Thanks buddy. I've opened a few up and s1 and s2 (the dry contact wires in question) go directly into the relay which is separated from the rest of the board. Are you saying there could be a risk of mains voltage jumping to these terminals? How isolated would they need to be?
    :S

    Thanks again.
    CF
     
  7. winston1

    winston1

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    Deleted.

    Unhelpful. Again.
    Barred from thread.
     
    Last edited by a moderator: 5 May 2021
  8. plugwash

    plugwash

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    You can't just connect a switch to a relay coil and have it turn a relay on, there has to be some form of power to the control circuit. Given the wiring diagrams show no external source of power to the control circuit power must be coming from inside the sonoff unit.

    It's possible that sonoff used a power supply that is safely isolated from the mains, but it's equally possible that they used a power supply that is not isolated at all, or that they used a power supply that is notionally isolated but is not approved to any safety standards. In the absense of documentation you have to assume it is not isolated.
     
  9. boringoldcodger

    boringoldcodger

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    At the risk of sticking my nose in where it doesn't belong.

    I believe:
     
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  11. SUNRAY

    SUNRAY

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    Let's get to understand some of the terminology.

    A dry contact (more commonly volt free contact or clean contact anongst other names) is the bit connected to the smart switch terminals S1 & S2. Basically dry contact means wiring and switch not connected to anything else, just as shown:
    upload_2021-5-4_17-42-33.png
    However, and just like any light switch, one side of that wiring will need to be connected to the power and the other will be connected to a load, such as this very standard domestic light arrangement:
    upload_2021-5-4_17-59-22.png
    The Sonoff unit will be replicating that in some way such as operate a mains voltage relay:
    upload_2021-5-4_18-14-10.png
    It may very well be working at a safe extra low voltage (SELV) such as this with 5volts DC (just as an example):
    upload_2021-5-4_18-4-24.png
    Equally instead of the relay it could be an input to a microprocessor but there will still be some sort of voltage involved.

    It's not the Sonoff that's 'Dry contact' but the Sonoff requires a 'dry contact' to be connected to it.
    Without appropriate paperwork or measuring voltage there is no way we can know what voltage it's working at and what standard of wiring is required. It doesn't help that the instructions I've found are about a useful s a chocolate teapot.

    Howeer I've found several references to screened cable which implies SELV, If, if, that is the case then the current arrangement is safe albeit I'd be very careful if it shares a switch enclosure with any 'conventional' circuits.
     
    Last edited: 4 May 2021
  12. CrazyFool

    CrazyFool

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    Thanks for the comments.
    Other people have measured the voltage and posted the results online. They measured 3.3v at S1 and S2 terminal.
    Is screened cable the same as unshielded cable inside a pvc jacket?

    Cheers

    ES
     
  13. SUNRAY

    SUNRAY

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    Thanks. In that case it sounds like an input to a microprocessor or something similar. The current cable arrangement will be electrically completely safe, however as mentionsed before I'd be wary of proximity with mains powered devices (at a 2 gang switch for example).

    There may also be an interference issue but I don't know what steps Sonoff have taken to mitigate such.
     
  14. plugwash

    plugwash

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    Sorry but no.

    The fact that the measured voltage between the two switch inputs is only 3.3V does not in any way imply what voltages may be present between the input terminals and other terminals, or the safety ratings of any isolation barriers.

    It's perfectly possible to have a 3.3V logic supply that is referenced to mains!
     
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  15. plugwash

    plugwash

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    Just found pictures of the internal PCB from the sonoff mini and as I suspected there don't appear to be any power isolation components. Looks like a bridge rectifier followed by a non-isolated buck converter to provide the logic supply.

    So you should definitely treat the switch wiring as mains wiring.
     
    Last edited: 5 May 2021
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  16. SUNRAY

    SUNRAY

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    OMG. Thanks PW, I can't believe I made such a stupid 'schoolboy mistake'.
    Again I'll say the limited instructions I found gave no mention of appropriate cable/voltages and I took the 3.3V on face value that others had made sufficient tests.

    As it happens since reading your correct reply I've been able phone to a colleague who has experience with the product and he states the voltage sits roughly half way between L & N so indeed the 'Dry contact' wiring does need to be treated as low voltage (mains 230V) and use of the earth wire for this is INCORRECT.
    In fact there is a requirement for an earth to be provided at the switch positions.

    Again I'll apologise for offering false information.
     
  17. CrazyFool

    CrazyFool

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    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:
    upload_2021-5-5_15-5-54.png

    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
     

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