Smart relays on ring mains

Hi, 

I have installed plenty of these devices, as well as their dimmer range. 

https://shelly.cloud/products/shelly-1pm-smart-home-automation-relay/

What I am seeing however, is a number of people within online forums, installing these smart relays on ring mains. What they are doing is spurring from a ring, in 2.5mm and then switching this spur with the relay (without any form of fusing down, via means of an FCU). 

I do not believe this is compliant, for the following reason: 

  • The relay is rated at 16A, and although it has overload protection, this is electronic and if the contact welds, the overload would not be broken by the relay itself.

If installed after a 13A fused connection unit, the device is not only protected by a 13A fuse (less than the contact rating) it is also then seen as some sort of appliance. 

Is there anything else to add to this? 

Would anyone here actually install these relays in-line with a ring main and if so, why/how does this comply with BS7671? 

A few people have stated that the likes of Click Smart have a similar setup in a double socket: 

https://click-smart.com/products/mode-smart-sockets

However these are covered by a product standard and not BS7671, so the application is very different. 

Thoughts? 

  • A customer had a rant at me, because I installed double pole switches upfront of the Smart switches he had bought for his immersion heaters, I explained they were there for safety to allow plumbers and electricians to work on his hot water tank and immersion heaters safely and refused to remove them, despite him saying they were not required and he didn’t want them.

    Bear in mind all of these switches are out of sight in his airing cupboard and I had to run two supplies through off-peak smart switches to get it to work, making it clear to anyone working on it in future what I had done.

  • It would certainly be cleaner with a 13A fuse on the supply side - not least because you can make it safe without killing power to the whole ring by taking that fuse out.

    The correct thing to do of course is to Email the makers, but it may be worth explaining what a ring final actually is - the company has offices in Bulgaria, Germany and the US, none of which expect sockets to be fed from 32A B type breakers.

    How it fails and if it is actually dangerous is unclear - it certainly should never be relied on as the means of safe isolation to work on something, and I can imagine the relay may either weld up as a dead short or blow open after an overload, so it can only be used for loads where that would be a nuisance but not a disaster.

    I'm not seeing CE marks in the photos, but they may be on the paperwork, or it may be  up to the end user to test and certify - which would be out of scope for most electricians.

    If the odd home enthusiast wants to try it on known low power loads, the real risk is low. I'd be surprised if any responsible sparks would be keen to adopt the responsibility however.

    Mike

  • If its controlling one double socket spured off the ring main then its no different to having a double pole switch in its place. A double socket is only rated at 13A culminative these days anyway so the relay can take that 

  • It rather depends what upstream protection the relay module is intended to be used with. You have to consider not just normal operation (if that was true we could omit all fuses and breakers back to the substation - I cannot imagine many folk recommending that) but also what happens during any credible failure modes, and if the relay unit is part of the fixed wiring, then that is faults both in the electronics pod of the relay, and up and downstream of it.

    I do not think it is clear cut - it may be internally designed to fail safe, but given the costs is less than £20, probably not.

    Maybe I'll ask the makers.

    M.

  • The relay is rated at 16A, and although it has overload protection, this is electronic and if the contact welds, the overload would not be broken by the relay itself.

    The regs get a bit messy with this sort of thing, as there are a lot of different characteristics to consider. Roughly speaking overload and fault current protection may be treated separately, with with the possibility of overload protection being downstream (or even implicit in the nature of the load), so often it's the fault current characteristics that need to be considered - which are often rather different to the simple long duration current rating of an item. It's the kind of consideration that lets us use what's basically 20A cable for an unfused spur from a 32A ring circuit.

    In the old days we'd sort of consider a 16A accessory as being roughly equivalent to a 16A conductor and calculate on that basis but that always was a bit of a kludge especially where the device is capable of making onto a fault or attempting to break a fault current - these days there's an entire section (536) demanding we consider all sorts of characteristics, but I think in most cases it boils down to 'ask the manufacturer' - either for stacks of data or just for their requirements for upstream protective devices for fault protection.

       - Andy.