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? 

Parents
  • 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.

Reply
  • 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.

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