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Din Rail devices in domestic home

Former Community Member
Former Community Member
I wish to install a Siemens Logo PLC+additional modules to control my home heating system. This is a DIN rail device does this it have to be mounted in a BS7671-3 compliant enclosure? If the enclosure also include a 10A MCB for protecting the input supply cable and 3A MCB for protecting the relays, would this change the answer? Only one mains electrical circuit will be going into the box, it will be providing the power of a small electric underfloor heating system, circlation pump for wet underfloor heating, and power for valves to provide zone based heating control.


It would have been nice to use a enclosure with a window so the display of the logo could be seen without opening the enclosure, or perhaps being able to ventilate the box as it will have a 1.3A 24V power supply in it to power the automation. Both of which do not seem allowed with a fire proof BS7671-3 enclosure.
  • 0
    By BS7671-3  I take it you mean the non-combustible requirements for consumer units (CU) introduced by AMD 3 of BS 7671:2008 (for info, we're currently on BS 7671:2018 AMD 1).


    The requirement only applies to consumer units and 'similar switchgear' - which is usually taken to mean things such a 3-phase distribution boards used in large domestics or switchfuses supplying distribution circuits. Smaller 'switch and fuse' devices - such as 13A a fused connection units (FCU) - although in principle very similar to a 1-way consumer unit - aren't considered to be covered by that requirement.


    So I think the question boils down to whether you (as the designer) consider your setup to be closer to a CU or a FCU. I think if it were me I'd go for the latter.


       - Andy.


  • 0
    Why is the input supply cable not protected in the CU? Alternatively, if it is spurred off a socket circuit, it could be protected by an FCU.
  • Former Community Member
    0 Former Community Member
    Its not directly wired back to the the CU, but rather on a spur from the ring main, I always like to have all spurs fused, as second owners can have a habit of replacing a single socket with a double or worse. Having an FCU wired into the wall would be my normal approach. But as the easily available sizes of fuses are 3, 5 and 13A. So if it were to blow (not that i think it ever would), the likely hood would be someone would shove in a 13A fuse. Hence I felt a 10A MCB might be safer option, as that is the rating on the relays for the LOGO, the extension unit relays are however only rated at 3A, hence why i said 2. Perhaps I will just do both being a belt and braces kind of person.
  • Former Community Member
    0 Former Community Member
    Thank you for explaining classification of devices considered to be "similar switchgear", I was not sure as to what they exactly referred to, and they last thing I want to do is have something installed into my house in a manner that was not compliant with the regulations. After all Safety is why I am using a "proper" PLC device and not some cheap relay board from china controlled by say a rasberry pi.

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