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Grid switch ratings enquiry

Farmboy
Had an interesting chat with a tech dept this afternoon after coming across some grid switches in a domestic kitchen.


Four grid switches (sharing a common backbox) are each fed via their own 16A cb's, and each feeds 1G sockets (for a cooker hood, fridge freezer, etc). Ignoring the cable sizes (the focus being on the grid switches), given the 13A plug fuses limits the loads in each cct, would you have the current rating of the grid switches equal to or higher than the 16A cb's (e.g. 20A), or lower (but equal to or higher than the socket rating e.g. 13A/14A)?


F
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    Any switch controlling a 13 Amp socket must be able to carry at least 13 Amps. That is what the 13 Amp socket can supply. A grid switch rated at 16 or 20 Amps could be used. It would be bad practice to use a grid switch rated at 6 or 10 Amps to supply a 13 Amp socket if the socket could be used by any unknown appliance. I suppose that the cooker hood is a very low consumption load and the control switch could be rated at 6 Amps., but why bother? 


    Z.
  • 0

    Had an interesting chat with a tech dept this afternoon



    Ah, this is going to be something "interesting" then - possibly something to do with the new regulations about co-ordinating switches etc with overcurrent protection (e.g. 536.4.2.3). Seemingly we can't just assume that a 20A switch is going to behave like a bit of 20A cable as far as faults are concerned (like we probably used to) - but we need the switch manufacturer's data in order to select fault protection - so I'm guessing that they've said something like a 20A MCB can't protect a 16A switch even if there's overload protection further downstream.


      - Andy.
  • 0

    Zoomup:

    Any switch controlling a 13 Amp socket must be able to carry at least 13 Amps. That is what the 13 Amp socket can supply. A grid switch rated at 16 or 20 Amps could be used. It would be bad practice to use a grid switch rated at 6 or 10 Amps to supply a 13 Amp socket if the socket could be used by any unknown appliance. I suppose that the cooker hood is a very low consumption load and the control switch could be rated at 6 Amps., but why bother? 


    Z.




    Ah, you've covered the switch being either rated higher than the cb, or lower than the socket rating, but the question is about the bit in the middle. Given in normal service each single socket won't overload, would you have the switch rating higher than the cb rating, or have the switch rating equal to or higher than the socket rating but lower than the cb rating eg. 13A socket rating, 13A switch rating, 16A cb??


    F

  • 0
    Bear in mind that a single socket could have a 2-way unfused 'cube' adaptor plugged into it, effectively turning it into a double socket, where 20A may flow (but never 26A, because that never happens, because Reasons),
  • 0

    wallywombat:

    Bear in mind that a single socket could have a 2-way unfused 'cube' adaptor plugged into it, effectively turning it into a double socket, where 20A may flow (but never 26A, because that never happens, because Reasons),




    Good point, so perhaps being sockets in this specific scenario aren't the best items to use as examples, the issue is more to do with the order of items based on current rating e.g. load, switch, ocpd, or load, ocpd, switch.


    F

  • 0
    Personally I'd base the switch rating on the breaker rather than the expected load, unless the load has its own overcurrent-protection and can't be modified by ordinary persons. And ideally not even then.
  • 0
    Just use 20 Amp grid switches for everything, then you are covered. Are there grid switches rated above 20 Amps?


    MK makes 20 Amp grid switches marked "Hob" and "Oven".

    https://www.tlc-direct.co.uk/Main_Index/Wiring_Accessories_Menu_Index/A_White_All/Grid_System/Grid_System_3/index.html?ad_position=1t2&source=adwords&ad_id=110077463957&placement=&kw=grid%20switch&network=g&matchtype=b&ad_type=&product_id=&product_partition_id=&version=finalurl_v3&gclid=EAIaIQobChMIpIDvlYyJ4wIVC5ztCh0c1AwhEAAYAiAAEgI7mvD_BwE


    Z.
  • 0

    Zoomup:

    Just use 20 Amp grid switches for everything, then you are covered. Are there grid switches rated above 20 Amps?


    MK makes 20 Amp grid switches marked "Hob" and "Oven".

    https://www.tlc-direct.co.uk/Main_Index/Wiring_Accessories_Menu_Index/A_White_All/Grid_System/Grid_System_3/index.html?ad_position=1t2&source=adwords&ad_id=110077463957&placement=&kw=grid%20switch&network=g&matchtype=b&ad_type=&product_id=&product_partition_id=&version=finalurl_v3&gclid=EAIaIQobChMIpIDvlYyJ4wIVC5ztCh0c1AwhEAAYAiAAEgI7mvD_BwE


    Z.




    Input appreciated but the OP is about what I've found in an existing installation, so it's not what i would do, it's what others have done (a 13A fused load, a 13A switch, a 16A cb), so i wondered in principle whether others would do the same thing i.e. have the switch rating equal to or higher than the downstream protective device but lower than the upstream device.

  • 0

    Zoomup:

    Any switch controlling a 13 Amp socket must be able to carry at least 13 Amps. That is what the 13 Amp socket can supply. A grid switch rated at 16 or 20 Amps could be used. 

    Z.




    If the socket has a BS 1363 plug fitted with a BS 1362 fuse rated 13A then it could be supplying 20 for a significant period (>3hours). Take a look at the fuse characteristics

    .8278a6e0c6707e10a3506006120401c6-huge-bs-1362-fuse-characteristic.png

  • 0
    true - 13A fuses and 16A breakers do not really discriminate,  (13A fused spurs might actually have closer protection if they were wired up with a 16A MCB instead)  but that is what we mean by a 13A circuit - one that can take 13A all day, but cuts off after some rather indeterminate time at 20A, and the cable rating reflects that (hopefully), in that a cable rated for X amps continuous, can carry a few times X for a short time.


    (fuses have the nice feature of not jamming or welding, and not having a problem with not getting faster with larger fault currents, so are better for dead shorts, - a disconnection is pretty much guaranteed, only the number of bits of busted ceramic to be swept up afterwards rises with the PSSC.)

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