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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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    (a 13A fused load, a 13A switch, a 16A cb)



    That's interesting - I've never come across a 13A grid switch - usually they're 5/6A, or 20A. 13A markings on BS 1363 accessories yes, but not grid. I wonder if the switch element had been re-purposed from a FCU (some brands seem to make them up from grid-like components).


       - Andy.
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    AJJewsbury:




    (a 13A fused load, a 13A switch, a 16A cb)



    That's interesting - I've never come across a 13A grid switch - usually they're 5/6A, or 20A. 13A markings on BS 1363 accessories yes, but not grid. I wonder if the switch element had been re-purposed from a FCU (some brands seem to make them up from grid-like components).


       - Andy.

     




    Crabtree, for example, do a 10A grid switch. These I've come across don't look exactly like a grid switch which mounts to a grid, but are modular in the same way and attach to a front plate. So, when I saw the switch and cb ratings, I wondered whether someone has decided to apply the same method 1 and method 2 principle now used to determine sizes of RCCB's and main switches, adopting the method 1 approach (in excess of total sum of downstream protective devices, but lower than upstream devices). So after talking to two tech helpers, the answer was don't know technically if this is permissible, but better to just rate switches higher than protective device.


    F

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    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




    Presumably the single 13 Amp sockets are behind the appliances like the fridge freezer, or at high level above the extractor hood, so are very unlikely to be used for additional appliances. That is why there are conveniently positioned grid control switches. Therefore this is a non-issue. A 10 Amp grid switch will not catch fire or blow up if carrying 11 Amps in most cases. A 20 Amp grid switch will be fine for other appliances like washing machines or tumble driers etc. Overloading is very unlikely if the single 13 Amp sockets are inaccessible. Stop worrying needlessly.


    Z.

  • 0

    Alasdair Anderson:




    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


     




    No it can't supply 20 Amps for hours. These are apparently single 13 Amp sockets for individual dedicated appliances like a cooker extractor hood and fridge freezer etc., so the loads are fixed. No overloading can occur.


    Z.

  • 0

    Presumably the single 13 Amp sockets are behind the appliances like the fridge freezer, or at high level above the extractor hood, so are very unlikely to be used for additional appliances. That is why there are conveniently positioned grid control switches. Therefore this is a non-issue. A 10 Amp grid switch will not catch fire or blow up if carrying 11 Amps in most cases. A 20 Amp grid switch will be fine for other appliances like washing machines or tumble driers etc. Overloading is very unlikely if the single 13 Amp sockets are inaccessible. Stop worrying needlessly.



    Ah, but you're only considering overload - what about fault currents?


      - Andy.
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    AJJewsbury:




    Presumably the single 13 Amp sockets are behind the appliances like the fridge freezer, or at high level above the extractor hood, so are very unlikely to be used for additional appliances. That is why there are conveniently positioned grid control switches. Therefore this is a non-issue. A 10 Amp grid switch will not catch fire or blow up if carrying 11 Amps in most cases. A 20 Amp grid switch will be fine for other appliances like washing machines or tumble driers etc. Overloading is very unlikely if the single 13 Amp sockets are inaccessible. Stop worrying needlessly.



    Ah, but you're only considering overload - what about fault currents?


      - Andy.

     




    What about fault currents? 559.5.1.204 allows a lighting circuit to be protected by an over current protective device up to 16 Amps, but we generally use 6 Amp rated light switches. There is normally no problem with that.


    Z.

  • 0
    Neither the light switch nor the grid switch has to survive after closing onto a fault, and probably won't - it just has to not blow up in a way that is injurious to the hapless user.

    -  in such a case if it goes bang and never works again, well it goes bloody bang doesn't it,  the switch is acceptable collateral damage.
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    What about fault currents? 559.5.1.204 allows a lighting circuit to be protected by an over current protective device up to 16 Amps, but we generally use 6 Amp rated light switches. There is normally no problem with that.



    But you can't just ignore all the other regulations because you've compiled with one of them.  Have a read of 536.4.2.3 that I mentioned earlier.

     

    Neither the light switch nor the grid switch has to survive after closing onto a fault, and probably won't



    Maybe not, but it might be nice if it survived if it was already closed when the fault occurred - and in any event it needs not to blow up to the extent of taking someone's arm off or eye out - how can you show that's the case if you don't have any consideration of the energy let-though by the protective device?


       - Andy.
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    An interesting thing about 536.4.2.3 (which may be relevant or not to the discussion) is that it doesn't seem to apply to all switching devices. It specifically mentions "switches complying with 60947-3 or 60669-2-4.....snip" which looking at the entry for switching devices in Table 537.4 are the only two suitable for Isolation. Now grid switches I believe are typically to 60669-1 (even the ones printed Cooker etc), so a) aren't suitable for Isolation, and b) not apparently within the scope of 536.4.2.3?
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    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),




    Being a little devilish ...


    Double socket-outlets to BS 1363-2 are now (to the current standards) rated at 13 A. Single socket-outlets are also rated 13 A.


    This leads to the question "Is it OK for a tumble dryer and a washing machine to be simultaneously plugged into a double socket-outlet?"


    Answers on a postcard to ...

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