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Plastic switch in external meter box

aligarjon
Can i fit a plastic switch fuse such as an 800kmf in the external meter box or does it have to be metal.   I am not interested in the dno view, they always seem to be happy with our switch gear in there around here so long as we leave them enough room.



Gary
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    It is raining......

     



    TPN boards, even where they do comply with BS EN 61439-3 can't be described in BS 7671 as consumer unit simply because BS 7671's definition of a consumer unit only applies to single phase equipment (i.e. having DP incomer).



    Of course, due to them being "similar switchgear" as opposed to Consumer Units. Principally, consumer units are a particular type of Distribution Board used in domestic [household] premises and, yes, they have to have a DP incomer, but the "similar switchgear", a distribution board, does not necessarily have one.


     
    As I read it, 421.1.201 isn't saying that the requirement for non-combustibility applies to BS EN 61439-3 equipment - rather it's saying that where ordinary persons are about (domestics) then it must both comply with BS EN 61439 as well as having an non-combustible enclosure - the BS EN 61439-3 is part of the requirement rather than part of the condition.



    Agreed. That is what was said.


     
    If a householder is following instructions to 'turn off at the main switch' surely the switchfuse is exactly the place they should be doing it. So really I don't buy that the existence of a downstream CU means that the householder wouldn't have any cause to operate the switchfuse.



    But there is not always a switchfuse. There may be a separate isolator, say a DNO supplied one. For a domestic [household] premises there will be a consumer unit. What is labelled "main switch" will determine where someone operates the main switch. Instructions for turning off the main switch are usually for isolating the consumer unit; there will be a label on it saying to do so before removing cover, for instance. There will not be a consumer unit without a main switch in the same enclosure in domestic [household] premises.


    Never say never, but the consumer unit main switch would be the "go to" device, it being the most obvious to a consumer.


    The point is "resetting a device" like an RCD or MCB in a consumer unit. Turning off a main switch is isolation.

    The consumer unit will have things that are in the off position if there is a fault or overload. That is consumer friendly, hence the moniker consumer unit for domestic [household] premises. 


    A 800KMF has a more " only an electrician should be fiddling with that" look than your bona fide, consumer friendly, consumer unit?


     
    I didn't know about the 63A limit for outgoing ways for CUs - and am a little surprised about that - since the BSI site in the overview of BS EN 61439-3 says rated current (Inc) of the outgoing circuits does not exceed 125 A and the rated current (InA) of the DBO does not exceed 250 A"



    Like I said "consumer units" to BSEN6143-9 are limited to 63 amp / way. I never said the standard stated that. Your "similar switchgear" are distribution boards and could be up to 125 amp/way, but I would suggest that at this point we have probably gone beyond a domestic [household] premises and 100 amp supply.

     
    and some examples of 80A/100A switchfuses appear to claim some sort of compliance with BS EN 61439-3 as well as 60947-3.



    I am glad the language used above as a descriptor of the particular wylex product does not infer absolute compliance with BS EN61439-3. 

    Is that due to the product being described as a Switchfuse? If it is manufactured to comply with BSEN61439-3 it is a distribution board as that standard  defines the specific requirements for distribution boards intended to be operated by ordinary persons (DBO). To my mind something built to comply with that standard is a distribution board. That Wylex product should be described as a one way distribution board.


    I note the literature posted a little way back from Wylex is 2015. They have their interpretation of "similar switchgear" in print; many had various interpretations at that time. If nothing else, it hopes there is a demand for a new product that is really only a distribution board and not a switch fuse.


     
    I'm still struggling to see why a 3x12 way TPN DB with 125A incomer and perhaps several 63A outgoing MCB is considered to be similar to a CU, but a 1-way SP distribution board with a 100A incomer and a 63A outgoing fuse isn't.



    Of course, a 1 way SPN  distribution board is "similar switchgear". As is the TPN when intended for domestic [household] premises.


     
    Fortunately building regs have their own definitions of things (and often didn't align with BS 7671 ones anyway) - as far as I can tell the notification requirement applies only to consumer units (not similar switchgear) - so perhaps there's no need to notify a TPN board replacement in a domestic?



    But the "similar switchgear" are distribution boards and when used in domestic [household] premises they are principally called consumer units. The notification requirement is for dwellings where there is a presumption of there being a consumer unit present. "Similar switchgear" is in BS7671 only, as the standard is not just for dwellings.


    I do not think we should be condemning the use of KMFs in domestic installations. There are many, many thousands installed, they are compact and manufactured as a switchfuse, usually for a specific purpose where a distribution board is not suitable.


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    Presumably something with a lot of high current terminal connections is, purely on probabilistic grounds, more like to to go whoosh than a FCU or fuse switch.

    From the pictures I've seen overheating tends to happen on high current terminals - either tails to incomer, incomer to bus-bar, bus-bar to high rated OPD, or outgoing terminal of the same, or on the N bar connections for the other side of the same circuits. Lower demand circuits - e.g. lighting circuits and most of the time ordinary socket circuits, individul MCBs for CH controls or alarm panels don't seem to draw enough current for enough time to pose as much of an overheating risk. So while your 16-way split load CU might look like a complete mess and full of potentially loose terminals, I suspect only a relatively small proportion are actually at a high risk of seriously overheating if loose - maybe those associated with the incomer and perhaps shower or cooker circuits. So a switchfuse carrying perhaps the entire load for an installation might well be somewhat lower risk by virtue of having fewer terminals - but perhaps only by maybe a factor of two or so rather than any order of magnitude.


    I could see the argument could be different for a large number of  long duration loads though - say an off-peak CU feeding storage heaters.


    DNO/supplier equipment overheating isn't entirely unknown either - so a reduced number of vulnerable terminals is no guarantee.


       - Andy.
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    I venture to suggest that whether a switch-fuse is a CU or not depends upon its function. If it protects long tails, i.e. a distribution circuit from the origin of a domestic installation to the (final) DB(s), I would say that it is a DB (with only one way).
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    Lets face it, the definition is a bit arbitrary. The intent is really focussed on "a part of the fixed wiring installation which the London Fire Service has noted goes up in smoke a lot and which tends to be installed in places like under stairs where going up in smoke is particularly unwelcome". Presumably something with a lot of high current terminal connections is, purely on probabilistic grounds, more like to to go whoosh than a FCU or fuse switch.
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    Reading the definition of "consumer unit" in Part 2 it mentions type-testing twice and the parts of the assembly being "suitable for such use". Assuming the "use" is as a "consumer unit", as opposed to a more general DB, would it make sense to ask the manufacturers of switchfuses whether they consider them to be consumer units? We would then have to take account of their instructions and act accordingly.
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    A few.


    One is called a consumer unit as it is done to a standard where it is intended for use by ordinary persons, but it's also the application; is that the only consumers equipment?


    DBOs [ consumer units] to BSEN 61439-3 are limited to 63 amp per way; note your example is 45 amp, also it is pre 61439-3, but I get the gist. KMFs and switch fuses can have HRC up to 80 amp /100 amp respectively. The KMF/switchfuse devices usually serve a full blown consumer unit to BSEN 61439-3, in that instance it is clear the consumer unit is the thing intended for use by an ordinary person.


    Those are the essential differences, but there are other things beyond BS7671 and the clumsy wording of 421.1.201,

    like the Building regs and particular requirements for dwellings.

    421.1.201  is only “what shall comply with 61439-3”. Firstly it is "consumer units". In my opinion the "similar switchgear" was to cover the potential use of TPN DBs for a 3 phase domestic or converted to single phase for lots of ways; it becomes a DBO when used in a domestic, most are now 61439-3 anyhow, but when the reg was published, some TPN distribution boards were not manufactured as DBOs. The Consumer Unit is specifically intended for use by ordinary persons, a DBO. Also the similar bit is for self assembled consumer units where you purchase an enclosure separately and the devices separately. I will go with a consumer unit like enclosure with a main switch with one or more ways/MCB’s that you can re-arrange yourself to not escape being 61439-3. But we have to be careful what we attribute “similar switchgear” to. It could lead to a bit of a conflict with notification requirements for building regs.

    Replacing a 800KMF with a new one is not notifiable works because it is not a consumer unit.

    I'm not sure I'm following the 'use by ordinary persons' thinking. As I read it, 421.1.201 isn't saying that the requirement for non-combustibility applies to BS EN 61439-3 equipment - rather it's saying that where ordinary persons are about (domestics) then it must both comply with BS EN 61439 as well as having an non-combustible enclosure - the BS EN 61439-3 is part of the requirement rather than part of the condition. I agree that the 'similar switchgear' was (at least in part) intended to cover TPN DBs in domestic situations - but the requirement is still that these need to comply with BS EN 61439-3 too. TPN boards, even where they do comply with BS EN 61439-3 can't be described in BS 7671 as consumer unit simply because BS 7671's definition of a consumer unit only applies to single phase equipment (i.e. having DP incomer).


    Likewise, in my mind if the fuse blows in a switchfuse in a domestic, the first thing the householder is going to do is to try to replace it (especially if they're of the generation that was taught to wire a plug) - and such a fuse can plausibly blow even if there's not a fault on the submain itself - either due overloading or poor discrimination with downstream MCBs on a fault after the downstream CU.  If a householder is following instructions to 'turn off at the main switch' surely the switchfuse is exactly the place they should be doing it. So really I don't buy that the existence of a downstream CU means that the householder wouldn't have any cause to operate the switchfuse.


    I didn't know about the 63A limit for outgoing ways for CUs - and am a little surprised about that - since the BSI site in the overview of BS EN 61439-3 https://shop.bsigroup.com/ProductDetail?pid=000000000030393941 says " rated current (Inc) of the outgoing circuits does not exceed 125 A and the rated current (InA) of the DBO does not exceed 250 A" - and some examples of 80A/100A switchfuses (e.g.  https://www.tlc-direct.co.uk/Products/WYDSF100M.html) appear to claim some sort of compliance with BS EN 61439-3 as well as 60947-3.


    I'm still struggling to see why a 3x12 way TPN DB with 125A incomer and perhaps several 63A outgoing MCB is considered to be similar to a CU, but a 1-way SP distribution board with a 100A incomer and a 63A outgoing fuse isn't.


    Fortunately building regs have their own definitions of things (and often didn't align with BS 7671 ones anyway) - as far as I can tell the notification requirement applies only to consumer units (not similar switchgear) - so perhaps there's no need to notify a TPN board replacement in a domestic?

     
    So an SFCU?

    Ah yes, fair point. Actually there are times when I do want to regard a FCU as a distribution board (e.g. when reading 314.4). It's perhaps a little easier to claim it's not similar to a CU though - if only in size - it's won't be employed to handle the sort of currents a CU could, or have a similar breaking capacity (6kA rather than 16kA) so less suitable for use at the origin of an installation and so on.


    Simple switches or RCDs in a box are easier to dismiss - since they don't even meet the definition of a distribution board (which is central to BS 7671's the definition of a CU).



    Seeing that apparently compliant solutions exist (e.g. that metal-clad Wylex switchfuse which meets BS EN 61439-3 as well as 60947-3) for domestic use, should we really be recommending the use of apparently non-conforming alternatives?


       - Andy.
  • Former Community Member
    0 Former Community Member
    Can i fit a plastic switch fuse such as an 800kmf in the external meter box or does it have to be metal.


    So it wasn't really a question.

    The box will be supplied and fitted by myself with switch fuse and tails already fitted for a changeover on the day. As i said in the OP the DNO guys round here are always very obliging and helpful.  The boxes are tight but they don't need to fit their isolator if i have fitted one already.  


    Regards


    BOD
  • 0
    UKPN:

    "I am not interested in the DNO view"

    You should be, because without a competency certificate signed by a DNO you are not able to work live on their equipment.

    An unauthorised "isolator" is not covered by the suppliers insurance.

    The fitting of an unauthorised "isolator" will loosen the meter tails irrespective of a withdrawn fuse so the meter terminals will have to be checked afterwards. This of course requires seals to be broken and replaced, again only persons with a competency certificate are able to do this.

    Your best course is to request a suppliers isolator via the customer. The suppliers meter operator will attend, fit, and you being there at the same time can reconnect the tails.

    Ascertaining whether a plastic isolator/sw/fuse is allowable under the "wiring regulations" would be futile anyway because that question has been thrown around for years and no one from the "institution has stuck their head above the parapet and given a definitive answer. 

    Regards, UKPN.


    The DNO will be attending.  The supply is being moved because of an extension and i have to extend the tails longer than 3 meters. The box will be supplied and fitted by myself with switch fuse and tails already fitted for a changeover on the day. As i said in the OP the DNO guys round here are always very obliging and helpful.  The boxes are tight but they don't need to fit their isolator if i have fitted one already.  



    Gary



     


  • 0
    AJJewsbury:


    What is the essential difference between a switchfuse and a 1-way consumer unit?


       - Andy.


    So an SFCU? Best change all mine for metal-clad types?


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    perspicacious:
    I reckon that both are my boxes on my land. My land! So where is the legal basis for the DNO "owning" one of them?


    On that logic, you own the DNO service cable, cut-out and meter Chris!


    As Simon Barker says, the householder (or builder) has to supply (and if necessary, replace) the boxes. It is, of course, a condition of the supply contract that the DNO and supplier may install their tackle in my box, on the garage wall, and in my ground; but that doesn't mean that they own the box or the space within.


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