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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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    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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    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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    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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    gkenyon:

    It could also be argued that an enclosure containing a single RCD, say for a shower, is not "similar switchgear" as it does not include for "distribution", it's a dedicated device (similar, the SRCD).


    So is a switch-fuse (+/- RCD) at the supply end of a distribution circuit distributing leccy or not? 


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    perspicacious:
    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


    So how did you come to that conclusion ?  ?


  • Former Community Member
    0 Former Community Member
    So how did you come to that conclusion ?


    No question mark in the OP  !


    Regards


    BOD
  • 0
    It is raining......

    Tell me about it... At least the garden likes it and the water butts are filling up again.


    I think we're generally on the same hymn sheet. So it seems that a 1-way DB (rather than a consumer unit) could be suitable for use by ordinary persons and comply with BS EN 61439-3 (not all will, but it's possible, if the manufacturer so chooses).


    BS 7671's requirement for a main switch (or main circuit breaker) is that it is as near as practical to the origin of the installation (462.1.201) - which is just what you want if you cut off power to the entire premises for whatever reason - so I still say (unless one is provided by the DNO) the the switch on a switchfuse should be used in preference to one on a downstream CU - especially by ordinary persons who are less likely to understand the distinction and/or be aware exactly where the submain cable runs for example.

     
    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.

    Now there I've got to differ - Consumer Unit got their name back in the day when rewireable fuses were still king - 'trip switches' (MCBs etc) are just the current fashion. We may well be returning to the situation were consumers may have to pull out and replace bits of their CUs to keep their installation in a serviceable condition if the current fad for SPDs (with replaceable cartridges) continues too.

     
    Of course, a 1 way SPN distribution board is "similar switchgear"

    So isn't a swithfuse just one example of a 1-way distribution board? They might (or might not) comply with different standards - but BS 7671 doesn't define things in terms of which standards they are constructed to meet, but in terms of their fundamental components and functionality. If it's "an assembly containing switching or protective devices ... associated with one or more outgoing circuits fed from one or more incoming circuits ... " BS 7671 seems to consider it a distribution board.

     
    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.

    I wouldn't condemn an existing one - any more than I'd automatically condemn a plastic CU or rewireable fuse box - but for a new installation, presumably claiming to comply with the last requirements of BS 7671 and where more obviously compliant options exists, their use seems dubious to say the least. (I'm not sure what you mean about 'where a distribution board is not suitable'  - do you just mean fitting into small spaces?)


      - Andy.

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    AJJewsbury:

    BS 7671's requirement for a main switch (or main circuit breaker) is that it is as near as practical to the origin of the installation (462.1.201) - which is just what you want if you cut off power to the entire premises for whatever reason - so I still say (unless one is provided by the DNO) the the switch on a switchfuse should be used in preference to one on a downstream CU - especially by ordinary persons who are less likely to understand the distinction and/or be aware exactly where the submain cable runs for example.


    I agree - and if it's the type where the fuse is easily accessible, take out the fuse and put it in your pocket. So is this now the main switch, and are you going to label it as such?


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    I have a friend i went to school with on facebook who is like you BOD.  He corrects every bodies punctuation and spelling .  He is an English teacher and rather annoying ?.  I'm not perfect but usually pretty good.  ?    Any way i took both with me and ended up fitting the metal Lewden switch fuse as it was easier to fit even though it was bigger.   I didn't know that you could fit the tails into the top or bottom of the main switch on them.  it made it a lot easier being able to run both cables into the bottom of the enclosure rather than trying to get a 16mm twin and earth up and round into the top of a KMF.  Before i get jumped on about rcd protection and under sized earth the cable went in under the ground floor floor boards.  No insulation and clipped direct as best as i could in the 2 ft void . I also ran a 10mm earth along side it.  :)



    Gary
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    It is raining nuisance callers now and my contacts on the phone are nearly up to T**tAvoidtimewaster567, though way behind notanotherconsultant2394567.

     
    So isn't a swithfuse just one example of a 1-way distribution board? They might (or might not) comply with different standards - but BS 7671 doesn't define things in terms of which standards they are constructed to meet, but in terms of their fundamental components and functionality. If it's "an assembly containing switching or protective devices ... associated with one or more outgoing circuits fed from one or more incoming circuits ... " BS 7671 seems to consider it a distribution board.

    No. I stand by FUSED SWITCH [ or switch fuse] like a KMF, not being like a DB. There is one neutral connection in, and one neutral connection out. Same for the earth conductor. There is no option for other neutrals or CPCs unless you are particularly rough.


    You left out a fundamental part of the BS7671 definition of what is a DB.
    "......together with terminals for the neutral  and circuit protective conductors"

    Consumer Units and the Distribution Boards that are similar switchgear to Consumer Units have neutral and CPC bars. The genuine old style one way distribution boards like wylex/ MEM had a rewirable or HRC fuse or plug in retro MCB, but also had an earth bar and a neutral bar.

     
    Now there I've got to differ - Consumer Unit got their name back in the day when rewireable fuses were still king - 'trip switches' (MCBs etc) are just the current fashion. We may well be returning to the situation were consumers may have to pull out and replace bits of their CUs to keep their installation in a serviceable condition if the current fad for SPDs (with replaceable cartridges) continues too.

    Perhaps the point was not clear. Consumer Units are a particular  DB for use in Domestics. A KMF  has a HRC fuse that is not that consumer friendly to remove unless you are familiar with them. There is no indication that they have blown; same for BS3036, but of course, you can pull this rewireable out and see if the wire is healthy or not. I may also add that a SPD does have an indicator on healthyness. All fairly consumer friendly, but not necessarily very helpful as nothing will ever be done about it.


     
    BS 7671's requirement for a main switch (or main circuit breaker) is that it is as near as practical to the origin of the installation (462.1.201) - which is just what you want if you cut off power to the entire premises for whatever reason - so I still say (unless one is provided by the DNO) the the switch on a switchfuse should be used in preference to one on a downstream CU - especially by ordinary persons who are less likely to understand the distinction and/or be aware exactly where the submain cable runs for example.

    We will start to run into a bit of difficulty with that in a block of Flats with metering all in a communal location, especially if there are off-peak distribution also where we are then in "fifty fifty or phone a friend " territory as all the labels are missing.

    Even when the KMF is used as an isolator, that does not qualify it as a DB.


    This thread topic is loaded with proverbial cul de sac's where something is always parked in the maneuvering space; see earlier replies about SFCUs for example. Careful what you wish for when what are product standard issues are levered into BS7671. [edited]


    Yes, KMFs fit in small places where your genuine one way distribution board will not, some of the time.

    A KMF for a submain in a box, outside, no problem. Inside, I would fit a one way distribution board?


    As a direct replacement for a faulty one [KMF], depends if space allows, but as you know, space is not likely to allow that in an existing as the small space was why the KMF was put there in the first place.

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