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Lights Supplied by 30 Amp Ring Final. B.S. 3036 Fuse.

Zoomup
HellOOOoooo All,


I came across a strange one today. I went to change a faulty touch dimmer switch for an ordinary light switch. I discovered that the two lights controlled by the switch did not originate at the lighting circuit, but from a 30 Amp wire fused ring final. The supply was connected to an old metal double socket box below the light switch, in an added conservatory, with a blank plate over it. I can not add a fused connection unit as the box is a double socket box. The blank plate is covered by a small easily removed panel convector heater. I was considering installing an inline fuse holder in the double socket box for the lighting circuit. The lamps are low energy types so overloading is very unlikely, but faults may occur.


Thoughts please.


Z.
  • 0
    Alan Capon:
    Zoomup:

    We are talking about a simple domestic situation. The fan L and switched L both originate from one B6 or B10. . . 


    As you have two fuses supplying your fan via its supply cable, you have dual supplies to the unit. The definition of a “circuit” will apply to each of your fuses. This would be considered dangerous in a commercial environment as it would not be expected, and would be labelled as being fed from two supplies. In a domestic environment it is equally dangerous, perhaps more so. It should be labelled as having dual supplies, or fed from a single fuse located before the light & switch as others have said. 


    Regards,


    Alan. 




    It is no more dangerous than if you assume two supplies feeding a three plate ceiling rose wired with a permanent L and a switched L. We don't label them with a warning label. Obviously circuit labels detailing items is a good thing which I adhere to. The 3 pole fan isolator totally disconnects the supplies to the fan if maintenance is required. Safety is maintained. And any sparks will check before touching potentially conducting live parts. Where is the risk? 462.2


    Z.


  • 0
    Zoomup:

    It is no more dangerous than if you assume two supplies feeding a three plate ceiling rose wired with a permanent L and a switched L. We don't label them with a warning label. . . 


    No, but using the definitions in BS7671, being fed from a single protective device makes it one circuit. Your two fuses feeding the fan makes it two circuits. 


    Regards,


    Alan. 


  • 0
    Alan Capon:
    Zoomup:

    It is no more dangerous than if you assume two supplies feeding a three plate ceiling rose wired with a permanent L and a switched L. We don't label them with a warning label. . . 


    No, but using the definitions in BS7671, being fed from a single protective device makes it one circuit. Your two fuses feeding the fan makes it two circuits. 


    Regards,


    Alan. 




    I can see no danger with the current obvious simple easily understood arrangement, and having two fuses and claiming two circuits from one single M.C.B. is illogical. Which reg. are you referring to?


    In a domestic situation at the front door, if there is a three gang light switch with lighting supplied over three separate circuits from three separate M.C.B.s, , would you affix a warning notice advising so? I have never seen such myself.


    Z.


  • 0
    I think most are of the opinion that having two fuses creates two further circuits - so with a common N that's technically a contravention of BS 7671. I agree that in this particular circumstance the actual danger is minimal - but as usual (as with the tails though thin steel plates) BS 7671 tends to have a blanket requirement based on a simple principle and doesn't bother distinguishing between different practical situations. If it was two different circuits from a CU or even two different FCUs with a common N downstream, it would maybe be a more obvious contravention but I think, given BS 7671's wording, the same rules apply here. Like I said, not an immediate calamity, but perhaps something to keep in mind if you're putting your name on a piece of paper claiming it did conform to BS 7671.


    From the point of view of a 3A fuse being there to limit the amount of current availble to start a fire (presuming we beleive the manufacturers) then maybe having two 3A supplies to the same smouldering box rather defeats the intention (depening on how the perhaps charred innards allow current to flow from both L and SL under the wrong conditions) - as a worst case it might be thought that two 3A supplies might be no better than a single 6A supply...


    As others have said, the usual solution is a single 3A fuse upstream of both the light, switch and fan - although granted that's usually far less convenient to retrofit.


       - Andy.
  • 0


    From the point of view of a 3A fuse being there to limit the amount of current availble to start a fire (presuming we beleive the manufacturers) then maybe having two 3A supplies to the same smouldering box rather defeats the intention (depening on how the perhaps charred innards allow current to flow from both L and SL under the wrong conditions) - as a worst case it might be thought that two 3A supplies might be no better than a single 6A supply...

     


    I have not read the manufacturer's reasons for requiring a 3 Amp fuse related to fire prevention, but I imagine that an embedded thermal cut out in the motor windings would be better suited to prevent a dangerous overheat of the motor and associated fire risk. Perhaps the 3 Amp fuse is to protect the fan's internal wiring. Fans with an integral 250/500mA or so on board ceramic fuse will be safer.


    Z.

     


  • 0
    AJJewsbury:

    I think most are of the opinion that having two fuses creates two further circuits - so with a common N that's technically a contravention of BS 7671. I agree that in this particular circumstance the actual danger is minimal - b


    Majorities eh? There have been many powerful majorities that were right in their own eyes over thousands of years of human history. But were they always right?


    Ignas Phillip Semmelweis comes to mind as being in a minority. He was bitterly opposed by the majority of his colleagues until Joseph Lister proved that his health claims were in fact correct.


    Z.


  • 0
    "Ignas Phillip Semmelweis comes to mind as being in a minority. He was bitterly opposed by the majority of his colleagues until Joseph Lister proved that his health claims were in fact correct. "


    Yes but did he have input into BS 7671?
  • 0
    The fuse in the fan posted by Zoomup earlier reminded me of a post by Paul Skyrme here https://talk.electricianforum.co.uk/topic/30686-protecting-2-ovens-on-1-radial-cicruit/?do=findComment&comment=420792

  • 0
    ebee:

    "Ignas Phillip Semmelweis comes to mind as being in a minority. He was bitterly opposed by the majority of his colleagues until Joseph Lister proved that his health claims were in fact correct. "


    Yes but did he have input into BS 7671?


    Perhaps not a direct input into the wiring regulations, BUT he may have saved the lives of early electrical pioneers, engineers and writers of early electrical safety requirements by his hygiene methods that saved many young lives. Their observations and engineering skills have been inherited by those following. He came just before the first wiring regulations of 1882.


  • 0
    OlympusMons:

    The fuse in the fan posted by Zoomup earlier reminded me of a post by Paul Skyrme here https://talk.electricianforum.co.uk/topic/30686-protecting-2-ovens-on-1-radial-cicruit/?do=findComment&comment=420792

     


    Is this requirement current?

    https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/950107/Guide-to-electrical-equipment-regulations-2016-tp.pdf


    Z.


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