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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
    davezawadi (David Stone):

    Z, what possible danger can there be requiring a fan isolator switch?


     


    You have confused me. I see no danger in requiring a 3 pole isolator switch as asked for by fan manufacturers. Fan manufacturers ask for the three pole isolator to be installed to isolate their fans. Isolators are specifically made for this purpose and labelled so. You need to ask them why David.


    Z.


  • 0
    I detect cross purposes. It is very handy to be able to cut off the power to the fan and keep the lights on, either to allow it to be cleaned, or as happens in our house at night when folk only want to go for a wee without the fan whirring away at 48dBi plus and waking the household for no good reason. (city dwellers used to traffic may not worry about this so much)


    The fusing is something else, and in many cases is probably is not needed at all.

    If it has to be  I'd rather there was a single fuse before the steady live and switched live split, rather than two afterwards, as it leads to confusion if just one fuse blows or is removed.

    I do not think there is a greater shock risk however than say a ceiling rose.  Folk taking electrical stuff to bits should look after themselves to the same level as they would unscrewing a socket or  light switch.

    In most designs I have seen the switched live is just a trigger, and high impedance, so does not draw any current worth mentioning at all, all the motor power is drawn from the steady live.


    Mike.

  • 0
    521.8.2 does not apply, as I am only dealing with ONE final circuit, that of the lighting circuit feeding a single timer extractor fan. No probs. here.

    BS 7671 definition of a circuit means that any overcurrent protective device starts a new circuit (see part 2) - in this case the L and SL are by definition different circuits as the are protected by different fuses. As they both directly connect to current using equipment they'd be classed as final circuits.


    Oddly enough there's nothing in the definitions that means that one final circuit can't supply another final circuit (despite the name).


       - Andy.
  • 0
    AJJewsbury:
    521.8.2 does not apply, as I am only dealing with ONE final circuit, that of the lighting circuit feeding a single timer extractor fan. No probs. here.

    BS 7671 definition of a circuit means that any overcurrent protective device starts a new circuit (see part 2) - in this case the L and SL are by definition different circuits as the are protected by different fuses. As they both directly connect to current using equipment they'd be classed as final circuits.


    Oddly enough there's nothing in the definitions that means that one final circuit can't supply another final circuit (despite the name).


       - Andy.


    Circuit definition. "An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s)." Notice the plural used here DEVICES.


    "Same origin." Yes the lighting circuit M.C.B., say a B6 or B10.


    "An assembly of electrical equipment."  Yes, Lights, shaver outlet and fan.


    We could argue about the fuses used to protect the fan are just there for fault protection and not overcurrent protection as the fixed fan load can not overload the supply cables in normal use.


    "Protective device(s)" can be included in one final circuit. So we can have more than one protective device in a final circuit.


    Z.


     


  • 0
    Treat bathroom fans with respect, they can and do catch fire.

    Extractor fan causes fire in Chester (cheshirefire.gov.uk)

    Faulty extractor fan fire in Limehouse | London Fire Brigade (london-fire.gov.uk)


    Z.
  • 0
    Zoomup:

    We could argue about the fuses used to protect the fan are just there for fault protection and not overcurrent protection as the fixed fan load can not overload the supply cables in normal use.


    What if the fan jammed? Would that not cause an overload?


  • 0
    The traditional inside out squirrel cage induction motor ones actually draw less current when stalled than when spinning, so a fuse will not do much. The problem is that even with the lower current there is then no cooling airstream so it overheats.

    Modern ones are supposed to have a thermal fuse or trip in the winding bundle, presumably not all do.

    With electronically driven  DC brushless motors, who knows how it is done, as many methods as designers. However, a fuse would need to be set a lot closer than 3A to prevent a fire - the 25watt soldering iron is enough in the wrong situation.

    M.

  • 0
    Zoomup:
    AJJewsbury:
    521.8.2 does not apply, as I am only dealing with ONE final circuit, that of the lighting circuit feeding a single timer extractor fan. No probs. here.

    BS 7671 definition of a circuit means that any overcurrent protective device starts a new circuit (see part 2) - in this case the L and SL are by definition different circuits as the are protected by different fuses. As they both directly connect to current using equipment they'd be classed as final circuits.


    Oddly enough there's nothing in the definitions that means that one final circuit can't supply another final circuit (despite the name).


       - Andy.


    Circuit definition. "An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s)." Notice the plural used here DEVICES.
    But your permanent Line and switched Line cables must be on different circuits because they are protected by different devices

    "Same origin." Yes the lighting circuit M.C.B., say a B6 or B10.
    DIfferent origins- 2x BS1362 fuses

    "An assembly of electrical equipment."  Yes, Lights, shaver outlet and fan.
    Fan only- supplied by two circuits

    We could argue about the fuses used to protect the fan are just there for fault protection and not overcurrent protection as the fixed fan load can not overload the supply cables in normal use.
     think you are confusing overcurrent (exceeding the rated value) protection with overload protection (which occurs in a mechanically sound circuit), a fault current can be an overcurrent if large enough, see definitions in part 2

    "Protective device(s)" can be included in one final circuit. So we can have more than one protective device in a final circuit.

     Yes, if in series, not parallel.


    Z.


     




    If you were fitting a boiler to an existing fcu on a ring final circuit, would you record the OCPD as 32A mcb or 3A fuse on any MWEIC?


    Also the label reg i was thinking of is 514.11.1 it does say not capable of being isolated by a single device, and you could argue the 6A mcb does this, but then so does the main switch and cut out. I think the intention is that it warns of a possible danger of live parts if one of the fuses blows and the fan dont work. 


    Back to the OP, I thought my idea of an fcu behind the switch i posted earlier was good, but seems to have been overlooked :)

     


  • Former Community Member
    0 Former Community Member
    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.


    The faulty touch dimmer switch was probably originally a switched fuse spur? Replace faulty dimmer with a switch fuse spur, job done no heaters need removing! :)

     


  • 0
    We could argue about the fuses used to protect the fan are just there for fault protection and not overcurrent protection

    Faults are one kind of overcurrent - overloads are another. The term overcurrent encompasses both.

    "Protective device(s)" can be included in one final circuit. So we can have more than one protective device in a final circuit.

    Indeed - the circuit goes all the way back to the sub-station after all. So your one of your circuits is protected by one fuse, a B6, maybe a submain protective device, the DNO's fuse etc (perhaps some of those devices rely on an upstream device for backup protection). While the other circuit is protected a different fuse, then the same B6 etc. Hence the two circuits aren't entirely protected by the same devices.

       - Andy.

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