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Fuse blows

  1. Jan 23, 2008 #1

    Adz

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    Hi, can anyone explain this:

    last night i switched on the hall light and the light blew. but it also blew the 5A fuse back at the dis board. My question is, why ?

    Ive heard this happen before but when someone asked why, i couldnt explain it....
     
  2. jcsd
  3. Jan 23, 2008 #2

    berkeman

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    A 5A fuse is pretty small -- this is at a distribution panel?

    My guess would be that the light filament failed in a way that mechanically left metal between the hot and neutral contacts for the bulb, in a way that survived long enough to pop the 5A fuse. Can you measure the resistance of the blown light bulb? Maybe the metal short is still there. Can you see in the bulb (is it clear?)?
     
  4. Jan 23, 2008 #3

    mgb_phys

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    Thats standard for a lighting circuit in the UK ( and presumably anywhere else with 240v)
    you tend to have a separate circuit for each floor so there aren't that many lights on the one circuit and at 240V you have 1.2KW total.
     
  5. Jan 23, 2008 #4

    berkeman

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    Ah, thanks. I learn something new every darned day here on the PF!
     
  6. Jan 23, 2008 #5

    russ_watters

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    Are receptacle circuits also 5A? In the US, standard residential circuits are 15A/120V= 1800w.
     
  7. Jan 23, 2008 #6

    Adz

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    Thanks for the replies. Yes, Berkeman, the 5A is at the distribution board. I was thinking along the same lines as yourself, about the metal short just after the bulb blew However, with the bulb hanging down, any loose metal would have fallen onto just glass. I know this has happened to other people and when they ask me why, i cant answer! Out of curiosity i measured the resistance, but it is just 'open circuit'.
     
  8. Jan 23, 2008 #7

    chroot

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    The filament is under tension. When the filament breaks, pieces of metal go flying about inside the bulb in a pretty violent fashion. It's possible some intermittent short existed for 100 milliseconds or so, producing a current spike large enough to push an already loaded fuse over the edge.

    - Warren
     
  9. Jan 23, 2008 #8

    mgb_phys

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    They are 32A if you have a ring main or 16A for spur circuits. I have a very old house without ring mains and it's difficult to get electricians to understand this.
    The maximum applicance fuse is 13A - so you can use a 3Kw kettle to make tea with properly boiling water!
     
  10. Feb 25, 2008 #9
    As I understand it, when the filament blows, an arc forms through the argon in the bulb.

    This can transfer down to the filament supports and can take up to 100A.

    Many lightbulbs have an inbuilt fuse to prevent this taking out the fuse in the fusebox.
     
  11. Feb 25, 2008 #10

    Adz

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    thanks! that sounds right to me!
     
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