1. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
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    I open this thread after some discussion arose in the 'tips and tricks' thread regarding appropriate fusing.

    So I open with my understanding of proper fusing and may be corrected if I am wrong. I will begin at the power input point all the way through to smaller circuits.

    RCD at input to device (i.e. mains) - fuse before rectifier and surge protection devices so that fuse blows first in event of surge damaging surge protection devices - fuse on each individual power rail after conversion to protect circuitry - output ports (i.e. Digital outputs) should have short circuit protection

    The tricky part becomes whether an RCD can be used. On larger power supplies i imagine the core losses to be large enough to simulate a large enough earth leakage (normally 30mA) to trip the device.

    What are your thoughts on this matter?
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Okay, folgoration is not in my dictionary. Please illuminate those of us who don't have the complete O.E.D. at hand.

    John
     
  3. jpanhalt

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    No apology needed. It's a good-sounding word. Google had a few hits, but they were related to heavy-metal bands and some home-built lasers -- no definitions or even enough context to make the meaning clear. I will try to use it today in at least one sentence.

    Thanks.

    John
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    A corollary to Murphy's Law is: Transistors will usually protect the fuses by going first.
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Cigarette silver paper makes the best sparks.

    Six inch nails start the biggest fires.

    If the lights in your old house keep blowing try rewiring the 5 amp carrier with 15 amp fuse wire. This makes rodents chewing through lighting flex glow especially brightly. It also stops the light dimming if you plug the smoothing iron into a side adapter in the lighting pendant.
     
  6. Audioguru

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    Dec 20, 2007
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    If somebody is stupid and touches a mains fuse and if the stupid person is grounded then of course they will be flogged (electrocuted) by trillions of electrons.
     
  7. thingmaker3

    Retired Moderator

    May 16, 2005
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    People in the UK do that too?
     
  8. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
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    O dudes, thanks for this thread. I've been giggling like a school girl now for 5 minutes.

    I can imagine the little italians running around with charged capacitors throwing them at the girls singing flogoration.
     
  9. jpanhalt

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    Don't know what flogoration means. Sounds bad, though.

    BTW: While the Italians may have given us a new word, folgoration, we have had fulguration as a legitimate English word for sometime. The latter word is usually used in the context of electrocautery to stop bleeding during surgery. There has to be a connection there.

    John
     
  10. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
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    That is correct Alberto, and also what I thought, but there was a comment in the 'tips and tricks' made that fuses are meant to blow _after_ the circuitry, so I just wanted to clear things up.

    Perhaps we should edit that post so we don't confuse anyone and have them intentionally design an unsafe unit? :)
     
  11. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
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    I imagine there may be a relation with the English version of a flogging. Flogoration could perhaps be the instrument used for beating one whilst re-sealing the wounds through electrocautery.
     
  12. studiot

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    Nov 9, 2007
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    My apologies, Skee. I never meant to cause such hilarity in a serious thread.

    Here is a quote from the other thread which I will explain further.

    Don't forget that fuses may be meant to work in conjunction with other protection measures such as earthing. They should be sized so that they will definitely disrupt and disconnect the supply if an external metal part becomes live for instance
     
  13. thingmaker3

    Retired Moderator

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    Said comment was humorous, not literal:
     
  14. studiot

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    OK so consider a few situations.

    Situation 1

    A metal clad electric heater should have the metal case earthed (by UK regs). If there is a fault in the heater such that the live feed becomes loose and contacts the case there is clearly a danger to any person touching the case. But the heater would otherwise function correctly.

    If the case is earthed and the live has a fuse, then a large current will flow to earth as soon as the live touches the case. This current will disrupt the fuse, disconnecting the faulty heater and rendering the situation safe.

    At no point will the fuse 'protect' the device.

    So the fuse satisfies my primary and secondary objectives, but not the third.

    Situation 2

    Consider someone who has spent £1000 on a loudspeaker. She would be well advised to place a fuse between the drive amp and the speaker. Some amps and / or speakers already contain fuses for this purpose.

    Here there is no risk to persons and I have not heard of fires being caused by the audio side of amps. Nor will a fuse be fast enough to protect the amp in the event of a cable short. However the fuse can be sized to protect the speaker from overdirving or a faulty amp.

    Situation 3

    Better makes of test meters containf fuses or cutouts in series with their test leads or terminals. This is intended to protect the device against users who try to measure voltage on a current range.


    Discuss away.
     
  15. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
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    Situation 1: Only the wiring is at risk here. Yes your upstream fuse should blow to keep the live wire from catching fire, but the circuitry is not in danger in the first place. It is only a convenient consequence that any of the power rail fuses blow, as their actual intent is to protect the circuitry.

    Situation 2: This is protecting a device or apparatus and fits the theme of this thread, that fuses are for protection of devices.

    Situation 3: Fuse is protecting device again.

    So in summation, all 3 situations seem appropriate to design fuses focusing on 'devices' and making sure none of your wires/tracks catch fire from over currents.

    The only situation which took into account human factors was earthing the chassis, which is in accordance with using RCD devices where possible to function as a circuit breaker if current is conducting via a path other than Live and Neutral. RCDs are very limited in high power applications as leakage currents and inrush currents in big bad capacitors are generally enough to trip them, so in these situations the only real solution is extreme care and lots of big warning stickers.
     
  16. studiot

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    Nov 9, 2007
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    Don't be silly.

    What I have stated is a requirement of the UK wiring regulations, and has been for a very long time.
    Furthermore they authorities state clearly the reason for the requirement to be as I have stated -To protect human life.

    Similarly, if you read the requirements of building and fire regulations you will see that various types of fusing are required to try prevent the large number electrically caused fires that occur in buildings every year, some with tragic loss of life.

    In both cases they really don't care two hoots what happens to the equipment itself.
     
  17. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
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    What you just replied with is in accordance with my reply.

    Human life is only jeapordized in any situation if the chassis to earth connection is broken. Otherwise fuses, although sized to protect circuitry/wiring, blow in the event of a fault and human safety is subsequently enforced because the source of the fault is cut before safety becomes a concern.

    If you were to try and fuse something to protect humans, your fuses could never allow more than ~15mA, as this is the point at which humans fail to be able to let go under high voltage AC.

    I think you are viewing from the top down and me the bottom up. Same solution, different perspectives.
     
  18. studiot

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    Not quite.

    What wiring do you think the fuse protects?
     
  19. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
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    For the purposes of this thread I was referring to the Live and Neutral wires. So in this situation the circuit breaker etc.. must be rated for less than live and neutral, typically 10A, in the event live or neutral are shorted to chassis (i.e. Earth). The earth wire must be suitably sized by the appropriate standard so it can easily handle the maximum live/neutral current.
     
  20. studiot

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    Now that you are not a student you have the time to think more deeply about the subject.

    Referring to UK practice:

    A typical house will be supplied via 100 amp fuse at the meter board.

    The type of heater I mentioned will be plugged into an outlet on a subcircuit, probably aring main supplied from a 30 amp fuse.

    None of this wiring needs protecting by the 13 amp fuse in the mains plug of the heater.

    Within the heater there may also be a fuse, perhaps 10 amps, and perhaps a thermal fuse or cutout.

    Why is this apparent duplication?

    Well suppose the flex from the wall outlet to the heater frays or is chewed through by the rabbit. (mine has done this several times).
    Then the fuse in the wall plug will disrupt, making the cable free of electricity and therefore safe. Since the cable is damaged it should be replaced, not protected.

    Now suppose someone drapes a towel over the heater so the heat cannot escape, although the device functions correctly from an electrical point of view.

    Then the thermal cutout will operate and prevent the towel catching fire and burning down the building.

    Now suppose, as I mentioned earlier, the line contacts the outer casing. The heater will still function normally in other respects.

    This is where fusing in conjunction with other measures, earthing in this case, operate together to ensure safety. Again as I mentioned earlier.

    In this situation the regulations are very very clear. The device must be removed from connection until the fault has been cleared, whether or not the device will still function or can protect itself.

    Relatively large currents flow rapidly to earth, disrupting the 10 amp internal fuse, to ensure this disconection condition is satisfied.

    This only ensures that the heater is not a safety hazard to humans. It does not protect the heater in any way. Nor does it protect the supply wiring, which does not need protection against currents of the order of 10 amps.

    It is interesting to further note ( and design correctly) the order in which fuses will blow. When a 30 amp ring main fuse blows it does not take all the lesser fuses on the ring with it in a correctly designed system.

    Think about it.
     
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