OT: Recommend a wireless card

Discussion in 'Home Networking' started by James, Aug 3, 2007.

  1. James

    Lurch Guest

    Fuse size has not a lot to do with the actual current draw of the
    appliance. The fuse is to protect the flex to the appliance. It will
    not protect the appliance itself. Have you actually looked at the
    fuses in the plugs on any appliances in your home? I'd imagine most of
    them would be 13A.
     
    Lurch, Aug 7, 2007
    #21
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  2. James

    Palindrome Guest

    The graphs I quoted figures from were for cartridge fuses - the same
    technology as fitted in 13A plugs. Dissipation only affects overload
    performance, it has little effect when very high fault currents are
    involved.
    So you say "nope" and then write "break a circuit" instead of "clear a
    fault"...
    Except that the physics is entirely different. Specialised fuses use
    exothermic or endothermic coatings, plus other means to "blow more
    quickly" - or more slowly. The 13A cartridge fuse is a very standard
    cartridge fuse with little more than a bit of tinned copper wire and
    some powdered glass within it.
    As explained previously, if the earth impedance is high, an insulation
    failure can result in the equipment case being at a dangerous voltage
    until such time as the fault clears - which could be tens of seconds.


    The pc case usually is. Take the situation of a fan heater plugged into
    the same extension as the computer. The user is sitting at his metal
    desk, with pc (on insulated feet) on top. The desk is touching a central
    heating pipe.

    The user goes to load a CD and knocks over his coffee - which goes into
    the fan heater. The insulation fails. The earth impedance limits the
    fault current so that the voltage on the fan heater case, and on the pc
    case (which shares the same socket earth rail) rises to 120v and stays
    there for some seconds. The user is touching the case with one hand and
    desk with other...
    13A isn't the problem. 40A+ is. You want to bet your life on a series of
    brushing contacts with 40A+ going through them. Dig out.
    Whatever you choose to call them - current imbalance devices are an
    excellent idea. Provided the fault current goes to earth. They don't always.

    Take the case above of the fan heater. This time it is double insulated,
    no earth connection. In goes the coffee. The coffee short circuits the
    live to neutral. The current imbalance device does not trip - the
    currents are still balanced. The only protection now is the fuse. But
    the fault current is limited by the loop impedance - and one of those
    wiping contacts has a couple of ohms resistance. Result - 3kW
    dissipating in that wiping contact for the next 10 seconds or more. Good
    luck.

    As you say, we'll just have to retain our different opinions. There
    isn't much point going over the same ground (joke) time after time.

    We can leave it to others to form their own opinions..
     
    Palindrome, Aug 8, 2007
    #22
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  3. James

    DCA Guest

    Now that is a new use for a fuse! It is not restricted to protecting cords.
    Dorothy was right in that it protects against live chassis faults too.
    In fact any radical increase in current above the fuses value really!!!!
    Yes - I have checked my appliance fuses. White goods -all 13A (but then
    they would be to cater for initial surge)
    All the small items such as transformers, cordless phone, laptop, hi-fi,
    freeview PVR, chargers, Modem, router and mobile/battery chargers are
    all 3A or 5A (albeit chargers have no earth connection at all)
     
    DCA, Aug 8, 2007
    #23
  4. James

    DCA Guest

    Dorothy - I feel as if I am banging my head against a wall and I'm sure
    person to person we would amicably reach agreed territory (perhaps some
    of this is misinterpretation)
    I disagree with your arguments - especially where coffee will now
    conduct so much available current on the chassis and conversely, earth
    impedance of an extra extension will not take the current???? - what one
    in a million nonsense!
    Lets look at an example where the earth of a premises is 0.5 ohms (which
    is probably far from reality). Over here I could achieve a theoretical
    current of 480A (I guess that is 240A over there). Now, if you extension
    lead added another 0.5 ohm which is very unlikely (it is far more likely
    the overused wall socket has a more worn and unreliable earth than the
    extension), you will still have 240A over here (and 120A over there). To
    drop it to 40A the earth would need an impedance of 6 ohms (3ohm in the
    US) and if it was that high there would be inherent H&S risks in the
    buildings wiring. That is NOT going to be as a result of a bloody
    extension lead - or two or three.

    I am equally concerned about your explanation of the current imbalance
    breakers.
    They do NOT measure leakage on the earth - and in fact operate the same
    without any earth.
    They purely rely on a current imbalance between L and N.
    Naturally, a leak to earth will achieve this but this effect is not
    exclusive to this as it is measured on the L/N imbalance.
     
    DCA, Aug 8, 2007
    #24
  5. James

    Jeff Gaines Guest

    Is Sue actually Dorothy in disguise, or are you suffering an identity
    crisis?
     
    Jeff Gaines, Aug 8, 2007
    #25
  6. James

    DCA Guest

    WTF did I get Dorothy from?
    That was rather bad given she even places Sue in the signature!
    Hands held high on that one :)
     
    DCA, Aug 8, 2007
    #26
  7. James

    Conor Guest

    Yes it will...
    Actually they haven't been for donkeys years so it's obviously been a
    while since YOU opened a plug.

    --
    Conor

    "Understeer" is when you hit the fence with the front of the car.
    "Oversteer" is when you hit the fence with the rear of the car.
    "Horsepower" is how fast you hit the fence.
    "Torque" is how far you take the fence with you. - Anon.
     
    Conor, Aug 8, 2007
    #27
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