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is third transformer hole an earth?

Discussion in 'Electronic Repair' started by john downie2, Nov 8, 2008.

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  1. john downie2

    john downie2 Guest

    my brother wishes wishes to take an Acer Aspire One mini-notebook to japan,
    from London, u.k. .

    He already has in his house a lead with a japanese type mains plug on one
    end and the normal type two pin femail type plug (the same as often goes
    into radios, etc) that will go into the computer transformer.

    But we see that the Aspire One has an additional *third* hole alongside the
    usual two hole plug that goes into the computer transformer. The three holes
    forming a triangle shape.

    Is it alright to use our existing two hole Japanese type lead and then only
    use *two* out of the three pins coming from the aspire transformer, please?

    My guess is that the third hole is only an *earth* connection. although this
    might be an additional safety feature, our main concern is whether it might
    do any damage to use the two pin only lead?

    I have emailed the technical services department and the spares department,
    but neither will answer the question, only want to sell me one of their
    three hole leads. Thanks for your advice.
  2. Whiskers

    Whiskers Guest

    No. If the power 'brick' has three pins, it has been designed to require
    three connectors.
    "Only" an earth? Sheesh!
    Assuming that your existing power brick can accomodate Japanese mains
    voltage and frequency, you will need an earthed Japanese plug to fit the
    Japanese mains socket. Which sounds like what the technical services
    department are trying to convince you of.

    I think Japan uses 100V mains at either 50 or 60 Hz depending on location.
  3. ian field

    ian field Guest

    If the "power brick" is a switch-mode converter (not an iron cored
    transformer) some types automatically adjust to anywhere between 100 - 240V,
    check the specification label as this information should be clearly marked.

    Most PSUs of this type are double insulated and do not require an earth,
    they only have a 3-pin plug because the regulations in the country of use
    say that all wall sockets must be 3-pin (there wouldn't be much point in
    supplying items with a 2-pin plug that didn't fit anything). Carefully
    examine the mains lead, if its round then it contains 3 wires and must be
    earthed, if the mains lead is flat so you can tell the outer covering is
    over a pair of wires side by side then there is obviously no third wire for
    an earth.
  4. Whiskers

    Whiskers Guest

    True enough.
    I took the OP to mean that the brick itself has three pins in the orifice
    to which the incoming mains lead is connected. That suggests that the
    makers of it expected it to be earthed.

    Both the laptop power bricks in use in this room (one HP, one Toshiba) have
    three metal pins for the mains lead to connect to - and /do/ require an
    earthed mains lead and socket. The arrangement is sometimes described as
    a 'cloverleaf' connector; more officially an IEC-320 type C5
    <> and
    <>. The two-pin type
    with no earth is a 'figure-of-eight' or IEC-320 type C7.

    Here in the UK, double-insulated appliances can be fitted with a mains
    lead that has a plastic 'earth' pin - whose functions are to open the
    shutters on the other two holes in the socket, and to ensure correct
    connection of the 'live' and 'neutral' pins. But mere presence of a metal
    earth pin on the plug cannot be taken as evidence of an earth connection,
    of course!
    Never seen a 'flat' three-conductor lead, or a round two-conductor lead?
  5. catchme

    catchme Guest

    yes, the 3rd hole is a ground, but while the voltage in Japan is
    similiar to North America (despite Japan only having 2 leads), i dont
    know what the voltage is in the UK.
    Japan runs 110v. on a 60a circuit, whereas Canada does 120.
  6. ian field

    ian field Guest

    I'd still take it that if the original mains lead was only 2-core, even if
    there is a 3-way connector on the power brick, then you can't have live,
    neutral and an earth if there's only 2 wires in the mains lead. 3 pin
    connectors do turn up every now and then with only 2 pins actually used but
    they're not very common.

    As long as the original mains lead is still with the power brick, its the
    best indication whether its safe to use without an earth or not. Of course
    with a detachable lead, if examination leaves any doubt as to whether its 2
    or 3 wires its easy enough to continuity test the earth pin.
  7. Whiskers

    Whiskers Guest

    UK standard domestic mains runs at 240V 50 Hz and most houses have been
    wired with "13 Amp ringmains" since the '60s. Our standard 13 Amp plugs
    and sockets have three pins; there are no standard 2-pin plugs that fit
    the 3-pin mains sockets - which require a third pin to open the shutters
    in the 'live' and 'neutral' holes before anything can be inserted into
    them (the earth pin being longer than the other two). Appliances which
    don't need an earth connection can be fitted with plugs having a plastic
    'earth pin' (which presumably saves manufacturers money).

    Japan's mains apparently runs at 100V, 50 or 60 Hz, and the plugs and
    sockets are only superficially similar to those used in the US. They seem
    to be rated at 10 12 or 15 Amps. (60 Amps would be pretty nasty if things
    went a little awry, even if a 6 KW fire might be useful). Three-pin
    earthed sockets are apparently not as common as might be desired. See
  8. hr(bob)

    hr(bob) Guest

    The answer to the original poster is yes, if the power brick will
    accept as low as 100V andat 50 or 60 Hz, then everything will be fine.
  9. The answer to the original poster is yes, if the power brick will
    accept as low as 100V andat 50 or 60 Hz, then everything will be fine.

    Many thanks to all. The power brick will take the lower japanese voltage.
    The japanese mains plug sockets only have two pins, so they dont have an
    earth connection.
  10. lars

    lars Guest

    Peter Hucker:
    The answer is no.
  11. Arfa Daily

    Arfa Daily Guest

    UK standard domestic mains runs at 240V 50 Hz and most houses have been
    Ringmain circuits in the UK are nominally rated at 30A, but have *outlets*
    on them rated to 13A. Bit of a 'picky' point, I know, but might be a bit
    misleading to non uk posters, the way you had put it.

  12. Arfa Daily

    Arfa Daily Guest

    Well, the cable itself may be rated to 32 amps, and the breaker may be
    either 32 amps or 30 amps in an older installation, but never-the-less, this
    type of installation has always been known generically as a '30 amp ring
    main circuit' and is generally accepted as being safely rated to 30 amps to
    include all ages of installation.


  13. ian field

    ian field Guest

    Some peoples knowledge is really rubbish.

    There are defined tables and graphs which accurately describe fusing
    capacity of fuses, it is known that a 5A fuse will carry 10A - but not for
    very long! If the kettle boils before the fuse wire works up to its melting
    temperature then the fuse wire cools again before it blows so you get away
    with it another day, but each time the fuse is used above its rated current
    it degrades a little and sooner or later will fail within the time it takes
    the kettle to boil.

    Actually a 5A fuse carrying exactly 5A has a finite fife expectancy - which
    actually isn't all that long, unfortunately I can't find the graphs to look
    up the exact life expectancy.
  14. Whiskers

    Whiskers Guest


    Our power sockets are only 3kW each - but it isn't the Watts that really
    hurt, or even the Volts; it's the Amps you really need to be careful with.
    60 Amps at a Japanese 100V would be a lot more dangerous than 30 Amps at a
    European 230V. (Although either could kill you, so the difference could
    be academic as far as the one grasping the wires is concerned).

    Notice that fuses and cut-outs are rated in Amps, and that "non-lethal"
    stunners for the cops are rated at tens of thousands of volts. In the
    right (or wrong!) circumstances, the current that will make a torch bulb
    glow can be enough to stop the human heart.
  15. Whiskers

    Whiskers Guest

    Well you go right ahead and play with the 1000 Amp wires if you want to,
    but wait for me to get out of sight and earshot first - I'm a bit
    It isn't. There's a threshold at around 50V at which the body's internal
    resistance drops considerably; after that, the resistance will depend on
    the path taken through and/or over the body and the amount of tissue
    damage caused - those things being dependent on the energy (ie Amps)
    available rather than on the voltage. AC and DC current also have
    different effects.
    Think of water pipes. A narrow pipe can be likened to a low current cable
    (few Amps) and a wide pipe is like a cable carrying a large current (many
    Amps). Both can be at the same pressure (Voltage) but one will cause a lot
    more damage when it bursts than the other. Even if the wide pipe is at a
    lower pressure, it can still deliver a lot more water than the narrow
    pipe; likewise, even at a lower voltage a cable carrying a lot of current
    will deliver a lot more energy than one with less capacity but at a higer
    I don't know what the regulations are for temporary electrical
    installations on construction sites in the UK, but I suspect they aren't
    much different from those relating to permanent installations. The use of
    110V equipment may have more to do with avoiding such stringent inspection
    and installation standards as apply to higher voltages - but the safety
    measures in use will be related to the current available, not the voltage.

    If there is "no potential to ground" then the system will only be safe
    with "double-insulated" appliances.

    To deliver a given amount of power, a low voltage supply has to carry more
    current (more Amps) than a higher voltage supply, so in that respect it
    can be more dangerous.

    Once the current is flowing, what matters is the amount of energy (Amps)
    not the 'pressure' (Volts) - and where the energy flows. A few milliamps
    will stop the heart, if that current flows along a path that disrupts the
    tiny nerve signals involved.

    Walking across a synthetic carpet can generate a charge of thousands of
    Volts - enough to create a spark when you get near another person or a
    metal fixture or piece of furniture. But there is very little energy

    See <>.
  16. Arfa Daily

    Arfa Daily Guest

    I've got to agree with Dave on this one. Much of what you say appears to be
    based on an incorrect understanding of Ohms Law, and how it applies to the
    potential for causing electrocution of the human body. The water and pipes
    analogy is good for some simple college explanations involving DC systems
    with fixed parameters, but its validity for explaining complex dynamic
    systems, is tenuous at best.

    A system carrying a lot of amps to the load that's drawing it, is no more or
    less dangerous than one that's only carrying a small current, except in as
    much as there is a greater potential for heat failure at connection points
    within that system. A high voltage system, irrespective of how many amps it
    is capable of carrying above a few milliamps, is far more dangerous to a
    human, than a low voltage system good for a few hundred amps. Given the
    (reasonably) constant resistance of any described path through the human
    body, a voltage of a hundred volts with a current availability of as little
    as 50mA, may be enough to kill under the right (wrong!) circumstances. On
    the other hand, a low voltage welding supply, will not have enough voltage
    behind it to push enough current through that same path to kill you, even
    though that supply is good for 200 amps or more.

    As far as your contention that a transformer isolated supply is only safe
    with double insulated equipment, that simply isn't true. The only way that
    you can drive current through the body from such an isolation transformer,
    is to hang yourself across both output terminals. Either terminal to ground
    will present no electric shock hazard at all.

  17. Whiskers

    Whiskers Guest

    So a charge of 10kV from walking across a carpet (DC) , or a "non-lethal"
    50kV "Tazer" (AC), are more dangerous than a 1kV power line? The danger
    from 'High Voltage' power lines comes from the vast amount of current they
    can deliver.
    What voltage does a 'low voltage' welder operate at? Don't they use
    capacitors to raise the supply voltage to at least 40kV? Or can welding
    really happen at 12V?
    Sounds too good to be true.
  18. Whiskers

    Whiskers Guest


    You can touch the 12V battery terminals reasonably safely, but don't try
    it with the 240V terminals. That voltage is well in excess of the body's
    50V limit for being a non-conductor, and the battery can deliver plenty of
    Amps to be lethal. It isn't the 240V that kill, it's the 21A.

    Turning fresh meat into smoke and ash can dissipate a great many Watts.
  19. ian field

    ian field Guest


    and scroll down to "Lethality of a shock".

    From the table there it is apparent that passing a current anything over
    about 0.1A is quite likely to be fatal, so it makes little difference
    whether the voltage source is capable of supplying 1A or 1000A.

    The lethality of any voltage source that can supply more than about 100mA
    will depend on there being sufficient voltage to pass lethal current through
    the body according to Ohms law (V/R=I).

    Typical estimates for human body resistance:

    Unbroken dry skin = about 100,000 - 500,000 Ohms
    Unbroken wet skin= About 1000 Ohms
    Broken skin= As low as 100 Ohms

    The higher the voltage, the greater the likelihood that localised burning
    will break the skin resulting in a rapid drop in resistance with consequent
    increase in current flow.
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