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Internal wiring of USA v UK mains plug

Discussion in 'Electronic Design' started by N Cook, Jun 27, 2007.

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  1. N Cook

    N Cook Guest

    This is someone's graphic of internal wiring of a UK line connector
    note the screw down, into captive hollows, for the bared leads , also the
    cord grip and also the internal fuse.
    This week I had to wire up a USA mains connector like this one
    I could not find an internal pic or graphic but it reminded me of the
    internal wiring of UK plugs of 50 years ago, before ROSPA and BS got
    involved - , wrap around screw terminals that can easily shed a loose wire
    filament, both of them, live and neutral surprisingly close together and
    what I find very odd , no cord grip/anchor and no fuse.
  2. Joel Kolstad

    Joel Kolstad Guest

    You can certainly get higher-quality plugs in the U.S., it just requires going
    to a real hardware/home improvement store rather than the supermarket or dime
    store. :)

    Keep in mind that, in the U.S., since we don't use ring circuits, a shorted
    plug is perhaps a skosh safer than the U.K. where the plug and circuit fuse
    are all in the same connector.
  3. N Cook

    N Cook Guest

    So the USA has separate fuse for each radial spur to each wall outlet ? each
    such wiring run requiring more than twice the weight of copper (110V v
    240V ) for the same kW delivery to the point of use?
  4. Tam/WB2TT

    Tam/WB2TT Guest

    I have never seen a house here in the US wired with stranded wire, except
    for one built in 1906. Generally #14 solid copper. BTW the 3 wire UK plug
    reminds me of what is used on a clothes dryer or stove here. Would you
    actually use one of these on a lamp?

  5. TT_Man

    TT_Man Guest

    Yes, fitted with a 3 amp fuse...

  6. Eeyore

    Eeyore Guest

    Yes, that's what's used. The previous BS 546 mains connectors had 15, 5 and 2 A
    versions but the 2A was rarely used anyway.

    Problem ?

  7. Joel Kolstad

    Joel Kolstad Guest

    No, it's just that each radial spur in the U.S. (which typically would serve
    one room, although I don't know what the actual law requires here) is limited
    to ~1.8W (120V, 15A), whereas my understanding is that each ring in the U.K.
    is generally fused at 30A or 32A, allowing a shorted outlet to pull as much at
    7.68kW before the ring fuse begins to think about blowing.
    Yes, this is true, and supposedly it was the cost of copper that drove the
    U.K. to use ring circuits.

    I don't know how much power cabling sells for in the U.K., but I have done
    some low-voltage (12V) wiring in New Zealand, and it was truly painful to be
    purchasing, e.g., 2.5mm cable compared to the prices in the U.S.

  8. N Cook

    N Cook Guest

    We have no choice in the matter, by law, we can use 1,2 or 3 amp fuses
    inside these plugs but thats the only choice

    The USA cannot have the equivalent of RoSPA (Royal Society for the
    Prevention of Accidents), AFAICS none of the USA ones have child preventers
    on them unless the mouldings on the wall outlets preclude that eventuality
    of small fiongers touching both pins.
    The other notable difference is the insulated pins that have been necessary
    refinement, again by law, for 20 years or so
    You can just see the orange plastic bits extending up the brass pins on the
    first pic on this wiki and the black bits on the one lower down on
    The phase and neutral pins on modern plugs have insulated bases to prevent
    finger contact with pins and also to stop metal sheets (for example, fallen
    blind slats) from becoming live if lodged between the wall and a partly
    pulled out plug. A downside to this prong insulation is that it may
    contribute to damaged sockets not making good contact with the prongs, which
    may even melt the latter. No such problems exist with healthy sockets.

    as an aside someone told me that per million houses there are more house
    fires in the USA due to wiring faults than any other country, partly due to
    a lot of timber construction and partly due to the higher current for a
    given KW of power transfered - is that the case?
  9. TT_Man

    TT_Man Guest


    Have you seen the way they connect their wires? They just twist them
    together and put a plastic cap over the bare wires!
    Multiply that by double current and it's no wonder they have fires :)
  10. Tam/WB2TT

    Tam/WB2TT Guest

    All heavy loads are connected across 240 Volts. 120 is basically used for
    things that can be moved from room to room. There seem to be two main causes
    for electrical fires in the US. One the improper use of extension cords; for
    example, a 10 foot length of 5 amp wire with a refrigerator and microwave
    plugged in at the far end. This comes about because older houses and
    apartments tend to have an insufficient number of wall outlets. Two, there
    was some problem with houses built about 30 years ago that uses aluminum
    wire; these require special connection methods. When a home owner replaces a
    switch or an outlet with a standard device, you have problems. Most home
    owners here tend to do their own electrical repairs.

    I think there is an inherent safety factor in a system where neither side of
    a 240V circuit is more than 120 V above earth potential. Never heard of a
    person being electrocuted who was not standing, or otherwise submerged, in
    water. This costs money. For instance the wiring to an electric clothes
    dryer will have two 20 Amp hot wires, a 20 Amp neutral wire (from center tap
    of transformer), and a 20 amp earth ground wire connected to a cold water
    pipe or ground rod. In some localities, the neutral and ground wires can be
    tied together at the appliance. I think the only reason for the heavy
    neutral and ground wires is to make sure the circuit breaker trip in case of
    a short. As recently as about 20 years ago, a much smaller earth ground wire
    was used.

    The electric use meter must be more expensive than an unbalanced single
    phase one.

  11. Neil

    Neil Guest

    A colleague in Maryland said that he'd attended a safety talk from what
    sounded like the equivalent of a Health and Safety guy, who advised that the
    best way to reduce the probability of fire in the house was to rewire the
    kitchen sockets using decent ($3) sockets rather than the $0.25 ones the
    builders use. Apparently it's something to do with quality of the bits of
    bent metal that make contact...
    And he also advised to not unplug appliances if possible, since that wore
    out the sockets faster.
    Out of curiosity, I bought a double socket for $0.44 at the local Walmart,
    and was somewhat dismayed by the apparent lack of robustness.
    No comparison with our 230V 13A sockets, but maybe that's why they are
    around $4+ each instead.
  12. Tam/WB2TT

    Tam/WB2TT Guest

    They are called wire nuts, and contain a threaded metal insert. Sort of an
    inverse of a self tapping screw. I understand the hesitance, but they don't
    seem to be a problem

  13. Ron(UK)

    Ron(UK) Guest

    We used to have a similar thing but made of ceramic, called a scruit
    (sp?) (pronounced screw-it) I understand that they may be outlawed these

  14. Gary Tait

    Gary Tait Guest

    Actually a typical US clothes dryer uses a 120/240V 30A circuit.
    And the ground wire always connects back to the services neutral, where
    things are grounded with rods and/or plumbing.
    They way is was, is that it was permitted to run just a grounded
    conductor (neutral) to the appliance, and bond that to the case.

    Since 1996 or so, that has been prohibited, so separate neutral and
    grounding conductors must be supplied to a 120/240V appliance.
    For the safety ground, yes. For neutral it is assumed, to the terminals
    in the appliance, that the appliance could draw the full current on the
    If you mean the US ones, 2-wire 120/240 ones are the norm, so are made
    in quantity enough not to be expensive.
  15. Gary Tait

    Gary Tait Guest

    The trick for the screws is to twist the strands anti-clockwise.

    Good plugs will have cord grips, and a mechanism you insert the (twisted)
    conductor into and screw down (although rather different from typical
    European/UK plugs)
  16. Gary Tait

    Gary Tait Guest

    For general purpose recepticle and lighting circuits, they are wired as
    radial, but one fuse or breaker per radial circuit.

    Yes, but not as much KWs are delivered to GP recepticles as there is in the
    230V world, so less copper is used. Yes, that is at the expense of not
    having 2.5 KW kettles and really funky coffee machines.

    Larger appliances that need more power have their own dedicated recepticle
    (or are hard wired) and circuit.
  17. Eeyore

    Eeyore Guest

    I fixed a Weller TCP iron a couple of weeks back that was running cold. A 'wire
    nut' inside was loose and a connection to the element had become iffy.

  18. b

    b Guest

    Sounds dodgy!

    I've spent time in the US and Japan, and I have to say that those flat
    blade sockets are an atrocious design. They suffer sloppy fit problems
    very easily . Those countries don't seem to have switched wall sockets
    either, which the Uk standard has, so you get more arcing if plugging
    in live equipment (which degrades the contacts even further).
    The UK plugs are more complex, and expensive, but a damn sight safer
    and a lot more sturdy and resist wear better -only ever had to
    replace one or two fittings over the years.

    I suppose all this is because it is a more recent standard - like the
    German PAL TV system - which, since it was introduced later, had the
  19. John Larkin

    John Larkin Guest

    Most US electric meters, at least the electromechanical kind, have one
    voltage coil (240 volts, l-l) and two current coils, one in each of
    the 120 volt phases. That computes power based on an assumption of
    voltage symmetry, usually a reasonable bet.

  20. James Sweet

    James Sweet Guest

    Done correctly, a quality wire nut is a very secure and long lasting
    connection. It's not simply a plastic cap, but a plastic casing over a
    threaded springy metal insert which grips the wires very well. I have some
    UK terminal blocks, and the problem with them is that there's no mechanical
    bond between the wires, the contact point is small, and they can and do work
    loose or corrode over time. They generally are ok, but neither method is
    greatly superior to the other.

    The double current isn't really much of an issue, our large loads are 240V
    too, it's handy to have both voltages readily available.

    You can get quality US style receptacles, problem is they're expensive so
    few houses come with them. I like many things about the UK plugs, but the
    thing I don't like is they're *huge* so things like power strips and
    multi-gang outlets are really cumbersome.

    Having discussed this in length with a friend in the UK, we've both come to
    the conclusion that both systems have many advantages and disadvantages and
    neither one is a clear winner.
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