Two-prong vs three-prong power plug for electrical equipment

Discussion in 'Electrical Engineering' started by alpha_uma, Jul 21, 2004.

  1. alpha_uma

    alpha_uma Guest

    Does anyone remember when manufacturers started using two-prong power plugs
    for TVs? For that matter, did they ever use three-prong plugs for TVs? What
    are the advantages and disadvantages--from a safety perspective, not from
    the cost-cutting perspective?

    They did that to the simple toasters a long time ago. Are they doing the
    same to microwave ovens, or have they done it already? How safe is it
    really? Are they simply assuming that all households would have been wired
    with electrical ground fault protection these days?

    Thanks
    alpha_uma, Jul 21, 2004
    #1
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  2. alpha_uma

    Beachcomber Guest


    >
    >I've never seen TVs with 3 prong plugs. Not even the old "transformerless"
    >tube TVs, although they used interlocks to prevent one from opening the
    >back while powered on, and they were at the time the only devices with
    >one blade wider than the other so the chassis was (hopefully!) at the
    >neutral's potential. (now most 2 prong plug devices have the wide blade)
    >
    >>They did that to the simple toasters a long time ago. Are they doing the
    >>same to microwave ovens, or have they done it already? How safe is it
    >>really? Are they simply assuming that all households would have been wired
    >>with electrical ground fault protection these days?

    >
    >Every microwave I've seen, including older ones, had 3 prong plugs.
    >
    >--
    >-Mike


    TV's generally have no exposed metal parts to shock the user and
    in-effect are the equivalent of double-insulated when it comes to
    safety issues, hence the two-prong polarized plug. Commercial video
    monitors may contain conductive rack mounts or metal frames and are
    almost always grounded.

    For a microwave oven, the grounding of interior conductive cage is an
    essential part of the shielding from microwave leakage and also serves
    as an electrical safety ground for the appliance. These are always
    grounded with 3 prong plugs. Not having a proper ground on a
    microwave oven might lead to a re-radiation problem under certain
    conditions.

    Just a note about your terminology which I am sure you understand but
    some readers may not. There is a difference between being wired with
    "Ground" protection and "Ground Fault Protection". "Ground"
    protection (in the form of 3 prong receptacles) is something that has
    been required in the US since the late 50's - early 60's for most
    places. "Ground Fault Protection" is a relatively new requirement
    that takes the form of a GFCI device, either at the breaker, on a
    receptacle, or sometimes on the power cord of an appliance itself.

    Beachcomber
    Beachcomber, Jul 21, 2004
    #2
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  3. alpha_uma

    alpha_uma Guest

    "Greg" <> wrote in message
    news:...

    > I have never seen a regular TV with a 3 prong plug. I have a "monitor"

    with one
    > but it was sold as an industrial unit.


    Me neither, or at least not I do not recall having seen one myself. However,
    all computer monitors that I have bought so far (all rated for residential
    use) have 3-prong power plugs. Does that mean computer monitors are less
    "doubly-insulated" than TVs? After all, many of the computer monitor
    manufacturers are the same companies that make TVs.


    > In fact most old 50s era tube TVs actually had the chassis connected to

    one
    > side of a non-polarized plug. That will wake you up if you lose a knob and
    > touch the stem.


    LOL. Does wonder to the hair too. Wonderfully lax electrical codes back
    then? :)


    > The same is true with consumer toasters and other kitchen appliances. They

    all
    > seem to be 2 prong although the ones made since the Johnson administration

    do
    > have polarized plugs. I think most are double insulated these days.


    Speaking of power plug and power socket "polarization", is it a world-wide
    practice? I know they do so in N. America. Do they do that in Europe and
    Asia? Other parts of the world?

    My Philips electrical kettle comes with a 3-prong plug. But I think I have
    seen some electrical hot pots (for boiling water or heating soup stuff) use
    2-prong plugs (I don't know if they are rated as "double-insulated" or not,
    though). Wouldn't that be dangerous (if the user is not protected by CFCIs)?

    How trustworthy is "double-insulation" for toasters, really?
    Al-U
    alpha_uma, Jul 21, 2004
    #3
  4. alpha_uma

    alpha_uma Guest

    "Greg" <> wrote in message
    news:...
    <snip>
    > >How trustworthy is "double-insulation" for toasters, really?

    >
    > It's fine until you stick the fork in there to retrieve a stuck begel.


    LOL.

    But wait a minute. Are toasters these days stamped with the "double
    insulation" rating or not? If "yes", then according to the standards of
    "double insulation", shouldn't the bread-slice chamber of the toaster be
    isolated from the live main power of the device? Not that I would want to
    stick a fork in there, but has anyone tried rescuing a bagel with a fork and
    live to tell it? :)

    Al-U
    alpha_uma, Jul 21, 2004
    #4
  5. alpha_uma

    alpha_uma Guest

    "alpha_uma" <> wrote in message
    news:I6yLc.72470$Mr4.56501@pd7tw1no...

    <snip>

    > My Philips electrical kettle comes with a 3-prong plug. But I think I have
    > seen some electrical hot pots (for boiling water or heating soup stuff)

    use
    > 2-prong plugs (I don't know if they are rated as "double-insulated" or

    not,
    > though). Wouldn't that be dangerous (if the user is not protected by

    CFCIs)?
    >

    <snip>

    Sorry for the typo. I meant GFCIs.
    alpha_uma, Jul 21, 2004
    #5
  6. alpha_uma

    SQLit Guest

    "alpha_uma" <> wrote in message
    news:FxuLc.77103$ek5.10158@pd7tw2no...
    > Does anyone remember when manufacturers started using two-prong power

    plugs
    > for TVs? For that matter, did they ever use three-prong plugs for TVs?

    What
    > are the advantages and disadvantages--from a safety perspective, not from
    > the cost-cutting perspective?
    >
    > They did that to the simple toasters a long time ago. Are they doing the
    > same to microwave ovens, or have they done it already? How safe is it
    > really? Are they simply assuming that all households would have been wired
    > with electrical ground fault protection these days?
    >
    > Thanks


    Double insulated tools are usually 2 pronged. All of my old power equipment
    has 3 prongs. The weed eater and my mower do not, just 2 prongs. I doubt
    that microwaves or anything that draws a lot of power over time will ever
    have 2 prongs. I guess hair dryers and curling irons are exempt cause they
    are never plugged into an non-GFCI outlet. (They never met my ex)
    SQLit, Jul 21, 2004
    #6
  7. alpha_uma

    alpha_uma Guest

    "Beachcomber" <> wrote in message
    news:...

    <snip>
    >>alpha_uma wrote:
    >><snip>
    >>Are they simply assuming that all households would have been wired
    >>with electrical ground fault protection these days?
    >>

    <snip>
    > Just a note about your terminology which I am sure you understand but
    > some readers may not. There is a difference between being wired with
    > "Ground" protection and "Ground Fault Protection". "Ground"
    > protection (in the form of 3 prong receptacles) is something that has
    > been required in the US since the late 50's - early 60's for most
    > places. "Ground Fault Protection" is a relatively new requirement
    > that takes the form of a GFCI device, either at the breaker, on a
    > receptacle, or sometimes on the power cord of an appliance itself.


    Yes, I should NOT have used the potentially confusing phrase "electrical
    ground fault protection" to mean "GFCIs". After all, there are different
    ways and levels of protection to decrease the possibility of an electrical
    fire or electrocution. For the sake of completeness, the commonly used ways
    (that I'm aware of) in house wiring are:

    (1) Fuse box (probably obsolete?)
    (2) Circuit breaker
    (3) Ground wire via 3-prong receptacles
    (4) GFCI (ground fault circuit interrupter)

    Are there other practised ways? Any known studies that compare the
    effectiveness of (4) vs (3) (assuming the presence of (2) in each) in saving
    lifes? Basically, I'm just curious whether or not manufacturers of
    electrical/electronic equipment are increasingly more in favor of achieving
    the "double insulation" rating than utilizing (3) via 3-prong plugs.

    Al-U
    alpha_uma, Jul 22, 2004
    #7
  8. alpha_uma

    alpha_uma Guest

    "SQLit" <> wrote in message
    news:u%zLc.114$FZ2.82@lakeread04...
    >
    > <snip>
    > Double insulated tools are usually 2 pronged. All of my old power

    equipment
    > has 3 prongs. The weed eater and my mower do not, just 2 prongs. I doubt
    > that microwaves or anything that draws a lot of power over time will ever
    > have 2 prongs. I guess hair dryers and curling irons are exempt cause

    they
    > are never plugged into an non-GFCI outlet.
    >
    > (They never met my ex)
    >

    LOL. AFAIC, Murphy's law trumps all other rules. Sooner or later, someone
    will stick a fork in the toaster or put a spoon in a microwave oven, and
    someone will use a blow-dryer while the sink or tub is full of water. I
    guess it is a good thing that the new codes require GFCIs be installed in
    bathroom circuits.
    Al-U
    alpha_uma, Jul 22, 2004
    #8
  9. alpha_uma

    Rowbotth Guest

    We had an old Admiral TV that Dad bought in the mid to late 1950s. A
    little 12" unit that had a picture that was only surpassed by satellite
    images of modern times, by the way.

    This thing had a metal case with a 2-prong plug, and it was eventually
    installed in the basement of our house some 20 years later.

    I learned very quickly that the knobs were plastic and were safe.
    Touching the case of the TV was a very dangerous occupation - not enough
    charge to do any physical damage, but enough to make you ay attention to
    the plastic parts of the TV. (I still recall the tingle running up my
    arm!)

    On a wooden floor, no tingle. But on that d*&^#d cement floor, watch
    out.

    HR.


    > TV's generally have no exposed metal parts to shock the user and
    > in-effect are the equivalent of double-insulated when it comes to
    > safety issues, hence the two-prong polarized plug. Commercial video
    > monitors may contain conductive rack mounts or metal frames and are
    > almost always grounded.
    >
    > For a microwave oven, the grounding of interior conductive cage is an
    > essential part of the shielding from microwave leakage and also serves
    > as an electrical safety ground for the appliance. These are always
    > grounded with 3 prong plugs. Not having a proper ground on a
    > microwave oven might lead to a re-radiation problem under certain
    > conditions.
    >
    > Just a note about your terminology which I am sure you understand but
    > some readers may not. There is a difference between being wired with
    > "Ground" protection and "Ground Fault Protection". "Ground"
    > protection (in the form of 3 prong receptacles) is something that has
    > been required in the US since the late 50's - early 60's for most
    > places. "Ground Fault Protection" is a relatively new requirement
    > that takes the form of a GFCI device, either at the breaker, on a
    > receptacle, or sometimes on the power cord of an appliance itself.
    >
    > Beachcomber
    Rowbotth, Jul 22, 2004
    #9
  10. s falke wrote:
    > "alpha_uma" <> wrote ...
    >
    >>Does anyone remember when manufacturers started using two-prong power plugs
    >>for TVs? For that matter, did they ever use three-prong plugs for TVs? What
    >>are the advantages and disadvantages--from a safety perspective, not from
    >>the cost-cutting perspective?

    >
    >
    > TV sets {'monitors'} with grounded cords are typically for "institutional"
    > duty; likely with a telltale odd lot of other assorted video/audio connectors.



    My TV, VCR, and component DVD player all have two prongs, and then all
    of my Stereo equipment has a two prong too. They're all polarized, with
    one blade being bigger than the others though.
    On the other hand, ALL of my computer has 3 prongs. CPU, Monitor,
    Printer. Although, they all also use interchangable power cords that
    can be detached from the unit, those cords with three holes that look
    like an inverted plug, I've noticed a ton of devices that use those
    standard cords, because all have the transformer in the device itself
    instead of on a brick for a cord.
    Anthony Guzzi, Jul 22, 2004
    #10
  11. alpha_uma

    Don Kelly Guest

    "alpha_uma" <> wrote in message
    news:uNyLc.72520$Mr4.30532@pd7tw1no...
    > "Greg" <> wrote in message
    > news:...
    > <snip>
    > > >How trustworthy is "double-insulation" for toasters, really?

    > >
    > > It's fine until you stick the fork in there to retrieve a stuck begel.

    >
    > LOL.
    >
    > But wait a minute. Are toasters these days stamped with the "double
    > insulation" rating or not? If "yes", then according to the standards of
    > "double insulation", shouldn't the bread-slice chamber of the toaster be
    > isolated from the live main power of the device? Not that I would want to
    > stick a fork in there, but has anyone tried rescuing a bagel with a fork

    and
    > live to tell it? :)
    >
    > Al-U
    >

    See those nice glowing heating elements. See the absence of any 1200watt,
    120/120V isolation transformer built into the toaster. The toaster elements
    are electrically hot and if you stick your fork in the toaster while
    touching a grounded sink or whatever- you can be in for a nasty surprise.
    The little wires holding the toast and other exposed metal parts are
    insulated from the heating elements. Attempts are made to make things fool
    proof but not damnfool proof.

    --
    Don Kelly

    remove the urine to answer
    Don Kelly, Jul 22, 2004
    #11
  12. In article <I6yLc.72470$Mr4.56501@pd7tw1no>,
    "alpha_uma" <> writes:
    >
    > Me neither, or at least not I do not recall having seen one myself. However,
    > all computer monitors that I have bought so far (all rated for residential
    > use) have 3-prong power plugs. Does that mean computer monitors are less
    > "doubly-insulated" than TVs? After all, many of the computer monitor
    > manufacturers are the same companies that make TVs.


    I don't think anywhere else much in the world uses US TV's, but the
    same monitors are used the world over, and hence conform to more
    different country's safety standards.

    > Speaking of power plug and power socket "polarization", is it a world-wide
    > practice? I know they do so in N. America. Do they do that in Europe and
    > Asia? Other parts of the world?


    The UK does, but most of Europe doesn't (in some parts of Europe,
    neither mains conductor is guaranteed to be near ground potential).
    The UK socket outlet (and it's round pin predecessor which is also
    polarised) are still used by most former British Empire/Commonwealth
    countries too.

    > My Philips electrical kettle comes with a 3-prong plug. But I think I have
    > seen some electrical hot pots (for boiling water or heating soup stuff) use
    > 2-prong plugs (I don't know if they are rated as "double-insulated" or not,
    > though). Wouldn't that be dangerous (if the user is not protected by CFCIs)?
    >
    > How trustworthy is "double-insulation" for toasters, really?


    Double insulated appliances are common in the UK, but it's difficult
    to make double insulated appliances with high power heating elements,
    so those aren't common. You won't find a double insulated toaster in
    the UK (not that I've ever seen anyway, nor anywhere in Europe AFAIK).

    --
    Andrew Gabriel
    Andrew Gabriel, Jul 22, 2004
    #12
  13. In article <>,
    (Beachcomber) writes:
    >
    > For a microwave oven, the grounding of interior conductive cage is an
    > essential part of the shielding from microwave leakage and also serves


    It can't be -- the impedance of the ground connection at 2.4GHz
    will be so high as to be completely useless. The shielding operates
    simply as a microwave reflector and doesn't need to be grounded for
    this purpose.

    > as an electrical safety ground for the appliance. These are always
    > grounded with 3 prong plugs. Not having a proper ground on a
    > microwave oven might lead to a re-radiation problem under certain
    > conditions.


    --
    Andrew Gabriel
    Andrew Gabriel, Jul 22, 2004
    #13
  14. alpha_uma

    alpha_uma Guest

    "Don Kelly" <> wrote in message
    news:g5GLc.77365$od7.46052@pd7tw3no...
    >
    >
    > "alpha_uma" <> wrote in message
    > news:uNyLc.72520$Mr4.30532@pd7tw1no...
    > > "Greg" <> wrote in message
    > > news:...
    > > <snip>
    > > > >How trustworthy is "double-insulation" for toasters, really?
    > > >
    > > > It's fine until you stick the fork in there to retrieve a stuck begel.

    > >
    > > LOL.
    > >
    > > But wait a minute. Are toasters these days stamped with the "double
    > > insulation" rating or not? If "yes", then according to the standards of
    > > "double insulation", shouldn't the bread-slice chamber of the toaster be
    > > isolated from the live main power of the device? Not that I would want

    to
    > > stick a fork in there, but has anyone tried rescuing a bagel with a fork
    > > and live to tell it? :)
    > >
    > > Al-U
    > >

    > See those nice glowing heating elements. See the absence of any 1200watt,
    > 120/120V isolation transformer built into the toaster. The toaster

    elements
    > are electrically hot and if you stick your fork in the toaster while
    > touching a grounded sink or whatever- you can be in for a nasty surprise.
    > The little wires holding the toast and other exposed metal parts are
    > insulated from the heating elements. Attempts are made to make things fool
    > proof but not damnfool proof.
    >
    > --
    > Don Kelly


    Yes, of course the toaster heating elements are electrically hot, and
    therein lies the danger. That is the main reason for my skepticism/sarcasm
    (in my previous message) on any toaster attaining any "double insulation"
    rating. For example, my toaster (with a 2-prong power plug) does not seem to
    have a "double insulation" rating--no "concentric square" symbol that I can
    see.

    Inside a toaster, the nichrome wires, which wrap around mica sheets, and
    which carry the current from the power plug, do not have much (physical)
    clearance to the

    bread slice chamber,

    which, IMHO, should be viewed as an

    "user accessible" area with many exposed metallic parts.

    My thinking is that I would feel more comfortable treating the typical $12
    toaster as a device which barely passes "basic insulation". And since
    toasters usually come with only a 2-prong plug these days, we can't even
    classify it as a Class I device. But I could be wrong as I only have a
    cursory understanding of electrical safety standards. So, I'll ask, again,
    if anyone has seen a toaster with a "double insulation" rating.

    Another thought. Why don't they use a (electrically) non-conducting sheath
    for the toaster like the kind they use for the electric stove to shield the
    nichrome wires? Too costly?

    Al-U
    alpha_uma, Jul 23, 2004
    #14
  15. alpha_uma

    Jimmie Guest

    "alpha_uma" <> wrote in message
    news:FxuLc.77103$ek5.10158@pd7tw2no...
    > Does anyone remember when manufacturers started using two-prong power

    plugs
    > for TVs? For that matter, did they ever use three-prong plugs for TVs?

    What
    > are the advantages and disadvantages--from a safety perspective, not from
    > the cost-cutting perspective?
    >
    > They did that to the simple toasters a long time ago. Are they doing the
    > same to microwave ovens, or have they done it already? How safe is it
    > really? Are they simply assuming that all households would have been wired
    > with electrical ground fault protection these days?
    >
    > Thanks
    >
    >

    Dont remember TV sets ever being 3 prong and I was a TV tech in the
    seventies.
    Jimmie, Jul 25, 2004
    #15
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