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

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. 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. 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. On the other hand I have never seen a microwave with a 2 prong plug. I assume it had to do with shielding.

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)

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

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

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.

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

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

It probably has more to do with noise isolation or simply perception of it.

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

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

?s falke

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

Sorry for the typo. I meant GFCIs.

| 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)

An old TV we had from long ago (made around 1949, cloth covered cord, half hemisphere like plug) did not have a polarized plug. I remember my dad saying not to touch the antenna wires on it because you'd get shocked (so of course I did, and did). But I fixed it by unplugging it and plugging it back in rotated 180 degrees. My dad thought I was a genius. Wanna guess what the chassis was connected to?

|>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.

It seems some things just have them, and some things don't. I bet in part it is due to the cost and where used. But now days they might be going back to ungrounded bcause of the number of GFCIs installed in kitchesn.

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)

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

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

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.

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.

I had the same problem when I started playing MP3s Bond the case of the A/V stuff to the PC with 14-12 ga copper, cheap at the Home Depot.

I can remember my younger days playing in a rock band. Many PA and guitar amp cords were two prong and while the chassis wasn't tied directly to the neutral (or hot if you got the plug backwards), many incorporated caps between the lines and chassis. Some even had toggle switches to switch the supply polarity (to reduce hum).

There were a few incidents where a guitar player would get a nasty shock when he grabbed a microphone. This was due to the PA amp's line polarity being reversed. There were also stories of one or two that were killed, but these may be urban legends.

Thank goodness I played drums. ;-) As far as computer monitors (and 'industrial' TV monitors), three prong plugs are used to provide a low impedance ground path to prevent the PC to monitor cable (or audio/video monitor cables) from carrying full ground fault currents in the event of an internal short, insulation failure, etc. Consumer audio/video products tend to be two prong, so a fault will pull all the equipment chassis to the same level and, with a high impedance back to the source, not much current will flow. Trouble is, people are mixing computers (as audio/video sources) with their grounded chassis with their hi-fi and TV equipment which isn't three prong. The day will come when a bad TV power supply will be grounded through that expensive multimedia video card with TV outputs. That's an expensive fuse.

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.

And it is time to say that toasters are probably NOT grounded because those little wires would spray sparks when the fork touched one of them AND the live element. Those sparks would cause physical injury when the toucher jerked back from the toaster and/or when the sparks (hot metal pieces) got on skin or in eyes. Probably that is why the toasters disconnect BOTH sides of the line from the element when not heating. (Check me on this, I think this is true.) And the case is floating, NOT grounded, for these reasons. Of course an isolation transformer at a toaster's VA level would be out of the question. --Phil

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.

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.

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).