why we are preferring 3pin plugs in some devices?

why we are preferring 3pin plugs in some devices (eg.in ups,extention boxes...) and why we are preffering 2pin plugs in some devices(eg.in ordinary night lamps)?

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Anything with a metal case probably should have a ground pin, although it is not always true.

On Jun 10, 6:33 pm, snipped-for-privacy@aol.com wrote:
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i think tri-pin is used in the case of heavy loads and double pin in the case of light loads as in heavy loads the earth fault current will be more when compared to light loads..

VIDHYA wrote:
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I would suggest that it is more to do with "heavy loads" having to get rid of a lot of energy - which makes complete double-insulation of any parts that could become live problematic. They usually have something that needs to come out of the innards, such as a (metal) rotating shaft, stream of fluid (water/air/refrigerant), wires, etc that could, potentially, either provide a conductive path or provide a hole through which a conductive path could be established. And/or, of course, "heavy loads" are far more likely to have metal parts, because of the temperature of operation or the amount of energy needing to be transferred.
Whereas light loads can get rid of the very small amount of energy that they consume through the layers of electrical insulation.
There is no automatic "earth fault current" with a two pin device. Such a current may not flow until a user touches it. At which point a fatal current could easily result. Whilst protection devices, such as GFCI, are certainly life-savers - the key thing about the design of two pin devices *should* be that any failure modes that present a shock hazard are extremely unlikely.
Yes, a "heavy load" is likely to have a much higher earth fault current capability. But it only needs a few hundred mA of earth fault current to kill - and that can come from a "light load" as easily as a "heavy load".
-- Sue

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Sue, fault current is determined by branch circuit size and the type of fault, not the attached equipment. I think "light loads" use 2 pin plugs, simply because they are cheaper to make. Here in the US there are plenty of desk and floor lamps with 2 pin plugs and metal construction. I don't know why they are not considered a hazard. We don't even have fuses in the plugs like you folks.

snipped-for-privacy@aol.com wrote:
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Where have I said otherwise? "Heavy loads" generally need higher rating infrastructure. Which, in turn is likely to result in higher earth fault current capability. A "heavy load" is likely to have a much higher earth fault current capability - because it is connected to a higher rated branch circuit. But my point was that this is irrelevant to whether the circuit needs a third wire, or not. There is more than enough capability for electrocution from a "light load" appliance.
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I consider them a hazard. Hence the *should* in, "the key thing about the design of two pin devices *should* be that any failure modes that present a shock hazard are extremely unlikely."
The UK used to allow similarly dangerous practices. I remember well plugging a smoothing iron into a two pin socket on a two way adapter plugged into a (ceiling) lamp fitting.
Never mind, I expect that the US will catch up with Europe, one day ;)
-- Sue

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We still allow unearthed appliances (we don't have a 2-pin mains outlet in common use, but plenty of appliances only have a 2-core flex). Such appliances have to be double insulated.
The concept of double insulated seems to exist in the US too, but the requirements for it don't seem to be the same, specifically don't seem to include the requirement for a double insulation barrier between live parts and a person, based on appliances I see marked as double insulated in the US which don't come close to being double insulated for Europe.

snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) writes:
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Can you give any examples? I'm in Canada, which has the same double-insulated appliances as the USA, and all the ones I've looked at really do have two insulation barriers. Tool housings are typically plastic. The output shaft or chuck is metal, but there's a non-conductive coupler between the motor armature and output gearsbox or shaft, or the armature is assembled with an insulating spacer between the armature shaft and the laminated armature magnetics.
    Dave

     snipped-for-privacy@cs.ubc.ca (Dave Martindale) writes:
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One that amazed me was a two slot toaster (for toasting sliced bread, just in case it's known as something else somewhere;-)
The mains lead was a 2-core zip cord (figure-of-8 as it's more commonly known here) with a molded 2-prong reversable plug. OK, that's common in the US, but being only a single insulation layer with no sheath, isn't allowed in Europe for mains voltage. I assume double insulated doesn't apply to mains cords in the US, even for appliances which claim to be double insulated?
The toaster case was metal. The element was a bare wire threaded over an insulating backing. There are a few stiff metal guard wires in front of the element to prevent the bread touching it. There's nothing to prevent the element breaking and contacting the guard wires (e.g. burning out, or more likely damaged by a piece of toast jamming and being pried out with a fork or similar). Out of curiosity, I turn the toaster over, and it's marked double insulated on the bottom. Well, it never could be in Europe, as there just aren't 2 layers of insulation between live parts and you. I don't think we have any double insulated toasters here. The designs are identical to this US one, but would always have a 3-core sheathed flex and 3 pin plug, so in the event of the case contacting a live part, the case remains safe.
Looking at the steam iron next, again, same single insulated zip cord and molded on 2-prong plug. It is conceivable that the element really is double insulated from the sole plate, but I can't check. I wonder how reliable that insulation might be when someone has filled the thing under the tap (fawcet) and got water spilled into all the wrong places? Again, I'm really dubious this would meet EU standards -- irons here are normally 3-core earthed, but I do have a small double insulated travel iron, so that is possible.

On 11 Jun 2008 22:17:39 GMT, snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote:
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The American toaster has always been the first hurdle in the Darwin champoinships. People stick all metal butter knives iin them

snipped-for-privacy@aol.com wrote:
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One assumes that when the timer operates and the toast pops up, the "hot" side of the mains is disconnected. This is not true with an un-polarized plug.
I've seen only a few toasters in which "both" conductors of the line cord, (flex), were disconnected when the bread popped up. Thus, even if the user moved the bar up and then attempted to remove a stuck piece of toast, he had a fifty-fifty chance of touching the element still connected to the hot side of the mains.
The lack of double insulation doesn't apply to most modern toasters which have plastic bodies.

| snipped-for-privacy@aol.com wrote:
|> Gabriel) wrote: |> |>> One that amazed me was a two slot toaster (for toasting sliced |>> bread, just in case it's known as something else somewhere;-) |> |> The American toaster has always been the first hurdle in the Darwin |> champoinships. People stick all metal butter knives iin them | | One assumes that when the timer operates and the toast pops up, the | "hot" side of the mains is disconnected. This is not true with an | un-polarized plug.
It seems one of the earliest electric toasters did have a polarized plug. See the 2nd image on this page:
http://en.wikipedia.org/wiki/Toaster
| I've seen only a few toasters in which "both" conductors of the line | cord, (flex), were disconnected when the bread popped up. Thus, even if | the user moved the bar up and then attempted to remove a stuck piece of | toast, he had a fifty-fifty chance of touching the element still | connected to the hot side of the mains. | | The lack of double insulation doesn't apply to most modern toasters | which have plastic bodies.
The toaster in the 2nd images of the above page doesn't have much insulation.

On Wed, 11 Jun 2008 21:53:00 -0400 snipped-for-privacy@aol.com wrote: | On 11 Jun 2008 22:17:39 GMT, snipped-for-privacy@cucumber.demon.co.uk (Andrew | Gabriel) wrote: | |>One that amazed me was a two slot toaster (for toasting sliced |>bread, just in case it's known as something else somewhere;-) | | The American toaster has always been the first hurdle in the Darwin | champoinships. People stick all metal butter knives iin them
Not me. Oh no. I always use a clean steak knife.

Andrew Gabriel wrote:
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The UL requirement for double insulation does require two barriers to live conductors. Of course different pieces of equipment accomplish this in different ways, some of which might not be immediately obvious on casual inspection.

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How does a metal cased US toaster manage this with exposed live elements? A European toaster is exactly the same, but could not claim to be double insulated.
Also, I think most (if not all) US mains flexs I've seen only have a single insulation barrier. That's not allowed in Europe, because it's missing the two barriers.

|> Andrew Gabriel wrote:
|>>> |>> |>> The concept of double insulated seems to exist in the US too, but |>> the requirements for it don't seem to be the same, specifically |>> don't seem to include the requirement for a double insulation barrier |>> between live parts and a person, based on appliances I see marked as |>> double insulated in the US which don't come close to being double |>> insulated for Europe. |> |> The UL requirement for double insulation does require two barriers to live |> conductors. Of course different pieces of equipment accomplish this in |> different ways, some of which might not be immediately obvious on casual |> inspection. | | How does a metal cased US toaster manage this with exposed | live elements? A European toaster is exactly the same, but | could not claim to be double insulated. | | Also, I think most (if not all) US mains flexs I've seen only | have a single insulation barrier. That's not allowed in Europe, | because it's missing the two barriers.
There insulation around the individual conductors, and insulation around the set of conductors. How many more layers are needed?

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The normal US NEMA 5-15 receptacle will either be on a 15a or 20a circuit.As you pointed out the potential for electrocution is the same. Any difference in fire danger is minimal. I suppose if the tombstones were there to justify a change in U/L listing practices we would see 3 pin plugs on everything. The 3 pin receptacle has been standard in new construction here for 40 years.

Palindrome wrote:
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The present USA sockets have two flat blades and a round ground pin. In fact the blades are of different widths with the "hot" lead on the narrow blade. Two blade plugs were meant to be fitted so that any possibly exposed parts were connected to the "neutral" though the wider pin. This is most often the base of an Edison socket. The socket center contact was supposed to be connected to the narrow pin through the switch.
Many cheaper products, including lamps with Edison sockets, have two blade plugs with equal width blades.
I worked as a "broadcast engineer" many years ago, before three pin plugs and sockets were common. The first thing we did when a remote broadcast set-up put hum on the telephone line back to the studio, was to: "reverse the plug".
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Parts of "Europe" still have two-pin sockets with no earth pin. In some places they are not even "polarized".
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I believe modern USA electrical codes have "caught up", unless you consider the 120/240 split phase and lack of loop wiring to be archaic!
The UK have done a splendid job of "catching up" since I traveled extensively there in the late 1960's. I plugged my two blade, (equal width), electric shaver plug into every conceivable kind of outlet using various "adapters". I remembered to set the switch to 240V and didn't destroy the shaver or myself. I changed rooms in a London hotel once, where they had the usual 240/120 isolation transformer, which I forgot to re-set to 120V after I'd set the razor to 120V. I "pulled the plug" just in time!
On a more serious note: I would like to see statistics comparing fatalities from electrocution versus those from electrical fires in both the USA and the UK.

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They've been mentioned here in the past. It's difficult to get directly compariative data as the same incident may be very differently classified in different places. Classifications are also not always reliable or what you might expect. Around 5 years ago, I took a detailed look at UK figures for incidents in the home (not elsewhere). Fires are often classed as electrical just because no other cause can be found in the remains, and one fire was classed as electrical which was an open chip pan fire which happened to be on an electric stove when it caught fire. Although chip pan fires are not normally classed as electrical in the UK - they have their own dedicated catagory here, being the number 2 cause of house fires after smoking. So we probably over-count electrical fires in the UK.
The UK average figures per year for the 10 year period 1992-2002 are:
Fatal Non-fatal Installations (electric shock) 5 576 Appliances (electric shock) 14 1700 Fires (electrical) 25 590
although they were dropping over that period, so they're higher at the start than at the end of the 10 year period.
I don't have the US figures any more, but they were much higher per capita. Actually, the US per capita figures for just home incidents was well higher than our total figures including public and industrial incients. Electrical fires was very much higher in US homes.
The reason I investigated the UK figures was to respond to the proposed change in UK legislation to bring in what's now known as Part P -- the requirement for inspection of home wiring, which previously didn't exist in the UK. Prior to this, the incidents in the UK had been steadily declining over the previous 30 years. A number of us thought the legislation was unnecessary, and would be counter-productive. However, I don't think any of us expected the extent to which we would be proved to be correct -- electrical fatalities in England and Wales instantly doubled with the introduction of Part P. So, from being the second lowest in Europe and hence one of the safest in the world, we are now up nearer to (but still less than) the US figures per capita.

On 11 Jun 2008 20:09:36 GMT, snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote:
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I really think most electrical fies in the US involve misuse of extension cords and cube taps or simply defective equipment plugged in but like you said, a lot of fires get pegged as electrical because they don't find any other obvious source. Unless arson is suspected most fire investigations are pretty superficial from what I have seen. Some guy at the fire department, trained investigator or not, checks a box on a form based on his first impression and that is "the cause". I do understand that when foul play is suspected and they call out the big guns, they can pinpoint the fire cause pretty well. That just doesn't happen that often, particularly if nobody got hurt.