why we are preferring 3pin plugs in some devices?

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.

There three classes of protection against electric shock listed in IEC/EN standards. See extracts from IEN/EN 60950-1 below:-

Class I - equipment where protection against electric shock is achieved by

- using BASIC INSULATION and

- providing a means of connection to the PROTECTIVE EARTHING CONDUCTOR in the building wiring those conductive parts that are otherwise capable of assuming HAZARDOUS VOLTAGES if the BASIC INSULATION fails NOTE CLASS I EQUIPMENT may have parts with DOUBLE INSULATION or REINFORCED INSULATION.

The typical electic stove, kettle etc used in Europe with a three pin plug.

Class II equipment in which protection against electric shock does not rely on BASIC INSULATION only, but in which additional safety precautions, such as DOUBLE INSULATION or REINFORCED INSULATION are provided, there being no reliance on protective earthing.

A typical electric shaver, hair drier etc, with a two pin European plug (no Earth)

Class III equipment in which protection against electric shock relies upon supply from SELV CIRCUITS and in which HAZARDOUS VOLTAGES are not generated NOTE For CLASS III EQUIPMENT, although there is no requirement for protection against electric shock, all other requirements of the standard apply.

Typical battery operated equipment.

BASIC INSULATION insulation to provide basic protection against electric shock.

SUPPLEMENTARY INSULATION independent insulation applied in addition to BASIC INSULATION in order to reduce the risk of electric shock in the event of a failure of the BASIC INSULATION

DOUBLE INSULATION insulation comprising both BASIC INSULATION and SUPPLEMENTARY INSULATION

REINFORCED INSULATION single insulation system that provides a degree of protection against electric shock equivalent to DOUBLE INSULATION under the conditions specified in this standard NOTE The term "insulation system" does not imply that the insulation has to be in one homogeneous piece. It may comprise several layers that cannot be tested as BASIC INSULATION and SUPPLEMENTARY INSULATION.

Hope the above helps.

BillB Leeds lad in exile

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.

On Wed, 11 Jun 2008 20:06:31 -0700 VWWall wrote: | 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 | | 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:

| 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.

I think this is an important point. Both of the most common 2-pin plugs have this problem. The Nema 1-15 seems to be almost universally of poor quality. At least some of the 3-pin versions, 5-15, 6-15,

5-20 and 6-20 are the ones I've used, are well made, though good ones tend to be expensive. 1-15 plugs almost always seem to fit the socket badly, and on anything drawing significant current, e.g. the iron in my hotel room, they often run warm, if not downright hot. The one on the desk lamp was so loose that it actually fell out of the wall a couple of times when I just touched the cable with my foot.

The 2-pin so-called 'Europlug' is about as bad. It's designed to fit several different European sockets, with slight differences in pin diameter and spacing. The result of this seems to be that it fits all of them badly. I Don't generally like plugs designed to fit a range of sockets, or sockets designed to fit a range of plugs. I have one particular horror, an unfused adapter designed to fit a BS 1363 socket (so already not a good idea) and which can accept a wide range of plugs, including 2 pin Europlugs, Swiss, Italian, French and German earthed plugs, though the latter two will not have an earth connection, Nema 1-15, 5-15, 5,20, 6-15 and 6-20, the Australian angled pin plugs, 2 pin and 3 pin 2A and 5A BS 546 and the current British shaver plug. There's not much metal or plastic in this thing, it's nearly all holes, it's unshuttered, and doesn't seem to make a good connection with any plug type.

Another problem with both Nema 1-15 and Europlugs is they do not adequately support the heavy so-called 'wall warts'. The laptop which I'm typing this on has such a power adapter connected via a Europlug, and it's a terrible fit in the socket.

At least the British BS 1363 13A plug is nearly always a good fit in its socket, and you seldom find one running warm, even under full load. Of course, they are quite similar to the older BS 546 2, 5, 15 and 30A round pin plugs

On Fri, 13 Jun 2008 14:16:11 -0700 (PDT) snipped-for-privacy@mail.croydon.ac.uk wrote:

| I think this is an important point. Both of the most common 2-pin | plugs have this problem. The Nema 1-15 seems to be almost universally | of poor quality. At least some of the 3-pin versions, 5-15, 6-15, | 5-20 and 6-20 are the ones I've used, are well made, though good ones | tend to be expensive. 1-15 plugs almost always seem to fit the socket | badly, and on anything drawing significant current, e.g. the iron in | my hotel room, they often run warm, if not downright hot. The one on | the desk lamp was so loose that it actually fell out of the wall a | couple of times when I just touched the cable with my foot. | | The 2-pin so-called 'Europlug' is about as bad. It's designed to fit | several different European sockets, with slight differences in pin | diameter and spacing. The result of this seems to be that it fits all | of them badly. I Don't generally like plugs designed to fit a range | of sockets, or sockets designed to fit a range of plugs. I have one | particular horror, an unfused adapter designed to fit a BS 1363 socket | (so already not a good idea) and which can accept a wide range of | plugs, including 2 pin Europlugs, Swiss, Italian, French and German | earthed plugs, though the latter two will not have an earth | connection, Nema 1-15, 5-15, 5,20, 6-15 and 6-20, the Australian | angled pin plugs, 2 pin and 3 pin 2A and 5A BS 546 and the current | British shaver plug. There's not much metal or plastic in this thing, | it's nearly all holes, it's unshuttered, and doesn't seem to make a | good connection with any plug type. | | Another problem with both Nema 1-15 and Europlugs is they do not | adequately support the heavy so-called 'wall warts'. The laptop which | I'm typing this on has such a power adapter connected via a Europlug, | and it's a terrible fit in the socket. | | At least the British BS 1363 13A plug is nearly always a good fit in | its socket, and you seldom find one running warm, even under full | load. Of course, they are quite similar to the older BS 546 2, 5, 15 | and 30A round pin plugs

I wonder if there is any hope of coming up with an all new outlet/plug design that could be made in such a way that it eliminates these common problems with legacy outlet/plug designs. The BS 1363 might have been a decent candidate, except that the way it is used in the UK would be a problem elsewhere (e.g. the rest of the world doesn't need a fused plug).

I'd more prefer a design such that there are 2 variants of outlets, one for 100-127 volt systems, and the other for 200-254 volt systems. And three different plugs such that a 100-127 volt one only fits the 100-127 volt outlets, and a 200-254 volt one that only fits 200-254 volt outlets, and a combination 100-127 + 200-254 volt plug that can fit either outlet variant for devices that can work fine from either voltage group.

Probably not. And making such an outlet design in a way that would prevent any existing plug from being forced into it would likely be impossible.

That's a good idea. Designing a plug which can fit more than one type of socket is more difficult than designing a socket which can take more than one type of plug. I think you'd probably need some sort of movable part, a pin which could be manually rotated to a different orientation or shifted to a different position, for example. That would probably cause problems for the requirement for being a good fit in the socket, without 'wobble'. There are adapters with two flat blades which can be rotated slightly to fit Nema or Australian sockets, and they are always terrible things, The 'Europlug' does it by mounting the two pins so they point in slightly towards the tips, and having just enough 'give' in the plastic moulding that they can be made to fit sockets designed for 4mm or 4,6mm (I think) an 19mm or something slightly different spacing. It's a horrible thing, can only be used with low-current devices, doesn't work well even with those, and then there's the 'wall wart' problem.

I recently discovered, when I bought the adapter set for my laptop, that the old British 5A 2 pin is alive and well in Korea. A Europlug will also (badly) fit these. Then there's the British shaver plug, which now has slightly fatter, closer-spaced pins. Again, the Europlug will, badly, fit a shaver socket.

Does anyone remember the 'Fit-All' plug? It was a clever idea, designed to fit 2 or 3 pin 5 and 15 A sockets as well as the new 13A ones. The pins retracted into the body and the required ones could be pulled out and rotated to lock them in place. I remember our chimney sweep having one on his vacuum. They were useful for contractors who traveled around, but the body was huge, and would often prevent it being plugged in to a socket. Most contractors just carried made-up adapters with them. They also had to be able to cope with things like D&S 13A round pin, and Wylex sockets.

| snipped-for-privacy@ipal.net wrote: | |> I'd more prefer a design such that there are 2 variants of outlets, one |> for 100-127 volt systems, and the other for 200-254 volt systems. And |> three different plugs such that a 100-127 volt one only fits the 100-127 |> volt outlets, and a 200-254 volt one that only fits 200-254 volt outlets, |> and a combination 100-127 + 200-254 volt plug that can fit either outlet |> variant for devices that can work fine from either voltage group. | | That's a good idea. Designing a plug which can fit more than one type | of socket is more difficult than designing a socket which can take | more than one type of plug. I think you'd probably need some sort of | movable part, a pin which could be manually rotated to a different | orientation or shifted to a different position, for example. That | would probably cause problems for the requirement for being a good fit | in the socket, without 'wobble'. There are adapters with two flat | blades which can be rotated slightly to fit Nema or Australian | sockets, and they are always terrible things, The 'Europlug' does it | by mounting the two pins so they point in slightly towards the tips, | and having just enough 'give' in the plastic moulding that they can be | made to fit sockets designed for 4mm or 4,6mm (I think) an 19mm or | something slightly different spacing. It's a horrible thing, can only | be used with low-current devices, doesn't work well even with those, | and then there's the 'wall wart' problem.

The design I was thinking of would, first of all, including a round "ground shell". That would be a metal shield (for arc safety) around the blades, and also serve as the protective ground. Think of the DIN connector used on older PC keyboards, but a bit larger, with 2 blades much like the UK plug. But the shell serves as the ground.

With the blades oriented left/right, there would be an indentation notch on the left and right sides for 100-127 volt systems, and on the top and bottom (away from the blades) for the 200-254 volt systems. A universal plug for equipment that can take either voltage would not have the indentation notch.

The outlet would have a ring hole to accept the shell. There would be a key hole to accept the indentation notch in the appropriate position for the particular voltage system it provides. The universal plug could fit either outlet. But the volatage specific plugs would only fit the appropriate outlet because the indent in the shell would block the entry of the plug. If you turn the plug to make the indent fit, the pins would be 90 degree off.

I'd actually thought of the same "ground shell" idea, but not in quite the same way. There is another advantage to it in that it prevents live pins being touched, and therefore removes the need for shrouded pins, as on BS1363. Also, if the shell was a tight fit in the socket it would prevent the 'wobble' problem, and the pins could be placed closer together to avoid increasing the overall size of the plug.

This sounds the wrong way round; surely a 'universal' plug would need notches in both positions?

I think you're talking about the notches being in the shell; is that right?

Ah, I think its the other way round, with the keys on the plug fitting into notches in the socket, is that correct?

It actually sounds like you've invented something not too far removed from the old BS196-1961 'Reyrolle' industrial plugs which were quite common at one time, but seem to have largely died out now. I think the last time I saw a totally new installation with them was on the Bank Extension of the Docklands Light railway, maybe 15 years ago. They were available in 5, 15 and 30A sizes, and in various keyings and colours for different Voltages, number of phases, grounding arrangements etc. I seem to remember that the use of these had been totally banned somewhere, Australia and/or New Zealand I think but I don't know why. There were fused versions available, where the fuses were the pins, as on the old D&S plugs, and could simply be unscrewed to replace, without having to open the plug.

[Snip]

The D&S plug was abandoned by my employer as it was found that the fused pin could become unscrewed and leave itself in the socket. This left a protruding live pin

The only place I ever saw these was in schools in London, There were still a lot of them in my primary school, and a few remained in my secondary school, in the early '70s. The large four pin three phase ones with the plastic shroud around the pins, and a locking screw, were still quite common in school workshops at that time. Whoever designed the BS196 plugs seems to have thought of this problem; the fuses for these plugs, unlike the D&S ones, had what looked like a washer, somewhat larger than the diameter of the fuse, screwed onto them. It wasn't a separate washer though, it was actually a solid part of the end cap of the fuse. This was held in place by a catch on the plug, released by pulling back a plastic lug above the pins.

On Sun, 15 Jun 2008 01:27:05 -0700 (PDT) snipped-for-privacy@mail.croydon.ac.uk wrote: | | | snipped-for-privacy@ipal.net wrote: | |> The design I was thinking of would, first of all, including a round |> "ground shell". That would be a metal shield (for arc safety) around |> the blades, and also serve as the protective ground. Think of the DIN |> connector used on older PC keyboards, but a bit larger, with 2 blades |> much like the UK plug. But the shell serves as the ground. | | I'd actually thought of the same "ground shell" idea, but not in quite | the same way. There is another advantage to it in that it prevents | live pins being touched, and therefore removes the need for shrouded | pins, as on BS1363. Also, if the shell was a tight fit in the socket | it would prevent the 'wobble' problem, and the pins could be placed | closer together to avoid increasing the overall size of the plug. | |> With the blades oriented left/right, there would be an indentation |> notch on the left and right sides for 100-127 volt systems, and on |> the top and bottom (away from the blades) for the 200-254 volt systems. |> A universal plug for equipment that can take either voltage would not |> have the indentation notch. | | This sounds the wrong way round; surely a 'universal' plug would need | notches in both positions?

The notches are a lock-OUT mechanism. The outlets would have a full round ring that would accept the presense of the notch for that voltage.

The other way around, with the universal having BOTH notches, then an outlet has to be able to accept BOTH notches for a universal plug to work. To prevent the single voltage plugs from being inserted, then the outlet design would have to block the lack of a notch. While this can work, it would require tighter construction tolerange to be sure someone cannot force the wrong plug in.

| I think you're talking about the notches being in the shell; is that | right?

Yes.

|> The outlet would have a ring hole to accept the shell. There would |> be a key hole to accept the indentation notch in the appropriate |> position for the particular voltage system it provides. The universal |> plug could fit either outlet. But the volatage specific plugs would |> only fit the appropriate outlet because the indent in the shell would |> block the entry of the plug. If you turn the plug to make the indent |> fit, the pins would be 90 degree off. | | Ah, I think its the other way round, with the keys on the plug fitting | into notches in the socket, is that correct?

These would be keys to block insertion for the wrong voltage plug in the design I suggested.

| It actually sounds like you've invented something not too far removed | from the old BS196-1961 'Reyrolle' industrial plugs which were quite | common at one time, but seem to have largely died out now. I think | the last time I saw a totally new installation with them was on the | Bank Extension of the Docklands Light railway, maybe 15 years ago. | They were available in 5, 15 and 30A sizes, and in various keyings and | colours for different Voltages, number of phases, grounding | arrangements etc. I seem to remember that the use of these had been | totally banned somewhere, Australia and/or New Zealand I think but I | don't know why. There were fused versions available, where the fuses | were the pins, as on the old D&S plugs, and could simply be unscrewed | to replace, without having to open the plug.

There are the larger IEC 309-1 and 309-2 plugs and outlets. These come in a number of different voltage and current configurations and have more complex keying to lock out wrong connections.

My idea is a smaller set of these, with the shell around the pins being metal and serving as the ground. One smaller pair for low current (15 or 16 amp) and a somewhat larger set (slightly larger shell, thicker pins) for higher current (30 to 32 amp). The shell might have a pin spacing of 1cm and a shell diameter of 2.4cm, while the next size up is 1.5cm and 3.6cm. Beyond that, go with the existing IEC 309-1/2 ones.

Note that my design allows for 180 degree plug rotation, and requires that no appliance connect either current carrying conductor to any human contactable metal surface (including the shell of an Edison style screw lamp base). These are requirements also seen in Germany for the rotatable Schuko (as well as the reversable Europlug). My design would also require anything taking 200-254 volts to accept it as 200-254 volts relative to ground (single ended) as is common in most of the world, or as 100-127 volts in two opposite polarities (double ended) to get 200-254 volts, as is common for 240V in North America.

My design does not address the 50/60 Hz issues, or the possibility for usage of DC. Potentially it could be made more complex to deal with those things.

We've known how to design connectors which don't generate arcing problems for 70+ years. It really isn't necessary to design arcing in -- you design it out.

An exposed ground can cause more problems.

On 16 Jun 2008 08:19:12 GMT Andrew Gabriel wrote: | In article , | snipped-for-privacy@ipal.net writes: |> The design I was thinking of would, first of all, including a round |> "ground shell". That would be a metal shield (for arc safety) around | | We've known how to design connectors which don't generate arcing | problems for 70+ years. It really isn't necessary to design arcing | in -- you design it out.

Could you give more detail? I've not heard of this. Is it some magic metal compound that prevents arcs? Why are utilities not using this on their switch stations?

|> the blades, and also serve as the protective ground. Think of the DIN |> connector used on older PC keyboards, but a bit larger, with 2 blades |> much like the UK plug. But the shell serves as the ground. | | An exposed ground can cause more problems.

Given that the frame of metallic appliances would be connected to this ground, how would this be a problem and it NOT be a problem for the appliance to be exposing it?

Look at almost any other country's mains connectors, except the US ones. This really is pretty much a US-only problem. At the most basic level, you need contacts with enough pressure to maintain low contact resistance and no excessive heating. I'm not sure why US manufacturers haven't resolved this. It may be that US mains plugs are too small to get sufficient hand grip to perform insertion/removal with contacts which provide enough pressure to prevent overheating. They barely have enough contact pressure to stop the plug falling out under the weight of its mains lead (certainly after the spring contacts have got hot and don't have as much spring in them).

Double insulated appliance.

If you do have a fault somewhere else, it may well be that gripping a well-earthed plug kills you by completing the current path. There's a good reason mains plugs do not have exposed conductive parts (and it's probably a regulatory requirement in some places).

The problem is less the design than the build quality. While I agree that many European plug standards are in some ways superior, a good quality US receptacle and plug will maintain contact without arcing or heating. The problem is that rather than buy $1.80 outlets and $4 plugs that are nicely made out of good quality materials, people buy 75 cent outlets and $1 plugs that are poorly made and flimsy. When I bought my house, it had many receptacles that would not reliably hold a plug, particularly big transformer plugs, but I replaced them all with commercial grade receptacles that have a nice precise fit with good grip on the plug.

It's a little late in the game to come up with a whole new design, just tightening the regulations on the quality of the parts would be beneficial and a whole lot easier.

Out of interest, anyone know where/when the original US 2-prong designed appeared? I'm guessing it's probably one of the oldest mains plug designs still in use, and some of this is probably down to the penalty for being one of the first.

I've seen original 2 prong receptacles in houses dating back to 1915 or so. I suspect they've been around longer than that, but probably not a whole lot longer. I've seen a few very early appliances with an Edison screw base plug. There is definitely a penalty for backward compatibility with older standards, but this backward compatibility is important, just look at the computer industry for a very clear illustration of this.

There is a list with images of all the EIA receptacles.

There are higher voltage models, and higher current models, and euro models. About 40 different plug/connector combos.

I don't have the link handy right now, but I dug through this sheet about a year or so ago.

Going with the standard is the way to go. anything else handcuffs your available sales region(s).