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AC Plugs!

Posted: Sat May 26, 2012 1:42 pm
by Lenp
Ok. Since things have been a little slow, I'll just stir the pot a little!...

In the beginning, there were 2 wire AC plugs, Neutral and feed. They could be inserted into the power outlet either way. Dangerous, if they powered a lamp or lighting device since the feed could be either on the lamp thread or the bottom button, who knows? Was there a shock hazard, well maybe!

Next came the polarized plugs and the inability to inset them incorrectly. Great, if all was wired well, the neutral was on the shell and the shock hazard was reduced. The neutral went to the un-switched side of the circuit so most of the device was un-energized when turned off.

Along about the same time came the 3 wire grounded system. Now not only did you have the benefit of the polarized plug, the metal case of the device was wrapped up safely to a ground point. Safety gets better again unless you used one of the 3 wire adapters that were hardly ever grounded!

Enter now the GFCI. Well this is a different breed of device since it provides it's safety value in either a grounded or ungrounded system with the plug inserted either way since it monitors the feed/neutral differential current and trips if it exceeds some value. Sometimes a nuisance but overall a good idea.

Now here comes the double insulated concept where the device has the normal insulation, let's say like in a motor winding, but then the motor is again insulated from whatever the user contacts. Here two points of safety must fail to become a hazard. It gets better again and it often has a polarized plug as well.

So, here's the topic for discussion. Why, except for the fact that they are probably cheaper because of the quantity, do they still use 2 wire polarized plugs on double insulated devices? Clocks, tools, some laptop and tool chargers, there are many. I do give a thumbs up to lighting applications because of the continued potential for a shock, but for the rest, does it make any other than economic sense?

Floor open for discussion!

Re: AC Plugs!

Posted: Mon May 28, 2012 12:17 am
by MrAl
Hi there Len,

So you are asking about reasons why manufacturers want to still use two prong plugs over three prong?
When i go to price three and two prong replacement plugs for 120vac, i see that the three prong are usually higher priced. That's the only reason why they would use two prongs.

I like two prongs myself because some things just dont need a ground, like if the case is plastic, and they are easier to plug in and deal with in general. When i think about the stuff i have that are two prongs and stuff i have that are three prongs, most of the stuff that have two prongs are light duty stuff like battery chargers, small lamps, glue gun, even a 120vac drill. But some of the more heavy duty stuff i've dealt with in the past have three prongs like circular saws, heavy duty drills, hand grinders, portable heaters, etc. It seems things that draw more current tend to have three prongs if that makes any sense. It could be that things with metal cases have to have that third ground prong.

They also use two prongs on most wall warts but the three prong ones hold it in the wall outlet better.

Most of the time i end up looking for a three prong to two prong adapter anyway, as many people do, and so the ground prong goes unused anyway. Ha ha, lot of good that does.

I think the main issue is they try to protect against shock after equipment damage. If we drop a microwave oven and internally the hot side of the 120vac line touches the case, when we plug it in it will either trigger a ground fault or blow a breaker or fuse. Otherwise we would be subject to touching the metal case that now has become live with the hot side of the 120vac line.
With a two prong plug the case would not have been connected to a true ground, so it could be more dangerous after equipment damage.

I would like to hear other ideas here too.

Re: AC Plugs!

Posted: Mon May 28, 2012 6:17 am
by Lenp
Hi Al,
I agree with your thoughts but the operative word here is POLARIZED 2 prong plug!

Re: AC Plugs!

Posted: Mon May 28, 2012 7:40 pm
by Dean Huster
Even though it may be a double-insulated device, the plug is still polarized because the hot wire is the one to be connected to the device's power switch, not the neutral. It's also the hot wire that's fused in the device if there's a fuse. It's just another safety step to help protect one who has the device open for troubleshooting and repair.

Re: AC Plugs!

Posted: Mon May 28, 2012 10:56 pm
by MrAl
Hi again Len,

Yes i see now the point of your question was about the polarized plugs over non polarized two prong'ers.

One guess would be that they are required to use these to pass some standard, or else they only stock those so they dont have to stock both types. It's harder to stock both polarized and non polarized and when you are sometimes forced to use polarized that means you MUST stock the polarized, so you may opt to not have to stock the other type as well. Saves space and confusion and accidentally using the wrong type plug.

Also as Dean said, they want to use the hot side for the switch, and also they want use the hot side for the center contact for a regular light bulb so that the user will find it harder to touch the hot lead when changing a light bulb. If they connected the hot side to the bulb screw metal, the user could more easily touch that when changing the light bulb.
I just checked and sure enough the lamp i just bought a week or two ago has a polarized plug too. I could check it and see if they did it right :smile:

Many wall warts are not polarized however. They have double isolation from the line so touching the output low voltage leads wont connect you to the line voltage no matter how it is plugged in.

Re: AC Plugs!

Posted: Tue May 29, 2012 5:34 am
by Lenp
Hi Guys,
I agree with the opinions but what triggered this post was when I looked at some new AC powered wall clocks.
No switch, no fuse, all plastic and polarized plugs....
Economics must dictate in some cases!

Re: AC Plugs!

Posted: Tue May 29, 2012 10:03 am
by MrAl
Hi Len,

Could it be that those plugs where the only thing available?
Or could it be that it is a UL standard? But then you could ask why that standard for something like a clock. Maybe so it becomes the common standard so there could never be a mistake with products that actually require it. For example, if they make clocks and toasters, maybe they might tend to use non polarized on toasters sometimes. More universally speaking, if one manu makes clocks and the other makes toasters and the clocks use non polarized, maybe the toaster manu's think they can get away with using them sometimes too.

And of course as i mentioned before, i would rather buy 100 polarized for 50 products that need it and 50 that dont then to have to buy 50 polarized and 50 non polarized and have to worry that the right plug gets onto the right product when it goes through assembly. It is so much simpler to use all one type on everything as long as it is allowed.

Re: AC Plugs!

Posted: Tue May 29, 2012 10:10 am
by Lenp
I just got it all figured out... :idea:
With the clock, if you could reverse the plug it would run backwards. :o
(you think! :???: )

Re: AC Plugs!

Posted: Tue May 29, 2012 4:00 pm
by haklesup
The third prong (safety ground) is only required on appliances with metal enclosures or metal hardware that could ever come in contact with the hot wire and the user if broken in any way. The idea is to provide a second change safety net for the hot lead to get back to neutral. In most cases, a failed appliance results in a blown circuit breaker.

Double insulated is just a fancy way of saying the appliance does not have any metal enclosure or large integral metal components like motor housings. while it does a better job of saying you shouldn't get shocked, I still wouldn't use them in the shower.

Cost of 2 prong under 3 prong is a main cost driver as is the gauge of the wire in the cordset. Its not just the cost of the molding of the prong but its 30% more metal in the cordset also making it bulkier for long cords. However there are some people who still do have 2 prong receptacles in places like bedrooms and hallways and having a 2 prong cordset makes things more universal for everyone.

There are exceptions for portable lighting apparently, I've seen my share of metal lamps with 2 prong cords but just about every metal luminary I have ever installed (perminant light fixture), there is always place to attach ground. The polarized plug allows the use of a neutral chassis (hot chassis are virtually unheard of and poor design but may have existed on tube TV sets) by guaranteeing only neutral can be connected to that chassis. While this is not common practice in consumer products, it is not uncommon in industrial equipment (which would normally also have a third prong connected to an outer enclosure) actually there are many options that work

Re: AC Plugs!

Posted: Tue May 29, 2012 6:40 pm
by Dean Huster
Many wall warts are not polarized however.
I hate polarized wall warts. Some even have ground pins. Polarization of those really limits how you can plug them into a receptacle. In fact, if you have two polarized wall warts and only one duplex receptacle, you're screwed unless you buy one of those 6X expanders or a short extension cord.
(hot chassis are virtually unheard of and poor design but may have existed on tube TV sets)
The "All-American Five" tabletop radios of the late 1930s through the early 1960s were hot-chassis designs and rarely had polarized plugs. Millions and millions of them were manufactured. They were insulated from the user by the cabinet, knobs and a Masonite or cardboard back, but require the use of an isolation transformer if you open them up for service if you want to live a long life.
... it [non-plarized plug] is not uncommon in industrial equipment ...
Whoa, Haklesup! I must have misunderstood something here. Industry follows standard NEC using grounded conduit, grounded motors, grounded enclosures and a third/fourth wire ground for literally EVERYTHING! It's the consumer products that often lack grounding, everything including televisions, radios, stereos, etc., especially low-cost items (they've eliminated the costly power transformer) from the 1950s and 1960s.

Re: AC Plugs!

Posted: Tue May 29, 2012 9:29 pm
by Robert Reed
I don't see the cost differential at all between a a polarized plug versus a non polarized plug. C'mon guys we are talking about a fraction of a gram of more metal on the one prong. I think that because it makes sense to use them on many appliances, they just stick with the polarized version for all devices. For my own use (and where they aren't really necessary), I don't like fiddling around with the polarized version trying to plug something in in a dark corner. I just want to push it in and go.

Re: AC Plugs!

Posted: Tue May 29, 2012 11:49 pm
by MrAl

Len, funny idea about plugging the click in backwards and having it run backwards. But what happened to me was even stranger...i plugged my clock in backwards and started going back in time. I ended up in 1970.

Let me just say this one more time...
If you have to stock two different items that cost almost the same but you could get away with stocking only one of those items because it would work for both products, it makes a lot of sense to just stock the one item. In this case we're talking about polarized vs non polarized two prong plugs. It's a lot simpler to stock only the polarized plugs. They work for both required by law products, and non required by law products. So use them on both and less worries.
If we stock both there is a good chance that a non polarized is going to end up on a product that requires a polarized plug, so by purchasing only polarized plugs we eliminate that problematic situation too. So we get at least three advantages there:
1. We only have to order one type of plug
2. We only have to have a bin for that one type
3. No chance the non polarized can end up on a required by law product.

Re: AC Plugs!

Posted: Wed May 30, 2012 8:47 am
by Lenp
Thanks for the clock story.
I now think my house is wired backwards.
I go two steps forward and three steps backwards.
Is that a 3 phase thing? :roll:

Re: AC Plugs!

Posted: Thu May 31, 2012 3:58 am
by MrAl
Hi Len,

Yeah, he he.
Instead of the clock I was going to use a worm hole but the apple wasnt big enough and neither was the worm, and unfortunately he wasnt even hungry at the time.

Re: AC Plugs!

Posted: Thu May 31, 2012 3:56 pm
by haklesup
Dean, the "It" I was referring to was not the plug but the grounded chassis used as a ground reference by the circuit. Perhaps my wording wasn't clear.

As for the cost difference, its not just the tab but the 6 feet or so of copper conductor in the cord that attaches to it. That's 33% more metal in the cordset, not just the prong. assuming the additional molding and the metal tab only add 5% or 10% to the cost, it could effect the cost of goods by almost 50%. Ground conductors are generally the same gague as the supply wires but not always.

That's similar reasoning to why romex cable with the extra hot conductor (3 cond plus gnd) is so popular. it reduces wiring by half because you can use the neutral and ground for two branch circuits (and since they are supposed to be out of phase, it does not double the current in the neutral conductor, it may even partially nullify it)

I once really did have a clockl I could run backward. It was the old schoolroom type. I simply removed a small metal clip that caused the motor to start in only one direction. Without it, plugging the clock in had a 50/50 chance of running either way. That clock has a synchronus motor which would not keep the correct time if not used at 60Hz, newer clocks have a fundimentally different mechanism that uses a quartz timed pulse to ratchet the second hand and gearset one second at a time and in only one direction.