What good is a polarized AC plug?

Posted by: tfabris

What good is a polarized AC plug? - 11/10/2007 17:39

Two Conductor AC plug (USA).

Once upon a time, both blades were the same size and shape. I should know, my old house is full of those kind of plugs. I have some that are polarized and grounded, but many are still the old symmetrical ones.

Nowadays, everyone has polarized plugs, where one blade is bigger than the other. Unless I dremel down one of the blades, those plugs don't fit in many of my outlets.

So I'm frequently dremeling down blades, cutting off ground pins, or using a ground adapter (with one of the blades dremeled down so it will fit a non-polarized plug).

They all work and I have noticed no ill effects from doing any of the above.

Anyway. My point being... If the AC power is just alternating current, what's the point of polarizing the blades on a two-conductor plug? All the appliances and equipment I've tried works fine the other way around. I can't imagine a situation where it *wouldn't* work the other way around. Is there a such thing as an appliance that must be plugged in with the correct polarity?
Posted by: andy

Re: What good is a polarized AC plug? - 11/10/2007 17:52

I don't know about two prong mains plugs (we don't have them here) and I don't know much about the US system. But in the UK we have three prong plugs, with Live, Neutral and Earth. The potential between live and neutral tends to be around about zero, whereas the potential between live and earth is around 220V.

When you are dealing with an appliance with a two prong figure-of-eight plug then polarity clearly doesn't matter, as you are free to plug it in any way around, but of course those devices aren't earthed.

I am always nervous when I use US or European plugs, they feel so unsafe compared to the chunky UK ones (whether there is any actual practical difference in safety I don't know). The US ones in particular feel like the prongs are made of tin foil, compared to the UK ones.
Posted by: tfabris

Re: What good is a polarized AC plug? - 11/10/2007 18:01

Quote:
When you are dealing with an appliance with a two prong figure-of-eight plug then polarity clearly doesn't matter, as you are free to plug it in any way around, but of course those devices aren't earthed.


The US plug I'm referring to is the equivalent of that one. Yet modern ones are polarized.
Posted by: robricc

Re: What good is a polarized AC plug? - 11/10/2007 18:02

I can't answer your quesiton, but I do have the same problem in my circa-1950s house. About half the outlets are 2-prong. Most/all are polarized, but some have been painted over so many times that the holes have become smaller.

It turns out that much of Japan uses non-polarized 2-prong outlets very similar to ours. Cisco part number 72-1925-01 is an IEC cord with 2-prong connector and a grounding wire that's about 4" long. It has come in handy on a few occasions and they're not too pricey.
Posted by: tfabris

Re: What good is a polarized AC plug? - 11/10/2007 18:05

*Nod*. I don't have any problem getting things plugged in, I'm just wondering what the point of polarizing an ungrounded AC power source is.
Posted by: mlord

Re: What good is a polarized AC plug? - 11/10/2007 18:07

Rather than destroying the safety features of all of your corded devices, perhaps instead you should simply be replacing those obsolete electrical outlets as need-be. A new outlet fixture costs about 0.79 each locally, or cheaper in bulk packs.

One purpose of the polarized 2-prong plugs is to allow devices that would otherwise have required a heavier 3-prong cord to use "double-insulated equivalency" construction along with the 2-prong plug.

The larger blade (that you've been destroying) is the neutral wire, and the smaller one is the hot wire.

If bunches of related equipment, say computers & attached (USB, Firewire, DVI-D, etc.. ) accessories, all use polarized wiring, then their PSU's will be in phase and maybe that means they'll have lower interference/emissions or some such thing -- pure speculation on my part.

Cheers
Posted by: mlord

Re: What good is a polarized AC plug? - 11/10/2007 18:11

One rather obvious safety feature of a polarized plug, is that it ensures that the outside screw conductor of a standard light bulb socket will not be connected to the hot voltage wire.

People are thus far less likely to accidently burn/electrocute themselves when changing bulbs.

Cheers.
Posted by: tfabris

Re: What good is a polarized AC plug? - 11/10/2007 18:19

Quote:
If bunches of related equipment, say computers & attached (USB, Firewire, DVI-D, etc.. ) accessories, all use polarized wiring, then their PSU's will be in phase and maybe that means they'll have lower interference/emissions or some such thing -- pure speculation on my part.

It would be interesting to know if there is any truth to this one. If so, I would consider it critical for the equipment in my basement recording studio.

Quote:
People are thus far less likely to accidently burn/electrocute themselves when changing bulbs.

So you're saying that touching one of the prongs will electrocute me, while touching another prong would be harmless?

How is that possible in an ungrounded AC system?
Posted by: mlord

Re: What good is a polarized AC plug? - 11/10/2007 18:20

Quote:
I'm just wondering what the point of polarizing an ungrounded AC power source is.


Ahh.. One side of your single-phase AC power soure *is* grounded at the source, meaning it literally is connected to the dirt outside your house. The other side is the actual AC supply, known as the "hot" or "live" wire. Only one of the two wires is hot.

Originally, electricity used a single wire distribution system in many areas, and each house had their own grounding rod(s) to complete the return loop back to the supplier. But house grounding systems were (and are) often suspect, so now a pair of wires comes in from the utility company: the new wire is the "neutral" wire, which is actually grounded out on the utility company end of things.

But just to continue with the safety trend, we now have three wires, where the hot and neutral are now supplemented by a local earth ground, which is connected to grounding rods at the utility pole and at the house electrical panel -- often just connected to a copper water supply in older homes, or to grounding plates/rods in newer construction.

Cheers
Posted by: mlord

Re: What good is a polarized AC plug? - 11/10/2007 18:25

Quote:

Quote:
People are thus far less likely to accidently burn/electrocute themselves when changing bulbs.

So you're saying that touching one of the prongs will electrocute me, while touching another prong would be harmless?

How is that possible in an ungrounded AC system?


The system *is* grounded.

You can only be electrocuted by inserting yourself into the current flow loop. Like standing on the kitchen sink in bare feet to change a light bulb --> perfectly safe when things are properly polarized because you are unlikely to stick your finger deep inside the empty bulb socket.

But if not polarized, then the hot wire could be on the outside shell of the socket instead of the inside contact, and you'll get fried.

Our neighbours here once called me over to their house next door, because they were getting sparks from their dining room light fixture. Turns out the hot and neutral wires were reversed on it, and electricity was flowing down the decorative metal hanging chain instead of through the correct wiring path.. Which is where the third, earth, wire comes in. It provides a mostly failsafe path for current to find the ground (mother earth) when the neutral wire is broken.

Cheers
Posted by: mlord

Re: What good is a polarized AC plug? - 11/10/2007 18:28

Quote:

Ahh.. One side of your single-phase AC power soure *is* grounded at the source, meaning it literally is connected to the dirt outside your house. The other side is the actual AC supply, known as the "hot" or "live" wire. Only one of the two wires is hot.


Here's your homework for tonight: Get out your trusty old voltmeter, set it to the 200VAC range, and stick one probe into some moist earth in your yard, and the other probe into the hot spade socket of an electrical outlet (that's the right-hand side one of a 3-prong outlet when the round prong hole is on the bottom).

Cool, eh? Tell us what voltage you measured. I'll go out and do it here right now.. gimmeasec..

Cheers
Posted by: wfaulk

Re: What good is a polarized AC plug? - 11/10/2007 18:32

Effectively, it's not that the hot wire forces electricity for 1/60th of a second and then the neutral wire does, it's that the hot wire forces electricity for 1/60th of a second and then sucks it back for 1/60th of a second.

In other words, if you stick a multimeter in an electrical outlet, you get 120V between hot and ground and between hot and neutral, but you get 0V between neutral and ground. You can stick your tongue in the neutral side of a socket and leave it there all day long. (Note: I do not suggest this.)

The reason that it has to be plugged in the right way around is for safety. If the 2-prong device you're using fails and shorts, it needs to make sure that its neutral connector is connected to ground, as that's how it's designed. If it tried to ground to the hot wire, it would fail to trip the fuse/circuit breaker.
Posted by: mlord

Re: What good is a polarized AC plug? - 11/10/2007 18:34

Quote:

Here's your homework for tonight: Get out your trusty old voltmeter, set it to the 200VAC range, and stick one probe into some moist earth in your yard, and the other probe into the hot spade socket of an electrical outlet (that's the right-hand side one of a 3-prong outlet when the round prong hole is on the bottom).

Cool, eh? Tell us what voltage you measured. I'll go out and do it here right now.. gimmeasec..



Here ya go. Red probe plugged into a single (hot) conductor of a 120VAC outlet, and black probe just stuck into the wet earth outside our house. You can read the meter voltage..

Posted by: mlord

Re: What good is a polarized AC plug? - 11/10/2007 18:41

Quote:

Here ya go. Red probe plugged into a single (hot) conductor of a 120VAC outlet, and black probe just stuck into the wet earth outside our house. You can read the meter voltage..



A variation on that experiment may also be instructional.. take a table lamp outside, and wire it with only one conductor going into the hot side of an outlet, and the other conductor stuck into the ground (do this with it switched OFF!!). Then turn on the switch, and watch what happens..

For an older home, the light will probably illuminate nice and bright.
With a newer home, probably nothing will happen.. can you guess why?

Posted by: mlord

Re: What good is a polarized AC plug? - 11/10/2007 18:51

Quote:

Get out your trusty old voltmeter, set it to the 200VAC range, and stick one probe into some moist earth in your yard, and the other probe into the hot spade socket of an electrical outlet (that's the right-hand side one of a 3-prong outlet when the round prong hole is on the bottom).

Cool, eh? Tell us what voltage you measured.


Then repeat the experiment, connecting the voltmeter to the neutral socket instead of the hot socket on the outlet. It will measure 0.0V --> that's the one you can safely lick, if (like Bitt) you're into that sort of thing.
Posted by: peter

Re: What good is a polarized AC plug? - 11/10/2007 18:56

Quote:
One purpose of the polarized 2-prong plugs is to allow devices that would otherwise have required a heavier 3-prong cord to use "double-insulated equivalency" construction along with the 2-prong plug.

I don't think that answers the question of why the 2-prong plug must be polarised. In the UK all items with 2-conductor leads must be double-insulated, but many such items (e.g. laptop PSUs) have non-polarised "figure-of-8" connectors (which I've just found out Wikipedia calls C7, having no evidence of their being called "Telefunken" as I had thought).

Peter
Posted by: mlord

Re: What good is a polarized AC plug? - 11/10/2007 18:59

No, the main reason is is that it ensures that the outside screw conductor of a standard light bulb socket will not be connected to the hot voltage wire.. Ditto for ensuring that switches cut the hot wire before the device in just about any product, and that internal fuses are on the incoming hot wire.

Cheers
Posted by: Redrum

Re: What good is a polarized AC plug? - 11/10/2007 19:00

Very good explanations.

Cool that you got your shoes to light as well.

I hope you didn't get them dirty. The wife will be mad.
Posted by: mlord

Re: What good is a polarized AC plug? - 11/10/2007 19:04

Quote:
Very good explanations.

Cool that you got your shoes to light as well.

I hope you didn't get them dirty. The wife will be mad.


Heh! SWMBO took that photograph!

We love those garden clogs, and it will be a sad day when they eventually wear out. All of the new ones have these stupid holes in them to let the garden muck dirty one's socks/feet, which kinda defeats the entire purpose of garden clogs..

Cheers
Posted by: wfaulk

Re: What good is a polarized AC plug? - 11/10/2007 19:10

No, the main reason is so that an internal short will go to ground.

Your chandelier example is a fine example of this. The chandelier was designed so that the neutral connector was connected to the metal body of the chandelier. This is required by electrical code (or some other law -- who knows?). If there was a short that connected the metal body to power, you need that power to go to ground, which the neutral conductor provides. If it is connected backwards, that short will remain live and people could easily get killed.

Of course, having the easily-touchable part of a lightbulb as neutral is not a bad side-effect of this.
Posted by: Robotic

Re: What good is a polarized AC plug? - 11/10/2007 19:10

Quote:
Effectively, it's not that the hot wire forces electricity for 1/60th of a second and then the neutral wire does, it's that the hot wire forces electricity for 1/60th of a second and then sucks it back for 1/60th of a second.

Just being nit-picky, but the period of the sine wave is 1/60th of a second- that's push + pull.
Posted by: mlord

Re: What good is a polarized AC plug? - 11/10/2007 19:13

Quote:
No, the main reason is so that an internal short will go to ground.

No, it will still do that even if the two wires are reversed. The purpose of polarization is to allow things like switches to be connected to the correct wire, so that when "off", there is no hazardous voltage present in the product beyond the switch. And etc..

Quote:

Your chandelier example is a fine example of this. The chandelier was designed so that the neutral connector was connected to the metal body of the chandelier. This is required by electrical code (or some other law -- who


No, neither the hot nor the neutral should ever be connected to the body of any fixture. That's what the third "earth ground" is for.

-ml
Posted by: wfaulk

Re: What good is a polarized AC plug? - 11/10/2007 19:13

A fine point.

s/60/30/g
Posted by: tfabris

Re: What good is a polarized AC plug? - 11/10/2007 19:26

Quote:
The purpose of polarization is to allow things like switches to be connected to the correct wire, so that when "off", there is no hazardous voltage present in the product beyond the switch. And etc..

Ah. This makes the most sense of all the reasons.

All good explanations in this thread, thanks everyone.

Follow up question:
So then why do things seem to work fine no matter which way I plug in the device? Why aren't old style plugs (or the English plugs) polarized?
Posted by: mlord

Re: What good is a polarized AC plug? - 11/10/2007 19:40

Quote:

So then why do things seem to work fine no matter which way I plug in the device? Why aren't old style plugs (or the English plugs) polarized?


Because AC current flows both ways, so the electronic stuff doesn't care at all. It's purely a safety issue.

-ml
Posted by: wfaulk

Re: What good is a polarized AC plug? - 11/10/2007 20:03

Think of it as pressured gas piping. On the power company end, there is a piston that continually moves back and forth in the pipe, forcing air out and then sucking it back in. On the other side, there's a giant balloon. At your outlet, there's a valve. Normally that valve is closed. Since the power company can't pull back and create a vacuum, and because it's far easier for it to just be sucking the air from other places where it is connected to the reservoir, nothing happens while the valve is closed. But when you plug something in, it connects the two pipes together, running through your device. Now all of a sudden, both pipes have air rushing back and forth. But if you were to manually open the power company side, you'd have air rushing out and being sucked back in. On the other hand, if you opened the balloon side, nothing would happen.
Posted by: AndrewT

Re: What good is a polarized AC plug? - 11/10/2007 20:10

Quote:
If bunches of related equipment, say computers & attached (USB, Firewire, DVI-D, etc.. ) accessories, all use polarized wiring, then their PSU's will be in phase and maybe that means they'll have lower interference/emissions or some such thing -- pure speculation on my part.


Switch mode PSUs take their bite equally on each half of the mains cycle so on a single phase supply it makes no difference...

They employ a full wave rectifier across the mains supply that feeds a big capacitor. The capacitor supplies a switching circuit that chops this rectified raw mains, at around 20kHz, into a step-down transformer. The capacitor is there to ensure the fast switching circuit has sufficient power in between the crests of the mains 50/60Hz sine wave.

Now, getting to my point here: I suspect you were thinking about power factor (PF)? This relates to where on the mains sine wave you draw your power. The utility co.'s want us all running with a PF ratio of 1 - an example of such a device is a simple resistive electric heater. With a PF of 1 we are presenting an even load throughout the cycle, anything less than 1 means were are 'unevenly' loading the supply. PF<1 is bad because it is more expensive to produce and distribute and it 'looks' to our electricity meters that we are using more kWH.

Switch mode PSUs are inherently bad here - they only draw current at the crests of the sine wave when the mains voltage exceeds the voltage in the capacitor. Natively, they have a PF of around 0.5 but nowadays internal correction components tend to make them around 0.7 or higher.
Posted by: AndrewT

Re: What good is a polarized AC plug? - 11/10/2007 20:24

Quote:
Why aren't old style plugs (or the English plugs) polarized?

But they are polarised? The 13A plugs can only be inserted one way and they have assigned Live and Neutral pins (link).
Posted by: tfabris

Re: What good is a polarized AC plug? - 11/10/2007 20:59

Quote:
But they are polarised?

I wouldn't know. I was just going by what Andy said above.
Posted by: AndrewT

Re: What good is a polarized AC plug? - 11/10/2007 21:07

Ah, right. He was talking about the figure of 8 "radio cassette" leads. No matter what mains connector you fit at the plug end, they are reversible at the appliance end.
Posted by: tman

Re: What good is a polarized AC plug? - 11/10/2007 21:28

Quote:
Ah, right. He was talking about the figure of 8 "radio cassette" leads. No matter what mains connector you fit at the plug end, they are reversible at the appliance end.

Those devices are double insulated so you should never able to touch anything which is live.
Posted by: tfabris

Re: What good is a polarized AC plug? - 11/10/2007 21:38

Okay, that all makes perfect sense. Thanks, everyone!
Posted by: oliver

Re: What good is a polarized AC plug? - 12/10/2007 00:01

We purchased a building about a year ago, which has a 120/220 single phase circuit, plus an additional 480v 3 phase delta circuit. After gutting all the unneeded stuff an rewiring everything to get a 480v 3 phase Y circuit, we noticed the old delta circuit had a hot ground to our water pipes. So you could literally stick the voltmeter into the ground and the other to any copper pipe in the building, and get a 120v circuit. Needless to say, we called and had the delta disabled that day
Posted by: LittleBlueThing

Re: What good is a polarized AC plug? - 12/10/2007 07:45

Quote:

With a newer home, probably nothing will happen.. can you guess why?




And for a really new home (with an RCD) all your lights and computers will go off and you'll have to reset your clocks and....

These Residual Current Devices replace the main (or sub) fuses/circuit-breakers and work by 'measuring' the current flowing into your house on the live wire and the current flowing out of your house on the neutral. If they don't match to within a few mA (typically 30), they trip. So any attempt to light bulbs using the live->earth->electric company loop will fail

Actually that's a 'cos anyone who's been miswiring plugs (or dremelling them down <shock>) will still not kill any innocent bulb-changing house guests who happen to be standing in the kitchen sink (Canada is a strange place...)

I must admit we have a lot more respect for the mains over here in the UK - I don't know anyone who would *dream* of doing what Tony did to a plug. Maybe the added lethality/pain of the 240V instils more caution?

As an aside - the RCDs do have a drawback - because the house circuit breakers only switch live and the neutral and local earths always have a slightly different potential you do get a tiny current flow if you short neutral to earth. So when you're rewiring bits of the house (eg changing wall sockets) you can still trip the main RCD if you brush the neutral against the earth *even after isolating the circuit at the fusebox*. This is much more annoying than a slight belt from touching the live with your fingers and weirdly leads to a far more bomb-disposal-like approach when you're threading the non-live wires throught the metal back-boxes etc..
Posted by: Roger

Re: What good is a polarized AC plug? - 12/10/2007 07:53

Quote:
it ensures that the outside screw conductor of a standard light bulb socket will not be connected to the hot voltage wire.


Ah, see, there's another cultural difference. In the UK, our light fittings/ceiling roses/etc. tend to be bayonet-fit, which means they have both conductors on pins inside the fitting.

Even the screw fit ones tend to have a plastic shell on the outside, so you have to put your finger in the fitting.
Posted by: tman

Re: What good is a polarized AC plug? - 12/10/2007 08:25

Quote:
I must admit we have a lot more respect for the mains over here in the UK - I don't know anyone who would *dream* of doing what Tony did to a plug. Maybe the added lethality/pain of the 240V instils more caution?

You'd have to be really determined to do that to a UK mains caltrop (Natural state of a UK mains plug in the dark and when you're barefoot is to be upside down. Closely followed by a 1x1 Lego brick for pain)

Not sure why but people just don't mess around with the mains like that in the UK. You wire the plug/socket exactly how they tell you to.
Posted by: mlord

Re: What good is a polarized AC plug? - 12/10/2007 10:04

Quote:

Even the screw fit ones tend to have a plastic shell on the outside, so you have to put your finger in the fitting.



It's rather easy to brush against the outside conductor of a partially unscrewed bulb (not the socket, but the bulb itself) while the bulb is still in contact with the ground lead of the socket. Thus the concern.

Cheers
Posted by: mlord

Re: What good is a polarized AC plug? - 12/10/2007 10:12

Quote:
Quote:

With a newer home, probably nothing will happen.. can you guess why?




And for a really new home (with an RCD) all your lights and computers will go off and you'll have to reset your clocks and....

... Residual Current Devices ...


Exactly. Except that over here, all that's likely to happen is that the outside outlet gets switched off by the device while the rest of the house keeps on ticking.

Most homes here have per-circuit devices (or even just per-outlet) that are required by code in certain rooms (eg. bathroom) and on outside (the building) circuits.

The industry and codebooks call them Ground Fault Circuit Interrupters, but yes they are just a form of Residual Current Device and they work exactly as described by David: they detect differences in downstream current flow between the hot and neutral wires, and trip (open the circuit) when there's a significant (micro amperes) difference.

The cool thing is that no earth ground is actually required, so these devices can be used even in really old homes that don't have the third (earth) wire present at all.

Cheers

-ml
Posted by: wfaulk

Re: What good is a polarized AC plug? - 12/10/2007 10:51

Quote:
Most homes here have per-circuit devices (or even just per-outlet) that are required by code in certain rooms (eg. bathroom) and on outside (the building) circuits.

Actually, a GFCI outlet will also protect any "downstream" outlet, too. Can be a pain when something trips and you have to wander all over the house trying to find which GFCI needs resetting. Less of a pain than getting electrocuted, though.
Posted by: hybrid8

Re: What good is a polarized AC plug? - 12/10/2007 11:54

Quote:
Actually, a GFCI outlet will also protect any "downstream" outlet, too. Can be a pain when something trips and you have to wander all over the house trying to find which GFCI needs resetting. Less of a pain than getting electrocuted, though.


Local code dictates what you can downstream from any single GFCI and how everything should be wired together. As an example, Ontario's electrical code has changed in the past few years with respect to kitchen and bathroom wiring, so even a relatively new home of 5 years old doesn't meet current wiring regulations. I'm sure code around the US has likewise changed in the past 5 or 6 years.

This home was built in 1999 and has only two GFCI outlets. One protects all the bathrooms and one protects all the exterior outlets, so it's easy to reset them if the need comes up. New regulations now require one in the kitchen and AFCI outlets are now required for bedrooms. I won't be upgrading the outlets before I sell the house (something I'm starting to try next week).