What is the point of polarized plug in home AC?

Question

Given the fact that with AC, electrons move back and forth, implying there is no real flow from A to B, what is the point of a polarized plug, since either way should do. The only explanation I have is that we know which side in the plug is live.

the neutral terminal in the plug is connected to earth ground at the breaker panel ... that should say `we know which side in the plug is live relative to ground` — jsotola, Jun 11 '22 at 00:31
One wire is nearly zero relative to earth, the other is plus/minus a large voltage. Touching either is a bad idea but the large voltage is much worse than than the nearly zero line. — user1850479, Jun 11 '22 at 00:41

hacktastical · Answer 1 · 2022-06-15T20:40:10.670

Why is the plug polarized if AC doesn’t care about polarity? tl; dr: Electrical codes require it for certain types of appliances. The polarization provides an assurance that 'hot' is wired to hot and 'neutral' is wired to neutral, to provide an extra measure of safety.

(Which electrical code? National Electrical Code section 422.40, which states "If the appliance is provided with a manually operated, line connected, single-pole switch for appliance on–off operation, an Edison-base lamp holder, or a 15- or 20-ampere receptacle, the attachment plug shall be of the polarized or grounding type. A 2-wire, nonpolarized attachment plug shall be permitted to be used on a listed double-insulated shaver.")

A two-prong receptacle allocates one leg as ‘hot’ (AC) and one as ‘neutral’ (near zero volts.) The ‘hot’ leg swings above and below ground and is the one that’s fused. Neutral carries the return; it’s not fused. In 120V countries neutral is the wider blade on the 2-prong plug.

So, what’s safer about the polarized plug? Devices with a single-pole power switch will be safer if the switch interrupts hot, leaving the rest of the circuit at neutral. Edison-base lamps will not only want their switch on hot, but the screw base also wired to neutral to reduce the possibility of shock from touching the lamp base. Finally, devices with outlets are effectively like extension cords, so they need to carry through with the polarization scheme to ensure the safety of something plugged into them.

What about non-polarized plugs? Some devices are allowed to use them. Specifically, modern AC powered appliances that are ‘double insulated’ (Class II), meaning they’re designed so that there is practically no possibility of either hot or neutral coming into contact with the exterior, and if they have a switch, the switch interrupts both hot and neutral (or, they’re a shaver. Weird, huh?)

Is this polarization foolproof? Not really. Old lamps and appliances (think early radios and amplifiers) weren’t ‘double insulated’, nor did they have polarized plugs. Worse, some appliances even had what’s known as a ‘hot’ chassis: a direct connection to one of the AC terminals to chassis without benefit of a transformer.

While upgrading an old appliance to a polarized plug can ensure that the chassis tie is to neutral, this is still inherently unsafe, given that the AC socket could be miswired or a bad extension cord or adapter used, resulting in full voltage on the chassis and a deadly shock if you’re unlucky (or foolish) enough to touch it. (The takeaway: be careful with vintage radio gear.)

Edison-base lamps (including old-school Christmas lights) have a similar problem: the exposed lamp threads can be tied to directly to line voltage if the plug is flipped the wrong way. Upgrading to polarized prevents this, again so long as the socket and extension also are polarized and correctly connected.

Old-school hot chassis, vintage light strings or modern lamps, the polarized plug provides improved safety as long as the plug and socket is wired correctly and it isn’t defeated by a bogus cord or adapter.

The grounded 3-prong plug and socket are even better than the 2-prong because they provide a protective ground as well as polarization. 3-prong sockets have been the standard in 120V countries since the mid 1970s.

Double insulated appliances are still very common, although the ‘hot chassis’ is a thing of the past, fortunately. Even though live and neutral wouldn’t obviously need to be treated differently, the electrical regulations (AU/NZ at least) call for the terminal designated as live to be treated differently in terms of creepage and clearance within the appliance. — Frog, Jun 11 '22 at 06:01
*"the polarized plug provides improved safety as long as the plug and socket is wired correctly"* True.. and **much less** safety if anyone ever wires it up the wrong way on a bad day. Never rely on the purported safety of keyed mains connectors! — tobalt, Jun 11 '22 at 06:15
@tobalt - yes, that’s a point I’m trying to make. It is something one needs to check. It’s a standard part of a home inspection for example. Statistically speaking however, the socket is far more likely to be wired correctly. — hacktastical, Jun 11 '22 at 06:20
@hacktastical thanks a lot. I understand all of it... but then, why a new crockpot from last year would have this? Since all benefits seems to be a thing from the past, I'm trying to figure out why mu crockpot from 2021 is like this. — Cherry, Jun 12 '22 at 14:29
The crock pot doesn’t qualify as ‘double insulated’ as spelled out in electrical codes. So it need to have a polarized plug. — hacktastical, Jun 15 '22 at 05:30

Harper - Reinstate Monica · Answer 2 · 2022-06-11T19:35:10.043

AC isn't naturally that way. We make it so.

Power flows in loops. In AC you have pole L1, pole L2, etc. (they're not called + and -, because of AC). They are just L1, L2, sometimes L3 and sometimes center-tap, depends on the transformer windings.

A power system doesn't have any reference to actual earth unless you establish it.

With AC building power, they "pick a pole" semi-arbitrarily, and they ground it to earth, for safety. Because electrical safety standards (NEC and the like) are data-driven research, and data shows this works best.

The pole they ground now has a new name: "NEUTRAL".

Neutral has a characteristic the other pole(s) don't have. Neutral is very near ground/earth voltage, so neutral is far, far less dangerous to humans than the other poles.

Knowing which one is "neutral" increases safety.

Think of a light bulb, it has a big metal "Shell" with a screw shape, and a contact at the tip, those are the 2 contacts. The socket has a matching contact "down in the hole" and the shell of course. When you're screwing in the bulb, the "shell" is in contact with the socket, and your finger might be in contact with the shell.

So the whole deal is a lot less dangerous if we make "Shell" connected to the Neutral wire, the one nearest earth.

And what's more, if we're going to switch a wire, for lamp control, we ought to switch the "hot" one, and leave neutral to be continuous to the lamp shell at all times.

Just one problem. What if we plug it in upside down?

Now the shell is always hot, and the tip is switched neutral. We must prevent this from happening!

So we key the connector, so it will only go in one way.

We can save money by not switching or fusing neutral

Now, we don't need a multi-pole switch and a common trip circuit breaker for every circuit, because neutral is innocuous.

Imagine you have a wet-dry shop vac and something goes wrong with the ball safety, and water gets into the motor. You want the off switch to fully de-energize the motor, so interrupting hot is essential. If you know which one is neutral, you don't need a double-pole switch.

Also, if a plug-in machine has an internal fuse or circuit breaker, again we want that on the hot wire. In fact there's a problem here. If the neutral fuse blew but not the hot, the load will now float the machine's internal neutral to hot voltage, and parts that never had voltage before are now at line voltage to ground. Whoops! So Code requires that neutral must be "Common Trip" with all hot phases, which requires a $10 circuit breaker. Polarization allows us to put a 10 cent fuse on the hot wire only.

... except reversing the wires would be bad. We must prevent that.

However, if your nationality has required grounded receptacles since, say, 1966... which describes all of North America and I believe the UK... this is a simple matter. First, sockets and wires use a very obvious color scheme. Second, if ground is present at the socket, it is trivial for the installer (or anyone, really) to test for correct polarity with a five dollar tester wired like this.

As a result, reverse-wired sockets pretty much don't happen. You can look at my reputation on diy.stackexchange to decide how much weight my opinion should hold, but I see a lot of bad electrical, and I find truly reverse-wired sockets to be extremely rare, I can't honestly remember seeing one.

What about the population of legacy 2-prong receptacles which pre-date grounding? Here, a 2-, um, -"pronged" attack is used. First, pre-1966 receptacles are not polarized and will reject a polarized plug. Second, people who want a polarized or 3-prong plug are given an easy path -- install a GFCI (human-safety-rated RCD) receptacle with or without ground! At this point you don't care if the polarization is backwards - the GFCI will interrupt any developing shock.

At the end of the day, the Code-making bodies (NEC, CEC, BS 7671) use a data-driven approach in rulemaking, because that reflects what's actually happening on the ground, and avoids the danger of "But sometimes" or "But maybe". Every fatality is investigated at length, and obviously, US fire chiefs share their data with the National Fire Protection Association, the curator of NEC.

So, the presence of hard data (or the absence of hard data) has a sharp effect on rulemaking. As an example, in the early days of blown insulation, they banned it with Knob & Tube wiring on the fear that a solitary conductor packed in insulation might overheat. When they had 30. years of hard data showing that never happened ever, they repealed the rule. This is how the sausage is made. And it's good sausage!

@Tobalt This issue is studied exhaustively by the Code making bodies in North America and the UK, and the are most definitely in favor of polarization, and swerved quite far out of their way to make it happen. Such decisions are based on a plurality of hard data, which often comes down to "likelihood of this happening vs the other based on field data". Do you have hard data they missed? Or is this an opinion of the ["But sometimes..."](https://www.youtube.com/watch?v=GiYO1TObNz8) variety... — Harper - Reinstate Monica, Jun 11 '22 at 18:36
you mention current going through a loop, but from my understanding, in AC electron move back and forth, which is what in my understanding make it useless. Then of course I understand the fuse and all... so then why my new crockpot from 2021 is polarized, but not my small oven? What defines such a decision from the makers? — Cherry, Jun 12 '22 at 14:33

tobalt · Answer 3 · 2022-06-11T10:59:03.603

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It is pointless

Large parts of europe use unpolarized plugs for decades and it is fine.

Polarized provides actually less safety if you relied on neutral being at earth potential! That is because you simply cannot rely on the neutral wire being at earth potential. A single wiring mistake would expose all " neutral" surfaces to lethal voltage. A dedicated Earth pin is anyway necessary.

On the other side, unpolarized plugs are timesavers. Just count the minutes of your life that you tried to insert USB-A plugs the wrong way around. The same happens for all polarized plugs.

edited Jun 11 '22 at 10:59

answered Jun 11 '22 at 05:59

tobalt

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It's only pointless in the large parts of Europe that use unpolarized plugs. In Ireland we use the British 13A plugtops which are both polarised and fused on the L pin. I know all the plug-in lamps in my house have switched live wires. There is some improvement in safety as a result. – Transistor Jun 11 '22 at 09:51
@Transistor are there never incidents where someone did the building wiring wrong by accident or even intention ? if not, how is this prevented? – tobalt Jun 11 '22 at 10:57
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@Transistor That is exactly the problem, you can get away with SPST mains switch on the L pin in a device if you have a polarized plug. But if the plug or socket is miswired then the mains switch will disconnect the neutral and device has live parts, maybe filter capacitance between live and chassis, which will give a shock. With unpolarized plugs it needs to be a DPST switch which disconnects both L and N so polarity does not matter and miswiring plug or sockect is irrelevant. Trust me, after one incident with extension cord with a single pole switch you will want double pole switches. – Justme Jun 11 '22 at 11:02
@Justme and I believe just using DPSTs always and everywhere - mindlessly - is more economic than using SPSTs plus paying the increased price for wiring quality control and making the installation part 100% foolproof – tobalt Jun 11 '22 at 11:07
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UL won't flunk your item if you do. However, the collected field data from North America is that reverse connected outlets are *breathtakingly rare*. This is a 1-2 punch of a) having required grounding since '66, and b) everyone owning a five dollar "3-light tester". @Transisitor remember continental Europe has 30mA RCDs on the whole house, so a reverse polarity shock won't do any more than stun. – Harper - Reinstate Monica Jun 11 '22 at 19:40

score 1 · Answer 4 · edited Jun 14 '22 at 23:17

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Some home electrical devices have 3 wire connection: phase, neutral and protective earth. Neutral connected to earth at source, usually transformer. Protective earth connected to earth close to panel and at load to all conductive chassis parts. In case of voltage leakage on chassis or external parts, current change the flow and may go through human body. Differential protection must disconnect the voltage source.That how user protected from getting injured by electrical current. Some devices have isolated body. They do not require polarized plugs, nor do they require the 3rd connection for protective earth.

edited Jun 14 '22 at 23:17

Elliot Alderson

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answered Jun 11 '22 at 01:01

user263983

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I own many devices that have only a 2-wire connection. The protective earth is **not** always connected. – Elliot Alderson Jun 14 '22 at 20:15
@elliotalderson briefly: double insulation. – user263983 Jun 14 '22 at 22:44

score 1 · Accepted Answer · answered Jun 15 '22 at 00:21

Some components are capable of retaining a state- capacitors for example. Everything has a little bit of capacitance and resistance so you can expect some small but actual current flow on the live rather than the neutral.

By switching it off on that live line, the appliance (which remains attached to neutral) is cut off from the Voltage swings that could be running power into and out of capacitor-like appliance. The swings remain in the wire up to the switch however and do spend energy there increasing and decreasing the voltage within the wire.

If you leave the live connected and switch off the neutral, the Voltage swings can power and de-power capacitor-like behavior in the appliance. Wire and appliance both incur voltage increases and decreases all the way up to the switch.

:. Unplugging appliances has a non-zero effect compared to switching them off, similarly it matters which side you break the circuit. The world is not so ideal and isolated as the circuit you envision.

If we switched to DC... there'd be another can of worms involving switching, and arcs.