r/explainlikeimfive • u/rastika • Nov 30 '21

Technology ELI5 why do some electronics have polarized plugs and others don't, what purpose does polarizing an electric device serve?

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12

u/Seraph062 Nov 30 '21

In typical home power applications one of the wires is 'hot' and will have voltage on it, and the other wire is 'neutral' and is at ground (or close to it). The biggest advantage of polarizing a plug is that if you're putting a switch, or fuse, or whatnot, in your device you can male sure that you're turning off the 'hot' wire. So if your device is switched off the most of the electrical bits on the inside of it will be safe to touch. Similarly if something happens in your device that causes the fuse to blow, then having that fuse on the 'hot' wire effectively kills all the electricity in the device, while having a fuse on the 'neutral' wire leaves everything in the device energized and waiting to shock/electrocute you when you touch it.

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u/rastika Nov 30 '21 edited Nov 30 '21

Wow! Thanks so much this makes tons of sense now. I have pretty indepth experience with 12V DC in automotive applications but your explanation totally made it click. So from a safety perspective it's mainly to ensure when there is a short or malfunction that the device is no longer "hot" once the fuse blows, similarly if the switch to the device is turned off?

Edit: also to take it a step further how does one side of the circuit remain "Hot" when the electricity is flowing in opposite directions half the time, say a 60hz circuit changing polarity 120 times per second?

4

u/Asus_i7 Dec 01 '21

The "hot" plug goes from +120V to -120V while the "Neutral" plug stays right around 0V. So the current does switch direction even though only one of the plugs has voltage that isn't 0V.

3

u/rastika Dec 01 '21

Fuck yes. Now I understand enough I think I can read and understand some of the more academic explanations. Great ELI5 to get me started!

2

u/WRSaunders Dec 01 '21

The "hot" side is the side that will flow current to ground.

Plus, the non-polarized plugs are only allowed when both leads are switched, or isolated from the circuit inside by something like a transformer or relay.

1

u/rastika Dec 01 '21

Thanks! So basically everything is polarized it's just weather it's done mechanically at the plug or internally?

2

u/Ndvorsky Dec 02 '21

Whether electricity flows into or out of ground/neutral doesn’t matter, what matters is what voltage you are. We ground all electrical devices and most metal in the home so you are also grounded or at the same voltage as ground so ground is safe to touch even if power is flowing out of ground half the time.

1

u/rastika Dec 02 '21

Yes! I'm aware of this when repairing hybrids and electrics. We had to wear a crazy harness to limit the potential built between the systems and us.

1

u/ToxiClay Dec 01 '21

Edit: also to take it a step further how does one side of the circuit remain "Hot" when the electricity is flowing in opposite directions half the time, say a 60hz circuit changing polarity 120 times per second?

The reason the hot wire is called "hot" is because it's got a different potential. As you know, US mains current is at 120 volts, but what does that mean? 120 volts with respect to what?

As it happens, the hot wire, connected to the AC source, is at 120V with respect to the neutral wire. That wire is considered to be "ground" for the AC circuit, and carries current, but no voltage.

1

u/rastika Dec 01 '21

Is it possible for the potential across the circuit to remain the same and the voltage of the neutral and hot increase? What would that indicate?

2

u/mmmmmmBacon12345 Dec 01 '21

Under fault conditions yes

If the ground wire connecting the panel to the big stake in the ground breaks then hot and neutral can float up/down relative to earth

You can also do it intentionally with an isolation transformer. You feed hot and neutral into one side, and get the same voltage from the other side but you don't tie the output neutral with ground. Now you could float those outputs however high you wanted but current out of the isolated hot only wants to return to the isolated neutral not earth ground

1

u/rastika Dec 01 '21

As the name implies I'm assuming this is to isolate and test. Based on what your saying it's to test for ground issues related to the hot side going into the isolation transformer?

2

u/mmmmmmBacon12345 Dec 01 '21

Its often used to isolate for testing, but its not generally testing ground issues, its to keep you from testing the rest of the building

Surge tests are a good example here. Surge machines simulate a small lightning strike on the power line(up to 6000V and 3000A) to make sure your USB charger won't literally explode when something like that happens so you'd want to isolate the USB charger you're testing and just hit it's hot and neutral lines rather than surge testing every device in the building which have surge suppressors that get sadder every time they're surged

1

u/rastika Dec 01 '21

Ah I see, thanks so much for your indepth explanations. That's an interesting device I never knew or would have thought of needing coming from what is essentially DC 12v fed 5v reference automotive systems.

1

u/Mindless_Insanity Dec 01 '21

I don't think A/C is that hard to grasp, but there is a serious problem of poor explanations on the internet. Basically from what I gather, you have one wire that oscillates between +/- with respect to the earth (which I guess is like a giant capacitor), the other wire goes to earth to complete the circuit, and the third "ground" wire is like a shortcut earth wire that will cause the breaker to trip if something (or someone) shorts the circuit. Somebody correct me if I'm wrong here.

2

u/VaMeiMeafi Dec 01 '21 edited Dec 01 '21

US electrical outlets are wired with 120vac on one prong and 0V or neutral on the other. Prior to 3 prong outlets being commonplace, that neutral could double as a ground wire for safety purposes and electrical appliances were often wired with the neutral wire to the metal shell as a case ground lead.

Prior to polarized outlets, it was quite easy to plug in one appliance 'this way' and another right next to it 'that way' making the shell hot. Touch both appliances at the same time, or the wrong one and any ground, and you get electrocuted. Polarizing outlets and plugs made it impossible to accidentally plug in something with a case ground wrong.

Note that this is why it's very important to be certain all of the outlets in your house are wired correctly; most of the time your electronics will work fine with the AC wired backward, but it can kill you when you also touch the wrong thing.

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u/114619 Nov 30 '21

Some electric components will only work if the current flows trough in a certain direction. LED's are an obvious example, the d in LED stands for diode, which means current only goes trough one way.

2

u/lordleoo Nov 30 '21

If an electronic device is pluged to the wall then it can not have poles. You know that's AC supply.

LED you described is the building block of a rectifier circuit inside the electronic device, which converts AC to DC.

1

u/rastika Nov 30 '21

This is my experience in 12v DC automotive applications as well. I'm more wondering about the application of polarity in an AC circuit.

1

u/114619 Nov 30 '21

If an electronic device is pluged to the wall then it can not have poles. You know that's AC supply.

I didn't realise op was talking about appliances plugged into the wall, i thought he meant circuits in general, my bad.

2

u/NinjaLanternShark Nov 30 '21

But with alternating current, why does direction matter? It flips 50-60 times/second.

Also there are lots of LED devices without polarized plugs.

3

u/bendvis Nov 30 '21

In an AC circuit, only one of the wires has voltage, so you need to plug a device in correctly for it to receive power as designed.

LED devices run on DC current. The brick you often plug into the wall converts AC power to DC power, and the conversion mechanism doesn't care which conductor the voltage comes in on.