The evolution of electric plugs

Factual Questions
1

I am just curious what is behind the following evolution.

When I was growing up in the 40s, all plugs consisted of, AFAIK, two identical prongs. The house I lived in until recently was built in 1942 and none of the sockets were either grounded or polarized. At some point grounded plugs became standard for most appliances and even the two pronged plugs were polarized. I had had to install new sockets all over or use “cheaters”, some of which I actually attached to a radiator. I recall that when I got my first computer in 1982, the head of our computer centre told me that if I had a problem and my computer was not plugged into a properly grounded socket, he would not even look at it.

But then came a lot of computer equipment that was not grounded, but did use polarized plugs. But more recently, small equipment (USB chargers for example, or the chargers for the new landline phones I just bought are neither grounded nor polarized. They are back to the plain two identical prongs of the 40s. Any explanation?

2

Back in the days of identical two prong plugs, if something had a metal case (as many things did back then), if the internal wiring shorted to the case, the user could get a nasty shock. So they made the neutral prong wider, since neutral was connected to earth ground. If you had something with a metal case, and something shorted to the case, now it would just blow the fuse.

But there was still a problem. If the neutral wire broke and you turned the device on, the case would no longer be at earth ground potential, and you’d have the full 120 volts on the case, limited in current by the internals of the device (the current would go though the device and then connect to the case since the case was connected to neutral, which was now broken). Again, a pretty serious shock hazard.

This evolved into our modern system, where we run a separate safety ground.

If the hot wire breaks, the device just stops working. No shock hazard.
If the neutral wire breaks, the device just stops working. No shock hazard.
If the safety ground wire breaks, the device keeps working, and still no shock hazard (though you’ve lost the safety at this point).
If the hot wire shorts to the case, it blows the breaker.
If the neutral shorts to the case, no real shock hazard, but if there is a GFCI present it will trip.
The safety ground is already connected to the case.

You can still end up with a shock hazard, but it requires more than one fault.

But what if you have a device that has a plastic case, and all of the electrical bits inside are insulated? There’s no real shock hazard no matter what shorts out or breaks. So in these cases, there’s no need for a polarized plug or a 3 prong plug. You can use the old fashioned two prong, non-polarized (both prongs the same size) plug and there’s no safety hazard.

There are rules that devices have to follow in order to use a symmetrical 2 prong plug, but as long as there is no shock hazard from them, they are safe to use.

By the way, the reason you attached to the radiator back then was that your water pipes were used as the home’s earth ground. These days, a separate copper rod (sometimes more than one) is driven into the ground to make the connection, since water pipes can be plastic and can’t be relied on any more. Metal water pipes are also connected to this copper rod, but that’s to make sure that the water pipes never become a shock hazard. Your home doesn’t rely on the water pipes any more for its earth ground.

3

I guess that the chargers you are talking about have a plastic, nonconductive case, and the two wires just feed a transformer or capacitive voltage dropper. Also, maybe residual current circuit breakers are more standard equipment in homes compared to last century.

4

Are you in the UK? :slight_smile:

In the UK, they are called RCDs (Residual Current Devices, or some variant like the above). In the US and Canada they are called GFCIs (Ground Fault Circuit Interruptors) or GFIs (Ground Fault Interruptors).

GFCIs in the US are required in kitchens, bathrooms, basements, any place where there is a chance of something wet creating a shock hazard. GFCIs are not required in bedrooms. If the home was built prior to the change in the National Electric Code (NEC) that required these (generally adopted in the late 70s and early 80s) there is no requirement to retrofit them.

Bedrooms require Arc Fault Circuit Interruptors (AFCIs), which protect you from arcing, i.e. things like a frayed extension cord. This is a more recent change to the NEC, and again, there is no requirement to retrofit them.

The use of 2 pronged devices is based on the device’s construction (insulation and failure modes), and has nothing to do with the use of GFCIs or AFCIs.

5

yep. if the device is “double insulated” it doesn’t require a polarized plug. things like USB chargers are just small switch-mode power supplies, and since they’re designed to prevent any shock hazard and don’t care about the incoming polarity, they just use two straight prongs.
Things with light sockets will always use polarized plugs in order to prevent the socket shell from being energized.

6

With alternating current, there is no real difference between the two wires since the current ‘alternates’ between them. The reason for differentiating between them is so that any switches in the circuit are all on the same side. If the switches are double-pole, it doesn’t matter, but if they only make or break one side, it is important that they all act on the same side.

In the UK, double pole switches are rare in domestic situations, but there is much emphasis on making sure that the so-called ‘live’ or ‘hot’ side is the same all the way through.

7

No, there is a definite difference between Hot and Neutral.
One side (“leg”) of the AC circuit is connected to Earth. That means that there is a potential difference between the other side of the circuit and any object (e.g. - plumbing) that is connected to Earth.

Why is this done?

To reduce the possibility of enormous potential differences between Earth and the power system. Remember that there is a DC potential of 100V per meter of altitude. Since power lines are often strung high up in the air, it would be dangerous to let them “float” and charge to thousands of volts. There are other reasons, too, but I won’t get into them right now.

8

I’ve been fried by a kitchen appliance (1950s manufacture) with a metal case and symmetric 2-prong plug - the residual current device sure came in handy, though I still got a brief shock - but once also got a shock from the metal case of a computer plugged into a polarized, grounded socket with faulty wiring.

9

Right, no wiring scheme will protect you from an incompetent electrician who doesn’t follow the scheme.

10

Just about everything that doesn’t operate on mains voltage- a laptop computer or a phone charger, is a switching power supply nowadays. Switching power supplies don’t care which pin is hot, so they make it so you can insert it either way for convenience- for a “wall wart” to always have it hang down no matter which way the outlet is oriented, or have it hang over the end of a power strip so it’s not blocking other outlets. Nor is there any conceivable way to get a shock from it since it’s in a plastic case so grounding isn’t needed (A few laptop power supplies do have ground, but for EMF shielding rather than safety).

There’s still a lot of things that require polarization, like a table lamp, or equipment grounding, like a microwave oven or desktop computer. So outlets haven’t reverted back to two pin unpolarized.

11

Once you get to three phase power there is a very big difference, (or in the US where you also get two phases in domestic systems.) One way this can make a big difference even in domestic installations is where two power outlets are run from different phases. There the neutrals will (should) be at the same potential, but the actives won’t, there will be a significantly higher potential between the two actives. 220 volts in 2 phase US systems, and 415 volts in 3 phase systems in 240 volt nominal countries.

Wire a 3 phase device in a delta and you don’t actually use the neutral at all. Which is why there is no (or a much thinner) neutral in power distribution lines. Your power supply company tries hard to have minimal current in the neutral.

What we see at the domestic power outlet is a small slice of a much more messy reality.

12

Some musicians have been shocked by ungrounded guitar amps. The steel strings on the guitar can deliver a shock.

It gets really bad if the amp has a microphone jack. Lips touch the mic. Not safe.

https://www.seymourduncan.com/forum/showthread.php?114593-Why-am-i-getting-an-electric-shock-from-my-guitar&

https://www.shure.com/en-US/support/find-an-answer/shock-problem-with-mic-and-guitar-amp

13

To be pedantic, the US system for residential and small commercial service is the single split-phase system, not two-phase. (However, true two-phase is used for residential service in some large apartment buildings, where the building has three-phase service and delivers two of the three to each unit.)

The single split-phase system steps down distribution voltage to 240 volts (not 220 volts). The output side of that transformer has two taps on each end of the coil providing 240 volts between them. Since they’re on opposite sides of the coil, they are 180 degrees out of phase. (Two-phase service has one lead 120-degrees out of phase from the other, netting 208 volts between them.)

There is a third tap equidistant between the two which is grounded and also connected to your house. That’s your neutral. Connect something between the two hots, you get 240v, between either hot and the neutral, you get 120v. Because the two hots are 180 degrees apart, they balance, so a shared neutral between 120v receptacles connected to different hots will only carry the difference in current between the two. This feature is often taken advantage of in kitchens which have a lot of high-current appliances; you can have duplex receptacles with their terminal bridges removed and connected to opposite hot wires, giving you more current capacity per receptacle without having to use thicker wiring. (If you do this, you also have to use the correct duplex breaker so both hots are cut in the case of one of them overloading.)