Treebeard's Homepage : Stumpers

Treebeard's Stumper Answer
10 January 2003

Plugged In

Many holiday toys need power from batteries or plugs. Batteries are DC direct current, and you better get the plus and minus right. Household electricity is AC alternating current with no plus and minus, but the plugs are still complicated. There are 2-prong unpolarized plugs that go in either way. There are 2-prong polarized plugs that only fit one way. And there are 3-prong grounded plugs that give no choice, along with GFI "ground-fault interrupt" sockets. People lived with simple unpolarized plugs for many years. Why do we need polarized and grounded plugs and GFI sockets now? Are the "cheater" adapters dangerous?

Left to right, the photo shows a few electrical plugs and sockets I found at home:

A 2-prong unpolarized plug will fit either way into a polarized or grounded socket or extension cord. But a polarized or grounded plug won't fit into an unpolarized socket unless you help it with a cheater. It says something that I couldn't find an adapter at home, but I know these common fixes:

Are these cheats really dangerous, even though AC power has no plus and minus polarity, and we've lived with unpolarized plugs and sockets for so many years?

I thank David Riehm for this stumper idea. As a bonus stumper, why do these all these plugs have holes drilled in the prongs?!

AC electricity doesn't have plus and minus like a battery, but it needs hot and neutral wires to complete a circuit. It's really arbitrary which is which, but for protection from lightning or static, one "neutral" wire is connected to earth ground. So are you. You could make a fatal connection just by standing! Polarized plugs only fit one way and guarantee that the exposed light socket is connected to the safer neutral wire. Grounded plugs and GFI add further protection. It's a safety thing, like seat belts and bike helmets and life jackets. Maybe you didn't need them yet, but don't ignore them!


I always figured computer CPU chips were complicated and my household AC power was easy. Now I know that AC power is complicated too. And I don't usually do stumpers that could be fatal if you or I get it wrong!

Suppose I run an un-grounded gas-powered AC generator to my house. Maybe it's sitting on a dry rubber mat off the ground, and so am I. I will have two interchangeable "hot" wires, meaning that I need to connect both wires to my appliances and lightbulbs to make them work, and it doesn't matter which is which. Electric power always needs two wires to complete a circuit to and fro, AC or DC.

I've never used a gas-generator, but I think I could touch either of those two wires without a shock, just like birds can safely roost on a single power line. Sure, I'd be in trouble if I touched both wires. My alternating current AC power would come from both lines, not one "hot wire" and "one neutral wire," so I'd have to touch them both to become part of the circuit. All my electrical stuff at home would still work just fine, at least for a while. But I would have a problem with static build-up from that spinning motor in the generator. The earth itself has a potentially shocking DC electrostatic charge, and a lightning strike can make it deadly. So one side of my power is usually connected to earth ground to prevent sparks.

Voltage is always measured as the relative potential between between two points, so the back-and-forth AC between hot and neutral doesn't care which is which, so you can ignore the red and black probe colors when measuring AC voltage with a voltmeter. But the difference to earth ground could be deadly if your appliance voltage to ground is different from your voltage to ground. You notice it when you get a static shock from a door knob or your car door. I figure it's like jumping into a waterfall 5 feet below the top. You get hit by the water that just fell 5 feet, but you'll be pounded if it's another 100 feet to the bottom!

Grounding one wire from the power line as neutral solves this problem in principle. But now you can become part of the circuit if you are standing on a floor connected to the earth and happen to touch the single hot wire. Grounding solves a problem, but it creates another danger. This is a hazard we live with. Polarized and grounded plugs reduce the danger by guaranteeing that the exposed parts of sockets and power tools that you are likely to touch are connected to the relatively safe grounded/neutral side of the circuit. But it only works if you follow the rules.

Three-wire grounded plugs and GFI/GFCI sockets add additional layers of protection in case a hot wire breaks loose and touches an exposed case, or an appliance falls in the bathtub. These safeguards detect current flow in and out and disconnect the dangerous circuit from the power source.

Graybear and Timothy Mallon sent really helpful long answers to this stumper. Here are the dangers as I understand it so far, but don't trust this! All your AC appliances should work fine with just two wires, but what happens if a wire comes loose inside and touches the case?

If you touch this defective appliance, you will complete the circuit to ground. The fuse may or may not blow depending on where the short is in the appliance and how much juice flows through you. Shocks much less than 120 volts @ 15/20 amps can still be fatal!
Grounding the case of that defective appliance with a 3-wire or polarized plug should protect you by completing the circuit and blowing the fuse or circuit breaker, at least if the short is closer to the grounded/neutral side of the circuit, and everything is wired right in your house.
There's still a problem if that appliance shorts closer to the hot side of the appliance, and it's not quite enough to blow a fuse. Then you could carry a potentially fatal current to ground. GFI/GFCI sockets monitor the current passing through the hot and neutral wires and shut off if they don't match within a few milliamps, eg if some of the current is going out through you to ground!

You don't appreciate those pain in the ass safety measures like seat belts and air bags and helmets and GFI sockets until you really need them. Please don't wait until after the fact. (It's been that kind of year.)

Don't mess with your household electricity without first consulting a licensed electrician!
Here are some Web links so you can at least talk the lingo...

Back to Stumper

Last modified .

Copyright © 2003 by Marc Kummel /