Polarized electrical sockets

About 4 years ago I was in Germany with my family, visiting an aunt and uncle who live there. My dad had recently bought one of those Digital8 Sony camcorders, which was meant for travel since the AC adapter could handle both 120 and 240 volts out of the box. The shape of the electrical outlets in Germany are the same as in North America, but there was a slight problem when we went to charge the camcorder battery. My aunt and uncle live in an older house, so the electrical outlets aren’t polarized (“polarized” meaning one prong is larger than the other, so non-grounded plugs can only be inserted in one direction).

My uncle goes and phones Sony Germany and asks if its OK to file down the larger prong so that it’ll fit in the socket. They give the OK, which makes sense since it’s AC. Neither prong is really a “positive” or “negative” so to speak, they alternate polarity some 50-60 times per second, depending which part of the world you’re in.

So the question that’s been bugging me on and off since then is why they bother making sockets and plugs polarized in the first place. I’ve periodically googled for the answer, but never come up with anything useful until now[1]. Even then, it’s a single page, only available from the Google cache, and it doesn’t come outright and explain why. It’s like some weird electrical engineering conspiracy to keep the secret of socket polarization under wraps.

The answer turns out to be obvious, but it never occurred to me before because I was always thinking in terms of how DC batteries work. They’re rated for a certain voltage, with one terminal positive, one terminal negative. That doesn’t mean the negative terminal is at 0V and the positive is at 1.5V (e.g. on an AA cell). It just means the difference between the two terminals is 1.5V. In fact, the negative terminal will most likely be floating somewhere above or below 0V, unless it’s pulled to ground by an external source.

So for some reason I always thought of AC outlets in the same way (i.e. the difference between the two “live” prongs would always be 120V/240V, with the positive and negative alternating at 50Hz/60Hz, but neither terminal would ever be guaranteed to be at any fixed value). I thought only the third “ground” prong was guaranteed to be 0V.

Not so. The larger prong is actually fixed at ground, and the smaller prong just carries a 50Hz/60Hz sine wave. So polarization is a useful safety measure because when a device is plugged in but turned off, it would be nice if the switch cut off the lead attached to the smaller prong. That way, any sort of energy storage components (e.g. capacitors, inductors) in the device will not get charged up and cause electrical shock if the device is opened and prodded. Makes perfect sense now.

I think it’s all kind of dubious as a safety measure though, especially in comparison to the direness of the warnings against filing down the larger prong and/or forcing the plugs in “backwards”. I mean, who opens and prods while things are still plugged in?

[1] This time around I was prompted to google for the answer while reading the manual to the APC SmartUPS 1400 that Tim got me for Christmas. The manual kind of touches on how the UPS can detect if the electrical socket is wired correctly.

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