ECN Forum Forums General Topics of the Trade General Discussion Area Wide Prong On Plugs

Cameramanbob,

Describing plugs and receptacles as "polarized" is a common and accepted terminology, but I think the word "polarized" not the best choice. Maybe a better description would be "keyed".

As pointed out by other posters, AC power has no polarity like direct current does.

What AC power has, and so receptacles and plugs have, is a high potential conductor/ slot (the hot) and a low potential conductor/slot (the neutral). The hot is at high voltage relative to earth potential and the neutral is lower voltage (close to earth potential in a correctly designed, built, and maintained system). Keep in mind that the current flowing in the neutral is the same as that flowing in the hot...and it's the current that kills you.

So it's safer to "key" the slots in the receptacle and the blades of the plug, so that the hot conductor will not be attached to the shell of a lampholder, and so that the hot conductor is switched.

So I think that your question comes up because of a mild misuse of the word "polarized". Is this clear as mud?

Cliff

Hi Bob,

Years ago many domestic radios and TVs were the AC/DC type, with one side of the supply cord connected directly to the chassis. Even though they were designed so that there was no exposed metalwork when properly assembled, it was still desirable to ensure that the chassis went to the neutral, just in case a control knob came off or something like that. Some of the cheaper designs also got quite a bit of intermod hum from an AC supply if the chassis went to the hot.

For something like a double-insulated power tool though, it isn't so important, and the motor will run perfectly well from an AC supply connected either way around.

Here in the U.K. quite a big deal is made over correct polarity, but in much of Continental Europe they have non-polarized (or non-keyed) plugs on all portable appliances, even those with grounding type plugs.

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