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Travel lighter with Apple's power adapter
Authored by: greed on Jul 08, '06 10:29:18AM

On North American AC wiring, the neutral wire leads to the centre-tap on the step-down transformer that provides 240V (nominal) house current. From the centre-tap to either hot wire gives you 120V, 'cause it's right in the centre. (The centre-tap is also grounded, but only at the transformer. Current flowing on the neutral return wire will produce a voltage compared to ground in your house wiring. The exact voltage depends on the current and wire gauge.)

The ONLY risk you run by reversing hot and neutral is having case components and cable shields float "hot". None of the electronic circuitry will be affected, UNLESS you connect two such devices together, in which case a current will flow on the cable shield connecting the two together.

This is a sign of an incredibly bad design, as it is possible to have hot-neutral reverse in a mis-wired receptecle, power-bar, extension cord, and so on. Relying on neutral to be the same as ground for safety applications is BAD. Either isolate mains from the case by a transformer, or don't connect the chassis to any mains wire at all. Use true ground (3rd pin) for chassis ground if needed. Heck, neutral is almost never true ground and you can get ground-loop currents on ungrounded devices that try and use neutral for shield or chassis ground.

You cannot identify a hot-neutral reverse without a ground reference. There is no way of having a fuse blow, or any sort of wiring fault indication, in a two-wire device. (Incidentally, if you have a "wiring fault" indicator on your power bar, that's what it is indicating--there is current flow from neutral to ground. A "wiring OK" indicator means that there is current from hot to ground.)

Lamps and devices like that are polarized so that the "hot" wire is always the little tiny one at the bottom of the lamp-socket, so that if you touch the lamp screw-threads while changing the bulb, you aren't going to get the full 120V to any ground you might be touching, just the 5-10V that tends to be the local difference between neutral and true ground (induced by current in the neutral cable in your house).

And yes, I have received electrical shocks from equipment with poor choices of chassis ground--and it isn't always because it's plugged into a mis-wired outlet. I can get a "tingle" on bare skin (it's summer, I'm wearing shorts) from the screws in the bottom of my iBook and the FireWire cable shield from my old Maxtor 80GB external drive, because the two devices use a different way to get chassis ground.



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