Power connectors

From: Eric Smith <eric_at_brouhaha.com>
Date: Tue Apr 20 19:04:31 1999

Steve Robertson <steverob_at_hotoffice.com> wrote:
> In the early days, it was common for manufactures to use the device chassis
> or frame as a conductor. They would tie the Neutral wire to the chassis
> (under the assumption that the potential was near 0V) and save a few
> pennies in the manufacturing process. The problem was, it was possible to
> put the plug in the socket upside-down. This meant the chassis was tied to
> the hot leg and you could get electrocution just touching the device. In
> the 50's and 60's it was quite possible to get shocked by your TV by just
> turning it on!

Of course, it is no longer considered acceptable for devices to be made this
way. They won't get UL listed; I'm not sure if there's any legal
prohibition, but any manufacturer with half a clue would not make a product
this way now due to product liability concerns.

> Today, most devices have polarized plugs so that can't happen.

Yes, but have you ever spent any time testing those polarized outlets?
An appalling number of them are wired incorrectly. Counting on the
polarization to make it safe is not a good bet.

If you're not sure, get one of those outlet-checkers at your local hardware
store or Radio Shack. The have three neon bulbs in a small housing with
a three-prong plug, and will detect most of the common wiring faults.

However, you have to be careful even with those. I've seen some of these
checkers that are labelled wrong! (Whoever produced those *deserves* to
be electrocuted.)

> The neutral line should NEVER be fused. If that fuse happened to blow and
> the one on the HOT(s) leg didn't, the device would still be hot (even with
> a blown fuse). This would cause an unsafe condition.

Which is one reason why you should never assume that just because the
switch is off that there is no dangerous high voltage present in a power
supply or the like. If you're *lucky*, everything might be wired correctly
so that it's at neutral, and neutral might be close enough to the safety
ground to be reasonably safe. But don't trust your life to luck. Unplug
it and be sure. And remember, there can still be enough charge on capacitors
to be dangerous, so you *still* have to be careful.

Since neutral and ground are *only* tied together at the entrance (and this
is very important!), at any given point that you might care to measure,
neutral might have a non-trivial potential. This arises because the neutral
is a current-carrying conductor, while ground is not, so the neutral line
has resistive losses.
Received on Tue Apr 20 1999 - 19:04:31 BST