system/36 awefully quiet...

From: Eric Smith <eric_at_brouhaha.com>
Date: Wed Nov 1 01:31:27 2000

> It wouldn't really be 220v center-tapped, though, at least not the
> way I'm used to thinking about it.

US Home wiring is usualy "split-phase". You are fed from a single phase
of the distribution system. You're on the secondary of a transformer,
which is 240V center-tapped. You get two hots and a neutral. Relative
to the neutral, the hots are 180 degress apart.

All of your 120V circuits are fed one hot and a neutral.

This is one reason why the ancient practice of tying an appliance chassis
to neutral has been disapproved for a long time. Suppose you have two
of these appliances close together, but fed from different outlets which
happen to be on the two different hots. If the outlets and the appliances
are wired correctly, you can touch both because they're both on the neutral.
But if anything's wired wrong, you'll get 120V or 240V through you.

And the neutral being safe is potentially a lie anyhow. It's a current
carrying conductor, so even though it's grounded at your service entrance,
it have a non-trivial voltage elsewhere.

Anyhow, your typical US residental 240V outlets, for ranges and clothes
driers, usually only give you the two hots and ground, but NO neutral.
The motors, heating elements, etc. get 240V and don't really have any
use for a neutral. But some equipment may need 240V split phase with
neutral. In this case the plug will have four pins (one for ground).
There are different NEMA standard plugs and receptacles for the various
configurations, with and without neutral.

Three-phase is more complicated. Low-end three-phase stuff is usually
208V three-phase delta, so you'd have four pins (one for ground).
This is fine for three-phase delta motors. But some things need
three-phase wye (Y), which adds a neutral for a total of five pins. A
lot of minicomputers that use three-phase power are wired this way. The
advantage is that you can put a 120V device between any phase and
neutral.
 
Older equipment (or receptacles) may not have a ground. This is bad,
especially if the equipment is not double-insulated.

I'd advise anyone who is trying to figure out how to run big iron that
doesn't simply plug in to an available outlet to consult an expert. And
I don't claim to be one. But I've spent some time figuring out how
to run PDP-11/70s and KL10s on other than the three-phase power DEC
intended. (Much easier on the PDP-11/70 than on the KL10.)
Received on Wed Nov 01 2000 - 01:31:27 GMT