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PICList Thread
'[OT] Re: AC Power Generation'
1998\07\25@153610 by Dwayne Reid

>To the PIC list,
>Sorry Fellas to bore you,
>But the power supply problem in the good old US of A interests me. I did
>not know that you guys had a such backward power distribution system.
>Down under in Aus we use a more civilised system. The power is generated
>by delta (triangular) wound alternators. 3 phase is distributed to the
>final point where the transformer is delta wound high tension primary
>and star ( Y ) wound low tension secondary. 415 volts across Phases and
>240 volts to the centre tap. This is the neutral wire which is also
>earthed. (grounded for the USA people). The four wire system is the
>retriculation system used for commercial and domestic consumption.

Exactly the same as in North America for industrial, except the voltages are
120 / 208 Vac.  The main difference for residential installations occurs at
the distribution transformer (pole pig).  The feeder to the distribution
transformer is 2 legs of the 3 phases (2.4 or 13.8 KV delta) *or* 1 leg and
ground (1.24 KV wye).  You don't see the wye setup much except in rural
areas.  The secondary of the distribution trasnformer is 240 Vac center
tapped (neutral), with the center tap grounded.  Each residence has 3 wires
entering: 2 insulated hots (called 'line') and a bare neutral.  The neutral
is bonded to the meter box and the breaker panel, where it then connects to
the local ground.  This ground is either 2 ground rods 10 feet long, minimum
of 6 feet seperation *or* a metallic cold water pipe entering the residence
from the city water supply.

This gives 120 / 240 Vac inside the residence.  Standard outlets are 3 wire:
earth, neutral, line.  An attempt is made to wire the residence such that
the loads on the individual phases balance.  It is ILLEGAL to intentionally
use the earth line for anything other than a safety ground.


>No wonder you guys in the USA are having problems with your PICS and
>RFI. With a floating neutral system and doddgy earthing system how do
>you get rid of the nasties in your main power supply?

no! No! NO!  We DON'T have a floating neutral!

The nasties in our power come from a wide variety of sources.  In
residential, brush type motors and lamp dimmers are big offenders.  Office
towers suffer from huge amounts of 3rd harmonic neutral currents from all
the peak rectifier type switch mode power supplies in older office and
computer equipment.  Certain parts of North America (US mostly) have
absoultely *incredible* numbers of lightning hits - many of these strike
above-ground telco and power lines.

I hope this helps.  The distribution voltages that I mentioned are used in
many parts of both USA and Canada but regional differences do exist.


Dwayne Reid   <>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(403) 489-3199 voice     (403) 487-6397 fax

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