Dear Rob: Of course I know what a three phase system is. In North
America, most high voltage (HV) is distributed using three phase. In a
residential area, a transformer's primary is connected to one of the phases
(with secondary producing what you call a 2-phase, 120VAC - 240VAC system).
At the next house, or group of houses, down the road another transformer is
connected to a different phase - and so on. Because the net power factor of
a residential area is inductive, periodically one will see a platform on a
pole holding three sets of capacitors each connected to a phase.

However, some businesses, farms, and essentially all industrial sites
receive all three phases mostly to avail themselves of the certain torque
and efficiency of three-phase-motors. [I told my students that the first
rule of 3-phase is never to lock the shaft of a new motor until one verifies
which way the shaft actually turns - because it will turn.]

It is most interesting to learn that in the Netherlands it is common to
deliver three phase to a residence. That is a surprise. My dormitory at
university distributed three phase with outlets on facing walls in some
rooms being on a different phase. This facilitated fast heating of hot-dog
sausages - not that I ever did such a thing. I have known about sqrt-of-3
for a very long time.

The use in the Netherlands of a fifth grounding wire - the green wire in
North America - is truly a good idea.

The "Wow" is prompted by the less than belt-and-suspender nature of a house
distribution system not using a green (earthing) wire. Steve pointed out
that things can go wrong. The system you described being used in the
Netherlands, while at 240 VAC, does not rely on all of the bonding staying
effective - and that is good. It also has the advantage of being able to
provide 3-phase to the few motors expected in a house.

Rob - many thanks for expanding my knowledge. Every engineer from the
Netherlands with whom I have worked has impressed me most favorably. Can
not say that for engineers from some countries.
73, Mac N8TT

"Rob" wrote in message ...

J. C. Mc Laughlin wrote:
Dear Jeff: Many thanks.

It is especially interesting to learn that the expectation of encountering
a
floating (no earthing) system is zero.

Expanding a bit on what you have communicated:
It appears that the transformer used between one phase of a HV 3 phase
line
and distribution "wires" is 440VAC, center tapped. The distribution
"wires"
to a house (or the like) are one side of the aforementioned transformer's
secondary and the transformer's center tap, which is labeled as neutral.
It
is expected that another house (or the like) is fed from the other side of
the transformer's secondary and its center tap.

This is not what a 3-phase system is. What you describe is a 2-phase
system.

A 3-phase system has 3 live wires, a neutral, and possibly a ground.
The 3 phases are 120 degrees apart, not 180.
Each phase has a voltage (230V here), and there is sqrt-3 times that
voltage between the phase wires (400V here). All three phases are
used in some electrical motors and in highpower applications like
electrical heating, and one of the phases is sufficient for a normal
house connection.

At the service entrance: one scheme connects the neutral to an earthing.
An alternative (called 'protective multiple earthing' (PME) ) does not
connect the neutral to an earthing, but connects exposed metal in the
house
to an earthing. Wow! The latter scheme is expected in new construction.

Why "Wow!" ?
It is the only safe system.

Here in the Netherlands we have the same situation. Power is distributed
over 4 wires (3 live phases plus neutral) and a 5th wire is locally
distributed for safety ground. It is connected to the safety ground
connections of all reeceptacles and to metal structures in the house,
like water and central heating pipes. The neutral is only connected
to ground at the substation, and usually has a couple of volts on it
when measured at the receptacles.


J. C. Mc Laughlin
Michigan U.S.A.
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