[Amps] Electrical Distribution Methods

[Manfred Mornhinweg]

There I go again... this time trying to help people to understand power 
distribution systems in 220V countries.

I will base this on the system used in Chile. This should be very 
similar to that used in most other 220V countries.

The main distribution throughout a city is made with three phase lines, 
without any neutral. Just three wires. In Chile this carries 23kV 
between phases. Of course, the voltage between any two wires is 120 
degrees out of phase with that between any other pair (that other pair 
by necessity shares one wire with the first pair, there being only 3 wires!)

Distribution transformers in cities are three-phase transformers. Their 
primaries have 3 coils wired in delta (triangle), so that each coil ends 
up connected to two of the three lines, and sees 23kV across it. The 
secondary windings are also three coils, each giving roughly 234V. These 
are wires in star configuration, that is, one side of each winding go to 
a common point, the other side of each winding is free. That common 
point is called "neutral", and is connected to earth ground right where 
the transformer is sited.

Four wires run along the streets, starting at the transformer. One 
connects to the common center point (neutral), the other three connect 
to each of the free winding ends (the three phases). Between each phase 
wire and the neutral wire there is 234V at zero load, dropping to 
perhaps 225V average under load, while between each two phase wires 
there is roughly 380V or slightly more. This is because two 225V 
voltages, with a relative phase of 120 degrees between them, add up to 
something a little over 380V.

Each home is connected to one of the phase wires, and to the neutral. 
These two lines are passed through the energy meter, ground fault 
interrupter, the phase or "live" wire is passed through circuit 
breakers, and then the two wires are routed to all outlets, lights (the 
  "live" passing through the light switches), and so on.

In addition each building has a grounding system, and there is ground 
wiring throughout the building, connecting to the center contact of all 
outlets.

The neutral is absolute not grounded in any way inside any building, but 
it is grounded right at the transformer. This ground at the transformer 
is called the service ground, while the ground at each building is the 
protection ground.

All electric devices plugged into home outlets must expect the live to 
be at either contact. That is, they must have the same degree of 
insulation applied to both input terminals.

Industries get all three phases and the neutral. Three phase outlets 
have five contacts, (the three phases, neutral and ground), and are 
polarized, so that it's known where the nuetral is, and also the phase 
sequence is known, so that three phase motors will run in the correct sense!

Rural distribution transformers are often single phase. They have a 
single 23kV primary winding, connected between any two of the three 23kV 
lines, and a single 234V secondary, that gets one side grounded at the 
transformer site, and called "neutral", while the other side is called 
the phase, even if there is just one phase in this case.

 From the point of view of a ham building or repairing an amplifier, 
this means:

- There is about 220V available under load;
- One side is very near ground (a few volts), but we can never know 
which side it is, as it depends on which way the amp is plugged into the 
outlet;
- The center contact is earthed, but we must not use it for anything 
except protection and RFI filters;
- Most definitely we must not ground any side of the 220V inside the amp!

And this is the same in cities or in rural areas. Even at my own home, 
powered from my very own microhydro turbine, I follow the same standard 
in my home! Just my distribution runs at 2kV split-phase rather than 
23kV three-phase, owing to the relatively low power I run. Since my 
transformer is inside my home, I have two separate ground stakes, one 
for service ground and one for protection ground. They are joined only 
through the physical earth, not through any wire.

One more thought about phasing: Since the three phase distribution 
transformers have delta connected primaries and star (wye) connected 
secondaries, the secodnary three phases are 60 degrees out of phase with 
the 23kV primary three phases! So, the 220v in a home will be at 60 
degrees to each of the two high voltage phases from which they came, 
which means that they will be at 180 degrees to the one phase from which 
they did NOT come! A rural home connected via a single phase transformer 
will instead be at zero degrees to the voltage between the two high 
voltage phases that power it.

In practice, of course, this difference is completely irrelevant.

Lastly, I cannot believe that in Thailand the 220V distribution is 
completely ungrounded and floating. That would let it free to float up 
and far away from ground potential, leading to flashover to ground at 
the weakest spot! Surely the neutral is grounded at the distribution 
transformer, just like in most (or all?) other 220V countries. Only that 
if there is no ground available in buildings wired with two-contact 
outlets, and with water pipes being plastic, in a tenth floor appartment 
it may be hard to get access to any ground, to be able to measure which 
is neutral and which is live! But in a house with a garden, it's easy 
enough to drive a stake into the soil, and use that as reference for 
measuring. Surely one side will end up just a few volts from ground, and 
measuring a low resistance to the ground stake - as long as the soil is 
wet and conductive!

If not, if really the distribution floats ungrounded, that would indeed 
be matter for concern. But nobody should ground one side at his home! He 
might upset the whole system, and draw huge currents through his ground 
wire. Using an isolation transformer to power the home, and grounding 
one side of the secondary, is fine in such a case, but not cheap.

Manfred


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