[sdiy] My PSU...

[Neil Johnson]

On Mon, 29 Jul 2002, Tony Allgood wrote:
> > The Neutral is also bonded to the SAME earth point as the house earth.
>
> But, I thought, perhaps wrongly, that neutral was connected to earth at
> the local sub-station (11KV step-down transformer). And your house earth
> was connected to earth at you house.

All house wiring that I've seen or done has the mains coming in from the
street/pole as a single-core lead-sheathed (or wire sheathed, depending on
the age) cable.  The sheath is bonded to the house earth rod, together
with the house earth and house neutral -- BUT ONLY AT THIS POINT!  Which
is the key here.

Indeed, for industrial three-phase wiring there is no "neutral" at all.
If one is needed (for single-phase outlets, for example) we generate a
neutral from the earth rod, with the same rated cable as the live wire,
whichever phase is used for live, or all three if its a large
installation, shared across the three phases -- which also explains why
you should NEVER connect an extension lead between two mains sockets,
especially ones from neighbouring houses!!

> The voltage between the two can get very high depending on how much
> current is taken by each phase on the substation's three phase outputs.
> I would typically get it to be around 0V to 6V when I measured it.

Yeah, its tricky balancing currents alright.  Even worse if you have large
motors (inductance) or spark eroders (capacitance) around as they
introduce massive phase shifts, which (a) suppliers don't like (so-called
wattless-amps) and (b) bad for transformers, generators and switchgear,
which like to see resistive loads.  That's why you often see capacitor
banks near substations in factories to balance out the inductance of all
the motors in the place.

Ah, the sweet, sweet 50Hz hum of a powerstation in action :)

Neil

--
Neil Johnson :: Computer Laboratory :: University of Cambridge ::
http://www.njohnson.co.uk          http://www.cl.cam.ac.uk/~nej22
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