[sdiy] My PSU (now certainly OT)

[J. Larry Hendry]

> ----- Original Message -----
> Neil Johnson <nej22 at hermes.cam.ac.uk> wrote:
> 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.

I'm sure that codes and practices vary along with voltage levels from
country to country.  But, a couple of things which are universally true are:
1- Only one earth ground at the house.  Multiple grounds at indistrial
locations that are all conected together.  2 - Earth ground at every utility
structure. Yep, neutral is tied to ground at every pole / tower / substation
ground grid.  What makes this alright is that they are all indeed bonded
together.  So, the voltage between the pole ground and the house ground
always very close to zero (difference = I drop across R from neutral flow).

> 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

While it is true most indistrial loads connect initial transformers with a
delta connection, utility ground is normally still supplied and connected to
control ground potential control voltage rise during short circuit
conditions.  Load may be all phase to phase, but statistically, faults most
faults start as phase to ground. Industrial grounding grids m,ust be
connected to utility ground at the supply point somewhere.

It is interested to discuss the differences in how residential loads are
distributed across the phases from one country to another.  I guess you
chaps in the UK actually may have 3 phase available in the house for larger
appliances.  In most of North America, each house gets single phase only
delivered to the pole mounted step down which then delivers 240 volts with a
center tapped neutral.  Heavy appliances are connected across 240.  120
loads are supposed to be balanced between the 120 legs.  So, in the US, you
have outlets in the same house which are 180 out of phase with other outlets
in the same house.

> 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.

IN the utility industry we call the two phase variants of current "WATTS"
for in phase current and VARS (volt-amps-reactive) to quantify the out of
phase current.  Obviously, we would prefer that all customers maintain lots
of WATTs and no VARs.  WATTs transmit well over wire, VARs to not (long
explanation I will not bore you with).  So, we used capacitors that can be
switched on and off near the phase lagging indictive loads to lower overall
current supplied to the location. The 90 degree leading current in the
capacitor adds / cancels with the 90 lagging current in the motor /
inductive loads.  Yes, the VAR load is bad for transformers / conductors
only in respect those amps subtract from the amps that are available to pass
WATTS and effectively reduce the capacity of the unit. I could teach a
semister course on balancing of VAR resources, so I better shut up now
before I get rolling. :)

Balancing supply of VARs is a separate but equally as important and
difficult task as balanceing supply and demand of WATTs.  Just as a shortage
of WATTS causes frequency decline, a shortage of VARS causes voltage
decline.

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

Well, OK.  You guys are slow.  <big grin>  North American frequency is 60.
Larry Hendry - old fart utility guy