Powerlines - test-drb@ccmp.vtda.org

26 Apr 1999

...
 2.  Phase.

As I understand it the usual practice in the US is take _either_ a phase and
...
 earth (neutral) _or_ two phases of the HV supply and
feed a single phase
transformer from this.  The LV side of the transformer is (say) 240V with a
centre tap.  The centre tap is earthed and provides the neutral connection, the
...
 outer two taps are then both hot at 120V, 180 degrees
apart.  This is true _even
...
 if_ the primary (HV winding) is connected between two
phases of the HV 3-phase
...
 system. 
Close but not quite right.  For single phase installations the power company
supplies 2 hots and neutral.  This is a center tapped secondary and the
neutral is earthed at the service entrance.  The primary of this transformer
is fed from only 1 high voltage phase.

...
 In the UK the usual practice is to connect all 3 phases
of the HV (almost
invariably 11kV phase to phase) to a delta-star 3-phase transformer.  The
secondary, the star-connected winding provides 3 phases and neutral. Neutral is
...
 earthed at the transformer.  Protective earth can be
provided by _either_ a
separate wire from the main earth at the transformer _or_ an earthing electrode
...
 at each customer's site _or_ the neutral is earthed
at a large number of points
...
 on the system and the customer's protective earth
is bonded to neutral at the
...
 point of entry of the supply.  In all cases the
customer sees 240V line to
neutral; if he gets more than one line (hot) connection, they will be 120
degrees apart. 

The delta-star is the same but with 120 phase to neutral and 208 phase to
phase.  In the UK it is the same but 240 phase to neutral and 416 phase to
phase.
You can also commonly find 480 3 phase delta- star 277 phase to neutral.
This is then used for lighting (277 florescent is common) and large loads
like air conditioning.  In these installations the customer has to provide
their own transformer to 208/120.

...
 In remote locations, only 2 phases of the 11kV system
are taken to the site.
...
 Small loads get a single phase 11kV/250V transformer
with one end of the LV
earthed; larger loads get a single phase 11kV/500V transformer with a centre tap
...
 earthed.  Some farm equipment (I am told - I have never
seen this) is rated for
...
 480V single phase. 
I don't think this is avail. in the US.
...

It is very unusual for a domestic installation to get more than one phase and
...
 neutral.  But 3-phase can be done - the supply company
don't mind putting it in
...
 if they think you'll buy a lot of electricity!

They will here also and I checked into getting it here but I am a few miles
...
 from the closest point that 3 phase is available for
distribution. I would either have to guarentee a large $ per month usage or pay them
to add the
additional lines.  Over $10,000 was mentioned.

Dan