The message <
[email protected]>
from "Josepi" <
[email protected]> contains these words:
====snip====
Since the centre tap of a supply transformer from a utility is almost
always
grounded (in N.America) and used for the reference, by default the two 120
volt outside legs are 180 degrees out of phase by definition.
Yes this is still single phase, and two phase is an incorrect
reference, but
the "phases" ("legs" if you will?) are always 180 degrees out of phase ***
WITH RESPECT TO GROUND OR CENTRE TAP ***
---------- clarified the implied reference-------------------
I'm getting heartily sick of the confusion over what is normally
referred to as a "Bi-Phase" supply. In the UK such supplies are normally
only seen on building sites where a 55-0-55 vac supply is used to power
tools in order to limit the maximum rms voltage with respect to ground
to that of any of the two anti-phase lives (or hots) thus greatly
reducing the electrocution hazard.
The tools are rated for 110 volt operation and receive exactly this by
using the two lives (hots). Effectively the supply is derived from a
centre tapped 110v secondary where the centre tap is earthed to prevent
an earth fault that would otherwise cause the voltage on the other leg
rising to 110v if the secondary were left 'floating'.
The stateside domestic dwelling supply is simply the same scheme but
with double the voltage to feed high wattage loads and the centre tap
connected to the neutral return (which is tied to the local substation
earth). Lighting circuits and low wattage appliances make use of just
one of the bi-phase lives (hots) and the neutral.
This has the advantage of a reduced electrocution risk and, as far as
GLS tungsten filament lamps are concerned, allow a more robust filament
for any given wattage to be used compared to the UK and european
standards of 240 and 220 volts respectively[1].
Essentially, for a given wattage, a 115v lamp can use half the filament
length at twice the CSA compared to its 230v counterpart. This gives the
choice of either the same life with a higher efficiency or a longer life
at the same efficiency compared to a 230 volt lamp.
[1] Although the EC have 'Harmonised'(tm) the domestic PSU voltage
levels to a nominal "230" volt, the allowable tolerances in each country
of the EC have simply been adjusted to avoid the need to actually make
any change to the original 240 and 220 volt supplies.
Most appliances designed for a nominal 230volt supply will work
perfectly fine on either voltage but tungsten filament lamps are an
exception to this rule since they are extremely sensitive to the effects
of voltage variation on their service life and efficiency and are
therefore designed for the voltage used in the region they are marketed
in. In this regard, the modern electronically ballasted CFL has the edge
over the traditional tungsten filament GLS lamp.