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Polarity vs Phase

Original poster: "Eric Davidson by way of Terry Fritz <twftesla-at-uswest-dot-net>" <edavidson-at-icva.gov>

Hi All,
    To make the idea of polarity a bit simpler, lets disconnect the grounded
center tap on the secondary of the distribution transformer.  Now the house
only has 240 volt,  2 wire service, no grounded neutral conductor.  I think
this is where the confusion is occurs.  The fact that we can have two 120 volt
circuits on one 240 volt secondary.  We still only have one phase.  If we
measure the voltage between L1 and L2 we see that it alternates between 240
volts and 0 volts.  For half of the cycle, L1 is positive with respect to L2
(the circuit is now isolated from ground, not safe), for the other half, L1 is
negative with respect to L2.  L1 and L2 alternate polarity with respect to one
another.  This is why transformer nameplates refer to additive and subtractive
POLARITY to determine correct phasing in 3-phase banks.  To me,  the idea of
referring to polarity as a single phase wave 180 degrees out of phase with it
self,  complicates a relatively simple concept.  I'm glad the early designers
of the electrical system didn't decide to tap the secondary in two places, or
we might be debating the presence of THREE phases on a single phase
line!...:o)  Hope this helps.  Coil safe.