RE: IS 240VAC two-phase - NO

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ian McLean" <ianmm-at-optusnet-dot-com.au> 

 >  In a 15K
 > NST,
 > if we want 7.5K to run a small coil, what do we do?  Utilize one high
 > voltage bushing (one half of the windings) for a hot, and the case,
 > which
 > again is connected to the center of the huge secondary, as a neutral.
 > Want
 > double the voltage?  Run off of both bushings for the full fifteen
 > grand.
 > Same with a distribution transformer.
 >
 > John Richardson"
 >

I didn't think you could leave a HV bushing on an NST 'open'.  The output
has to 'go' somewhere.  i.e. The secondary must be connected.  This is the
other reason we have safety gaps on the NST, other than as a last wall of
defense to protect from RF transients that might make their way back to the
NST's, they ensure that the NST's are not voltage stressed by ensuring that
if something is wrong, like an open circuit, cap blown open-circuit (usually
blow closed though), incorrectly set spark gap, etc., that the NST's output
is still allowed to dicharge somewhere.

If this is the truth of the matter, then surely it is not safe to use just
one side of a 15kV centre tapped NST.

Certainly not the same as a distribution transformer.  These can be powered
without the secondary connected.

Or so I have been led to believe ...

I believe the opposite is also true.  You can completely short an NST's HV
bushings to each other or the centre tap.  The tranformer will get warm but
will survive because it is current limited.  I have even heard of people
running NST's this way for a few hours to help heat up the bitumen potting
to help remove it :-o

Of course shorting a distribution transformer (esp. without ballasting) is
likely to blow breakers from here to the substation, or boil the interior of
the transformer until oil starts squirting from the safety valves and/or the
transformer goes booooommm.

Someone please correct me if I am wrong.  An explanation as to why this
is/is not the case might be good.

Rgs
Ian