mixed: turn on surge, transmission line resonators

As noted, a transformer, variac or other can have a notable turn on surge.
I bounced off this with some 15A nominal variacs tripping 15A brakers
with no-none-not-any-at-all load.  I have scoped power supply transformers
doing the same thing.  Worst case first half cycle is 10x nominal and i have
(had...) the scope photes to go with it.

My understanding of the cause is a little different than some theories
i have seen here.  Specifically, it was proposed that it takes a finite amount
of time for the mag field to penetrate (build up in) the core of a cored
trasformer.  This means that for the first 'fraction' (Undefined) of a cycle,
the transformer 'thinks' its an air core coil.  Close randomly on the AC line.
Close at zero voltage and all is mellow.  Close at a voltage peak and the
immediate current is 0 (inducive load), then it gets 'rather high', then settles
to a value dependent on connected load, if any, etc.  Breakers can be had with
varying delays, in part for this reason.

(For grins we verified that, in the power supply case, 'charging current
into the filter caps....)

'...transmission lines are flat...."


IF used as transmission lines, in the usual mode, AND TERMINATED in their
nominal impedance, yeah.  IF NOT, then they get real resonant.  I once
(no longer) had use of a sweeper and spectrum analyzer.  Plug on a tee,
with an UNTERMINATED chunk of coax  (or balun to 'twinlead' and sweep.
NICE display of fairly narrow resonances at 1/4 wave (electrical), 1/2,
3/4, full, 1.25 .........