In article 1106658377.6477361036c3636354041bdde6782eb8@teran ews,
"Paul King" writes:

Yes. Those dips are called "brown-outs" and are caused when the transformer
tappings are changed. Since the switching contacts are "make before break"
(otherwise you'd get a very momentary blackout) the transformer experiences
a momentary "shorted turns" episode. This causes *HUGE* currents to flow
within the (shorted) transformer windings, creating the brown-out.

Not for at least 50 years (the age of some of my supply distribution
books). When mechanical switches were used, a second coil allows for
smooth switching with no shorting. SCR's have been used for on load
tap switching for perhaps couple of decades now, and their diode
properties mean there's no shorting current even without the secondary
coil. They do zero voltage switching in any case (not that this
mechanism is used to prevent shorting).

--
Andrew Gabriel