OK. since the subject has come up I'll toss one in. This wasn't a "show stop," but it did produce a rather long
intermission.
I was working as TD and LD at a summer
theatre doing operettas, mostly Gilbert & Sullivan, some Strauss, Offenbach, Lehar,
etc.. The
theatre was in a (surprise!) converted barn. About a 300 seat
house, 20'
proscenium, not much in the way of
flies (old hay loft, maybe?) and what looked like a home-brew light board. The supply was 120-240 V single
phase AC.
The board was in two pieces. The first
unit had three 6kW autotransformer dimmers, each with a 3-position
switch. The "DOWN"
switch position
fed the
dimmer output to load connectors on the board, the "CENTER" was OFF, and "UP"
fed the
dimmer output to the other part of the board. Dimmers 1 and 3 were
fed from the "A" side of the 120-240 feed;
dimmer 2 was
fed from the "B" side of the supply. (I know; not a balanced load, but not my design.)
The other part of the board consisted of 3 banks of six 1 kW autotransformers. Each
dimmer also had a 3-position
switch controlling the
power to the
dimmer; "DOWN" was CONSTANT (dimmers alternating between "A" and "B" sides of the supply), the "CENTER" was OFF, and "UP" was powered by one of the three master dimmers. On bank one (dimmers 1-6) the constant
power supply alternated between sides of the supply, "A"-"B"-"A"-"B"-"A"-"B", while
power for all six dimmers from the bank's master
dimmer was from the "A" side of the supply. A similar situation existed on
dimmer banks 2 and 3. The result was that in each bank three of the
dimmer's
power switches had the full 240V across the two
switch positions. I did not know of this situation at the time I took the job, but I found out soon enough. The switches were of a type that did not have a mechanical pause between positions.
Fast forward to the activity. First show of the season. I was using the "A" master
dimmer in its "Independednt" mode for
house lights and its "Master" mode for the
FOH DS areas (5) and
curtain warmers (1) which also served as DS side lights. As the show begins:
house down,
overture,
curtain warmers down,
switch curtiain warmers from CONSTANT to MASTER & set first
cue level,
switch master
dimmer from
house to MASTER, curtain, all
FOH comes up with curtain. Then the rest of the show and pretty much the reverse at the end of each act. This procedure worked fine for five of the six perfrormances.
Fast forward to the end of Act I on closing night. All
FOH is now @ 10 on master
dimmer. Now, I usually switched the
curtain warmers from MASTER to CONSTANT at this
point, to leave them up on the curtain as the rest of the
FOH stuff goes down on curtain close. This was usually done at a convenient
point in the last number, usually a big chorus number of some kind. Apparently I had been lucky for the preceding five performances and always managed to make this
switch somewhere near the zero-crossing of the AC
power. Not so this time, when I managed to
switch at the peak of the AC cycle, which resulted in a nice 240V arc across the
dimmer's
power switch. This arc, which did not extinguish itself, began throwing flame out of the front and back of the board. After what seemed like a half-hour of this (but was probably not more than 5 seconds) I pulled the board master
power, which not only killed the arc but also killed everything else. And as the
house lights were
fed from the board, when the board lost
power the
house emergency lights came on. (Nice little battery-powered jobs, one at each corner of the room.) Our very unflappable
conductor never lost a
beat and finished the last pages of the score in total darkness except for the
spill from the
orchestra stand lights. After applause died he announced that
intermission would be longer than usual and urged everyone to visit the patio bar.
I spent
intermission tearing into the board, disconnecting the burned
switch and wiring the
circuit to a couple of switches nailed to a hastily made 1x3 frame sitting on top of the board. We finished the performance (after a long intermission--the bar did well that night) and I spent the following weekend replacing the
switch and re-wiring half of the board so that nothing switched from one side of the 240V supply to the other. I still have the burned-up
switch in my collection of oddities.