"Dave Plowman (News)" wrote in message
...
In article ,
SH wrote:
They are bigger. Still used in theatres.
^^^



The reason for that is if a plug had a fuse in it and it blew during a
live performance, it was not a quick or safe job to get a maintenance
man out to climb into the suspended walkways in the dark, find the plug,
unplug it, take it apart, replace fuse, reassemble the plug, plug it
back in and vacate the suspended walkways.

Err, theatre lighting doesn't use a ring main. Most usually a radial with
dimmer control. You don't want such circuits confused with a GP mains
supply - hence different connectors.

Ah, I wasn't sure whether lighting circuits in theatres were spurs (with
only one route from the supply to the lamps) or a ring (with two alternative
routes from the supply to the lamps). Either way, they are fused at the
lighting board rather than the plug, and usually the supply goes via a
variable resistor or triac dimmer and switches so several circuits can be
switched on/off or dimmed in synchrony.

I remember working as a lighting operator for school plays, perched 10 feet
above one side of the stage on a gantry with a row of about 10 vertical
wire-wound dimmers which could be switched between lighting circuits - eg
we'd put all the lights that we wanted to dim in sync on adjacent dimmers so
a 2-, 3- or 4-dimmer length of wood could be used to move all the dimmers in
sync. Once that effect was over, we'd switch circuits to other dimmers to
put ones for another synchronous effect on adjacent dimmers. The fuses were
per dimmer rather than per lighting circuit, so if a fuse blew, you switched
the circuit to another dimmer while you rewired the blown fuse. I remember
the "blackout" switch that killed all lights everywhere when a total
instantaneous blackout was needed: it was a surprisingly small insignificant
switch which operated a humungous relay nearby which always made a loud
clonk.

The big daddy dimmer was the one for the house lights. That was a flat box
about 3 feet square with load of ventilation holes all over it and a big
wheel on the front that made a loud screeching noise as the contacts moved
over the wire-wound coil. The instruction there was the move it smoothly,
quickly and fully from one end to the other, not leaving it half-on or
almost-but-not-quite off/on for any longer than necessary because it got
very hot. I think there were 24 house lights, each with a 500 W bulb shaped
like a very large 60 W bulb, so that was 12 kW that had to be dimmed - hence
the need to go from off to on as quickly as possible, because the wire-wound
coil didn't like 12 kW passed through it.


The school also had a lecture theatre with a modern triac dimmer system in
the projection booth and ceiling-mounted lights. But it had no proscenium
arch and no wings, so all the fancy lighting couldn't be used for a stage
play where actors had to enter or exit. The only way to do it was to open
one of the fire doors either side of the "stage" and have the actors enter
from outside in the playground.

What intrigued me about the lecture theatre was that the fluorescent tubes
could be dimmed from very dim to fully bright - they couldn't dim right to
extinction so there was a sudden switch-off once they got very dim. But I'm
not sure how you dim fluorescents while maintaining the striking voltage.
Did they feed them with a crude square wave that remained above the striking
voltage for an gradually-decreasing portion of each mains cycle, a bit like
you'd dim LEDs?