ETC Dimmer Rack
All of the dimmer circuits throughout the school in the Market Square, Cafeteria and Drama room are connected to the central dimmer rack backstage. The dimmer rack is what allows you to very the brightness of each of the circuits by changing the voltage on the circuit. An electronic dimmer rack uses circuits and semiconductors to achieve this task (as opposed to a mechanical or relay based one). The dimmer rack receives 3 phase power (3x 120V) as far as I know, and is split across all of the lighting loads. We've never had an issue with it tripping, and it's rated for over 1 kW per channel (our most powerful lights are 1 kW), if it does, the power is controller from the big disconnect in the electrical room in the back of the office, which staff and student's aren't allowed in.
Controlling the Dimmer Rack
The dimmer rack in turn, is controlled by one of the lighting boards using DMX. There are 4 DMX ports (I'm aware of) in the school that are wired in parallel, allowing a lighting board to be connected. 1 in the Drama Room (by the speaker amp, across the drama office), 1 in Radio L'Am (under the microphone/speaker plugs), 1 in the Market Square (the panel by the cafeteria door) and one backstage (side of the dimmer rack). Since all of the ports are wired in parallel, only 1 of them can be in use at a time, otherwise, the lights will flicker continuously.
When you open the front panel, you can see the full panel. On the left side, there are the dimmer number that the slot corresponds to. These can be found on the individual slots (I'm pretty sure all the outlets for the lights are labelled, I added some missing ones). This also corresponds to the DMX channel that the port is controlled by. For example, a light plugged into plug #8 will be controlled from DMX channel 8, and is dimmed via the 4th slot.
Since all the slots are the same, I'll describe one. Going left to right, the next thing we encounter are the switches. These are disconnects that can be used to turn off one of the circuits. This might be useful if you're plugging in/swapping out a light. If the switch is in the ON position (left), then it is controlled normally. If it is in the OFF position (right), the dimmer is turned off, and there won't be any voltage on the circuit (ideally).
Next, in the center, is ventilation for the coils responsible for the voltage adjustments.
Finally, in the right, we have a signal light. This light shows the result of the DMX or signal input going in to the dimmer. If the light is turned on, then the signal light should be on. This is useful for debugging. If the signal light is off, then the dimmer rack is not receiving the signal for the light to be on. However, if the signal light is on, but the actual light isn't, then it's a problem with the dimmer or the lamp (statistically, probably either the bulb or forgetting to turn off the disconnect switch).