In Sydney, the 'zell' found your old fusebox is a throwback to when Prospect County Council monopolised the supply of electricity. The zells were supplied to PCC and installed in residential meter boxes along with two meters, a general rate and the off-peak rate which was activated by the zell, through a hard non fused hot water isolation switch which was separate to the one marked main switch(!), fed through a regular old ceramic fuse and then it went off toward your hot water tank.
When your hot water tank died, a new 315 litre tank would take up to 48 hours to supply hot water, so plumbers used to unceremoniously cut the county council security tags running through the screws on the zell and flick to switch to on so the tank would start heating immediately. The next off-peak signal cycle would cause the zell to just act as normal. From about '92? onwards the zell supplied was a new and improved model. With this one, you could flick the breaker switch inside the zell all you like but the thing would turn off again after about 20 minutes. Fuckers!
Commercial premises had a similar arrangement in that while there was no zell, there were separate meters for Process Heat. There were different rates offered and it was intended for kitchens with deep fryers and big ovens. Hand dryers in the dunnies were permitted also if I remember correctly.
The other annoyance with zells was if a lightning strike killed the zell, your tank could take a day or two before you realised the heater wasn't working. You'd ring your electrician, who would come out only to tell you the zell needs changing. So then, you'd have to contact the govt-run Prospect CC and ask them nicely if they could kindly replace the zell at their convenience any time in the following fortnight. Maddening. Some later HWT's had booster elements in the top connected to general rate, and when the zell quietly shit itself, your power bill skyrocketed.
Off peak signal injection has played havoc on audio equipment from the early '70s with early transistor amps and rudimentary power supplies. You'd hear that 1050hz sound all the time. The valve gear that I had didn't seem to pick up as much or maybe I didn't notice it.
This annoying signal still plagues Sydneysiders to this day as it causes LED downlights to 'dip' and speed controlled fan motors to be affected. Whilst I have systematically gone through my place and replaced or filtered audio gear that picks it up, my air-con fan sounds like darth vader at times during the night.
I saw a vid once of a car sized signal mechanical injector at a power plant. I'll see if I can find it.
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