I’m sure you’ve seen it in a movie-someone turns a switch, and a series of lights make a “thump” noise as they are turned on, sometimes in a sequence in a hallway. What causes that noise, or is it just a movie effect to sound impressive?
In large facilities like stadiums or warehouses, all the power to run those lights can’t run through that little dinky wall switch.
Instead the wall switch controls a device called a “relay” if small, or a “contactor” if large. Which is essentially a large high-powered electric switch itself controlled by the flow of electricity. Up until recently, those were always mechanical devices that make a loud clunk when turning on and off. See Contactor - Wikipedia.
In modern (last 20-40 years) installations they are often electronic devices and silent. Although lots of mechanical contactors are still made and used every year.
Separately, it’s not uncommon that when switching a large total load like a warehouse full of lights, to break that up into a set of smaller sections. And then to arrange the control of that so the sections turn on in rapid succession, not simultaneously. This reduces the size of the surge in the upstream wiring by spreading the turn-on spike across a couple / few seconds versus instantly. A couple of seconds is an eternity in electrical world.
You’re right it’s also sorta dramatic-looking in film or TV. But it’s totally rooted in reality.
If you watch modern traffic lights carefully you’ll notice that the various lamps do not turn on or off simultaneously. e.g. if 4 lanes with 4 signals are going to all go green at once, you’ll see each of the 4 fixtures change a fraction of a second after the other. Once noticed, this effect can’t be unseen. You won’t find it at every intersection, but you will at many. I don’t know that it’s for the same load-spreading purpose, but I suspect it is.
I once worked in a warehouse that had the thunking noises and the staggered start like on TV. Plus, if they were switched off for some reason and immediately turned back on, it would be a minute or two before they came back on again. Annoying (to multiple people) when somebody accidentally flips the wrong light switch in a row of them. (I don’t recall specifically if I was ever That Guy.)
(Of course, this is unrelated to how in cartoons a light bulb comes every time somebody has thunk something.)
For those kinds of loads you may see what looks like an ordinary old-fashioned light switch, except that instead of the toggle handle pointing up or down, it’s pointing straight out. If you move it to the up or down position and let go, that triggers the contactor(s) to the appropriate on or off state, then a spring in the switch returns the handle to the neutral straight-out state.
The High Intensity Discharge (HID) lights that are used in stadiums and warehouses need to cool down before the arc that produces the light can be reignited.
I remember watching baseball games when one or more lighting towers would go out, there would be a 15 minute delay to get them back on. The flashing multicolor light shows possible with modern LED stadium lights always seems miraculous to me even though I understand why they rare now possible.
Modern contactors are both physically small and pretty quiet. I’ve come across 50+ year old ones that are so loud that they sound like a gunshot, and as big as a toaster.
Moot these days with LEDs.
Still real on a lot of traffic lights.
They replaced the bulb(s) with LEDs, but didn’t replace the (expensive) logic running the stop lights. So that still calls for a split-second delay in starting up the various fixture lamps. The need for that may be moot with LED bulbs, but the logic still does it.
When you’re switching high voltage, you want to do it as quickly as possible, because it will arc.
You may also hear a similar thunk when the compressor of your home’s AC turns on. There’s a contactor in there that looks like this:
If the compressor doesn’t turn on, a common cause is this contactor is failing. Another failure symptom is that it continually makes the thunking noise rather than just making it once.
'Zactly.
It’s actually easiest to see that effect on the intersections with retrofitted LED fixtures vs the older incandescents because the ramp-up time on the LEDs is so short. Instead of e.g. all 4 incandescent lamps blossoming pretty much in sync subject to the vagaries of lamp age, etc., you see the LEDs execute a rapid fire “On, On, On, On” cascading across the intersection.
Back when Token Ring networking was a thing, the MAU (more or less the equivalent of an Ethernet switch) would make a loud CLICK when you plugged in a cable, because TR included power. For fun, we’d hand a newbie a TR cable, point to a MAU, and tell them to plug it in. They were always sure they’d done something bad when they’d hear the CLICK, often jumping a foot in the air!