Does the cable modem really need to be unplugged for 30 seconds?
The 30 second number comes from a chain of people high-balling the number to make sure the next person gets it right. When I worked for a cable ISP, I’d tell the head of IT that the modems had to be unplugged for 5 seconds to reset them (a quick in-out sometimes left them in a weird state, and 5 seconds is a reasonable wait.) He would tell the customer service managers to unplug them for 10 seconds, to make sure they got at least the 5 seconds. The managers would tell the customer service reps to unplug for 30 seconds, to make sure they got at least the 10 seconds, and so on; I heard a rep at one point suggesting a full minute. I think this is a natural reaction; each person assumes the number they heard is the bare minimum required for the reset to work, so they add some padding before they pass it along.
Some of these devices (notably the typical home router type things) also have a master reset button — usually recessed that you must press with the end of a paper clip or something. The design typically requires that the reset button be held for some time period (5+ seconds) in order to prevent accidental erasure, after which some copy of firmware in ROM is copied to flash, or the entire flash is zeroed, or whatever “master reset” means for the device. I’m willing to bet that the “unplug for ‘n’ seconds” meme grew out of an incorrect application of this knowledge — that you must hold down the master reset button for ‘n’ seconds, therefore you must also unplug the device for ‘n’ seconds. From ‘n’ it increased to 30 for the reasons already pointed out in the thread.
