Today I learned. I always thought it was some “electricity magic thing” like additional heat generated within the power strips causing issues between the connected devices, but this makes a lot more sense than whatever I was thinking of.

Although I do think I might have mixed some things up between regular power strips and those outdoors/industrial ones with a long (double/triple digit meter) rollable cable which my dad was a big user of back when he used to work in construction. Basically back when I was little he used to tell me never to plug power tools into a rolled-up “power wheel”, and I think that when I was later heard you shouldn’t daisy chain power strips I must have made that (wrong) connection.

Surge protectors do have one magical electrical thingy in them: metal oxide varistors (MOVs). They're what shunt current in an overvoltage transient and they do age with usage.

I've always known surge protectors age and eventually stop protecting from surges. But what I always wanted to know is how can you tell when it expires? I'm assumings it's based on how good/bad/stable the electricity is in your area. But still, is there any way to know when it's time to replace?

Typically surge protectors have a little light to let you know if the surge circuitry is still good. Others will fail safe, meaning the won't power on if the circuitry is bad. Cheap ones may do neither.

Either way, if your house has had a surge and other equipment has died that wasn't surge protected, probably a good time to replace all surge protectors in the house, they're not really meant to survive multiple large surges. They shunt the power destructively, just somewhere you don't care.

> they're not really meant to survive multiple large surges. They shunt the power destructively, just somewhere you don't care.

Is this a concern when buying used rackmount power conditioners (like used for live music setups?), to protect home IT gear? Can they be worn out without a sign that they are?

What I've read is that their main failure mode is that, as they age, their trigger voltage gets lower and lower, and at some point the normal line voltage is enough to trigger them all the time. And when they overheat, either due to being triggered all the time or due to diverting a large surge, they fail open and no longer have any protective effect on the circuit. High quality surge suppressors would have fuses physically touching the MOVs, so that when a MOV overheats and fails, the fuse opens and cuts power to the now unprotected output.

It's about heat. Cable has some resistance and emits heat. This heat has to dissipate somewhere. If cable is rolled out, it'll dissipate heat to the air. If "power wheel" is not rolled out, cable will heat cables around. Outer cables will dissipate heat to the air, but inner cables will not. So with enough current and enough time, this thing will melt.

You probably won't have issues charging iPhone from this thing or powering something for few seconds, so no need to go crazy about it, just something to keep in mind.