3 ms·

Really nitpicky, but not really a GFCI. The USB connector is detecting a short circuit--a direct connection between the two pins--and cutting power while the c

Really nitpicky, but not really a GFCI. The USB connector is detecting a short circuit--a direct connection between the two pins--and cutting power while the condition is present. If you wanted to put this in terms of household electrical, this would be closest to a self-resetting breaker. (And actually, most devices you have already _have_ this. There's usually a self-resetting fuse/PTC thermistor protecting your USB ports from over-current conditions. The only novel thing here seems to be that it's done in software.) A GFCI, meanwhile, works by detecting in imbalance between the hot and neutral lines. The amount of current flowing through one should match the amount flowing through the other. If power is being drawn through the hot pin but not returning through the neutral, it has to be going _somewhere_ and it's not where we intended it to go. It cuts the power off rather than let it continue to flow out and find a path to ground through your kitchen sink/etc.

ajross 3y ago
Even more nitpicky: that's totally a GFCI. You "detect a short circuit", by definition, by detecting current flowing between pins that shouldn't carry current. GFCI is the same thing, just for the special case of three-wire power delivery where all it needs to do is test one ground line.
- burnerthrow008 3y ago
  Gelman amnesia and all that… Even more nitpicky: it’s not a GFCI because GFCI does not detect current leakage between pins that shouldn’t carry current. It measures current imbalance between pins that should be carrying current. The distinction is that a real GFCI (which this USB port is not) does not care whether the imbalance is returning through a third conductor on the power cord or some other path. If this were a real GFCI, it would be able to detect when current flows out through the case, to your hand, though your body and then back though the puddle of watch you’re standing in. That’s not what this is doing. It is measuring unexpected current between the pins within the port for the purpose of avoiding galvanic corrosion (the hint is in the daemon name reported by macrumors).

ajross 3y ago

Even more nitpicky: that's totally a GFCI. You "detect a short circuit", by definition, by detecting current flowing between pins that shouldn't carry current. GFCI is the same thing, just for the special case of three-wire power delivery where all it needs to do is test one ground line.

burnerthrow008 3y ago
Gelman amnesia and all that… Even more nitpicky: it’s not a GFCI because GFCI does not detect current leakage between pins that shouldn’t carry current. It measures current imbalance between pins that should be carrying current. The distinction is that a real GFCI (which this USB port is not) does not care whether the imbalance is returning through a third conductor on the power cord or some other path. If this were a real GFCI, it would be able to detect when current flows out through the case, to your hand, though your body and then back though the puddle of watch you’re standing in. That’s not what this is doing. It is measuring unexpected current between the pins within the port for the purpose of avoiding galvanic corrosion (the hint is in the daemon name reported by macrumors).