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When I worked at an NOC for a fairly large ISP they told me a story of an intermittent outage that occurred randomly on weekends. Something was injecting insane

When I worked at an NOC for a fairly large ISP they told me a story of an intermittent outage that occurred randomly on weekends. Something was injecting insane amounts of RF interference on the lines. They tracked it down to the node, then the neighborhood. And then finally they found the culprit. What was knocking the network out of whack? It was some guys edge trimmer. The motor in it was injecting crazy signals somehow. The ISP bought him a new trimmer and promptly took pictures of this one and destroyed it. Never had an issue with RF again.

lucaspiller 9y ago
On a related note it’s interesting how electric shavers (which could easily suffer from the same issue) are allowed on aircraft, but mobile devices need to be in flight mode. Are the frequencies of RF noise from an electric motor in a different range, so they don’t cause issues with aircraft?
- danaliv 9y ago
  It's not the aircraft they're worried about, it's the cellular network. The phones-in-airplane-mode rule is an FCC thing, not FAA.
- throwanem 9y ago
  The problem with electric motors is back EMF, not so much RF - motors don't spin that fast. (Switch-mode power supplies do switch that fast - into the hundreds of kilohertz - which is why cheaply made and thus poorly suppressed ones do tend to produce radio noise that you can hear, in harmonics at least, in the lower AM band. It'd be rare to find a tool motor that spins much over 30K RPM, and even that's on the high end.) So the noise is too low frequency to qualify as RF, but you're still dumping inductive voltage spikes into the supply, which can destabilize other equipment on the same circuit or one nearby - in this case, while the neighborhood concentrator might not have been (probably wasn't) on the very same circuit as the trimmer, it probably was on the same transformer, or else the circuits ran parallel in a conduit somewhere and the noise was getting capacitively coupled into the concentrator's supply. This wouldn't be a problem with a battery-powered trimmer because it isn't connected to the aircraft's hotel supply - or if it is, it's through a wall wart, which will contain a transformer and at least some suppression circuitry, which will dissipate any EMF that makes it out of the tool. And in general, the way you suppress this kind of inductive noise is with capacitors and chokes, which any reputable manufacturer of a line-powered motorized tool will include in the design. But there are a lot of disreputable manufacturers out there, too.
- failrate 9y ago
  Another thing they don't want on take off or landing is a bunch of dense wedge-like objects flying around in case of a fuck up.
digi_owl 9y ago
Reminds me of someone having trouble getting their Tesla to charge after the power company installed "smartmeters". The reason was that the model they had picked was one that used the power lines to transmit the readings, and the added noise tripped the Tesla's sensitive charging circuit.
throwanem 9y ago
> The motor in it was injecting crazy signals somehow. It was too cheaply made to have an interference suppression capacitor across the supply leads, is how. Bet he got it from Harbor Freight...

lucaspiller 9y ago

On a related note it’s interesting how electric shavers (which could easily suffer from the same issue) are allowed on aircraft, but mobile devices need to be in flight mode. Are the frequencies of RF noise from an electric motor in a different range, so they don’t cause issues with aircraft?

danaliv 9y ago
It's not the aircraft they're worried about, it's the cellular network. The phones-in-airplane-mode rule is an FCC thing, not FAA.
throwanem 9y ago
The problem with electric motors is back EMF, not so much RF - motors don't spin that fast. (Switch-mode power supplies do switch that fast - into the hundreds of kilohertz - which is why cheaply made and thus poorly suppressed ones do tend to produce radio noise that you can hear, in harmonics at least, in the lower AM band. It'd be rare to find a tool motor that spins much over 30K RPM, and even that's on the high end.) So the noise is too low frequency to qualify as RF, but you're still dumping inductive voltage spikes into the supply, which can destabilize other equipment on the same circuit or one nearby - in this case, while the neighborhood concentrator might not have been (probably wasn't) on the very same circuit as the trimmer, it probably was on the same transformer, or else the circuits ran parallel in a conduit somewhere and the noise was getting capacitively coupled into the concentrator's supply. This wouldn't be a problem with a battery-powered trimmer because it isn't connected to the aircraft's hotel supply - or if it is, it's through a wall wart, which will contain a transformer and at least some suppression circuitry, which will dissipate any EMF that makes it out of the tool. And in general, the way you suppress this kind of inductive noise is with capacitors and chokes, which any reputable manufacturer of a line-powered motorized tool will include in the design. But there are a lot of disreputable manufacturers out there, too.
failrate 9y ago
Another thing they don't want on take off or landing is a bunch of dense wedge-like objects flying around in case of a fuck up.

digi_owl 9y ago

Reminds me of someone having trouble getting their Tesla to charge after the power company installed "smartmeters". The reason was that the model they had picked was one that used the power lines to transmit the readings, and the added noise tripped the Tesla's sensitive charging circuit.

throwanem 9y ago

> The motor in it was injecting crazy signals somehow. It was too cheaply made to have an interference suppression capacitor across the supply leads, is how. Bet he got it from Harbor Freight...