alt Hacker News> Because the alarm wire carried current only during a fault, an open 25-inch strap produced the same electrical state at the receiving end as a healthy power plant.
I’m a building automation and controls tech in Manhattan, and for this exact reason digital feedback points and alarms are always normally-closed, and analog points are 4-20mA. This allows differentiation between nominal zero values and a fault in the loop.
Disasters like this are what our best practices are borne from.
Replies
stilldavid • today at 5:36 PM
My company sells a 4-20mA sensor and I always wondered why. Thanks!
> I’m a building automation and controls tech in Manhattan
Hi - sorry if this is not your expertise, but I thought I might as well try asking. I live in ~central Manhattan, in a ~40th floor apartment with a corner view with large windows meanings lots of glass through which sound will travel. For around a year now there's been an annoying high tone coming from somewhere nearby, I would suspect perhaps some large AC fans, or maybe generator coil whine? It's persistent, around both ~2587Hz and ~2475Hz peaks, so a quite high pitched tone that's been driving me insane. The moment I open a window it's just excruciatingly "there", and even with windows shut it's still faintly there in background.
I don't know how somehow no one else has complained/gotten it fixed in this long. I'm overall use by now to manhattan noise, but this kind of persistent high pitched tone is another level of noise annoyance. I know the general direction it's coming from, and have few ideas for which buildings/devices it might be based on just seeing from my window, but pinpointing this kind of tone is difficult.
Again, might not be your expertise at all, but if you know of how I could proceed with it, to either find the source, and/or report this somehow for an investigation (311 ? some other agency? ) ? Thanks upfront!