Subj : 3.5 weeks to being la
To   : Spectre
From : Arelor
Date : Sun Jul 24 2022 07:17:02

  Re: 3.5 weeks to being la
  By: Spectre to Arelor on Sun Jul 24 2022 08:03 am

 >  Ar> It is trivial to detect if the main has power and cut off via
 >  Ar> relays/switches whatever. You can do that with off-the-shelf
 >  Ar> hardware purchased in the electrotechnics supply store.
 > 
 > I find it hard to wrap my head around that... how is it going to detect an
 > outtage if you're feeding the same stuff back in there? You could figure it
 > out on startup easily enough but once you're connected it'd be a positive
 > feedback cycle... <boggle>
 > 
 > 
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Duckduckgo or Startpage your way to UPS diagrams for a basic understanding :-)

There are lots of ways to do it. The most basic is using a switched relay (or
whatever that is called in english) that stays in a given position when there
is current in a given line, and moves to another when there isn't.

Say you have a Hospital which takes its power from the grid, and which has a
diesel generator for emergencies. The basic state is for the switch to stay in
position A so power passes from the grid to the radiodiagnostics equipment. THe
switch stays in position A as long as the main grid is providing tension.

WHen the lights go out, the switch moves to position B, which means the
HOspital is cut from the grid, and the generator turns into the new power
source (I hope you have an automated means to start it up, instead of sending a
maintenance guy to the celar to power it on). If the power from the grid comes
back in, the sitch changes back to position A, cutting off the generator from
the hospital as to avoid conflics (and hopefully something powers the generator
off).

Then again, actual critical installations are more complex than that. A diesel
generator takes a good bunch of seconds to kick in so generally you want a set
of inline batteries somewhere to provide power while the generator is warming
up, for example.



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