[time-nuts] Power monitoring

David Forbes dforbes at dakotacom.net
Thu Apr 17 14:25:08 EDT 2008

Hal Murray wrote:
> My power went out the other day.  That reminded me that I've always been 
> slightly curious about that area.

Our telescope suffers from flaky mountain power, so we have to do this sort of 

The problem is sending a message that the power went out to the university 
campus from the mountaintop over a network that may not be working because the 
power went out. Many steps of communication depend on UPSes to hold up for a few 
minutes to get that email out.

We have a big honkin' Liebert 40 KVA UPS to run all the telescope's computer 
systems between when the power dies and the big generator kicks in. It has the 
nice byproduct of AC mains voltage and current monitoring over RS-232. I expect 
you can buy a separate box to do this, but it has to be installed by an 
electrician to read current draw and probably is not cheap due to the required 
agency approvals.

For monitoring whether there is mains power or not, a 110VAC coil relay plugged 
into the non-UPS outlet tells a computer that there is no mains power. 
Dirt-simple and reliable unless someone unplugs it or puts it into the UPS 
outlet. The relay's power cable has been wrapped in red tape and labeled 
prominently to help prevent tampering.

The computer that does the monitoring is on a UPS behind the Liebert UPS, so it 
will work for a longer while after a power failure. A battery-operated data 
acquisition system would be better for long outages.

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