« Reading List: No Second Place Winner | Main | The Big Three Automakers »

Wednesday, August 31, 2005

Fourmilab on Generator Power; UPS Sensitivity Settings

Due to maintenance on the high-tension lines which feed the quarter megawatt Fourmilab electrical substation, the power grid feed was cut for eight hours today. To ride out the interruption, ENSA provided a diesel generator and scheduled a cut-over to and from it with at most a 15 minute outage during the transition. As 15 minutes is uncomfortably close to the battery capacity of the most heavily loaded UPS here, I negotiated a five minute "best effort" transition and decided to use this as a test of the graceful system shutdown mechanism if it came to that which, happily, it didn't.

After the transition to generator power, several of the UPSes began to flip back and forth between mains and battery power several times per minute. After a little ratiocination, I observed that this only affected UPSes installed on or before approximately 1998. All APC SmartUPS units have a "sensitivity" setting on the back panel; I set the sensitivity of the older units to the minimum value and they were subsequently happy with the generator power. None of the newer UPS units had any problems at all running on the generator, and the transition from the generator back to grid power was smooth; the UPSes easily rode out the short interruption.

Power from the generator doesn't meet the Swiss precision of that which normally issues from our power outlets. Here's what the UPS reported on the generator:

MAXLINEV : 241.9 Volts
MINLINEV : 234.7 Volts
OUTPUTV  : 234.7 Volts
LINEFREQ : 53.2 Hz
This is what we're used to seeing when on the grid:
MAXLINEV : 240.4 Volts
MINLINEV : 233.2 Volts
OUTPUTV  : 233.2 Volts
LINEFREQ : 50.0 Hz
After the transition back to grid power, I reset all the older UPSes to high sensitivity; they've run that way for years with no problems, so there's no reason to change now.

Posted at August 31, 2005 23:09