Reminder: Never connect a generator to home wiring without transfer switch

Lamar Owen lowen at
Mon Aug 30 20:13:31 UTC 2021

On 8/25/21 11:26 AM, Dave wrote:
> Back feed is a significant problem but bringing a generator that is 
> not synchronized to the grid can have dramatic results, typically only 
> once 

This, IMO, is a great thread, lots of good reading here.

My $dayjob is at a site where the previous occupants did indeed operate, 
under PE supervision and special permit from the electric cooperative, 
grid-synchronized generators.  Most of the required switchgear is still 
here, including the dual-incandescent-bulb sync indicators.  Since we 
don't have the required 24x7 PE (that's licensed Professional Engineer, 
by the way) supervision, we don't do this, but over a period of several 
years had normal ATS setups with isolated generation installed, with a 
more distributed setup with only critical loads on UPS.  But grid-sync 
generation is a form of UPS.

The process of going from grid being offline, then adjusting the 
generator governors to sync-in and closing the main breakers (three of 
them, 2,500A each) at the very instant of sync, was apparently quite the 
sight to behold, with the sync indicators blinking and pulsing, until 
they locked in....  At the 2,500A level it is definitely not a pretty 
sight to reclose out of sync.

Synchronous generators were more reliable and less expensive in those 
days than battery backups, especially in the megawatt class, and if you 
only required intermittent uninterruptable power.  The largest battery 
backup on site was 500KVA and was a Piller motor-generator with a huge 
battery on the DC bus.  The total site drew a bit over 2MW on a normal 
day, and so loads were prioritized and during critical operations only 
the required generator capacity was brought online in synchronous mode.  
The main breakers were set up with power-loss instant trip; when we went 
ATS and regular generator operations (much much less load for us) we 
disabled the power-loss trip functions on the three 2,500A mains.

I have some friends who work for the local electric cooperative, and all 
of them have backfeed stories.  Around here, which is very rural, it's 
not at all uncommon to have a single house isolated on a distribution 
spur; nor is it at all uncommon for some people to have the 'suicide 
dryer cord' 'ATS' in use.  One story I heard was that one individual had 
been a repeat offender, and several line workers had gotten bit (none 
were injured, thankfully) by his 4KVA generator; so after the repair of 
one outage, a line worker taught the individual a lesson by hitting the 
recloser without letting the individual know ahead of time (they had 
been letting the individual know that power was coming back on) and 
watching the individual's generator, out in his yard, explode from the 
out-of-sync condition.

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