U.S. Energy Information Administration - EIA - Independent
Statistics and Analysis Today in Energy Note: Normal system
frequency of 60 cycles per second appear as blue. Green and red
colors indicate higher than normal frequency. A large region of
electric systems, called the Western Interconnection, responded
to the San Diego blackout on September 8 in less than a minute.
A YouTube video vividly illustrates how changes in frequency
across interconnected alternating current (AC) electric systems
ripple across such a large area in a very short time. The video
shows the variation of frequency (the number of times per second
that the electric charge reverses direction) in the Western
Interconnection associated with the San Diego blackout on
September 8. An event, which has still not been fully
characterized, drove system frequency below 60 cycles per second
for about 25 seconds. Interconnected electric systems in North
America are designed to operate within a very narrow frequency
band around 60 cycles per second