|[AMRadio] High Voltage and shocking revelations|
rbethman at comcast.net
Wed Jan 21 13:03:56 EST 2009
I've been following the "threads" on these "shocking" occurrences. I've
chuckled under my breath at some, and silently shaken my head at others.
I want to first say that I had been involved in HV generation and
distribution prior to becoming licensed in 1980. We worked with three
phase 4160VAC, referred to as 5KV, 13,800VAC, referred to as 15KV, and
voltages up to 125KV.
There is an "expectation" of "safety" *IF* one follows all the
appropriate procedures of opening circuit breakers, racking out circuit
breakers and the like. Those that don't know what "racking out" is, it
is the disconnecting a "large" circuit breaker by an electric motor that
lowers the breaker from even being in the circuit at all. Insulating
"shutters" slide into place over the opening the breaker WAS occupying.
This is done by motor, as these weigh from 800 lbs and up.
This expectation is NOT realistic. I and my fellow workers have
received shocks from these voltages AFTER all these procedures were
followed. One instance involving the 15KV hit required several EEs to
determine HOW it could possibly occur.
I caution everyone NOT to expect interlocks to protect you. They ARE
supposed to work, BUT, the nature of electricity is that where one MUST
expect Murphy's Law to rule! Anything that CAN go wrong WILL go wrong!
If there has been an occurrence of arcing, anticipate TRACKING.
Tracking is where the voltage becomes present in places where it should
NOT be, due to carbon from the arcing incident allowing a path for the
voltages to travel.
That specifically is how I "met" with 5KV. We were sent in to assess a
generator's damage and determine what was going to be required to repair
it. The circuit breaker was racked out, and even removed from the
cabinet. I began looking into the control panel for voltage regulation
and voltage settings. While removing the "tap change" board, I got hit
with 5KV. The ratchet went flying about 30 feet, with me following.
It turned out that the "event" that damaged the unit left a lot of
carbon inside the breaker cabinet assembly. The unit was still
connected to the power grid by hard cable. The voltage traveled through
the carbon residue from the "event". I informed my Supervisor of the
situation. He did NOT believe me, since we had taken ALL procedural
steps. He the attempted to work on the SAME component. He managed to
draw an arc of over a foot, while the ratchet was in contact with the
frame, grounding that end of the arc. I grabbed a CO2 extinguisher and
quenched the arc.
He gave me the rest of the day off, along with the rest of the team.
While he tried to figure HOW.
It is NO fun!
Bob - N0DGN
Bob - NØDGN
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