[AMRadio] High Voltage and shocking revelations


rbethman 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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