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Mid-point Ground

 >> Are you saying that if I stick my finger on the center point of this
circuit I won't feel hardly anything? <<

 That is correct.  As John stated, it would probably have a slight potential
on it if there is not a perfect balance between the two halves but it is a
regular demonstration in my shop.  Even those fearless sign installers think
I'm crazy (when I grab the bare wire on the trode) but I just know the
principles involved.  Each tube down the line will drop the voltage if you
measure it at the trodes.  When you get to the middle, it is very near zero. 
Picture a sine wave.  The point in the middle is the node where there is no
voltage movement up or down.  Of course if you break one of the connections
all bets are OFF.

 >> Now this is a point I am confused with too.  Why would "grounding" this
central part of the circuit be better than letting it find its own ground?  Is
this in case a tube fails the circuit will have less footage at high
potential? <<

 As I said,  the only reason to ground the midpoint of the circuit is if you
have long runs of GTO out to the ends of the tubes (like in a border job that
doesn't return to the transformer).  You can put the home run on the short
side and send the grounded GTO out to the long end without having to worry
about the long GTO interacting because of the full potential on it. The other
reason is because of the Code.  Referencing the secondary assures that
voltages will not exceed 7500V with respect to GROUND (and like John said,
they are looking at failure modes).  It is possible with a secondary that is
not center tapped to go into an unbalanced condition where one side would be
say 9,000V and the other 6,000V relative to ground and this would be more of a
hazard (in the eyes of the NEC) because of the higher potential to a person in
the field.  In theory,  since the secondary is midpoint grounded inside the
case this should not happen. That is why housings are rated for 7500V and not
15,000V.  This whole midpoint ground thing is one of the most discussed and
misinterpreted aspects of neon installation I can think of.  So if you are
confused by it, you are certainly not alone.



  E-mail from: Tom Biebel, 16-Dec-1995
 

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