>
> You just can't assume anything when working with these hv circuits.
>
Amen! I remember working on a TV transmitter a few years ago ...
Had a 30 KV power supply in which the B+ was grounded through a
metering resistor and the B- tied to the cathode (typical for a
klystron). The shorting stick had a large braided cable bolted
to the cabinet.
I was working in the bias cabinet (cathode end) trying to figure
out why the transmitter would not show beam current or make any
power. As typical when the cabinet was open, the shorting stick
was hung on the top of the bias string (B-) in case of a problem
with the built in shoring bar. I reached into the back of the
cabinet to check resistances on the filament transformer and
woke up (fortunately) a minute or two laying across the room!
The problem was that the metering resistor had opened up! The
discharge path for both the shorting bar, the safety stick,
and the bleeder resistors were through that metering from the
B+ to the chassis of the external power supply.
To make matters worse ... I was working on the transmitter alone
at 2 AM, there was nobody else on the site (policy of the company
- they didn't want to pay for the "extra" staff ... I left them
shortly after that).
Fortunately, the transmitter beam supply had been off for several
minutes, the capacitors (40 uF worth at 30 KV) had bled down a
bit and I had made contact with a point well "down" the bias
divider which limited the current. Other than the momentary
disorientation I was uninjured.
I am fortunate to be able speak from hard experience:
1) check, double check, and check again!
2) Never work around HV when tired
3) always have someone else (trained in CPR) present when
you are working on HV
4) teach those around you where the breaker is and how to
pull it if you get in trouble
73,
... Joe, K4IK
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