Jim Brown has stated a very important point. I think the folks at
Astron are not thinking of some problems that bonding the negative of
the DC supply to chassis can introduce, and in fact were introduced by a
colleague who did bond the negative to AC and chassis ground on a club
station Astron 20 amp supply.
What happened was a lightning strike to the Antenna tower and especially
to a vertical Lo Band VHF ground plane sitting atop a beam array for HF,
VHF, and UHF. (Yes I know, a rotating ground plane, but that is another
story, as the Low Band belongs to our host organization for the club
station.)
The strike sent induced current along shields of the coax to the station
where the coax was bonded to the building red iron roof girders, which
was bonded to both earth ground, water pipe ground, and electrical
conduits to 3 wire outlets. It appears that the surge that was induced
in the conduits traveled to various radio gear and in this case through
the AC ground of the Astron, to the primary of the power supply where it
blew out the MOV across that line. The current in the chassis also
traveled through the negative lead from chassis and AC ground, and
destroyed the 3 terminal voltage regulator. In addition, there was
enough chassis and negative lead current to enter a dual band radio and
vaporize the negative power trace by arcing to the chassis, while
completing a circuit to the shield of the coax connected to the tower
and its ground rod which was bonded by no 6 copper to the AC and water
pipe ground point some 10 feet from the tower.
Current was spreading to and dissipating in many directions and paths.
The over voltage crow bar tripping risk here was handled in the
rebuilding by putting a disk ceramic cap across the plus and minus DC
output.
By the way, each radio chassis, and tuner chassis was bonded by braids
to a copper/ bronze wide bus bar that ran behind the radio desk and
around the room to the exit point for grounds. It served as the
grounding point for telephone switch protectors in that room, as well as
the radio station coax arrestor panel.
By tying the negative of the Astron to the chassis, we ground looped
ourselves into possibly causing more damage than might have been
experienced if the DC buses had been floating. The VHF radio tied to
the Astron was the only radio that suffered internal damage. And it was
solely the vaporized negative trace. The lightning may have hit the VHF
beam coax connection, but the Low Band ground plane was the highest
object on the tower. That antenna has a double shielded coax feeding
it, and after the event, no shorting was found in any of the beams nor
the ground plane, or any coaxes. With its folded unipole construction,
the low band antenna probably shunted much current to the coax shield.
Of course, this was a weekend storm event, and no one was at the club
station to see what exactly transpired.
-Stuart Rohre
K5KVH
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