Hi Alex,
I used to install radio stations, two way communications systems, and CATV
systems. We almost never had failures from lightning even with hundreds of
sites, many of which suffered direct hits. The key is more in how the site
is wired than in fancy gimmicks.
> Naturally, we are going over our grounding system to ensure that we do
all
> that
> can possibly be done in an effort to avoid a recurrence.
If you want to be safe, bring ALL cables that enter the building through a
metal bulkhead. Use grounded connectors at the bulkhead, and protect
telephone and power mains wires with MOV's.
Every cable entering the bulkhead should have a solenoid winding using a
few turns of conductor, spaced a few feet away from other cables and
suspended in air. The cables should be buried for some distance before
entering the building.
If you can not bury the feedline, at least add a single large loop of coax.
DO NOT ground the tower grounds to the building grounds, unless they are
closer than 50 - 100 feet to each other. If they are close, bond the tower
to the station ground and install a large ground mat of heavy copper wire.
Number eight AWG wire is fine buried in straight lines away from the
building and tower.
Use multiple deep rods, spaced several feet apart connected with wide
copper flashing. NEVER use braiding for ground leads. Your little (even
though wet) ground rod is like having nothing at all for a ground!
Remember, everything (and that includes plumbing or any other conductors)
must enter the building near the bulkhead, and be securely grounded to the
bulkhead with wide copper flashing. It is critical that coax connectors be
properly installed, grounded to the tower where they leave the tower (at
ground level) and tightened properly. Adding lightning surge protectors is
a waste of time and money on coaxial lines IF they are properly installed
and maintained.
If you can get a copy of the NAB Broadcast Handbook, it details proper
station design.
> watering to keep the soil around the copper rod always wet. The 7/8-inch
> heliax feeder was also grounded to the tower at the top and also at a
point
> further down where it bends to leave the tower to enter the transmitter
room.
I hope the heliax does not leave the tower above ground level, unless you
have it grounded at multiple points after it leaves the tower!
> Since the lightning strike, we have checked all connections to ensure
that
> contacts are good and have now installed a 10mm copper cable diretcly
from
> the
> lightning arrester to the copper in the earth.
Probably a waste of time. The tower is already a good wide conductor for
lightning.
> We have also ordered a surge protector which will be installed on the
mains
> coming into the building.
Good idea. Just make sure the power lines, feedlines, telco lines, and even
plumbing all enters at the same area and ALL are securely bonded to each
other. DO NOT run a separate ground into the building from the tower to a
rack!!!!! Everything inside the building can be grounded to the bulk head,
and the bulk head grounded to earth. But never ever run any isolated lead
that is not tied to the bulk head outside the building!!
73 and HNY,
Tom
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|