Les
I have read the message you posted on the lightning protection forum.
First, we must recognize that no level of protection will totally protect
electronic equipment from lightning damage. We can however reduce the
probability of damage and the extent of the damage by taking selected steps.
Just think of what could have happened if you had not taken the steps you
did!
It sounds like you have taken some excellent precautions but unfortunately
you failed to realize the necessity to bond the CATV cable to the ground
system. I am not sure what is meant by the term SPG.
We must protect each structure individually. EVERY conductive material that
enters the structure or maybe an enclosure must be effectively bonded
together with the shortest and straightest conductors as is practical. It
is preferable if this is done at a single or nearly a single location. This
would generally be at or very close to the electrical service entry point of
a structure. The impedance of the bonding conductors used to bond the
various conductors entering/exiting the structure together is critical as
significant voltages may be developed between the bonding points depending
on the length of the bonding conductor. Remember that a 1/4 wavelength or
odd multiple of a 1/4 wavelength is a high impedance. So, if the bonding
conductor is 8 feet long and there is lightning energy in the strike at
about 28 MHz then the bonding conductor is a high impedance and a voltage of
several thousand volts may be developed between the ends of this conductor.
The solution is multiple conductors of various lengths. Remember that
lightning energy acts like RF, it is conducted on the surface. So, large
surface area is important. Not braid, but flat copper strap. Braid is not
desired because the weaving of the conductors increases the impedance of the
conductor and the small conductor size means it is easily broken or corroded
and broken.
Suitable surge suppression and the bonding of all components together is
more important than how well the structure is "connected" to the soil or
earth. The better the connection to the earth then the quicker the charge
introduced in the structure will be dissipated but those connections must be
made at the correct locations. Consider a communications site that is on a
rock mountain. There is no effective connection to the soil/earth but those
sites generally survive lightning strikes with no or minimal damage. But,
the protection becomes expensive to some. To others it is cheap insurance
and just the cost of doing business.
We also must remember that during a lightning event a significant magnetic
field is established around the lightning path as well as in conductors that
carry the lightning energy to the earth. This magnetic field can be very
damaging to sensitive electronic equipment that is not suitably protected.
If you desire to protect equipment from the electromagnetic pulse then all
of the I/O ports of the equipment must also have suitable surge protection
devices (SPDs) installed. As an example your alarm system has conductors
routed to sensors within the structure and external to the structure. These
conductors are great antennas! The electromagnetic energy from a nearby
event is coupled into the wiring throughout the structure and it seeks a
point where this can be equalized. Typically through the sensitive devices
within the alarm panel. The only way to protect the alarm panel is to
install a suitable SPD on each conductor entering and exiting the alarm
panel right at the alarm panel. I have seen fire alarm panels in large
buildings destroyed by lightning activity. I recommended suitable SPDs be
installed and to my knowledge those folks have never experienced a failure
of those panels since.
You mention using aluminum materials as conductors or bus bars. Remember
that you may be attaching copper conductors to the aluminum material and
these two metals are not friendly. This practice generally results in some
corrosion and degrading of the electrical connection. This can be
acceptable if properly installed and maintained but that is rarely the case.
I have seen connections that looked just fine but had several ohms of
resistance and exhibited carbon deposits resulting from lightning currents
through the conductors.
You mention you have some SPDs installed. It is important that the bonding
conductors be kept as short and straight as possible. The SPDs should be
tested or replaced once exposed to lightning activity to ensure continued
protection. You should not use any SPD that is not UL listed for the
application, excepting maybe RF transmission lines. This mainly applies to
AC and control line products. Use an SPD suited for the application.
Consider the operating voltage of the circuit to be protected and select an
SPD that has an operating voltage suitably above that point but not so high
that the clamping voltage of the SPD exceeds the breakdown voltage of the
device you are protecting. Your antenna, rotor control, other
control/communications, telephone, CCTV and CATV MUST ALL be protected by a
suitable SPD. For RF lines you desire a SPD that DC isolates the input and
output such as a Polyphaser device. Yes, Polyphaser does make suitable
devices for CCTV and CATV cables. Many of the inexpensive SPD available do
not perform to our expectations. The problem is we don't find out until
after the fact. Also, we might purchases something at a hamfest that is
used. Unless we evaluate the product we don't know if it will be effective
in handling the surge energy we expect it to handle. Try to avoid used SPD
products. Purchase new from quality vendors. The problem is they are not
inexpensive.
I hope this provides some food for thought.
Regards
Bruce, W3YVV
Bruce Carpenter Consulting, LLC
P.O. Box 97
Dayton, MD 21036-0097
410-531-0200
bruce.carpenter@earthlink.net
http://www.bruce-carpenter-consulting.com
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