Scott it sounds like you have a tiger by the tail in that you are in a
particularly hot
lightning location. Your efforts to date sound reasonable. I will expand on a
few
statements.
First, your system is a good deal like a telephone system and many of the
techniques used in
telephony may help you. You might like to check out the US Printing office for
REA documents
relating to lightning protection.
For the 70 volt outputs I am thinking of using gas tubes to earth where the
wires
enter the building supplemented by MOV, Tranzorb, or other devices near the
amplifier location. I welcome suggestions as to device types.
Tranzorbs are a very good product. They also come in a wide range of voltages
which make them
applicable to almost all applications. They are fastest devices available and
the only ones
that stand a chance of reacting to the leading edge of the current pulse.
MOVs are the heavy lifters but are generally too slow to offer much leading
edge lightning
protection. They are labeled transient protectors as opposed to lightning
protectors for this
reason.
Gas protectors fall in the middle for speed and are only available for higher
voltages.
All of these devices will fail in time if subjected to repeated lightning
strikes. Diodes
(Tranzorbs) fail shorted. Gas tubes fail open. MOVs may fail open or low
resistance.
Ordinary silicon signal diodes can be used to protect microphone inputs. Fast
shockly power
diodes can be used too. Remember the bigger the diode the slower it reacts.
All of the above components are network devices meaning they are not designed
to absorb the
total energy of the lightning strike by themselves. They are designed to work
in conjunction
with other components in the protection network. The protection components,
particularly
diodes, need to react with some resistance or inductance to limit current. The
copper
resistance of the wire and transformers may be enough but adding a few ohms
more, say 4.7
ohms, 1 or 2 watts, in series with each side of the line, will usually allow
the diodes to
survive a direct hit.
Inductors and inductive resistors (wire wound) sound like a neat idea since
inductance will
slow down the wave front and provide an impedance for the protection devices to
react with.
In reality inductors do not survive well. This is because magnetostrictive
reaction,
particularly in solenoid wound resistor or inductors, can explosively shatter
the coil form.
However, I have seen dual segment wound, toridal, common mode chokes
successfully used. The
ferrite cores do not shatter.
Good luck,
Fred Townsend
Scott Lacey wrote:
> To the group,
>
> I am seeking advice as to the best methods of protecting a Public Address
> system
> against recurring lightning damage. The system uses several commercial PA
> amplifiers, each driving several speakers at indoor and outdoor locations.
> There are
> also several locations where microphones can be plugged in. The longest
> speaker
> wires may be up to 250 yards long. It is believed that the charge is being
> coupled to
> the speaker wires where it then returns to ground at the amplifier location
> within the
> building, destroying the solid state devices within the amplifier. A
> technician has
> added fuses to all external microphone inputs and speaker outputs. While these
> have blown several times during storms without obvious damage to the
> amplifiers it
> is my belief that fuses are generally too slow to protect semiconductor
> devices. I am
> seeking advice as to surge suppression devices.
> System particulars are as follows:
>
> 1) The PA amplifiers have 70 volt outputs. All speakers are transformer
> coupled.
>
> 2) All microphones use standard XLR connectors. They plug into metallic
> conduit
> mounted jacks at locations inside and outside the building. The outside
> microphones
> are unplugged during storms.
>
> 3) The amplifiers are located on the second floor of the building. Each
> amplifier is
> dedicated to a set of speakers at one location. The amplifier driving the
> longest wires
> is the one which most often has to be replaced.
>
> 4) The building is in a location known to be susceptible to lightning
> activity. Electrical
> appliances have been destroyed on at least two occasions.
>
> 5) All protective grounding efforts to date have been made to the conduit. I
> am
> recommending that this be supplemented by driven rods.
>
> It is pretty easy to buy commercial surge suppression devices for the ac
> lines. I feel I
> need advice as to the best methods to protect the audio inputs and outputs.
>
> For the 70 volt outputs I am thinking of using gas tubes to earth where the
> wires
> enter the building supplemented by MOV, Tranzorb, or other devices near the
> amplifier location. I welcome suggestions as to device types.
>
> For the microphone inputs I am thinking of using semiconductor transient
> voltage
> suppressors near the amplifier. Again, any sugges
