Lightning Revisited [RE-wrenches]

[Drake Chamberlin]

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Bill,

I'm glad we are continuing the dialogue on lightning.  It would be good to 
arrive at definitive answers to this ongoing question.  In Colorado, 
lightning is of major consequence, especially in mountainous areas.

At 06:45 AM 4/13/02 -0700, you wrote:

Drake,

One thing to make very clear is that the effects of most lightning occurs by
induction during a typical strike away from the energy system (direct
strikes can do whatever they feel like doing).

*************************************************************************

By "induction," I'm assuming that you mean that the lightning strike 
creates a magnetic field, which as it rises and falls, creates voltage, and 
therefore current in the wire.  I.E., a varying magnetic field across a 
wire will create current in the wire through "inductance."

There are certainly a lot of scenarios that happen when lightning 
strikes.  As you say, a direct lightning strike will do as it pleases.  The 
mega volt lightning bolt will not even "see" the 600 volt insulation on the 
conductors.

Let's look at a scenario where two wires in the same conduit have 
electrical potential induced into them by a lightning surge.  If one 
conductor is grounded, the grounding electrode will shunt the high 
potential to ground.  If the ground is good, the grounded conductor will 
remain close to ground potential.  The induced potential will be mainly in 
the ungrounded conductor.  Therefore, there will be a large voltage 
difference between the two wires.  The internal components of the system 
will "see" a high voltage.

If neither of the wires are grounded, then the voltage will rise and fall 
equally on both wires.  Since there will be little voltage difference 
between the two wires, the system components will not "see" high voltage.

How lightning protection works is controversial.  It used to be believed 
that lightning rods functioned mostly by conducting lightning safely to 
ground.  There are instances where this appears to have been the case.

The theory that seems most plausible to me, at least in most cases, is that 
grounded metal in the air equalizes static potential between the atmosphere 
and ground, before it reaches flash potential.  Electrical potential seeks 
ground.  Although it is often said that lightning strikes from the ground 
upward, it is still an equalizing effect.

When lightning strikes in the vicinity, substantial potential will be 
conducted to ground.  The more static that can be equalized in the 
atmosphere in the vicinity of an installation, the less likely a close 
strike will be.

But say a strike occurs a quarter mile away and an installation is exposed 
to a surge, how will the energy dissipate?

If the circuit conductors are ungrounded, they will have little direct 
attraction for the surge.   The electrical potential will be conducted down 
the frame ground.  If voltage is magnetically induced into the wiring, both 
ungrounded conductors will rise and fall in their potential at 
approximately the same rate.

But, if one of the conductors is grounded, it could create a differential 
in the wiring.  If it is bonded to the metal frame, the grounded circuit 
conductor will be at the same potential as the frame.  The frame will be in 
direct electrical connection with the charged atmosphere, with absolutely 
no insulation to protect the wire from the elevated voltage.

This appears to be a major root of the present question.  Does atmospheric 
potential get into a system through exposed metal bonded to a circuit 
conductor, or are all the effects magnetic?  Hikers often report hair and 
beards reacting to static before strikes.  The rule is to hit the ground 
immediately in these situations.  Does this type of voltage get into a PV 
or wind system?

I still believe that the best way to protect against lightning is to ground 
the array frame to its own rod.  If possible, leave all wires ungrounded, 
and do not connect the array frame to any other part of the system.

If it is necessary to ground the negative wire (as it usually is), make 
that single DC bonding connection at the system DC disconnect in the power 
room.  I realize that this may not always comply with current code.

I've had good luck "bullet proofing" systems that have experienced surge 
damage, using this method. For high voltage systems, it may be more 
important to provide grounding against electrocution hazards than to 
provide the best lightning protection.

-Drake

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