Lightning Revisited [RE-wrenches]

[Bill Brooks]

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). Current and Voltage is
induced in conductors due to the strike and no direct connection is needed
at all. These are the surges that do the majority of damage to electronics
and can be completely invisible from the outside of IC chips and other
components. The grounded conductor is already at ground potential and will
rise in voltage with the whole system--however, it does not need a surge
arrestor because it already has a solid connection to ground. The poor
ungrounded conductors are at the mercy of the surge arrestors and are
therefore much more likely to be unable to dissipate the charge induced by a
nearby strike.

One spectacular example of this induced current happened many years ago at
the Solarex Solar Breeder facility in Frederick, MD. A lightning bolt struck
the ground 100 yards from the building and the lightning induced so much
current in parts of the array that it melted the wiring and modules. The
problem, that was subsequently fixed, was that wiring was done in large wire
loops for the series strings. Each series loop had a cross-sectional area of
several hundred square feet. These wire loops were eliminated and they never
had another problem.--Moral of the story--if you have a large number of PV
modules in series, NEVER create an inductive loop. Always run the return
wire along with the other wiring in a series circuit.

Bill.

-----Original Message-----
From: Drake Chamberlin [mailto:solar at eagle-access.net]
Sent: Thursday, April 11, 2002 8:13 PM
To: RE-wrenches at topica.com
Subject: Re: Lightning Revisited [RE-wrenches]


Lightning protection is an issue that is not nearly resolved to my
satisfaction.  My logic says that if the "grounded circuit conductor" is
bonded to large amounts of metal, that is exposed to surges, the "grounded
circuit conductor" will become temporarily energized.

In a system that depends on ground rods, that are located on the premise,
there will be a fairly high resistance to ground.  This would mean
substantial potential on the "grounded circuit conductor."  (A system's
"grounded circuit conductor" connected to the utility's "grounded circuit
conductor" would have much less resistance).

"The circuit between the grounded and ungrounded conductors will complete
itself through various equipment.  It would seem that a high surge on the
grounded conductor could create damaging current in the system.

By Code, systems over 50 volts must have one current carrying conductor
grounded.

It is often hard to avoid grounding a conductor on low voltage systems
because of other equipment in the system that will need to be grounded.

 From the 2001 NEC Handbook:

690.41 System Grounding.

For a photovoltaic power source, one conductor of a two-wire system with a
system voltage over 50 volts and the reference (center tap) conductor of a
bipolar system shall be solidly grounded or shall use other methods that
accomplish equivalent system protection in accordance with 250.4(A) and
that utilize equipment listed and identified for the use.
Low-voltage systems that are not grounded must have overcurrent protection
in each of the ungrounded conductors, as required by 240.21.
Other methods that employ available equipment may be used to achieve
objectives contained in 250.2(A), thereby providing protection for the
photovoltaic power source circuits equivalent to solid grounding. "

-Drake

At 07:52 PM 4/11/02 -0600, you wrote:
>Bill,
>I see a possible problem with your recommended solution. The separate
>structures provision also requires a separate single-point bond in each
>"structure" between grounding conductor and a grounded conductor. On the
>three-phase wild AC of many wind generators, what would be the grounded
>conductor (the equivalent of AC neutral or DC negative)? Have I missed
>something here?
>Allan at Positive NRG

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