and LIGHTNING PROTECTION|
By Windy Dankoff
Lightning and related
static discharge is the number one cause of sudden, unexpected failures in PV
systems. Lightning does not have to strike directly to cause damage to sensitive
electronic equipment, such as inverters, controls, radios and entertainment equipment.
It can be miles away and invisible, and still induce high voltage surges in wiring,
especially in long lines. Fortunately, almost all cases of lightning damage can
be prevented by proper system grounding. Owners of independent power systems do
not have grounding supplied by the utility company, and often overlook it until
it is too late.
My own customers have reported damage to inverters, charge
controllers, DC refrigerators, fluorescent light ballasts, TVs, pumps, and (rarely)
photovoltaic panels. These damages cost many thousands of $, and ALL reports were
from owner-installed systems that were NOT GROUNDED.
means connecting part of your system structure and/or wiring electrically to the
earth. During lightning storms, the clouds build up a static electric charge.
This causes accumulation of the opposite charge in objects on the ground. Objects
that are INSULATED from the earth tend to accumulate the charge more strongly
than the surrounding earth. If the potential difference (voltage) between sky
and the object is great enough, lightning will jump the gap.
your system does four things: (1) It drains off accumulated charges so that lightning
is NOT HIGHLY ATTRACTED to your system. (2) If lightning does strike, or if a
high charge does build up, your ground connection provides a safe path for discharge
directly to the earth rather than through your wiring. (3) It reduces shock hazard
from the higher voltage (AC) parts of your system, and (4) reduces electrical
hum and radio caused by inverters, motors, fluorescent lights and other devices,
and not least . . .
GROUNDING IS REQUIRED by the NATIONAL ELECTRICAL CODE
(NEC)(r). Photovoltaic systems are included in Article 690 of the Code. Low voltage
systems are NOT exempt from grounding requirements or from the NEC.
achieve effective grounding FOLLOW THESE GUIDELINES:
A PROPER GROUNDING SYSTEM:
Minimal grounding is provided by a copper-plated
ground rod, usually 8 ft. long, driven into the earth. This is a minimum proceedure
in an area where the ground is moist (electrically conductive). Where the ground
may be dry, especially sandy, or where lightning may be particularly severe, more
rods should be installed, at least 10 feet apart. Connect or "bond"
all ground rods together via bare copper wire (#6 or larger, see the NEC) and
bury the wire. Use only approved clamps to connect wire to rods. If your photovoltaic
array is some distance from the house, drive ground rod(s) near it, and bury bare
wire in the trench with the power lines.
water pipes that are buried in the ground are also good to ground to. Purchase
connectors approved for the purpose, and connect ONLY to cold water pipes, NEVER
to hot water or gas pipes. Beware of plastic fittings -- bypass them with copper
wire. Steel well casings are super ground rods. Drill and tap a hole in
the casing to get a good bolted connection. If you connect to more than one grounded
object (the more the better) it is essential to electrically bond (wire) them
to each other. Connections made in or near the ground are prone to corrosion,
so use proper bronze or copper connectors. Your ground system is only as good
as its weakest electrical connections.
If your site is rocky and you cannot
drive ground rods deeply, bury (as much as feasible) at least 150 feet of bare
copper wire. Several pieces radiating outward is best. Try to bury them in areas
that tend to be moist. If you are in a lightning-prone area, bury several hundred
feet if you can. The idea is to make as much electrical contact with the earth
as you can, over the broadest area feasible, preferably contacting moist soil.
You can save money by purchasing used copper wire (not aluminum) from a
scrap metal dealer, and stripping off the insulation (use copper "split bolts"
or crimped splices to tie odd pieces together. If you need to run any power wiring
over a distance of 30 feet or more, and are in a high-lightning, dry or rocky
area, run the wires in metal conduit and bond the conduit to your grounding system.
WHAT TO CONNECT TO YOUR GROUND SYSTEM:
THE METALLIC FRAMEWORK of your PV array. (If your framework is wood, metalically
bond the module frames together, and wire to ground.) Be sure to bolt your ground
wires solidly to the metal so it will not come loose, and inspect it periodically.
Also ground antenna masts and wind generator towers.
|This article is somewhat dated. For low voltage systems (12, 24 volts),
code now allows negative to NOT be bonded to ground if double pole breakers are
used. WE actually advocate floating negative on low voltage systems as
in automobiles and RVs. |
The main goal is to reduce the surge voltage between
the power wires: if the wires are run parallel and both are ungrounded; even a
1000 volt surge relative to ground will not create much voltage between the two
parallel wires. If one wire is grounded though, there could be a full 1000 volts
between the grounded and ungrounded wires.
GROUND THE NEGATIVE
SIDE OF YOUR POWER SYSTEM, but FIRST make the following test for leakage to ground:
Obtain a common "multi-tester". Set it on the highest "milliamp"
scale. Place the negative probe on battery neg. and the positive probe on your
ground system. No reading? Good. Now switch it down to the lowest milli- or microamp
scale and try again. If you get only a few microamps, or zero, THEN GROUND YOUR
BATTERY NEGATIVE. If you DID read leakage to ground, check your system for something
on the positive side that may be contacting earth somehow. (If you read a few
microamps to ground, it is probably your meter detecting radio station signals.)
Connect your DC negative to ground ONLY IN ONE PLACE, at a negative battery
connection or other main negative junction nearby (at a disconnect switch or inverter,
for instance. Do NOT ground negative at the array or at any other points.
YOUR AC GENERATOR AND INVERTER FRAMES, and AC neutral wires and conduits in
the manner conventional for all AC systems. This protects from shock hazard as
well as lightning damage.
PV ARRAY WIRING should be done with minimum lengths
of wire, tucked into the metal framework, then run through metal conduit. Positive
and negative wires should be run together wherever possible, rather than being
some distance apart. This will minimize induction of lightning surges. Bury long
outdoor wire runs instead of running them overhead. Place them in grounded metal
conduit if you feel you need maximum protection.
SURGE PROTECTION DEVICES
bypass the high voltages induced by lightning. They are recommended for additional
protection in lightning-prone areas or where good grounding is not feasible (such
as on a dry rocky mountain top), especially if long lines are being run to an
array, pump, antenna, or between buildings. Surge protectors must be special for
low voltage systems, so contact your PV dealer.
SAFETY FIRST!!! If you
are uncertain of your ability to wire your system properly, HIRE AN ELECTRICIAN!