How Lightning Protection Works
by Robin Gudgel
comes into wiring by common mode. What common mode means is that the
PV positive and PV
negative as well as AC positive and AC negative rise at the same
level from the near lightning strike.
just talk about PV input circuits at the moment knowing the battery
and AC react the same. You have long wires coming from the PV array
to the controller, sometimes very long. These wires act as an
antenna. When a surge comes in the vicinity of the wires, it induces
voltage. The closer the strike, the higher the voltage induced into
the wires. Since the surge is raising the voltage equally in the
plus and minus wires, there is no danger to the low voltage
transistors in the unit until the surge finally finds a path to earth
ground. So what happens is the voltage builds up on the PV+ and PV-
input or battery plus and battery minus. It keeps on building higher
and higher. This all happens very quickly of course.
all UL and ETL listed products are required to pass a hi-pot test at
the factory. This test is typically 1000VAC plus twice the peak
working voltage. We convert this to the equivalent DC voltage which
is 1.414 times higher than the AC requirement. There are sometimes
advantages in using this higher DC voltage due to parasitic
capacitance that can make the product fail when using AC.
the case of our charge controller, we are required to connect all
four of the terminal block wires together, PV+, PV-, Bat+ and Bat-.
All four of these connections are connected at the same time to one
lead of the hi-pot machine. The other lead of the machine is
attached to the casting. The casting represents ground since all
equipment enclosures are supposed to be connected to earth ground.
Once these two connections are made, we push the GO button on the
hi-pot machine. It slowly ramps up from zero volts to 2300 volts DC.
It holds the voltage at 2300 volts for one second. The intent here
is to check the insulation between all of the electrical circuits
inside the controller to ground (the casting). If the unit
withstands this 2300VDC for one second without drawing more than 3
milliamps of current, the test automatically terminates. If the
product under test does not make it, the machine stops, records the
voltage where breakdown occurred and turns on loud buzzers and bright
lights. You then have to go find out what was built wrong, fix it,
and run the test again. A failed hi-pot test does not happen very
often and depending on what went wrong, it can damage the product.
will be one point where the insulation broke down or there was
insufficient distance from a circuit board trace to a casting stand
off or something like this. When a product leaves the factory and
has a UL or ETL label applied, you can be assured the product has
passed this test. The test is required to insure we are building a
safe product. You do not want any of the live circuits breaking down
to the enclosure case (the casting). This would be dangerous for
anyone touching the case which is supposed to be at ground potential.
does lightning come into play with this hi-pot test? We know that
the product was tested to be good to 2300VDC in the case of our
controller. Remember, we tied the plus and minus wires together and
applied a very high voltage to the circuit. Because we had them all
tied together, it did not cause a problem with the low voltage parts
in the product. Even though there was a high voltage applied, the
difference between leads of the individual components is zero. That
is until there is a breakdown in the insulation or air gaps designed
to withstand this high voltage. When there is a breakdown, the
hi-pot machine will only put out 3 milliamps. This usually does not
hurt the product. The breakdown point gets corrected, the product
gets tested again and hi-potted again.
when a real near lightning strike is induced into the wiring of a
system, a very similar thing happens as the hi-pot test. The
lightning will continue to rise sometimes to 100,000 volts. There
are no consumer products made that can withstand that high of induced
voltage. They typically have about 10 to 20% margin of safety over
and above the 2300V requirement. It costs money and space to have a
higher breakdown voltage, so design margins are typically slim.
now that the lightning has been induced into the wiring and it
reached 3000 volts, we can estimate that someplace inside the
controller is going to breakdown and arc over to the case which is
connected to earth ground. The lightning is not limited to only 3
milliamps of current. It can go well over 3000 amps.
thing happens as the circuits break down. The lightning will
continue rising and the circuits may breakdown at 100 different
places at the same time. Now you can imagine the mess that comes
from thousands of amps flowing in 100 different places inside that
metal casting. The product will be toast. There is no way to
rebuild a unit that has been subjected to this type of damage.
let's add an SPD300 to the input of the controller. When lightning
gets induced into the PV wires, it starts rising, but when the
voltage reaches 385 volts, the SPD begins conducting. The SPD has
thermal metal oxide varistors connected from PV+ to ground and from
PV- to ground. Depending on how close the strike is, there could be
100,000 amps behind the surge. The MidNite surge arrestors are good
for up to 115,000 amps. The higher the current, the higher the
voltage will rise even though the SPD is trying to clamp the voltage
down. We have measured about 900 volts of clamp with a 3000 amp
surge. UL uses 3000 amps as their simulated lightning surge. Since
each controller was tested at the factory to be able to withstand at
least 2300 volts, a 900 volt/3000 amp surge will not hurt the unit.
If the lightning strike was a direct hit, there may be no saving the
product or SPD. It all depends on where the lightning entered the
Resistance of the wiring is your friend in this case. The resistance offers
impedance to the surge and gives the SPD a chance to act. We were
able to test our SPD's with a 40,000 amp surge at 22,000 volts. I
did not think you could get 40,000 amps through 12AWG wire, but I was
terribly wrong. The lightning surge is so fast that the wire just
passes the high current right on through.
Click here to learn more about MidNite Solar Surge Protector Devices (SPD).
Robin Gudgel at MidNite Solar