Transients: Lightning Strike
Copyright © 1996 Francis J. Martino
Two 30 HP, 480 Volt pump motors, each controlled by across-the-line magnetic starters, were
located two-hundred and fifty feet from the control house. A surge suppressor rated for three phase,
480 Volt distribution systems, was installed in the control house. The rating was 1700 peak
Let-Through Volts for the 6000 Volt, 3000 Amp impulse test, and 2400 peak Let-Through Volts for
the 20,000 Volt, 10,000 Amp impulse test.
Lightning hit a pine tree that was located sixty feet from the control house. The pump motor that
was operating at that time incurred a fault at the moment of the lightning strike.
Transient voltages were induced by the lightning into the two-hundred and fifty feet of wire. The
transients damaged the motor winding and caused a fault condition.
The surge suppressor did reduce to a safe level the following transients that were induced by
the lightning strike:
a) Transients entering the control house that were induced by the lightning strike into the
two-hundred and fifty feet of wire
b) Transients induced into the control house wiring
c) Transients entering the control house from the incoming power system
d) Transients entering the control house caused by the motor fault current surge
Why was the motor not protected? The answer is certainly obvious, but lets go through the
calculations to consider the motor leads as being an effective power supply that brought the
transients to both the motor and suppressor.
The suppressor was rated with six inches of lead length so that, at the point of connection
with six inches of leads, the Let-Through Voltage will be as noted above.
However, due to the high frequencies of transient voltages and the capacitive effects of long
wire lengths, the Let-through Voltage of any suppressor will be increased by 9.5 Volts per inch
of lead length.
What was the Let-Through Voltage of the suppressor at the motor terminals with the effective
two-hundred and fifty foot lead lengths to the motor? With a total of five hundred feet of wire, or,
6000 inches, the Let-Through Voltage of the suppressor was increased by 9.5 volts times 6000, or,
57,000 volts. Using an average length for the leads, we can assume the "power supply" was at the
mid-point and that the motor and suppressor were in connected in parallel at the 125 foot point.
The peak Let-Through Voltage can then be more accurately valued at 23,500 volts.
At a 23,500 volt let-through level, the suppressor was incapable of passing to ground any
significant amount of the transient, and therefore the motor had no protection from the lightning
strike which introduced transients into the wires between the suppressor and the motor.
The suppressor could only have protected the motor from transients that entered through the
wires of the incoming power system. Those incoming transients would have been reduced to a safe
level at the point where the suppressor was connected to the distribution panel. To protect from
the lightning strike, a suppressor should have been installed in the pump house in addition to the
one in the control house.
A typical unit that could have been used is rated for 1550 peak Let-Through volts with the
6000 V, 3000 A Impulse Test. Surge Amps total is 58,000 with 2520 joules dissipation. The same
unit will allow 708 peak Let-Through Volts with the 1000 V test.
The unanswered question that remains is: "Just how many surge amps did the lightning strike
deliver?" When purchasing surge protection, you take your best guess at the needed surge amp
protection. That guess will be based on the probabilty of lightning strike, the cost and inconvience
of equipment failure, and the amount of money you are willing to spend.
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