Bonded penetration analysis for a severe lightning strike to a facility

Abstract

Lightning strikes pose a serious threat to facilities and their subsystems. If a facility takes a direct strike, large amounts of pulsed electromagnetic (EM) energy can radiate into the interior of the facility. This energy can couple into electronic systems causing failures. Often, proper shielding of the facility can reduce the radiated energy by an order of magnitude. In an attempt to reduce pulsed EM energy, facilities are built to resemble a Faraday cage. However, most facilities have several imperfections which limit the effectiveness of their shielding capabilities. Penetrations into the facility are a type of imperfection that allows EM fields to be produced in the interior of the facility. Therefore, penetrations must be connected to the Faraday cage through bond wires to maintain the shield's integrity and protect sensitive components. Finite element computer simulations have been performed to determine the effects of bonded penetrations, using 6 AWG bond wires. In an attempt to offer guidelines, which optimize the facility's shielding effectiveness; several bond wire configurations have been investigated. Bond wire lengths, bond wire orientation, single and multiple bond wire configurations and varying the angle between bond wires have been investigated. Simulation results have shown that multiple bond wires result in greater than 40dB reduction of pulsed EM fields in the interior of the facility and a spacing of greater than 45° is optimum for bond wire spacing, for the simulated facility. In addition, the penetration current diverted by the bond wire was monitored. For severe direct lightning strikes, i.e. Ipeak=200 kA and dI/dt=400 kA/μs, the simulation suggest greater than 90% of the lightning current is diverted through the bond wire into the Faraday cage for the configurations examined. The high current nature of the severe lightning pulse produces large Lorentz forces on the bond wire. Laboratory experiments are being developed at the LLNL pulsed power lab to ensure that bond wires maintain proper connection when exposed to high currents, ensuring desired shielding throughout a direct strike.