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Ggroundinstrumentlightning surgecableV2V1+–V3surgeR1cable resistanceSA1V1V3V2SA2SA3D1Gprotecteddeviceelectriccircuitry+–PPGSS7Induced lightning is a current source which tries to flow the current. This current is the lightning surge current. When current does not flow smoothly, it generates high voltage and forces to let the current flow. The high voltage generated at that time is lightning surge voltage.Now, let us explain the fundamentals of lightning surge protector. Here we explain them with an exam-ple of a most commonly used model, the MDP-24-1. The diagram below is a simplified circuit of the MDP-24-1. On the cable, lightning surge voltage V1-V3 is generated to let the current flow. We call V1, interline surge voltage, and V2 and V3, line to ground surge voltage.The surge protector absorbs V1 at the discharge element SA1 and V2 and V3 in SA2 and SA3. SA1-SA3 can be considered as voltage-dependant switches. When the voltage is high, an arc is discharged and they suppress the arc discharge of V1-V3 around 30V. It is important to limit the surge voltage as low as possible. Especially, V1, which is applied directly to the protected instrument, is eliminated by the discharge element SA1 and the voltage limiting element D1.SA can bypass high current, however, its firing potential is high and the setting voltage varies widely. In order to compensate these disadvantages, it is combined with D1 of which firing potential is low and the setting voltage does not vary much and suppresses the surge voltage to a low level. D1, which does not tolerate high current, is also protected by R1 which limits the surge current (Figure 7-1).The discharge element and the voltage limiting element together are called the surge absorber element. Since there is no one ideal surge absorber element, i.e. small size, small setting voltage variation, fast response time, high discharge withstand current rating, no failure in shortcircuit mode, the surge absorb-er circuit is designed with combinations of components to bring out their good characteristics.Discharge breakdown (V2, V3)Lightning surges cause a very high potential (volt-age) difference between two conductors and the ground, and a discharge occurs between some part of an electronic circuit and those electrically con-nected to the ground, such as metal housings. Electronic components become damaged because some of the discharge current flows through the electronic circuit (Figure 6-1).Figure 6-1Figure 7-1Fundamentals of surge absorbing

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