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DISCUSSIONGGsignal cablesurgeprotectorinstrumentV2V1+V2iRV1 = RilightningsurgeGGsignal cablesurgeprotectorinstrumentV2V1+V2lightningsurgeZV1iGGcrossover wiringsignal cablesurgeprotectorinstrumentlightningsurgeRV2V2i i : Surge protector discharge current Z : Wiring impedance R : Surge protector grounding resistance V2 : Surge protector max. surge voltage25QUESTIONHow much resistance is appropriate when grounding the M-Rester?As for lightning protection, any grounding is OK. However, for safety reason, D grounding (100Ω) or more is recommended.10ANSWERANSWERThe important thing to remember is that device ground should be connected with a crossover wire. Then ground is at the poten-tial lightning site. As a result, between the signal line and ground terminal (G) of an instrument, discharge voltage (V2) of the surge protector can be added. When appropriate crossover wire is made, instantaneous electric potential is established be-tween the surge protector and the instrument and the volume of the ground resistance does not affect lightning protection func-tion.Separate wiringA common-mode voltage V1, ground resistance (R) (lightning surge current (i), is added to discharge voltage V2 at the surge protector between the signal line and a ground terminal of an instrument. In this case, ground resistance need to be very small (less than sever-al ohms). Otherwise, the surge protector does not protect the in-strument from the lightning.Crossover wiringBecause lightning surge current does not flow to crossover wire, only clamping voltage (V2) of the surge protector is added between signal termi-nal and grounding terminal (G) of the instrument. Full lightning protection of the surge protector is performed.Wiring at instrument siteBecause lightning surge current flows in crossover wire, voltage (V1) which figure is calculated by wiring impedance (Z) (lightning surge current (i), join together with clamping voltage (V2) of the surge protector and added to between signal terminal and ground-ing terminal of the instrument. Lightning protection lessened when crossover wire is long.

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