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DISCUSSIONcrossover wiringsurgeprotectorinstrumentgroundgrounddistanceincreasedground potentialsignal line4 – 20mA DCpower supplylinelightninglightning rodsurge currentinstrument panel 2instrument panel 1GGsurgeprotectorGGGinstrumentGgroundsurge currentsurge current24QUESTIONAn instrument was blackened by a lightning strike via a lightning rod. Is there a reason for this?Because ground potential is elevated, discharge breakdown occurred to the instrument. Set a surge protector. Also, grounding of the lightning rod and surge protector, an instrument, should not be shared but ground individually.9ANSWERANSWERWhen lightning rod is struck by the lightning, high current flows to the ground and voltage at grounding point rises. When instru-ment panel 1 is within the shield angle, grounding of instrument panel 1 is near the lightning rod and voltage at the grounding of instrument panel 1 becomes high. Therefore, relative electric potential difference is generated between the instrument panel 1 and the instrument panel 2 which is set away from the lightning rod and ground potential is low. Then, discharge breakdown oc-curs between enclosure of instrument panel and the circuit.To prevent this to happen, set surge protectors at both instrument panel 1 and 2 and bypass surge current and absorb relative electric potential difference. If the grounding of surge protector and instrument panel 1 are shared, most of the lightning current possibly flows to the surge protector and exceeds discharge withstand current rating. Be sure to set grounding separately.If grounding of surge protectors are shared...Grounding resistance of surge protector is at least about 1Ω. When lightning current is 200,000A, voltage generated accordingly is 200,000V. If lightning rod and grounding are shared, this voltage flows backward from G terminal and destroy the instrument.have enough distance separation between them.Recommended models: • Model MDP-24-1 (4 –20mA DC signal line) • Model MA-100 (100 –120V AC, 2A power supply) • Model MA-200 (200 –240V AC, 2A power supply) • Model MAX-100 (100 –120V AC, 5A power supply) • Model MAX-200 (200 –240V AC, 5A power supply)

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