Grounding of the device cable shield
Why must the cable shield not be grounded on the instrument field device?
Cable shielding protects the signal transmitted by the conductor from external interference. The cable screen blocks external interference and noise and then directs it to the ground.
The screen must be grounded to only one source. Grounding is usually done in the equipment panel or cabinet, while the field equipment point is left ungrounded and isolated. This eliminates the ground loop current that can occur if the shield is grounded at both ends. In this case, the ground loop current becomes noise for the signal wire itself.
The following wiring diagram shows the ungrounded instrument cable shield on the operating device.
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