Communication ports in protection relays have usually isolation barrier against circulating
currents between devices connected to the same communication bus. These circulating
currents can be caused by potential differences between devices because of the long
cables or poor grounding of the device. For this reason RS485 communication port in
Vamp relays is also isolated.
Because of this isolation barrier it is critical that communication ground and cable shield
ground are connected to their correct separate places. Communication cable shield is
designed to prevent disturbances entering the communication line and the inducted
voltages to cable shield can be rather high. For this reason communication cable shield
must not be connected to communication ground.

Failure to follow this guide may cause high voltage entering communication ground which
may damage the isolation barrier and RS485 converter leading communication failure to
SCADA. 

refer RS485 grounding schema.jpg

Released for:Schneider Electric India