It is necessary to use shielded cable on the STOA / STOB terminals in order to meet SIL level 3 requirements.

If there is damage to the insulation on an unshielded cable, the potential exists for an external source of power to come in contact with the conductor of the wire connected to the STOA and/or STOB terminals, which could interfere with the normal operation of the safety circuit.   For this reason, it is necessary, in order to meet SIL level 3 requirements, for a shielded cable to be used with both ends of the shield grounded.  If the insulation of this wire is damaged, it is likely that any external source of power will make contact with the shield first and be shunted to ground before making contact with the conductor and potentially interfering with the safety function.
Note: It is possible for a ground loop to be created by connecting both ends of the shield to ground.  If this happens, it is acceptable to connect the shield to ground on the drive side only.

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