Switch amplifier with 1-channel isolated barrier, dry contact or NAMUR inputs, relay contact output, line fault detection (LFD). It features reversible mode of operation. Power rail or terminals 14, 15-connection. This isolated barrier is used for intrinsic safety applications. It transfers digital signals (NAMUR sensors/mechanical contacts) from a hazardous area to a safe area. The proximity sensor or switch controls a form-C changeover relay contact for the safe area load. The barrier output changes state when the input signal changes state. The normal output state can be reversed using switch S1. Switch S3 is used to enable or disable line fault detection of the field circuit. During an error condition, the relay reverts to its de-energized state and the LEDs indicate the fault according to NAMUR NE44.