Protection relays have one critical function, anything additional is secondary. Fault recorders are designed specifically for monitoring protection operations without other priorities.

• In the unlikely event of a relay failing to issue a trip command there may not be any record of the event in the relay to diagnose the problem.
• Because comprehensive trigger criteria can be set independently of the protection function, it is possible to record even very slight anomalies in the supply system.
• Identification of malfunctioning transducers, bad switching contacts, breaker restrikes or ferroresonance.
• Simultaneous recording with fast sampling (waveform) for detailed fault analysis and slow sampling (RMS) for the identification of power swings and for the performance of stability evaluations.
• Recordings include information on breaker and battery performance.
• Additional continuous recording modes monitor power system operation comprehensively, proof of how a power system is operating can be provided at any time, even if no disturbances were detected during the period in question.
• Evaluation of power quality, including harmonic and flicker analysis to IEC 61000-4-30, IEC 61000-4-7, IEC61000-4-15.
• One sub.net can monitor the performance of many relays.
• The communications link to a relay is a critical point for cyber security. A monitor has no control function so is not a significant risk.

A protection relay is not a substation monitor and sub.net is not a relay. The two devices should be treated as complimentary where both are required to protect and monitor an electricity network.