The purpose of a safety monitoring relay is to monitor a safety circuit and enable the output contactors when the safety circuit is closed and reset.  Depending on the Safety Category (Cat 1-4) or Performance Level (PLa-PLe) the input circuit can be single or dual channel and may require cross wire detection.  Similarly on the output side it’s possible to connect single or double contactors and should include auxiliary feedback relays on each.  A safety monitoring relay is designed specifically to detect wiring faults that my cause incorrect operation and fail in a known safe way. 

 

Fig 1: Example of Wiring to Category 4

It is possible to wire multiple inputs in series, but this is only possible if the inputs are designed for series wiring.  Also, series inputs will lower the diagnostic coverage and therefore lower the possible safety category or performance level.  Generally, it is not possible to wire semiconductor devices in series or devices with a combination of normally open and normally closed channels.  In these instances, or to maximise achievable Safety Category/ Performance Level a separate input should be used for each device. 

Wiring up to 4 safety devices is possible with Schmersal SRB-E-240ST.  This provides all the functionality of a single input safety monitoring relay but can accept 4 inputs with cross wire detection, edge reset and feedback monitoring.

 

Fig 2: Example of Wiring 4 Devices to Category 4

To monitor more than 4 devices it is possible to wire multiple devices in series, however, this reduces the ability to detect some faults and careful consideration should be used with wiring devices in series. For best diagnostic coverage, devices should be wired individually to a safety PLC or multiple safety relays cascaded together depending on the complexity of the safety function(s).

When cascading relays in a Category 4/PL e system, it is critical that the system can detect a single fault.  This includes faults between the relays.  Since the SRB-E-204PE outputs are semiconductor, input crosswire detection will not work, however the SRB-E-204PE had output monitoring and will detect any faults such as short between the output channels.  On the detection of such a fault, the SRB-E-204PE will fault and turn off both outputs.  The SRB-E-204ST will loose the inputs to the corresponding input and will remove the output to the safety contactors.  The fault on the SRB-E-204PE will not reset until the detect fault is addressed.

Fig 3: Example of Wiring 12 Devices to Cascaded Safety Relays Category 4

This combination of SRB-E-204PE and SRB-E-204ST can achieve at least 16 safety input devices providing there is only one simple safety function.  If there is a more complex safety the Schmersal Protect PSC1 safety PLC allows for up to 272 Input/Output and is fully programable for complex safety functions.

Contact one of our application engineers to discuss your safety system requirements.