Monitoring methods depend on the type and complexity of the safety device.

In certain situations, it is critical to use self-monitoring safety devices to ensure proper operation, but it is also important to verify the normal functioning of the control elements between the machine and each safety device that are directly responsible for stopping the hazardous motion.

This can be accomplished by the monitoring input to the safeguarding system. This monitoring feedback function is commonly called External Device Monitoring (EDM).

The following are typical styles of monitoring:

• Single Channel: monitoring contacts are connected in series.
• Dual Channel: monitoring contacts are verified individually.
• Power Monitoring: a series-parallel connection of monitoring contacts provide feedback about machine status. Power is supplied to the module only when both contactors work as they should.