External Device Monitoring
March, 2023
Author: Arshdeep Singh
External Device Monitoring (EDM) is used by safety devices such as safety relays to monitor the state of external devices connected to them. External feedback from the positively/force-guided contacts of the Machine Primary Control Element (MCPE) or contactor is added to the start condition of the safety relay.
What are positively guided contacts?
Positively guided contacts are mechanically linked together in such a configuration that Normally Closed and Normally Open contacts can never be closed simultaneously.
If a Normally Open contact gets Welded, the Normally Closed contact will not be able to return to the closed position when de-energized (Figure 1).
Figure 1
If the external device contacts get welded due to fault, the Normally closed contacts will not close when the device is de-energized. Since the normally closed contacts of the device are monitored by the safety relay, they will not allow the safety relay to Reset. Figure 2 shows typical reset/monitoring circuits.
Figure 2
External device monitoring can be used on a variety of different safety devices such as Light Curtains, Safety Relays, Safety Contactors, etc.
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