Switching units are used to monitor the status and health of safety edges or safety mats, they monitor the 8k2 resistors and all cables in the circuit. If the resistor is shorted out or becomes open circuit, the safety relays will open and stop the machine or gate.
Multiple safety edges or mats are connected in series with an 8k2 Ohm terminating resistor as the final link in the circuit. By monitoring the quiescent current in the circuit the switching unit determines if one or more of the safety edges has been triggered, in this condition the safety relays are opened on the switching unit. Using the same technique, the switching unit also detects cable breaks and will report a system fault and again open the safety relays.
The wireless solutions also monitor the 8k2 resistor in the link and report the status back to the receiver. If the one of the safety edges are triggered, the safety relays on the receiver will open and provide the stop signal to the safety system.
While multiple edges can be monitored in each channel, BBC Bircher Smart Access recommends that no more than four safety edges are monitored by each channel of the switching unit. It is also important to note that any category 2 switching unit should be checked during the operating cycle to ensure correct operation and conformance. Category 3 switching units are self monitoring devices.