Safe current monitoring including Emergency Stop shutdown to PL d with a 3SK2 safety relay and two current monitoring relays
The current consumption of a fan motor, for example, rises as the filter becomes clogged. This is why drives have to be shut down when a set limit value is exceeded. The current consumption is monitored with the aid of two current monitoring relays with diverse technologies (3UG4 and 3RR2) and one 3SK2 safety relay. An emergency stop device is additionally monitored for shutting down the motor in an emergency.
It is possible to achieve up to PL d with the redundant use of two current monitoring relays with diverse technologies. A specific current limit or current range (upper and lower limit) is set here on both 3UG4 and 3RR2 current monitoring relays. These monitor the current consumption of the motor continuously and indicate via relay contacts whether the range or the limit is maintained or exceeded.
The safety relay in turn monitors the signals of the current monitoring relays, as well as the two emergency stop contacts, for discrepancy and cross circuit. If the current consumption of the motor exceeds the current limit or range, or if the emergency stop device is actuated, the motor is immediately shut down in a safety-related manner.
When the current consumption of the motor has fallen below the current limit again, or is within the current range again, if the emergency stop device is unlatched and the feedback circuit is closed, the Start button can be used to switch on again.
If two redundant monitoring relays are used in the sensor circuit to detect process variables, this can result in one monitoring relay detecting a limit overshoot before the other. This can be caused by setting or measuring deviations of the devices.
In the example given above, one monitoring relay could detect the limit overshoot shortly before the other in the case of a continuous increase in current. In this case, the power supply to the drive is switched off. Due to the necessary cross-comparison of the inputs in safety-related evaluation, the discrepancy error remains active. The application can only be switched on again after zero crossing of both channels. In this case, the monitoring relays must be checked and manually reset.
This behavior can occur when monitoring slowly increasing process variables. One possibility for avoiding a discrepancy error, for example, is an empirical calculation of the setting parameters for synchronizing the monitoring relays.
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