Siemens 3UG3521-1AL20 Control Monitoring Relay

Siemens 3UG3521-1AL20 Control Monitoring Relay is a compact, 22.5 mm wide digital monitoring relay from the SIRIUS 3UG4 family that protects electrical loads by watching current levels and signaling when they depart from a setpoint. It monitors current in the 2 to 500 mA range (AC/DC) and switches a single changeover contact (SPDT) when the threshold is exceeded or undershot, with wiring via screw terminals for robust control-panel connections, and is designed for 230 V AC, 50/60 Hz control circuits. The device supports adjustable response characteristics, including an ON delay and handling of transient spikes, with a typical delay window from 0.1 to 20 seconds and selectable hysteresis to define the trip band. While the 3UG3521-1AL20 has been superseded by newer models in the same family (notably the 3UG4621-1AW30), the core function remains: digital current monitoring to verify load operation and enable fault signaling, including open-circuit monitoring in automation applications.