ABB OTM_C21D Compact Automatic Transfer Switch

The OTM_C21D automatic transfer switch can be used as a source transfer switch in a three-phase or single-phase networks. Monitored conditions are, no-voltage, phase-loss, overvoltage and un- dervoltage detection, transfer delays, generator start and stop, and remote test function.

Source transfer can be performed using a manually operated handle, locally using push buttons or fully automatically. The automatic mode includes several operating methods: Line 1 priority, no line priority and manual back switching mode. OTM_ C21D can achieve communication function by using the optional external Modbus RTU module.

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  • Reasonable structure, small volume, nice appearance, provided with protective shield, safer and more reliable power supply.
  • Complete protective functions, including short circuit, overload, open phase and loss-of-voltage protection.
  • Noiseless, energy saving, simple installation, easy operation, reliable and stable performance.


  • Fire-fighting equipment
  • Smoke Extract Fans
  • Sprinkler pumps
  • Emergency lights


Automatic transfer switch Parameters
Rated operational voltage Ue[V] Single-phase (2P)

Three-phase (3P/4P)


220~240 V AC 50~60 Hz


Frequency 50Hz/60 Hz
Operating voltage range 0.7~1.3 Ue
Measuring accuracy ±3%
Operating angle 90° ( O-I, I-O, O-II, II-O)

180° ( I-O-II, II-O-I)

OFF time 0,6 – 0,7 s
Total transfer time 2.5 s
Output relay utilization category 3A, AC1, 250V
Electromagnetic compatibility Class B
Ingress Protection Rating IP20, front panel
Rated impulse withstand voltage Uimp 8 kV (6 kV for control circuit, disconnect the power line of the control circuit before carrying out the dielectric voltage withstand test)
Operating temperature -25~55oC
Transportation and storage temperature -40~70oC
Altitude Max. 2000 m