VMD421H-D-3 Bender

The AC/DC sensitive residual current monitor RCMA420 is designed for monitoring earthed 

power supply systems (TN and TT systems) where smooth DC fault currents or residual 

currents continuously greater than zero may occur. These are in particular loads containing 

six-pulse rectifiers or one way rectifiers with smoothing, such as converters, battery chargers, 

construction site equipment with frequency-controlled drives. Currents in single conductors 

can be monitored too.

The prewarning stage (50…100 % of the set response value IΔn2) allow to distinguish between 

prewarning and alarm. Since the values are measured with measuring current transformers, 

the device is nearly independent of the load current and the nominal voltage of the system.

• AC/DC sensitive residual current monitoring in earthed two, three or four conductor 

systems (TN and TT systems)

• Monitoring of variable-speed drives, UPS systems, construction site equipment, printing 

machines, battery systems, laboratory equipment, wood working machines, MF welding 

systems, furniture industry, medical electrical equipment, etc.

• AC/DC sensitive current monitoring of, in the normal case, de-energised single conductors 

(e.g. N and PE conductors)

Once the supply voltage US is applied, the start-up delay is activated. Measured values 

changing during this time do not influence the switching state of the alarm relays.

Residual current measurement takes place via an external measuring current transformer 

of the CTUB100 series. The currently measured value is shown on the LC display. In this 

way any changes, for example when circuits are connected to the system, can be recognised

 easily. If the measured value exceeds the set response values, the response delays 

ton1/2 begin. Once the response delay ton1/2 has elapsed, the K1/K2 alarm relays switch 

and the alarm LEDs AL1/AL2 light up. If the current falls below the release value (response

 value plus hysteresis), the release delay toff.