A brief look at the operating principle
An MRCD (Modular Residual Current Device) is a modular protection arrangement that combines residual current detection with tripping a compatible circuit breaker. The measuring device, sensor and tripping element must be assessed together; an alarm contact alone does not provide the same function.
Application scope of MRCD and RDC-DD
An MRCD provides modular residual current protection together with a suitable circuit breaker. RDC-DD describes a DC residual current detecting arrangement for Mode 3 AC charging under IEC 62955. The detection and switching chain of RDC-M modules is assessed separately from an industrial MRCD protective combination; the application scope of each product is determined by its manufacturer documentation.
How do RCM alarms differ from MRCD protection?
An RCM monitors residual current and provides measurement and alarm information. An MRCD is a modular arrangement used for protection together with a suitable circuit breaker. Measurement, evaluation and tripping functions may be located in separate components. This makes it possible to form a protection chain without routing the high operating current directly through the monitoring device's terminals. Connecting an RCM relay to an arbitrary circuit breaker does not automatically make the combination a verified MRCD solution.
The sensor and circuit breaker belong to the same chain
Bender specifies a suitable CTUB102–CTBC measuring current transformer combination and a circuit breaker with an undervoltage release for the MRCDB425. Suitable sensors for the MRCD410R are listed in the CTAC group. These examples show why selection cannot rely solely on sensor aperture or the MRCD designation. The current waveform to be detected, measuring electronics, sensor connection and circuit breaker's tripping behaviour must be considered together. The overall response includes the entire chain from the measuring device to disconnection of the load circuit. This approach is assessed for protection arrangements in industrial installations accessed by authorised personnel, where components are verified as a combination.
Prewarning supports maintenance; tripping provides protection
Suitable Bender MRCD models can provide a prewarning before the tripping level is reached. The MRCDB425 has separate relays for prewarning and main alarm, plus Modbus RTU to transfer measurement information to a supervisory system. This helps track changes in operating conditions. Prewarning and protective tripping are different events; prewarning is not a promise of uninterrupted operation that postpones a protection requirement. Thresholds, delays, frequency characteristics and circuit breaker compatibility are set according to the application. The same parameters or sensors must not be assumed to apply to different models.
| Task | RCM | MRCD arrangement |
|---|---|---|
| Measurement | Residual current monitoring | Residual current detection with a compatible sensor |
| Alarm | Model-dependent thresholds and indications | Model-dependent prewarning and protective tripping |
| Protection chain | The RCM monitoring function does not replace the RCD protective function | The evaluator, suitable sensor and suitable circuit breaker must be verified together |
Bender models for a modular protection chain
MRCD models are assessed together with the current waveform to be detected, compatible measuring current transformer and circuit breaker to be tripped.
LINETRAXX® SmartDetect MRCD410R
Evaluates AC and pulsating DC fault currents in earthed TN and TT systems. Prewarning and protective tripping arrangements are formed with suitable components.
Completed with a suitable CTAC sensor and circuit breaker with an undervoltage release. It is not presented as a Type B solution for smooth DC detection.Explore the model and documents
LINETRAXX® SmartDetect MRCDB425
MRCDB425 is a modular protection device sensitive to AC and DC fault currents in TN and TT systems. It can transmit prewarning, main alarm and measured values via Modbus RTU.
The manufacturer requires a suitable CTUB102–CTBC combination and circuit breaker with an undervoltage release. Protection relies on the verified combined operation of these components.Explore the model and documentsAn MRCD model alone is not the circuit breaker that disconnects the load. Select the complete measurement and tripping chain according to component compatibility and response conditions. Product images belong to Bender.
Bender sources for these models
Bender technical explanations
This English summary is based on the Bender technical sources listed below.
- Bender — Function and possible applications of MRCDs ↗
- Bender — SmartDetect MRCDB425: sensor and circuit breaker compatibility ↗
- Bender — SmartDetect MRCD410R: measurement and protection combination ↗
Sources checked: 2026-10-22. Diagrams belong to Bender; Turkish explanations summarised by ADEO Teknik.