RCD Type MI: Modular residual current protection solution for DC residual currents

Protection against dangerous body currents is a central concern in electrical installations. Residual current devices (RCDs) are commonly used for this purpose. RCD Type A is widely used and, according to device standards, must tolerate a DC residual current of up to 6 mA to ensure its protective function.

The problem: Saturation at higher DC fault currents
However, challenges arise with higher DC residual currents. In such cases, RCD Type A can enter magnetic saturation. This saturation impairs the device’s ability to detect and respond to fault currents properly. In the worst case, this leads to a complete failure of the protective function – a phenomenon known as “blinding.” As a result, personal protection for all downstream circuits is no longer guaranteed.

Specific loads require specialized protection measures
Certain electrical equipment generates fault currents that deviate from the pure sine wave of the mains frequency (50/60 Hz) and may include smooth DC components. Examples include frequency-controlled drives in fans, stage and truss motors, or electronic ballasts in lighting systems. For such applications, an RCD Type B is required. It can detect and disconnect both AC and DC fault currents and covers an extended frequency range.

The limits of downstream Type B

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It is a common misconception that installing an RCD Type B downstream of a residual current device of Type A, F, or unknown design achieves the desired protection objective. This is not the case, as the function of the upstream RCD can still be impaired by potential DC fault currents. Equipping mobile electrical installations with Type B RCDs across the board is often associated with significant costs and, outside of construction sites (see DIN VDE 0100-704), is not strictly necessary in many cases. Moreover, the issue of unknown supply sources remains unresolved. In many of the applications mentioned, an all-current sensitive protection that ensures safe disconnection at DC residual currents ≥ 6 mA is sufficient.

The solution: The INDU-ELECTRIC RCD Type MI

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INDU-ELECTRIC has developed an answer to these challenges: the new RCD Type MI. It offers a cost-efficient solution for all-current sensitive protection, especially in mobile applications and at unknown supply points – in other words, wherever it cannot be guaranteed that the upstream residual current device is of Type B.

Technical advantages of the RCD Type MI:

• Compatibility: Enables operation downstream of RCDs of Type A, F, or at unknown supply points.
• Cost-efficiency: Offers a significant cost advantage compared to widespread use of Type B RCDs.
• All-current sensitivity: Ensures detection and disconnection of DC residual currents from 6 mA upwards.
• Integrated personal protection: Includes a built-in 30 mA personal protection feature, sensitive up to a frequency of 1 kHz.
• Independence from mains voltage: Tripping occurs independently of the mains voltage.
• Flexibility: Ideal for mobile use at various locations.
• Integration: Seamlessly integrates into INDU-ELECTRIC power distribution units.

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Conclusion:

‍The INDU-ELECTRIC RCD Type MI represents a modular and economical solution to ensure residual current protection even in the presence of DC fault currents – particularly in environments where the type of upstream protection is unknown or a Type A or F is used. It thus closes an important gap in personal protection within mobile power distribution.

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