Differential relays

Residual current relays are modular protective devices associated with a separate detection torus, designed to monitor leakage currents on high-rating circuits beyond the capacity of standard residual current devices. Their tripping threshold and time delay are adjustable to ensure selective coordination between several levels of protection. This equipment meets the requirements of Annex B of the IEC 60947-2 standard and of NF C 15-100, with polyamide housings mounted on DIN rail.

Operation and construction of industrial residual current relays

Measurement principle using a separate detection torus

The residual current relay consists of an electronic processing unit and a separate toroidal core that encircles the live conductors of the monitored circuit. This separate detection torus measures the imbalance between the phase and neutral currents, revealing any leakage current to earth. Our standard practice favours this principle for high-power industrial outgoing circuits where an integrated residual current device would be technically impossible or economically disproportionate.

Setting the threshold and time delay for selectivity

The tripping threshold of the residual current relay is set according to the target application, generally between a few tens of milliamps and several amps. The adjustable time delay, which we frequently see implemented at the head of the installation, allows response times to be staggered between the main residual current relay and downstream protections. This coordination ensures that only the device closest to the fault operates, preserving continuity of service on healthy outgoing circuits.

Applications and implementation in electrical panels

Adjustable residual current protection for high-current industrial outgoing circuits

Feedback from installers confirms that residual current relays are used on high-power outgoing circuits supplying industrial machines, generator sets or secondary panels. The polyamide DIN-rail housing integrates into the electrical panel without modifying the existing main protective device, whether a thermal-magnetic circuit breaker or a switch-disconnector. The detection torus, sized to match the rating of the busbars or cables, is positioned upstream of the outgoing circuit concerned.

Residual current selectivity in the main panel and downstream coordination

Vertical selectivity is one of the most decisive use cases for choosing a residual current relay at the head of the installation. We recommend setting the relay with a sufficient time delay to allow the residual current circuit breakers on the outgoing circuits to react first. Our installation feedback shows that this architecture, compliant with NF C 15-100 requirements, avoids cascading nuisance tripping that would paralyse the whole installation.

Standards and technical mounting characteristics

Compliance with IEC 60947-2 and NF C 15-100

Residual current relays fall under Annex B of the IEC 60947-2 standard, which defines the specific requirements for insulation-monitoring devices and residual current protective relays. Compliance with NF C 15-100 guarantees their admissibility in French low-voltage electrical installations. Our commissioning tests show that checking the threshold and time-delay settings is an essential step before energising the circuit.

Hardware construction and panel integration

The polyamide housing provides mechanical and thermal resistance suited to industrial electrical panel environments. Its standardised DIN-rail mounting makes installation and maintenance easier. The separate detection torus, available in several bore diameters, adapts to the conductor sections of high-current outgoing circuits. In the field, we find that this separation between the measurement function and the switching function offers valuable design flexibility for design offices.

Frequently asked questions

What is the difference between a residual current relay and a residual current device?

A residual current device combines the measurement function and the mechanical switching function in a single device, with limited ratings. The residual current relay separates the detection torus from the control relay, allowing very high-rating circuits to be monitored and protection to be associated with an existing circuit breaker or contactor.

How should the time delay of a residual current relay be set to ensure selectivity?

The time delay is set to a value higher than the tripping time of downstream residual current protections, generally between 0 and several hundred milliseconds depending on the range. Our installation experience confirms that a staggering of at least 40 ms between two successive levels is a sufficient margin to avoid simultaneous tripping.

Can a residual current relay alone switch off the faulty circuit?

No, the residual current relay has no switching device of its own. It sends a command to a circuit breaker, contactor or shunt trip that opens the circuit. This architecture requires the compatibility between the relay and the associated switching device to be checked in terms of voltage and coil current.

Which detection-torus ratings are available for industrial outgoing circuits?

Separate detection tori come in bore diameters suited to the busbars and cables of industrial installations, from outgoing circuits of a few tens of amps up to distributions of several thousand amps. The choice of the torus's inner diameter directly determines the maximum conductor section that can be monitored.