Differential Relays

Re.val.co International, located in the municipality of Rho (near Milan) manufactures special electrical elements called Differential Relays, used within various types of systems.

The main purpose of differential relays is to detect a difference between the incoming current and the outgoing current within the circuit, activating as a relay to command the opening of a power line disconnecting element in order to prevent damage to the system or to something, or someone, interfering with it.


Differential earthing relays

Differential relays consist of an amperometric relay and a toroidal summing transformer and are used in LV networks with alternating current for TT, IT, TNS systems, ensuring protection against indirect contacts (complementary protection to direct contacts) and against fire hazards (since the modest currents to earth cannot trip the magneto-thermal current device).

CEI 64.8 states that the differential relay circuit breaker is considered as additional protection and therefore in addition to the protective measures indicated in the standard, not as the only means of protection against direct contacts.

All conductors of the single-phase or three-phase line, including the neutral, must pass through the toroid in such a way that it detects the resulting residual current; the device intervenes when, due to an insulation defect, the vector sum of the currents in the conductors shows a differential resultant.

Insulation control relay

A relay used for insulation control in single-phase or three-phase networks, with or without an earth-insulated neutral.

This device operates on the principle of applying a direct voltage between the network to be controlled and earth

The relay will detect the current that the system absorbs as a result of the application of the above DC voltage.

The actual value of the system’s insulation resistance is given by the ratio between the applied voltage and the detected current.

Toroidal current reducers

These current reducers are combined with differential relays and consist of a magnetic core that allows even very low fault currents to be detected.
The connection of the toroid to the differential relay must be made with shielded cable in certain cases.

  • Differential threshold < 100 mA
  • Toroid installed at distances > a 10 m
  • Signal cable installed less than 30 cm from power cables

If openable toroidal measurement transformer are used, it will be necessary to ensure that the core contact surfaces are perfectly clean and that the coupling screws are tightened securely before closing them.


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