Differential residual circuit breaker

This post is also available in: Portuguese (Brazil)

A differential residual device (known as DR in Brazil) acts through according the difference between two variables (in our case, currents). There is a toroid inside the enclosure which has a coil for each input. Among these, there is also a auxiliary coil connected to a cut relay.

If there are difference between all currents entering this device, there will be a residual magnetic flow into the core (toroid), a voltage will be induced to the auxiliary coil and the cut relay will act. Some of these devices can also protect against overloads.

Its connection must be made with all phases and neutral wire (if exists). Remember that the earth conductor must not enter the device and only leakage current higher than the device’s sensibility will excite the core to induce enough voltage to the cut relay.

There are some exceptions about the use of this device:

• TT: you can use it in TT grounding systems, because the earth conductor exists separately to the neutral wire.
• TN-S: the same as TT.
• TN-C: its effectiveness is reduced because the earth and neutral wire is the same.
• IT (with impedance): you can use but the first fault could be not detected, because of its sensibility.
• IT (without impedance): in this system, this device probably will not act, because the leakage currents are almost zero.

Another exception for this device is that it does not detect electric shock caused by a phase to phase or phase to neutral contact (ever since these phases and neutral belong to the same circuit).

I have heard that this device detects short circuits between coils of a motor. It will depend the nature of the short circuit: if it is between only two coils, the leakage current will return through another phase or neutral wire and there will be no differential current. But, if the short circuit is between a coil and the mass of the motor, the device could act because the leakage current will return through the ground.

This device should be installed in wet areas such as bathrooms and kitchens, and also into circuits that reaches somewhere outside the edification.

In IT systems, the use of this device is not mandatory when continuity is essential to safety of someone (operating rooms).