From the image above, we can see that the three-phase power line and neutral line from the circuit breaker to the electrical equipment pass through the zero-sequence current transformer, and after the secondary winding of the differential current transformer is processed by the RCD detection and control device, to control the circuit breaker to trip. This is how the leakage switch RCD works.

Note: Pay attention to the above picture, the case of the electrical equipment is directly grounded and not connected to the power grounding electrode, so this grounding method is called TT grounding type.

Let the three-phase currents be la, lb, and Ic, respectively. Since the phase difference between them is 120 degrees, if their amplitudes are equal, then under normal circumstances, the current phasor sum of the three is equal to zero, that is:

We also know that the N-line current, that is, the neutral line current, has the following relationship with the three-phase current:

It is natural for the N-line current to be equal to zero, as this is a direct result of the balance of the three phases.

Now, there is leakage in the system, which is assumed to be phase A current.

Since the A-phase part of the current flows to the PE line, and then returns to the power supply through the ground grid, the A-phase current becomes larger. But the current in the N line is still the comprehensive value of the three-phase balance current, so there are:

Here is called residual current, it will drive the RCD to perform leakage protection operation.

It is worth noting that: in the TT grounding form, the grounding grid has a certain impedance, so the grounding current is not large. If the circuit breaker is used for trip protection under TN, the circuit breaker may not operate. Therefore, under TT, IEC60364 requires that RCD must be installed.

Action Problem of Residual Current in Leakage Switch RCD

Residual current includes two different types of leakage current, one is the leakage current caused by the breakdown of the insulation of electrical equipment, also known as equipment leakage current; the other is the leakage current that flows from the human body when a direct electric shock occurs to the human body.

The former is of great significance to the fire protection and equipment protection of the low-voltage power grid, while the latter is of great significance to the protection of the human body. When the human body comes into contact with the live conductor, if the current flowing through it is 40~50 mA and the maintenance time is 1 second, it will cause electric shock damage to the human body. In the IEC60364 standard, multiply the human body electric shock injury current by a factor of 0.6 to obtain a current of 50x0.6=30 mA, and define this current as the critical current value for human body electric shock injury. The residual current action protector that prevents the human body from being electrocuted is our common RCD leakage switch.