Why does RCCB/RCD trip?

The problem

Permanent leakage current

Unfortunately, sometimes it happens that RCCB/RCD trips even with no faults or accidental contacts thus requiring a manual reset. Let's try to understand why the circuit breaker does operate even in absence of any fault.

A small leakage current flowing to ground is always present in every electrical facility and is almost never due to the quality of cables insulation but is mainly related to the extension of the system itself. The larger the system, the greater the leakage current.

Electrical cables along the whole facility introduce a parasitic capacitance proportional to their length. The specific capacitance of the wires are quite low (few pF per meter) but usually an electrical facility does contain many meters of this cables and a typical residential installation can reach a capacitance of tens of nF, as a reference.

The current flowing through this parasitic capacitance is far from be negligible compared to the tripping threshold of RCCB.

In addition to this current there are leakage currents due to built-in EMI filters of modern electronic devices. The total leakeage current can be very close to the limit thus reducing considerably the tripping marging of the breaker.

At first sight the problem seems unavoidable, but there are good news!

The solution

Offset the capacitive leakage current

Since the leakage current flowing to ground is a capacitive current (90° phase advance to the voltage) it could be recognized and canceled by injecting the same current counter-phase. It's exactly what we do, role reversed, when we compensate the power factor of an inductive load.

In a similar manner we can cancel the capacitive component of the residual current by introducing an adequate inductive current.

Keep in mind that we're talking about the small leakage current flowing to the ground and not the bigger load current flowing through the phase and neutral conductors.


B-Stop can fix the problem

B-Stop is a device that injects an inductive compensation current. This greatly reduce the permanent capacitive leakage current to ground regaining the nominal RCCB tripping threshold.

Doing so the unwanted tripping events (for no apparent reason) are greatly reduced without degrading safety since the RCCB will continue to operate regularly as expected in the event of any fault to the ground or accidental human body contact.

An Example

Let's take an example

Let's suppose we have a capacitive leakage current to the ground of about 17mA (it's a very common situation) and the installed RCCB having a nominal operating current of 30mA.

RCCBs can trip starting from a current as low as half the nominal value by standard requirements. We do assume an effective tripping threshold of about 20mA (also a very common situation). The tripping level is just 3mA above the permanent leakage current thus the margin is about a merely 3mA!

Futhermore must be considered that the actual threshold may change slightly  over the time and environmental conditions such as temperatura, humidity and voltage. A margin of just 3mA does not ensure that the threshold could not be exceded under certain unfavorable conditions.

The initial  leakeage current of 17mA can be lowered to just 7mA by adding a B-Stop configured to inject an anti-capacitive current of 10mA.


Initial situation: 17mA => margin of  3mA = (20mA - 17mA)

Final situation: 7mA => margin of 13mA = (20mA - 7mA)

Margin has been increased of about 4 times!

The safety

Safety first of all

Beware of those who offer solutions that can lead to exclude the RCD or increase the tripping threshold. Remember that the legal limit is 30 mA in civil electrical installations!

Adding B-Stop may eliminate the problem of nuisance tripping when you cannot or you don't want to modify the existing plant. Doing so you still maintain the same level of protection against accidental contacts.