Residual Current Circuit Breakers without Overcurrent Protection

An electrical installation is deemed as safe when it is made so as to protect the human body from the electrocution phenomena, that is the patho-physiological effect generated by a sufficient rate current flowing through it. At 50-60 Hz, most of human beings barely sense current rating below 0.5 mA approx, although such current flows through their body.

The effects of a current flowing in the human body mainly depend on the current value and lasting, with major affections on the heart and the breathing apparatus.
Diagram 1 shows the different danger areas depending on the electrocution current and its permanence time within the human body:

AREA 1: usually no reaction
AREA 2: usually no physiologically dangerous effects

AREA 3: usually no permanent organic damage. Possible muscular contraction and respiratory difficulties; reversible heart troubles, including atrial fibrillation and temporary heart attack without ventricular fibrillation, which increase with current intensity and duration.
AREA 4: besides the effects detected in area 3, the possibility of ventricular fibrillation occurrence increases by 5% approx (curve c2), by 50% (curve c3), more than 50% over curve c3.

The electrocution may be originated by either a direct contact with an alive circuit part or by indirect contact with an accidentally alive metal part due to a circuit fault. Direct contacts must be opposed primarily by guaranteeing the insulation of the alive parts and using cabinets or boxes featuring a minimum IP20 protection degree.

The RCCB Residual Current Circuit Breaker with high sensitivity (IΔn≤30mA) is supplying an additional protection, by timely switching off the electric circuit in case those above mentioned are not efficient.
Indirect contacts protection is achieved by a proper coordination between the earth-circuit resistance value (conductors + earth plate) and the RCCB operating value.

According to the installation Norm CEI64-8, the mathematical formula to comply with is the following:

Where Rt stays for earth resistance, IΔn for rated residual current of the RCCB. Example: when installing a 30 mA RCCB, the RT must be lower than 1.666 ohm. The above means that a suitable earth-circuit and RCCB will contain within 50V the voltage presence on metal parts connected to the earth-circuit.
The RCCB duty is to switch off the electric circuit when earth faults are detected in order to readily prevent dangerous situations.
The internal amperometric circuit is continuously computing the vector cross product of the single or three-phase current circuits and, when such value exceeds the pre-set apparatus value, the RCCB trips.

According to the wave form of the leakage current they are sensitive to, the RCCB can be classified as follows:
• AC TYPE: suitable for all electric installations which may include devices featuring a possibly sinusoidal alternate fault current.
• A TYPE: suitable where the connectable electric devices have electronic rectifiers or electronic circuits regulating the current wave form because it operates in presence of fault current either of alternate or pulsating type.
• B TYPE: they are recommended in installations with presence of equipment with electronic circuits which may generate fault DC currents and/or high frequency. Examples of applications are installations with motors with variable frequency, rectifiers, frequency converters, PV systems, charging stations for electrical vehicles, etc.

Depending on their intervention sensitivity, the RCCB can be classified as follows:
• Low Sensitivity: (IΔn = 0,1A – 0,3A – 0,5A) not suitable to protect from direct contacts; they are instead suitable for the indirect contacts, provided a proper coordination with the earth-circuit is made as explained above.
• High Sensitivity: (IΔn = 0,01A – 0,03A) suitable to protect from both direct and indirect contacts.

A complex electric circuit can be protected against earth leakage by applying different RCCB, each protecting a single circuit shunt and one as a general RCCB, above all others. This solution is avoiding a generalized switch off, while in presence of a simple local fault.
This objective can be obtained in two alternative modes:
a) more simply but with lower selectivity performance, by installing a general RCCB featuring at least a triple selectivity than the following ones (amperometric selectivity). Depending on applications, quite often such a solution does not sufficiently comply with requirements.
b) a total selectivity is obtained by installing specific apparatus (RCCB type S) having the differential release mechanism that, in presence of earth leakage current, starts tripping upon the full switch-off of the following RCCBs in the circuit (chronometric selectivity).

Residual Current Circuit Breakers without Overcurrent Protection

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