Residual current selectivity

How to get selectivity between residual current protection devices (RCDs)

With its many functions and types, the residual current circuit-breaker can be defined as follows:
a device sensitive to earth currents, able to open an electric circuit within a certain time when the current to ground exceeds the preset value. It is used to protect people and things against direct contacts (a device with high sensitivity, it is an additional protection), indirect contacts, and loss of insulation.

The professional rule for the electrical installation always imposes, except for special plants, the presence of an earthing system, both in civil and industrial buildings. Furthermore, the Standard IEC 60364 makes the use of a residual current circuit-breaker compulsory in many cases for protection of people, giving prescriptions referring to the trip time and currents in relation to the installation voltage, to the existent distribution system, and to the places of installation.

Good protection of the installation should provide:

  • A main residual current type of circuit-breaker so as to ensure protection against the faults that may occur between the main circuit-breaker and the distribution;
  • Protection of each individual shunt with a residual current device.

Thus, there is the need to study carefully the selection of devices suitable to guarantee selectivity, and prevent an earth fault in any point of the distribution circuit from causing downtime for the whole installation.
In general, two residual current devices are selective for each current value if their trip zones do not overlap. This condition is obtained by respecting the following points:

  • The residual current trip threshold of the device on the supply side must be higher than or equal to double the residual current trip threshold of the device on the load side:

                    I∆nSupply side ≥ 2xI∆nLoad side

    This relationship is necessary for taking into account the concept of rated no trip residual current, which is the maximum current value for which the residual current circuit-breaker definitely does not trip. The Standards indicate a current value of I∆n/2 and, below this value, the device does not have a definite behavior, i.e. it may trip just as it may not trip.

  • The minimum no trip time of the circuit-breaker on the supply side, for each current value, must be higher than the maximum trip time of the circuit-breaker on the load side:

                    Tminsupply > Ttotload
For residual current circuit-breakers complying with the Standard IEC60947-2 (CEI EN 60947-2), the prescriptions regarding the trip curves for residual current without delay or for the delayed type are given in Annex B of the Standard.
The differentiation of the trip time can be realized more easily by using delayed type residual current (∆t = time limit of no trip in ms or if ∆t=60ms) with definite time or with inverse time, where tripping can be delayed according to a selectable time. These apparatuses are generally installed on the supply side of other general type residual current devices and it is advisable to have a relationship of 3 between the trip thresholds.

Function G

Protection against earth faults can be realized using the function G available on the electronic trip units installed onboard the moulded-case or air circuit-breakers.
The trip characteristics can be adjusted in current (from 0.2 to 1xIn) and in time (inverse or definite time), depending on the different versions. Realizing protection against indirect contact with this type of function requires a careful analysis of the distribution system and of the value of the earth fault current.

With Emax 2 circuit-breakers, it is possible to realize zone selectivity for function G according to the same philosophy described for function S. This makes it possible to reduce the trip times between two residual current protections in series, increasing the margin of safety for any fault on the load side of the supply-side circuit-breaker, since its trip time is not as high as it should have been to obtain selectivity towards the load side with the classic method for time selectivity.

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