For correct residual current protection, the more important aspects are linked to tripping times: protection against contact voltages is only effective if the maximum times indicated on the safety curve are not exceeded. If an electrical system has user devices with earth leakage currents which exceed the normal values (e.g. presence of capacitor input filters inserted between the device phase and earth cables) or if the system consists of many user devices, it is good practice to install various RCDs on the main branches with an upstream main residual current or non-residual current device instead of a single main RCD.
Horizontal selectivity
The non-residual current main circuit-breaker provides “horizontal selectivity ” preventing an earth fault at any point on the circuit or small leakage from causing unwanted main circuit-breaker tripping, which would put the entire system out of order. In this way, however, section “k” of the circuit between the main circuit-breaker and the RCDs remains without “active ” protection. Using a main RCD to protect it would lead to problems with “vertical selectivity ”, which require tripping of the various devices to be coordinated, so that service continuity and system safety are not compromised. In this case, selectivity may be amperometric (partial) or chronometric (total).
Amperometric or partial selectivity
Selectivity may be created by placing low-sensitivity RCDs upstream and higher-sensitivity RCDs downstream. An essential condition which must be satisfied in order to achieve selective co-ordination is that the I .1 value of the breaker upstream (main breaker)is more than double the IΔ2 value of the breaker downstream. In this case, selectivity is partial and only the downstream breaker trips for earth fault currents IΔ2 < IΔm < 0.5*IΔ1.
Chronometric or total selectivity
To achieve total selectivity, delayed or selective RCDs must be installed. The tripping times of the two devices connected in series must be coordinated so that the total interruption time t2 of the downstream breaker is less than the upstream breaker ’s no-response limit time t1,for any current value. In this way, the downstream breaker completes its opening before the upstream one. To completely guarantee total selectivity, IΔ value of the upstream device must also be more than double that of the downstream device in accordance with IEC 64-8/563.3, comments.
For safety reasons, the delayed tripping times of the upstream breaker must always be below the safety curve.
