Technical Papers: Low voltage selectivity with ABB circuit-breakers

Selection of the protection system of the electrical installation is fundamental both to guarantee correct economical and functional service of the whole installation and to reduce the problems caused by abnormal service conditions or actual faults to a minimum.

Within the sphere of this analysis, the coordination between the various devices dedicated to protection of sections of installation or specific components is studied in order to:

·         guarantee safety of the installation and of people in all cases; rapidly identify and exclude just the area involved in the problem, without indiscriminate trips which reduce the availability of energy in areas not involved in the fault;

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·         reduce the effects of the fault on other integral parts of the installation (reduction in the voltage value, and loss of stability in rotating machines);

·         reduce the stress on components and damage to the area involved;

·         guarantee service continuity with good quality power supply voltage;

·         guarantee adequate support in the case of malfunction of the protection delegated to opening;

·         provide the personnel in charge of maintenance and the management system with the information needed to restore service to the rest of the network as rapidly as possible and with the least interference;

·         achieve a good compromise between reliability, simplicity and cost-effectiveness.

In detail, a good protection system must be able to:

·         perceive what has happened and where, discriminating between abnormal but tolerable situations and fault situations within its zone of competence, avoiding unwanted trips which cause unjustified stoppage of a sound part of the installation;

·         act as rapidly as possible to limit the damage (destruction, accelerated ageing, etc.), safeguarding power supply continuity and stability.

The solutions come from a compromise between these two antithetic requirements– precise identification of the fault and rapid tripping - and are defined according to which requirement is privileged.