Page 25 - SACE Tmax XT - Technical Catalogue

INSTALL ATION 1/21
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Coordination

Coordination between circuit-breakers
Selection of the protection system for electrical installation is fundamental both to guarantee correct
economical and functional service of the whole installation and to reduce to a minimum the problems
caused by abnormal service conditions or actual faults.
The coordination between the various devices dedicated to the protection of sections of installation
or specific components has to be studied in order to have a protection system able to:
– detect what has happened and where, discriminating between abnormal but tolerable situations and
fault situations within its zone of competence, thus avoiding unwanted trips which may cause unjusti-
fied stoppage of a properly operating part of the installation;
– act as rapidly as possible to limit the damage (destruction, accelerated ageing, etc.), thus safeguard-
ing power supply continuity and stability.
In order to achieve these goals, it is necessary to know the operating coordination between an up-
stream circuit-breaker and the downstream one and choose the best combination that fits the installa-
tion specific needs.

Selectivity and back-up coordination
There are two different type of coordination between circuit-breakers.
If the priority of the installation is to assure service continuity, then selectivity is the coordination
strategy. Selectivity, according to IEC 60947-1, is the coordination between the operating characteris-
tics of two overcurrent protection devices. In the event of an overcurrent within established limits, only
the circuit-breaker installed on the line affected by the fault (downstream) will trip. If the overcurrent
value exceeds the selectivity limit, then also the upstream device will trip.
If the priority of the installation is to achieve economical savings of the components, then back-up is
the suitable coordination. Back-up is the coordination between the operating characteristics of two
overcurrent protection devices, so that the upstream protection supports the trip of the downstream
protection in case of a short-circuit with a higher value than the short-circuit capacity of the down-
stream circuit-breaker. In this way, it is possible to optimize the breaking capacity of downstream pro-
tection devices by downsizing them.