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Characteristic curves

To design a low-voltage switchboard in accordance with the valid rules, the system planner needs to dimension the protection settings of the molded case circuit breakers.

The settings selected for the trip unit of a molded case circuit breaker depend on the type of equipment to be protected, e.g. switchboard and applications.

Tripping characteristics up to a tripping time of ≥ 1 ms are represented graphically. In order to ease the coordination of different protection devices, the current is specified as a multiple of the current setting value and the time is specified in seconds.

Tripping characteristics for thermal-magnetic trip units

Characteristics are displayed graphically in the double-log coordinate system (cf. IEC 60947‑2, paragraph 4.7.4 and IEC 60269‑1). The current : time ratio is 2 : 1.

Tripping characteristics for electronic trip units

Characteristics are displayed graphically in the double-log coordinate system (cf. IEC 60269‑1). The current : time ratio is 1 : 1.

Response threshold of the long-time delayed protection, thermal

Delay of the short-time delayed protection

Response threshold of the long-time delayed protection

Response threshold of the instantaneous protection

Delay of the long-time delayed protection

L

Overload range

Response threshold of the short-time delayed protection

S

Short-time delayed short-circuit current range

I2t characteristic ON/OFF of the short-time delayed protection

I

Instantaneous short-circuit current range

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