Đường cong cắt MCB và giới hạn cáp

How this is calculated#

A miniature circuit-breaker does not trip at one exact time for a current. Two breakers of the same type and rating trip at different times, so a maker draws a band: the earliest and the latest each can trip. This tool draws that band for a Type B, C or D breaker, draws the cable's limit beside it, and reads the latest trip time at the fault current you enter.

Three figures come from AS/NZS 3000. A breaker operates within its conventional time at 1.45 times its rating. Table 8.1 takes it as tripping instantly, within 0.1 s, at its mean tripping current: 4 times the rating for Type B, 7.5 for Type C and 12.5 for Type D. The largest loop impedance for a 0.4 s disconnection follows from that current. The rest of the band, the edges of the instant region and the thermal part, is a typical band. AmpSize does not hold AS/NZS 60898, and the result says so.

The cable's line is its adiabatic limit: the time it can carry a fault current before its insulation is damaged, from the k factor of AS/NZS 3008.1, through the fault rating tool. The circuit passes when the breaker's latest time is no longer than the cable's time at that current. Check it at the far end, where the fault current is least and the trip slowest, and give the fault current at the board too: there the current is highest, the cable's time shortest, and the worse of the two ends decides.

Worked examples#

• A C20 lighting circuit in 2.5 mm² with a 300 A fault trips in 0.1 s at the latest, well inside the cable's 0.86 s. The Table 8.1 instant trip current is 150 A.
• A B20 circuit in 2.5 mm² with a 150 A fault trips in 0.1 s against the cable's 3.42 s. The Table 8.1 instant trip current is 80 A.
• A D32 motor circuit in 6 mm² with a 500 A fault can take up to 1.6 s on the slow edge of the band, just inside the cable's 1.77 s. Table 8.1 counts it as instant above 400 A.
• A long B16 circuit in 1.5 mm² with only 60 A of fault current fails: the breaker can take 27.74 s, longer than the cable's 7.7 s, and 60 A is under the Table 8.1 instant trip current of 64 A.
• The same C20 circuit with 3000 A of fault at the board fails at the board: the band shows 0.1 s there, and the cable lasts 0.01 s at 3000 A, though it takes 0.86 s at the far end's 300 A. The maker's let-through energy decides it.
• A C32 circuit in 4 mm² close to the board, 2000 A of fault fails on the band: it shows 0.1 s, and the cable lasts 0.05 s. The maker's let-through energy decides it.

Limits#

The band is typical, not a maker's curve. A real breaker clears a large fault in much less than 0.1 s, and limits the energy it lets through; the maker publishes that energy as I²t. Where the band fails a cable at a high current, compare the maker's I²t with k²S² before you change the cable.

gG fuses are not drawn yet. Their curves are in IEC 60269, which AmpSize does not hold.

See also: Loop impedance, Fault rating and IDMT protection.