Protection Coordination Calculator

Discrimination (selectivity) means that on a fault only the protective device nearest the fault opens, so the rest of the installation stays on. AS/NZS 3000:2018 clause 2.5.7.2 requires it where loss of supply would cause danger or serious inconvenience, and deems it met by ratios between device ratings in some cases. The calculator checks it by comparing the downstream device's slowest clearing time with the upstream device's fastest trip at every current up to the fault level.

The curves are the generic zones the device standards allow (AS/NZS 60898.1 for MCBs, IEC 60947-2 for MCCBs and air circuit breakers, IEC 60269 for gG fuses), not a particular manufacturer's curves. Real devices sit inside those zones, and in the instantaneous region, where both breakers trip in a few milliseconds, only the manufacturers' selectivity tables can confirm discrimination. The calculator marks that region as undetermined rather than guessing.

Miniature circuit breakers with B, C or D curves; thermal-magnetic MCCBs with adjustable Ir and Im; electronic trip units with long-time, short-time (with I²t on or off) and instantaneous stages on MCCB or ACB frames; gG fuses from 16 A to 1250 A; and custom devices from points you enter, either a minimum and maximum curve or one nominal curve with a tolerance.

From the adiabatic equation t = (kS / I)², where S is the conductor cross-section and k comes from the K table for the conductor and insulation in the cable's edition (AS/NZS 3008.1.1:2025 Table 5.1, or AS/NZS 3008.1.1:2017 Table 52), starting at the insulation's maximum operating temperature. The protective device must clear every current from about 5 s down to the fault current faster than the cable reaches its limit, and its let-through energy at the fault current must stay below k²S².

Yes. Anyone can run a study of two devices without an account. A free account saves one study to a project; a subscription saves as many as you need, with up to eight devices each.