A 16 A curve C circuit breaker allows a maximum earth fault loop impedance of 1.92 Ω at 230 V. The tool reads it from AS/NZS 3000:2018 Table 8.1. A measured or calculated Zs at or below that value disconnects a fault in time.
The case
The circuit is 30 m of 2.5 mm² copper PVC cable with a 2.5 mm² earth, on a supply with Ze of 0.35 Ω. R1 + R2 is 0.54 Ω, so Zs is 0.89 Ω, well inside the limit. The longest run that still meets the limit is 86.94 m.
What changes the answer
The curve sets the limit. A 16 A curve B breaker trips at a lower multiple of its rating, so it allows 3.59 Ω, and the same cable can run 180.01 m.
A higher Ze shortens every run. Measure Ze at the board before you rely on a calculated Zs.
See also: AS/NZS 3000 Table 8.1 maximum Zs explained and Earth fault loop impedance and disconnection time.