Chapter 43: overcurrent and the adiabatic equationHow long can a cable survive a fault?

Chapter 43 is protection against overcurrent — overload and fault current. First, what a fault actually is:

Appendix 3 — the time/current curves

Appendix 3 holds the time/current characteristic graphs — the "graphs of doom". Find your device type and rating along the top, find your fault current along the bottom, follow up to the curve, read the disconnection time off the side.

434.5 — how long the cable can take it

The adiabatic equation answers "will the conductor survive until the device operates?"

Table 43.1 gives k. You pick your row by insulation type, initial temperature, conductor material and whether the csa is above or below 300 mm²:

t = k²S² ÷ I²
t = 115² × 1.5² ÷ 360²
t = 13 225 × 2.25 ÷ 129 600
t = 29 756 ÷ 129 600
t = 0.23 seconds

What it means: the 1.5 mm² conductor can carry 360 A
for 0.23 s before it is damaged.
Check in Appendix 3 that the device opens faster than that.
A type B of 6–20 A needs only 30–100 A to open in 0.1 s,
so at 360 A it opens well inside 0.23 s.   Cable protected
(Compare t with the device's operating time, not with Table 41.1.)

Chapter 43 · protection against overcurrent 434.5 · adiabatic equation t = k²S²/I² Table 43.1 · k values (copper 115, aluminium 76) Appendix 3 · time/current curves and characteristic tables