Time to trip for an inverse-time overcurrent relay following IEC 60255 normal/very/extremely inverse curves.
Formula
$$ t_{op} = \frac{k \cdot \text{TMS}}{(I/I_p)^a - 1} $$
Symbol legend
| Symbol | Meaning | Unit | Source |
|---|---|---|---|
| $I$ | Actual fault current | A | CT secondary × ratio |
| $I_p$ | Pickup current | A | relay setting |
| $k, a$ | Curve constants | - | IEC 60255 |
| $TMS$ | Time multiplier setting | - | relay setting |
| $t_{op}$ | Operating time | s | result |
Sources
- IEC 60255-151 - Measuring relays and protection equipment.