Electrical Safety Level 2 Practice Test

Instantaneous is the part of the trip curve that uses the magnetic element to trip almost immediately when the current spikes beyond the pickup. Because arc flash energy is roughly the current squared times the time the arc lasts, clearing the fault in a fraction of a cycle drastically reduces the energy released, even though the current is high. The long-time delay region relies on heating and takes longer to operate, letting the arc continue and increasing energy exposure. The short-time delay adds some delay beyond instantaneous, so the arc lasts longer than instantaneous but shorter than thermal-only, still resulting in more energy than instantaneous. Thermal only trips are slow, governed by temperature, which also means more arc duration and higher incident energy. So the instantaneous region minimizes the energy by clearing the arc almost immediately.