Which part of a protective device's trip curve responds to very high fault currents within milliseconds?

• A. Long-time delay
• B. Short-time delay
• C. Thermal trip
• D. Instantaneous

Answer: (D)

When a fault current is extremely high, the protection needs to open almost instantly to limit damage. The instantaneous portion of the trip curve is designed for that. It uses a magnetic mechanism that reacts to the big surge in current and causes the breaker to trip in just a few milliseconds, independent of any heating effects. This is different from the thermal path, which relies on heating from current and therefore takes significantly longer to trip, and from the short-time and long-time delay paths, which intentionally delay tripping to handle transients or coordinated upstream protection. So the instantaneous path is the fast-response element that activates at very high fault currents within milliseconds.