How does inverse-time overcurrent protection differ from definite-time overcurrent protection?

• A. Inverse-time trips after a delay that shortens as current grows; definite-time trips after a fixed interval once pickup is exceeded.
• B. Inverse-time trips after a fixed interval; definite-time trips after a delay that shortens as current grows.
• C. Inverse-time operates on voltage; definite-time operates on current only.
• D. Inverse-time travels faster than the speed of light.

Answer: (A)

This question tests how current-based protective relays respond over time to a fault relative to the pickup current. The key difference is in how the trip time behaves as the fault current changes.

Inverse-time protection is designed so that the time to trip decreases as the fault current increases. In other words, the higher the current beyond the pickup level, the faster the relay trips. This creates a curve where light faults near the threshold produce longer delays, while severe faults clear rapidly to limit damage and maintain system reliability and coordination.

Definite-time protection, by contrast, uses a fixed delay after the current exceeds the pickup. Once the fault current crosses the threshold, the relay will trip after a preset, unchanging interval, regardless of how much the current exceeds the pickup.

The option describing inverse-time as delaying a fixed amount regardless of current is incorrect, as is the option claiming inverse-time depends on voltage (these protections monitor current, not voltage). The idea that some mechanism travels faster than light is not relevant to how protective relays operate.

So the correct concept is that inverse-time protection trips after a delay that shortens as current grows, while definite-time trips after a fixed interval once the pickup is exceeded.