Electrician Red Seal · Question
A substation is protected by a set of overcurrent relays. Which type of relay is typically used for inverse time overcurrent protection, where the operating time decreases as the fault current increases?
An Inverse Definite Minimum Time (IDMT) relay provides inverse time overcurrent protection, meaning its operating time is inversely proportional to the fault cu
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Question: A substation is protected by a set of overcurrent relays. Which type of relay is typically used for inverse time overcurrent protection, where the operating time decreases as the fault current increases?
Answer options:
- Definite time relay
- Instantaneous relay ✅ Inverse definite minimum time (IDMT) relay
- Under-voltage relay
Correct answer: Inverse definite minimum time (IDMT) relay
Explanation: An Inverse Definite Minimum Time (IDMT) relay provides inverse time overcurrent protection, meaning its operating time is inversely proportional to the fault current magnitude, becoming faster for larger currents. A definite time relay operates after a fixed time delay regardless of current magnitude, and an instantaneous relay operates with no intentional time delay.
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