Overcurrent Calculation Formula for Relay Protection

Overcurrent relay settings are calculated using the rated current, plug setting multiplier (PSM), and time dial setting (TDS) to ensure selective and reliable protection of electrical circuits.

Key Concepts

Overcurrent Relay (OCR): A protective device that trips a circuit breaker when current exceeds a predefined threshold, preventing damage to equipment and maintaining system stability . Plug Setting Multiplier (PSM): The ratio of actual fault current to the relay pickup current. It determines the level of current at which the relay operates and is typically expressed as a percentage of the rated current . Time Dial Setting (TDS) / Time Multiplier Setting (TMS): Controls the delay before the relay trips, allowing coordination with downstream relays to ensure selective fault isolation . Pickup Current: The minimum current at which the relay begins to operate, usually set above normal load to avoid nuisance tripping .

Step-by-Step Calculation

1. Determine Rated Current (I_rated): Identify the full load current of the equipment or feeder to be protected.
2. Select Plug Setting Multiplier (PSM): Choose a PSM based on desired pickup current. Common values are 50%, 75%, 100%, 125%, or 150% of rated current .
3. Set Time Dial (TDS/TMS): Adjust the time delay to coordinate with upstream and downstream relays. This ensures only the nearest relay trips first .
4. Calculate Relay Pickup Current (I_pickup): $$I_{pickup}=PSM\times I_{rated}$$ For example, with a rated current of 1250 A and PSM of 150%, the pickup current is: $$I_{pickup}=1.5\times 1250=1875\text{ A}$$
5. Determine Relay Operating Time (t_operating): Using IEC/IEEE standard time-current curves (Standard Inverse, Very Inverse, Extremely Inverse, Long Time Inverse), calculate the operating time: $$t_{operating}=\frac{PSM\times I_{rated}}{TDS}$$ For a TDS of 0.25, the operating time would be: $$t_{operating}=\frac{1.5\times 1250}{0.25}=7500\text{ A·s (or adjusted per curve)}$$
6. Adjust for Coordination: Ensure the relay settings allow downstream relays to trip first for faults within their zones, while upstream relays provide backup protection .

Practical Considerations

• Selectivity: Only the relay closest to the fault should trip to prevent unnecessary outages.
• System Stability: Proper settings prevent cascading failures in the network.
• Equipment Protection: Correct settings protect transformers, motors, and feeders from overcurrent damage.
• Standards Compliance: Use IEC 60255 or IEEE curves for inverse time characteristics to ensure reliable operation .

Summary

Calculating overcurrent relay settings involves defining the rated current, selecting an appropriate PSM, setting the time delay, and using standard time-current curves to determine operating times. Proper coordination ensures selective tripping, system stability, and equipment protection, making it a critical step in electrical system design and maintenance .