Calculation of Overcurrent Setting Value for Relay Protection

Overcurrent relay settings are calculated using the pickup current, plug setting multiplier (PSM), and time multiplier setting (TMS) to ensure selective and reliable protection of electrical systems.

Key Components of Overcurrent Relay Settings

1. Rated Current (Full Load Current, FLC): The normal operating current of the protected equipment, such as a motor, transformer, or feeder .
2. Plug Setting Multiplier (PSM): Determines the multiple of rated current at which the relay will operate. Common values include 50%, 75%, 100%, 125%, and 150% of rated current .
3. Time Dial Setting (TDS) or Time Multiplier Setting (TMS): Adjusts the relay's operating time to coordinate with upstream and downstream relays, ensuring selective fault clearing .
4. Current Transformer (CT) Ratio: Converts high primary current to a measurable secondary current. The CT ratio must match the system load and fault levels to avoid saturation .

Step-by-Step Calculation

1. Determine Pickup Current: The pickup current is the minimum current at which the relay begins to operate. It should be above the maximum load current but below the minimum fault current. The formula is: Pickup Current = PSM × Rated Current / CT Ratio .
2. Select Relay Curve Type: Choose an IEC 60255 inverse time characteristic curve: Standard Inverse (SI), Very Inverse (VI), Extremely Inverse (EI), or Long Time Inverse (LTI). Each curve has constants that affect operating time .
3. Calculate Operating Time: Using the IDMT (Inverse Definite Minimum Time) formula: Operating Time (s) = (0.14 × TMS) / (PSM^0.02 - 1) This formula varies slightly depending on the relay curve type and ensures proper coordination .
4. Adjust for Coordination: Ensure downstream relays operate first for faults in their zone, while upstream relays provide backup protection. The coordination time interval (CTI) is the time difference between upstream and downstream relay operation .

Practical Example

• System Data: CT Primary = 400 A, CT Secondary = 5 A, Plug Setting = 0.8, TMS = 0.1, Prospective Fault Current = 5000 A.
• Step 1: Calculate PSM = Fault Current / Pickup Current = 5000 / (0.8 × 400) = 15.625
• Step 2: Calculate Operating Time = (0.14 × 0.1) / (15.625^0.02 - 1) ≈ 0.240 s . This ensures the relay trips quickly for faults while maintaining coordination with other protective devices.

Tools and Calculators

Online calculators simplify these calculations by allowing input of rated current, CT ratio, PSM, TMS, and relay curve type to automatically compute pickup current, operating time, and coordination intervals . These tools are particularly useful for complex industrial or utility systems.

Summary

To calculate overcurrent relay settings:

• Determine the rated current and CT ratio.
• Select an appropriate PSM above full load but below fault current.
• Choose the relay curve type and calculate operating time using TMS.
• Verify coordination with upstream and downstream relays.
• Use calculators or software tools for accuracy and efficiency. Properly set overcurrent relays protect equipment, prevent nuisance tripping, and ensure selective fault isolation in electrical systems .