The rated current of a protective relay is calculated by multiplying the relay's rated current by the plug setting multiplier (PSM) and adjusting for time dial settings (TDS) and system conditions.

Key Concepts

Rated Current (I_rated): The nominal current of the equipment or circuit being protected, often based on the motor or line full-load current. Pickup Current: The current at which the relay begins to operate. It is typically expressed as a percentage of the relay's rated current and is adjusted using the Plug Setting Multiplier (PSM). Plug Setting Multiplier (PSM): Ratio of actual fault current in the relay coil to the relay's pickup current. Common values range from 50% to 200% depending on the relay type and protection requirements . Time Dial Setting (TDS) or Time Multiplier Setting (TMS): Adjusts the relay operating time to coordinate with upstream and downstream protection devices .

Calculation Formula

For an overcurrent or overload relay, the relay setting can be calculated using: Relay Setting (A) = (PSM × Rated Current) / TDS Where:

• PSM = Plug Setting Multiplier
• Rated Current = Full-load current of the protected device
• TDS = Time Dial Setting, which adjusts the relay's operating time

Example

Suppose a motor has a rated current of 1250 A, the relay PSM is 150%, and TDS is 0.25: Relay Setting = (1.5 × 1250) / 0.25 = 7500 A This ensures the relay trips appropriately during overcurrent conditions without unnecessary tripping during normal operation .

Additional Considerations

1. Motor Characteristics: Include service factor, starting current, and ambient temperature when setting overload relays to prevent nuisance tripping .
2. Relay Curve Selection: Choose the appropriate inverse-time characteristic (Standard Inverse, Very Inverse, Extremely Inverse, or Long Time Inverse) according to IEC 60255 standards to coordinate with other relays .
3. Coordination: Ensure downstream relays operate first for faults in their zone, while upstream relays provide backup protection .
4. Ambient Temperature Compensation: Adjust settings for high ambient temperatures to account for reduced cooling of motors .
5. Relay Type: Thermal, electronic, or electromagnetic relays have different response characteristics; settings must match the relay type and application . By following these steps and using the formula above, engineers can accurately calculate the rated current and settings for relay protection, ensuring reliable operation and equipment safety.