Relay protection overload factor

The overload factor in relay protection defines the multiple of rated current at which an overload relay will trip, protecting motors from sustained overcurrent and thermal damage.

Definition and Purpose

The overload factor is a key parameter in motor and equipment protection, representing the ratio of actual current to the motor's full-load current (FLA) at which the relay operates. It ensures that the motor is protected against sustained overcurrent conditions that can cause overheating, insulation breakdown, or mechanical stress, without tripping during normal startup currents or transient loads.

How It Works

Overload relays, whether thermal, electronic, or solid-state, operate based on the I²t principle, where the relay calculates the thermal energy accumulated over time. The relay has a pickup current, often set as a percentage of the motor's FLA, and a thermal time constant that determines how long it takes to trip at a given multiple of the rated current. This allows the relay to distinguish between short-term inrush currents and harmful sustained overloads.

Calculation and Settings

Determine Motor Full-

Load Current (FLA): Use the motor nameplate or calculate based on motor rating, voltage, and efficiency. Select Overload Factor: Typically, the overload relay is set 5–15% above the FLA to account for normal operating variations. For example, a 20 A motor with a service factor of 1.

15 may have an overload relay setting of 27. 6 A. Apply Thermal Curve: The relay's tripping time is determined by the thermal curve, which specifies how long the relay will take to trip at different multiples of the FLA.

This ensures protection against prolonged overcurrent while avoiding nuisance trips during motor startup. Standards and MultipliersIEC 60255-8 and IEC 60947-4-1 define the thermal overload characteristics and I²t tripping curves for motor protection. Plug Setting Multiplier (PSM): Indicates how many times the relay secondary current exceeds the pickup current, affecting the trip time on IDMT curves.

Higher PS

M results in faster tripping. Time Setting Multiplier (TSM): Scales the base operating time from the relay curve to coordinate upstream and downstream protection.

Practical Considerations

Undersetting: If the overload factor is too low, the relay may trip frequently, causing process interruptions. Oversetting: If the factor is too high, the relay may fail to protect the motor, risking thermal damage or winding failure. Motor Service Factor and Ambient Temperature: Adjust the overload factor to account for motor service factor and environmental conditions to ensure accurate protection.

In summary, the relay protection overload factor is a critical setting that balances motor safety and operational reliability by defining the current threshold and timing for overload tripping, guided by thermal principles and international standards.