Which phase current is used for the ANSI 49 (Thermal Overload) thermal replica calculation?

Issue

The thermal overload function is tripping, and the customer wants to verify which current value is used in the thermal model.

Product

MiCOM Series Protection relays

Environment

MiCOM protection relays
ANSI 49 Thermal Overload protection
Current measurement and thermal replica calculation

Cause

Clarification is required regarding how the relay determines the input current for the thermal overload function, especially during unbalanced load or fault conditions.

Resolution

The MiCOM relay family automatically uses the largest phase current among the three phases for the ANSI 49 thermal replica calculation. This ensures worst‑case heating is always considered in the thermal model.