What Causes LR9G Overload Relays To Trip?

Issue:
Understanding LR9G overload relay operation: tripping as designed under load conditions

Product Line:
TeSys™ Giga Series

Environment:
IEC Contactors and Overload Relays

Resolution:

There are four current–based protection functions provided by the overload relay:
• Thermal overload protection
• Phase loss protection
• Phase imbalance protection
• Ground–fault protection

The thermal overload condition causes the overheating of the motor. Thermal overload conditions occur:
• During the starting phase when the starting time is too long, or if there is stalling conditions.
• During operation if there is jam condition or any condition that results to increase abnormally the currents flowing in the motor.

Phase loss. The overload relay calculates the rms current value in each phase continuously. When the rms current value in one of the three phases is lower than 0.1 Ir and the rms current value in another phase is greater than 0.8 Ir, the overload relay triggers a tripping in 4 +/- 1 s.
NOTE: The phase loss protection cannot be disabled.

Phase imbalance. The overload relay calculates the imbalance current ratio in each phase as given below:
• Ii1 = (| I1 - Iavg | x 100) / Iavg
• Ii2 = (| I2 - Iavg | x 100) / Iavg
• Ii3 = (| I3 - Iavg | x 100) / Iavg
Where Iavg = (I1 + I2 + I3) / 3, and I1, I2 ,I3 are the current values in L1, L2, L3 phases.
The network imbalance ratio Iimb is the maximum imbalance current ratio calculated: Iimb = Max (Ii1, Ii2, Ii3).
When the imbalance ratio limb exceeds 40%, the overload relay triggers a trip in 5 +/- 1 s.

Ground Fault. The overload relay calculates the ground current Ig continuously. When the ground current Ig exceeds more than 10%, the relay trips in 1±20% s.

Released for: Schneider Electric USA