Overload Relay

I think what you are doing is calculating the top end of the curve using the I²t formula. It doesn't work that way.

The IEC 60947-4-1 formula is simply that for Class 10, it will trip in between 4 and 10 seconds at 7.2X I_e, with I_e being the Set Current, and is based upon a COLD relay.

So if you set the current at 6.0A and inject 43.2A, it will trip in not more than 10 seconds, but not less than 4 seconds. Then if you want to determine the trip time at some value less than that, you plot an I²t curve with the other end of the curve being 1.05X I_e as the "not trip" value. So the issue is, you don't start the curve by squaring the current setting, you CREATE the curve FROM that fixed point ON the curve and only look at the points BELOW it. Did that make sense?

SolarEagle,

This is an example of the difference in IEC vs NEMA trip curves by the way. IEC says 7.2X at a cold state, NEMA says 6.0X at a warm state. IEC then goes on to say that in a warm (operating) state, the trip values WILL BE reduced to as low as 25%. So it CAN BE as low as 1.8X. NEMA makes no such allowances but assumes there is no point in calculating a protection device trip time in a cold state because that will never happen. However in a warm state, the pickup point of the curve is generally 1.25X, which is higher than the IEC lowest point. This is because both curves are based on typical motor thermal damage curves. NEMA designed motors are much more robust than IEC designed motors. When IEC component mfrs apply for UL listings, UL acknowledges that a lower trip value from an IEC device will mot harm the motor, and when the IEC mfrs test at 6X from a warm state, they always pass because they trip faster than the NEMA curves.