Overload for Dummies

Thanks for clearing my confusion :)

Your knack of using the right words to express things is fantistic!

A very good reply that makes the issue crystal clear.

Keep up the good work. I really wonder what the lumberjacks, whom you did not join, are missing!

one question. Is the motor 99% or 100% loaded?

Not sure what type of overload protection you are looking at or working with but there is a drastic diffference between setting up a simple MCC bucket starter overload relay and setting up the overload protection for the distribution feeder breaker that supplies incoming power to the MCC buss where the bucket is located.

The distribution breaker protection allows a much broader control of all trip protection parameters.

This allows differentation between "long time" and "short time" overload trip points which may be why you see "loads running fine for hours at fla and others tripping as soon as they cross fla." (It depends on the type of device, where it resides in the circuit protection scheme, and where the pickup and trip parameters are set.)

Also; Other reasons for trip time variance are:

During acceptance testing (bench test) of overload relays and breakers the trip time quite often varies significantly from device to device.

It is alarming how many protective devices new, right-out-of-the-box do not meet the trip time window requirement.

It is also common for in-service protective devices to not function correctly after interrupting a high fault current.

These and other operating dynamics are why the OEM of the protective device (usually) only warranty the device to operate one time and recomend testing of the device after a fault trip before returning it to service and/or replacing the device if it fails to meet test parameters.