Overload Curve

[timm33333]

When coordinating a typical motor circuit (LV breaker, contactor, and overload relay) with upstream device, should we plot the TCC of LV breaker only? Or should the TCC of overload relay be drawn as well? If we draw the TCC for overload relay, then should it overlap the thermal part of the LV breaker?

 

[ScottyUK]

Generally the breaker used in a combination starter will be a mag-only type which provides short-circuit protection. You need to plot the breaker and the OLR curves because they will intersect somewhere around 8x - 15x motor current depending on how the breaker is set.

 

[timm33333]

Thanks. The magnetic only circuit breakers are usually IEC style breaker. In American style (thermomagnetic) breakers, the breaker also has an overload element which becomes redundant when a starter with separate overload relay is added. In this case, should two overload curves be plotted (one for overload element of breaker and the other for overload element of the starter) ?

 

[unclebob]

I would plot both curves. If possible, the overload relay could be adjusted to coordinate with the thermal curve of the breaker by lowering its time delay and/or current setting. But in the end, both devices keep the faulty circuit at the starter.

Bob

 

[dpc]

The magnetic only circuit breakers are usually IEC style breaker.

This is not true. Instantaneous trip only molded case circuit breaker are commonly used in the US for combination motor starters and have been for at least fifty years. The term "motor circuit protecter" or MCP refers to a molded-case circuit breaker with no thermal element. Thermal-magnetic breakers can be used also.

 

[jraef]

If you use a Thermal Magnetic breaker in a motor starter circuit, you would run your TCC coordination with the thermal curve of the breaker, no need to run the OL relay because by the very nature of a starter circuit, the overload relay trip will ALWAYS be lower than the TM breaker thermal trip curve.

If you use a factory built starter with a Mag. Only breaker, then you WOULD include the OL relay thermal curve, because THAT then becomes the long-time over current protection for the circuit.

"Will work for (the memory of) salami"