understanding trip curves

[Hobbyman2]

Just thought I would toss this in in hopes it helps some one understand more about how electricity works at motor start up . https://www.c3controls.com/white-paper/ ... ip-curves/

Why do we need different trip curves?
Circuit breakers must trip quick enough to avoid equipment or wiring failure, but not so fast as to give false, or nuisance trips.

To avoid nuisance trips, circuit breakers need to be sized appropriately to compensate for inrush current. NEMA defines instantaneous peak inrush as the momentary current transient that occurs immediately (within half an AC cycle) after contact closure.

Inrush current is what causes the lights to dim in a house when a motor, such as that on a clothes dryer or vacuum cleaner starts up.

Figure 2 (below) is an example of the inrush current for an AC motor.

As the graph shows, the inrush current caused by switching the motor on is 30A. It is much higher than the operating, or steady state current. The inrush current peaks, then begins to decay as the motor spins up.

We need different trip curves in order to balance the right amount of overcurrent protection against optimal machine operation. Choosing a circuit breaker with a trip curve that trips too soon can result in nuisance tripping. Choosing a circuit breaker that trips too late can result in catastrophic damage to machine and cables.

[Hobbyman2]

I would add there are other factors that could trip a breaker , wire size , length of wire ,, current draw , corrosion on wires / terminals , grounding etc.