Circuit Breakers

Seimens bought Gould years ago,.   Breakers that fit our box are getting harder to find since newer homes use newer breaker boxes that use more modern breakers.   Several years ago I noticed the shop lights flickering.   A breaker in one of the breaker positions in our box, had arced over making it difficult to get a good contact.   I put i na new breaker in a different position to handle the circuit.  
I have also added several circuits (mainly in the workshop) and also noted that when we upgraded our house AC, the breakers are now too large.  

NEMA defines instantaneous peak inrush as the momentary current transient that occurs immediately (within half an AC cycle) after contact closure   This inrush current is much higher than the operating current of the same device.   This is most commonly seen in circuits with electric motors.  
The breaker must tollerate the inrush, without a nuisance trip, but if the high current is sustained for too long, the breaker should trip.   This length of time that high current is allowed versus the amount of overcurrent comprises the trip time curve.   Allowing the high current to continue for too long a period of time, can damage the device on the line as well as the wiring itself.    

Breaker Trip Curve determines how long a breaker takes to trip when overloaded.   This is usually shown as a graph:

We replaced our house AC several years ago.   The old AC was small and typically needed very high current (50 Amp 220 Volt breakers), the new AC was high effeciency (only drew 26 Amps) that had a max breaker spec of 40 Amp 240V.   I needed to replace the 50 Amp breakers with 40 Amp breakers on this line.