At what amperage is a breaker designed to trip at?

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kwired said:
I conceded to that, but at some point in time the current has to rise or we would never have any current to operate the motor. And when it rises it rises at a rapid rate. This time is apparently so short that without using specialized measuring equipment it seems to be almost instantaneous.

I was told there is no "inrush" current when a motor starts. Exactly how you look at this is all that is being mentioned as well as some terminology being thrown in the mix. Bottom line is even it it isn't at the same instant the circuit gets closed, there is a rapid rise in current at some point and sure looks like it is some point less then a tenth of a second, which is a long time with 60 Hz being the measuring stick.

Bottom line - there is a surge of current that flows at some point during the process of starting that motor that is higher then it's normal running current.

And I am fairly certain that it often is higher (though only for very short time) then what the motor would draw steady state at locked rotor condition.
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The rapidity with which current rises through an inductance is adjustable through its time constant L/R.
 
I will agree that "starting current" may be more of a proper term to use then "inrush current" - but is only so if you have specific definitions of those terms for the application. With general language definitions - both are suitable to some extent.
 
kwired said:
I will agree that "starting current" may be more of a proper term to use then "inrush current" - but is only so if you have specific definitions of those terms for the application. With general language definitions - both are suitable to some extent.
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Say, take six to eight times rated current as starting or locked rotor current and more than that and of much shorter duration inrush current.
 
180530-1208 EDT

Inrush certainly has a muddied definition, especially as used by electricians.

Getting back to the original question of loading a circuit breaker, or other protective device.

Any manufactured product will have nominal specifications for how it works or fits. There will be variations in making the components of the product. Thus, the actual operation of an individual product may operate at some other point than that at which the specifications imply.

So in the breaker curve shown at post #2 there are a range of values, red part of curve, at every time point where within that range it is indeterminate whether a particular breaker will trip, further a particular breaker may have variance in its operating value on repeated trials.

Not shown is how ambient temperature of the breaker changes the curve.

To the left of the red band you should never get tripping and to the right tripping should always occur.

More later.

.
 
180530-2138 EDT

ritelec:

I think that a possible answer to the question you posed in the thread title is:

The design trip value vs duration of load current is roughly at the mid point horizontally of the red band of the curve in post #2. The width of the red band allows for manufacturing tolerances. From a working perspective you need to be considerably below ( to the left ) of the curve to prevent unwanted tripping.

The shape of the curve vs time is probably more a matter of chance with available materials and shapes, than designable.

When you get to the 1 cycle, 16.6 mS at 60 Hz, point other factors come into play.

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gar said:
180530-1208 EDT

Inrush certainly has a muddied definition, especially as used by electricians.

Getting back to the original question of loading a circuit breaker, or other protective device.

Any manufactured product will have nominal specifications for how it works or fits. There will be variations in making the components of the product. Thus, the actual operation of an individual product may operate at some other point than that at which the specifications imply.

So in the breaker curve shown at post #2 there are a range of values, red part of curve, at every time point where within that range it is indeterminate whether a particular breaker will trip, further a particular breaker may have variance in its operating value on repeated trials.

Not shown is how ambient temperature of the breaker changes the curve.

To the left of the red band you should never get tripping and to the right tripping should always occur.

More later.

.
Click to expand...
Yes, whatever you want to call the temporary rise then fall in current when a motor starts - the overcurrent devices need to be able to carry it without interrupting the circuit.
 
180531-1247 EDT

Because concepts and definitions get watered down.

If a dam bursts, then there is a fast and very large flow of water for a short time (same with transformer, incandescent, or capacitor inrush). If there is a major storm and the dam overflows, then a greater than normal flow of water goes down the river, and lasts substantially longer than in the case of the dam burst.

In both cases the flow is great than normal.

.
 
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