Instantaneous setting on Motor Circuit Protector Breaker

[EddyWirbelstrom]

How is the instantaneous element of a current limiting circuit-breaker such as a motor circuit protector set ?
eg. If a motor with a LRA of 6 x FLA and a starting power-factor of 0.2 has an initial symmetrical rms inrush current of 2 x LRA = 12 x FLA, then half a cycle after motor energization the motor asymmetrical rms current could be ( worst-case assuming motor start at voltage zero crossing ) 15 x FLA and the peak (crest) current would be 26 x FLA.
Is the current limiting circuit-breaker instantaneous element calibrated ?
Is the required instantaneous setting :
15 x FLA / 0.8 for 20% minus tolerance of Instantaneous element
or
26 x FLA / 0.8 for 20% minus tolerance of Instantaneous element
See attached spreadsheet for calculations.

 

[electricpete]

From the end of your spreadsheet: "Is it [instantaneous element]calibrated in rms or peak ( crest ) current ?"
That is an easy one to answer. When we say the instantaneous is set at some number like 1000 amps, that refers to rms value of a sinusoidal test current. That's how you test it.... with a sinusoidal current.

That does not necessarily mean that the breaker responds to the rms value of a non-sinusoidal waveform. A conservative assumption (with respect to preventing trip) would be to set it as if it responds to the peak.

But conservative with respect to preventing trip does not always equal compliance with NEC. NEC provides some fairly tight limits, in the neighborhood of 17*FLA max for energy efficient motors when lower settings prove unsuccessful, with allowance to round to the next higher setting (alll of above from memory.... could be a little off).

One thing to note is that analytical calculation is a tough thing to do. You have brought up some factors. Another is response of the trip element to anything other than sinusoidal current as mentioned above (not defined by standards). Yet another is possibility for non-simultaneous contact closure which will boost your max peak instaneous far above 2*sqrt(2) times locked rotor current.

So in the end imo, it is an empirical thing. The theory of the NEC is supposed to be empirical values that allow you to set it high enough so it doesn't trip. In my experience, NEC is sometimes not quite high enough even at the top of the range.


Search the forum for "instantaneous" in the subject line and you should find a ton of discussion.




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(2B)+(2B)' ?