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- #2

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You need to know only the rated (nominal) current of the breaker.

- #3

Staff Emeritus

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Isn't there a spec sheet for that specific model?

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Thanks

I have MCCB data as follows:

MCCB Rating = 40 Amps (i.e. Maximum Continuous Ampere Rating at 40 degree C)

Thermal Range = 32 - 40 Amps

Magnetic Range = 200 - 400 Amps (i.e. Instantaneous Pickup Range)

Horizontal axis = Multiple of current setting (i.e. Multiples of 40 Amps - Breaking Rating).

Based on this info, what will be the maximum instantaneous fault clearing time (secs)?

I came up with:

1) 5.5 secs corresponds to 10X (400 Amps)

2) 4.6 secs corresponds to 9X (360 Amps)

which one of the above two values in (secs) are close to being correct or else?

Note: Not to confuse you regarding 9X (360 Amps) value I choose in the graph.

I selected 9X because I selected this MCCB to protect a Motor whose Lock Rotor Current = 143 Amps, the Service Factor of this Motor is 1.0, therefore I used 360 Amps magnetic setting > (2.5 X LRC) = 357. 5 Amps.

Regards

- #6

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As for the instantaneous(see note below) - you need to make sure that the LRA hits that part of the curve, but noting some motors ( particularly high efficiency) can hit LRA on hard line starts.

The motor OEM will often provide the same format time current curve to help with this coordination.

Often the Locked Rotor coordination really is referred to a Short Time (not instantaneous) - in general terms Long Time is for Motor overload, Short time ( locked / seized motor) has a small fixed amount of time to allow for starts, and true Instantaneous is for electrical short circuits / faults ( so there is no intentional time delay on the trip).

- #7

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- #8

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If the LRA is 143A = 3.6X ( of the 40A) --- as I read this you are up to 40 Secconds - a small motor like this probably can not survive 40 Sec locked rotor. Another way to think of this for a 480V 3PH motor...( 143 x 480 x Srt(3) )= 119 KW of heat ! being dumped into the windings of the motor!...

I would only want LRA to exist for 1-2 Seconds at most in a typical application.

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