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How to select a breaker?

Before breaker's selecting for your electrical system, you need to calculate value of expected short circuit current at the place of breaker's installation. Then you need to calculate value of heat pulse and 1s current (expected value of current during one second). After that you need to calculate power of breaker and finally, after all, you can select appropriate breaker. Values of characteristics of selected breaker need to be higher from calculated values of characteristics of your power system.

You can calculate operational current of breaker using this expression:

After that, you need to calculate expected value of surge current:

After that, you need to calculate expected value of heat impulse:

And finally, you need to calculate 1s current (expected value of current during 1s):

So, current of interruption of your breaker and power of interruption of your breaker are:

Additional expressions that you can use during your calculation:

where are:
ti-time of interruption
Inp-operational current of breaker
SnT-rated power of transformer
Un-rated voltage
kud-surge coefficient
Tae-time constant of aperiodic component of short circuit current
Iud-surge current
A-heat impulse
I0"-short circuit current in subtransient period (generators are in no-load conditions)
I'-short circuit current in transient period
Td"-time constant of subtransient component of short circuit current
I1s-current during one second
Ii=expected value of current of interruption of your breaker
Si=expected value of power of interruption of your breaker
Ze"-equivalent impedance of power system in the place of fault (subtransient period)
I"-short circuit current in subtransient period (generators are in full-load conditions)
I'-short circuit current in transient period
Ze'-equivalent impedanse of power system in the place of fault (transient period)

For a branch circuit feeding a single pump, you would generally size the circuit at 125% of the pump's full-load amperage. If you're not using a variable frequency drive or soft starter (which have built-in overload protection), you would use a Motor-circuit protector (MCP) breaker that has both thermal and magnetic trip capability. Sizing would be according the breaker manufacturer's recommendations for a motor of a given horsepower, but not larger than would be required to protect the circuit conductors.

"The total load of an area" is much too ambiguous to answer. If you have lighting and receptacles, you're going to need a different type of breaker than if you have motors or mixed types of load. There is no general approach. Circuit breaker types are very specific to the application.

Safety should not be taken lightly. Installing the wrong type of breaker could result in equipment damage and/or physical harm.

There are instantaneous breakers as well as time delay breakers. For time delay breaker, for example, you go 250% maximum of the rated current based upon the HP of a motor (look in the NEC), not on the nameplate label. The nameplate current value is for overload protection. Also try to size the breaker so that the conductors are protected.

As we know all there are many types of " Discrimination or Selectivity ", where the most using are:
- Based Current Selectivity
- Based Time Selectivity
and the choosing of suitable type is depending on many parameters:
- Kind and setting of Circuit Breakers
- Kind of Loads
- Kind of Selectivity that should be realized " Thermal, Magnetic, or both"

Noting that :
- We can use both types " by Current & by Time " in the same network
- The Selectivity study is not easy, because some upstream & downstream cables' sections should be changed, that means the calculation of short circuit currents should be also re-done and the settings of circuit breakers especially " Magnetic or Short-time " will be changed
- Realizing of " Based Energy Selectivity " is not easy, because we should have :
- " Non-tripping Curve Energy " for Upstream circuit breaker
- " Limitation Curve Energy " for Downstream circuit breaker
- These curves are only done by the manufactures of circuit breakers
- If there are in the network many circuit breakers equipped with " Earth Fault Protection ", a Selectivity study of " Earth Faults " should absolutely be done to define the kind, setting, and timing delay of each of those protections.

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