Maximum and Minimum Short Circuit Current

I'm not sure that the term "minimum short circuit current" is meaningful; it could be arbitrarily close to zero. Your circuit breaker or fuses need to be able to interrupt the maximum possible short circuit current, and they also need to interrupt smaller overcurrents according to the breaker/fuse time/current curve, which needs to be less than the motor damage curve.

There is no such thing as Maximum and Minimum Short Circuit Current, similarly time-current curve setting for circuit breakers corresponds to low voltage CB's only where as higher voltage CB's are dependent on separately incorporated protection relays.

The CB's, fuses etc. are rated to protect the system from short circuits effectively.

As an example, a protection relay in a medium voltage motor feeder will have three current settings designated by symbols "I", "I>" and "I>>"

You have to enter value corresponding to locked rotor current in chapter "I>" while "I>>" is set as a back up protection element. The element "I" is set in accordance with rated motor current but please note that during starting when the motor draws 6-8 times the rated current this element is blocked by relay logic for a selective time duration to avoid tripping during starting inrush.

Similarly current grading of section CB's may be done to accomplish selective tripping of part of a long feeder.

You are correct.Minimum fault current calculations shall affirm that what extend the medium voltage circuit can be extended within the range of protection system estabilised

Short ckt current is of two type, breaking & making current. When a breaker is on in normal condition and an abnormality arise then the breaker break the ckt in a faulty condition witha fault current known as breaking current.

When already a faulty condition exists and breaker get put on then it has to withstand the fault current known as making current and then break the ckt. Making current= 2.55 x breaking current.

U may call these two currents as min and max SC Current.

I am afraid that you are grossly incorrect and are mis-guiding the Op and other readers of this forum.

What you are talking is the Making Current and Breaking Current and they, definitely are NOT the maximum and minim short circuit currents.

Reference is drawn to Clause 2.0 of IS 13234 (IEC 60909) wherein it is stated that:

"There are two different short-circuit currents to be calculated which differ in their magnitude:

- the maximum short-circuit current which determines the capacity or rating of electrical equipment;

- the minimum short-circuit current which can be a basis, for example, for the selection of

fuses and for the setting of protective devices and for checking the run-up of motors."

Also, in Clause 5.0, it is stated that "the calculation of maximum and minimum short-circuit currents is based on the following simplifications:

1) For the duration of the short circuit there is no change in the number of circuits involved,

that is, a three-phase short circuit remains three phase and a line-to-earth short circuit

remains line-to-earth during the time of short circuit.

2) Tap changers of the transformers are assumed to be in main position.

3) Arc resistances are not taken into account.

While these assumptions are not strictly true for the power systems considered, the

recommended short-circuit calculations have acceptable accuracy."

Also, the voltage factors to be applied for both are different.

Even while you calculate the maximum and minimum short circuit currents, in both cases, you must calculate the RMS Current and the Peak Current.

I wish you read the Standards thoruoghly and understand yourself first, before choosing to answer such critical questions.

The short circuit calc's are needed to design a proper "cascade" of breaker action. Pf there is a short in a desk lamp cord you want the action to happen in the local distribution panel rather than the main beaker of a 20 story office building. It is also needed to determine the level of safety equipment to be used if the system ever needs to be worked on while energized (hopefully never). An arc flash WILL ruin your day and, perhaps, your life!

Okay, I'll be referring to IEC 60909 as a basis, but the ANSI method is equally applicable. The calculation of short circuit currents is essentially an estimate based on the set of power system parameters that you've defined. You also have to remember that the typical short circuit current is modeled as a decaying sinusoidal waveform (also with an exponentially decaying dc component). Therefore, the fault current can be characterised by several current values at various times during the short circuit.

IEC 60909 outlines a method for calculating the following short circuit currents:

1) Initial short circuit current - the highest short circuit current at the point of fault, i.e. time t = 0

2) Peak short circuit current - the highest short circuit current, which usually occurs somewhere in the first half-cycle of a fault. This value is used to size the short circuit withstand capability of equipment, e.g. busbar fault rating, circuit breaker max fault rating, etc

3) Symmetrical breaking current - is the current that occurs at the time the circuit breaker will trip (you choose this time based on your installation, usually t = 0.02s, 0.05s, 0.1s, etc). Obviously this value is important to specify the circuit breaker's interrupting capability.

4) Max and Min steady state short circuit - after some time, the short circuit current decays to some steady state level. The maximum and minimum values are calculated here. I'm not entirely sure what the point of the max steady state current is, but the minimum current tells you where to set your fault pickup level. After all, you want the protection to trip when there's a fault. This is perhaps more important in situations where the minimum fault level can be quite low, e.g. faults at the end of long lines.

The term "minimum fault current" is a poor term to use to describe the intended parameter. A better term to use would be "far end fault current" which would be the expected potential fault current for a fault at the far end of the distribution. The "far end fault current" circuit would include the line impendance to that point. So, the "far end" fault current would be less than an expected fault current near the power source.

The far end fault current is not a minimum fault current because theoretically, the far end fault current would be lessened by any kind of additional impedance (squirrel) in the circuit at the time. Fault currents could be considered to be those currents in excess of the sum of motor starting currents and other connected loads. I therefore prefer to use a term other than "minimum fault current".