Circuit Breaker Calculations

I also want to know somebody help us.

One question.... How long is 'momentary' ?

Circuit breakers have several rated currents as defined in IEC. There are 1-sec and 3-sec ratings which will help a system designer in properly co-ordinating the system to ensure no unnecessary power outage in the event of a fault. Please study 'discrimination and cascading' in this connection. Links to a couple of good papers from Schneider Electric...

ECT167

ECT 201

The 1.6 figure is unfamiliar to me. The IEC specifies some multiples of RMS .. 2.1, 2.2 etc to be the asymmetrical peak of the short-circuit current for testing. Is this what you mean? By the way, all this is for LV which i know..your reference to 3 sec implies MV or HV..if so, lets hope some experts will clarify more. Nevertheless, the principles of discrimination, cascading etc will remain valid for MV/HV also i think.

Hi

Short circuit current has three states 1) Sub transient, 2)transient and 3) steady state

when fault occurs, the initial impedance to the fault current is always very less and by the time it increases and becomes constant in steady state region of fault current

Breaking capacity of switching devices (1 sec or 3 sec) implies its fault current withstanding capacity (Ekectrical as well as mechanical fources), During breaking capacity test in the labs, fault current also has to be of all three states and breaking of current should occurs in steady state region after 3 secconds of fault current start flowing from the devices.

regarad

Divyesh patel

a) Any short circuit current imposes two types of stresses on the electrical equipment & conductors. One stress is the thermal stress proportional to the product of the Root Mean Square Value of the Short Circuit Current and the duration of the short circuit (I2t). Another kind of stress is the elctro-dynamic stress (sort of explosive stress) which is proportional to the square of the peak current (i2). In a pure sinusoidal wave, the peak value of current will be Root 2 times the RMS Current. But, since the initial shortcircuit wave form is asymmetrical, due to the decaying DC Component of current, this factor of asymetry has to be taken into consideration, while determining the peak current and hence the electro-dynamic forces. This decaying DC component magnitude and duration is dependant upon the ratio of System Reactance to System Resistance (X/R Ratio). The total peak current will be the product of this Asymmetry Factor, Root 2 and the RMS Current. As a guide, standards give a multiplying factor of Root2 & asymmetry factor, depending on the magnitude of the RMS Short Circuit Current and the Network Power Factor. This value could be anywhere between 1.5 and 2.2 as per IS 13947-part 2. In your application you could have seen this multiplying factor as 1.6.

b) A correction. Braking current is just a magnitude only; it does not have a duration. What you must be meaning is the "Short Time Withstand Current (Icw)", which does have a current rating permitted to flow for a given duration (e.g.) 50kA for 1 sec. In time discriminatory protection systems, it may so happen that the uppermost stream breaker might clear the fault, if the lower stream breakers fail to clear the fault. In such a case, all the breakers in series have to carry the full short circuit current till the fault is cleared by the uppermoststream breaker. This capacity to tolerate carrying short circuit current for agiven duration is the Short Time Withstand Current. Obviously, the more is the Short Time Withstand Current Duration, the more will be the discrimination possibilities. So, 3 sec rating is better than 1 sec rating.

Thanks to many for their valued comments.

sks

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