Calculation of the masterbuilt 28 qt electric turkey fryer manual precise rms value of an asymmetrical current at any time after the inception of a short-circuit may be very involved.
Contact-parting (interrupting) network.Related article: terms TO know, asymmetrical short-circuit current: A fault current whose waveform is asymmetrical to the zero axis.This precise method may be used, if desired; however, simplified methods have evolved whereby the DC component is accounted for by simple multiplying factors.In Part 2 (December 1995 issue we'll use a simple example that shows the necessary steps needed to determine the appropriate reactances and resistances of the power system apparatus for the first-cycle (momentary) and contact-parting (interrupting) networks.Energise HV side by LV supply.With the ever-increasing kva capacity of power systems, the possible short circuit currents are becoming so high that it is frequently necessary to consider the effect of these short circuits on the heating of the cables.First-cycle (momentary) network.It is usually expressed in ohms and consists of inductive and capacitive parts.Types of networks to calculate symmetrical rms current.Accurate decrement factors to account for the DC component at any time are required, as well as factors for the rate of change of the apparent reactance of the generators.The conductor size must be large enough to carry the short circuit current for a sufficient length of time to permit the circuit breakers to open before the conductor is heated to the point where it damages the insulation.These networks only differ from one another by the assignments of constant reactances for the machines.Short LV phases and connect these short circuited terminals to neutral.The sum of these two components at any time instant is equal to the total asymmetrical short-circuit current at that instant.
This network is used to calculate the contact-parting (interrupting) symmetrical rms current for circuit breaker minimum contact-parting times.5 to 4 cycles after the inception of the short-circuit fault.
Sub-transient reactance: The value of reactance that determines the amount of short-circuit current during the first half cycle after the fault occurs.Now with the help of variac applied voltage is slowly increased until the ammeter gives reading equal to the rated current of the HV side.The magnitude of fault current from these sources is limited by the impedance of the machine itself as well as the impedance between the machine and the fault itself.Performing short-circuit calculations requires an understanding of various system components and their interaction.The other types of unbalanced short-circuit faults are important in selecting the time-current characteristics and settings of phase-overcurrent and ground-fault protective devices to provide selective coordination.Here, the rotating machine sources of short-circuit current are represented by different constant reactances than the first-cycle (momentary) network, according to the entries in the second column of Tables 4-1 and 4-2 of the 1993 edition of the ieee Red Book (or Tables 24 and.1, we see a 3-phase synchronous generator, previously unloaded, that has been subjected to a balanced, 3-phase fault across its accessible terminals.This network is used to calculate the first-cycle (momentary) symmetrical rms current.
The diagram also shows that this waveform is a combination of two components: a unidirectional (DC) component (upper left waveform) and a symmetrical (AC) component (upper right waveform).
The chart at right shows the maximum currents to which various size copper conductors can be subjected for various times without injuring the insulation.