Current short circuit

To avoid damage to equipment and harm to personnel, electrical components of the facility power system must be selected to withstand available short circuit currents and to isolate facility circuits quickly. 

All electrical protective equipment (e.g., circuit breakers, fuses, bus bars, and motor starters) is rated for maximum short circuit currents by NEMA standards. Proper selection of equipment must be based on available short circuit currents. 

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Line Starters and Combination Starters A line starter consists of a contactor (motor-starting switch) and motor-overload relays, (Contactors are capable of canving and interrupting normal motor-starting and -running currents they are not, hovv ever, normally capable of interrupting short-circuit currents. They must be backed up by fuses or a circuit breaker for this function,... 

These reactances are measured by creating a fault, similar to the method discussed in Section 14.3.6. The only difference now is that the fault is created in any of the phases at an instant, when the applied voltage in that phase is at its peak, i.e. at Vni- so that the d.c. component of the short-circuit current is zero and the waveform is symmetrical about its axis, as shown in Figure 13.19,... 

In certain cases, where a long delay may be necessary for the protective scheme to operate, it may be desirable to use the maximum steady-state short-circuit current V2 /j, for a more appropriate setting, rather than the maximum transient current i2 /, as by then the fault current will also fall to a near steady-state value, /sKr.m s.i-... 

Table 13.11 Multiplying factors to obtain the momentary peak (maximum r.m.s. or dynamic) values of the short-circuit currents including the sub-transient d.c. component at different power factors (FI/Xl)...

The electrical rating on a fault or short-term high current thermal efllecls, expressed by / c sc. ke., short-circuit current (I q) and its duration Use - I or 3 seconds). 

This test is conducted to verify the suitability of the equipment to withstand a prospective short-circuit current that may develop on a fault. It may also be termed the steady slate symmetrical fault current or the short-time (withstand current) rating of the equipment. When the equipment is an interrupting device, it is referred to as its symmetrical breaking current. 

It is permissible to lest just one panel of a multi panel-assembly so long as the construction of other panels is similar and busbar arrangement and supports are the same. The value of the prospective short-circuit current may be determined from a calibrated oscillogram. The test current in any phase should not vary by more than 10% of the average in the three phases and must be applied for a predetermined time of I or. 1 seconds. Unless specified otherwise, this should be considered as to be I second. 

From the oscillogram, shown in Figure 14.4 one can easily determine the average r.m.s. value of the short-circuit current, its duration and the momentary peak current. For easy evaluation, this oscillogram has been divided into ten equal parts (I to lO) and is redrawn in Figure 14.5 for more clarity. The short-circuit commences at point D and concludes at point A2, A A2 being the original zero axis. At the instant of short-circuit, the zero axis shifts to B A2- B) B is the initial d.c. component that decays to zero at A2 at the conclusion of the test. 

Inductive interference the production of electrical potentials in conductors due to the induction from alternating magnetic fields arising from short-circuit currents or operational currents in high-voltage power lines. 

The field strength, E, induced by grounding short-circuit currents in high-voltage overhead power lines or railway power lines, is basic in calculating the pipeline potentials (see Section 23.3.2). The field strength, E, follows from Refs. 2 and 13 ... 

The electrical distribution system design and equipment selection must consider requirements of the utility company for protection and metering. Available short circuit currents from the utility distribution network to the primary of the facility s main transfoiTner must be considered in selecting circuit protection devices for the facility distribution system. 

Kurzschluss, m. (Elec.) short circuit, -ofen, m. short-circuit furnace. -Strom, m. short-circuit current. 

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