Short circuit current Instantaneous

In AC grids, the initial symmetrical short-circuit current, IK", defined as the rms value of the symmetrical AC component at the occurrence of the short-circuit, the peak short-circuit current, hr the maximum instantaneous value of the short-circuit current, and the steady state short-circuit current, Ik, defined as the rms value of the short-circuit current after fading of all transients, shall be differentiated in general. For far-from-generator short circuits, Ik" = ... 

Rated instantaneous short-circuit current Rated control voltage 1 AC 50-60 Hz Dimensions Weight... 

The time constant Ta is important because it influences the lower envelope of the short-circuit current wave form to such an extent that the current can fail to cross the time axis until several cycles have been completed. This is demonstrated in 7.2.10 and Figure 7.1 shows the result. The behaviour of the instantaneous current imparts a heavy duty on the stator circuit breakers. Should this be anticipated in practice, from preliminary design studies, then the equipment involved should be specified accordingly. 

Short-circuit protection. To provide an instantaneous tripping on a short-circuit delay of, say, one or two cycles may be introduced into the tripping circuit to bypass any transient currents and avoid an unwanted trip. 

To isolate the circuit after the lest is over and to also interrupt the test in between, in case the test piece fails. The breaker must possess an instantaneous capacity of more than the test current and the short-circuit MVA of the feeding generator. To achieve the desired voltage it must be suitable to perform the duties of repeated short-circuit tests. 

A CT is provided in series with the spark gap to sense its operation during a line fault. As soon as there is arcing, it provides an instantaneous command to a short-circuit relay. The relay, in turn, closes the bypass breaker, within 3 to 5 cycles, leaving only the natural line impedance in the faulty eircuit. Now = 0, which limits the fault current to the natural level of the system, as if the capacitors were not connected. The shorting device is restored to its original status as soon as the fault condition is cleared. The device must be capable of interrupting the line fault... 

Short circuits that do not involve earth, and which are within the length of the cable, can be detected by setting the instantaneous elements of the overcnrrent relays to a value of current calculated at the receiving end of the cable that flows into a zero-impedance fault. Customarily this fault is a three-phase fault for which the calculations are straightforward. If the fault is beyond the cable for example in a consumer then the fault current will be less and shonld be cleared by the consnmer protective device. The feeder cable relays will then act as a back np to the consumer relays. 

To overcome the disadvantages of these previously described systems, the HELM tracker system allows continuous measurement of the current flow in every cathode in an electrolytic tankhouse. The system allows for instantaneous detection of the magnitude and specific location of electrode current distribution anomalies such as short circuits and poor contacts, which is displayed to the operators through the systems HMI. The system requires no electrical or physical contact with the cathodes, and can be easily retrofitted into an existing plant, without interfering with the operation. 

In a short-circuit condition, magnetic action, accomplished by an electromagnet in each breaker pole, causes the breaker to trip instantaneously. In a magnetic-only breaker, the thermal sensing bimetal is left ont. Sohd-state breakers utilize current transformers and soUd-state circuitry in place of the thermal and magnetic trip units. 

Generally in a nail penetration test, an instantaneous internal short would result the moment the nail is tucked into the battery. Enormous heat is produced from current flow (double layer discharge and electrochemical reactions) in the circuit by the metal nail and electrodes. Contact area varies according to depth of penetration. The shallower the depth, the smaller the contact area and therefore the greater the local current density and heat pro-... 

A schematic of the apparatus is shown in Figure 2. The series resistor senses current. When the overall circuit RC (R and C are circuit resistance and capacitance, respectively) time constant is short compared with the voltage developed on the sensing resistor is proportional to the instantaneous... 

The recloser is another overcurrent device that automatically trips and recloses a preset number of times to clear or isolate faults. The concept of reclosing is derived from the fact that most faults are of the temporary type and can be cleared by de-energizing the circuit for a short period. Reclosers can be set for a number of operation sequences depending on the action desired. These typically include instantaneous trip and reclose operation followed by a sequence of time-delayed trip operations prior to lockout of the recloser. The minimum pick-up for most reclosers is typically set to trip instantaneously at two times the current rating. 

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