Interrupting devices

Rated short-time current rating or fault level of a system (breaking current for an interrupting device) Duration of fault... 

Rated momentary peak value of the fault current (making current for an interrupting device)... 

To increase the impedance of the network, a series resistor or reactor is sometimes used to contain the fault level of a system within a desirable limit. This may be required to make the selection of the interrupting device easy, and from the available range, without an extra cost for a new design as well as an economical selection of the interconnecting conductors and cables. Such a situation may arise on HV >66 kV or EFIV > 132. kV transmission networks, when they are being fed by two or more power sources, which may raise the fault level of the system to an unacceptable level. The cost of the interrupting device for such a fault level may become disproportionately high, and sometimes even pose a problem in availability. 

We have mentioned two systems, I second and 3 second. A choice of any of them would depend upon the location and the application of the equipment and criticality of the installation. Generally speaking, it is only the one-second system that is in practice. The three-second system may sometimes be used for low fault level networks, where V3 / c, would fall within the capability of the available interrupting devices and at reasonable cost. 

It is observed that there may be asymmetry in the system as long as the short-circuit condition lasts, as illustrated in Figure 13.20, i.e. up to the opening of the interrupting device. (For opening times of interrupters, refer to Table 19.1.) But the content of the asymmetry may be quite feeble after three or four cycles. Flowever. if the short-circuit condition still prevails, such as when... 

The fault ctirrents also develop electrodynamic foi ces, Fii, as in equation (28.4) due to the sub-transient d.c. component. These forces play an important role in the meehanical design of the interrupting device, the load-bearing and mounting structuies for the interrupter and the bus system, and the hardware used in a switchgear assembly. All such mechanical parts, supports and hardw-are should be adequate to withstand such forces when they arise, A procedure to arrive at the ideal size of the current-carrying components, mounting structure, type of supports and hardware cte, is discussed in detail in Example 28.12. 

If the short-time lating of the interrupting device is higher than the fatilt level of the system, which is the case with modern interrupting devices, the fault level of the system alone will prevail for the busbars, components and hardwaie. For example, for a system fault level of. SO kA. if the interrupter used is of 65 kA short-time rating, the bus system and all associated components w ill be designed for 50 kA onlv. 

Thus, only three of the five interrupting devices can be switched at a time as required without causing a parallel operation between any of the two incomers. 

For a larger number of supply sources, each having two interrupting devices, one as incomer and the other... 

For impulse withstand voltages across the contact gaps of an interrupting device, refer to lEC 60694 and lEC 60298, Wet or dew test is a weatherproof test (Section I4..5.I0) and is meant for outdoor assemblies. 

Interrupting devices - switches, breakers, MCCBs, power contactors and fuses... 

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. 

The oscillogram also reveals the following vital information for an interrupting device, if used in the circuit, to make or break on fault ... 

This is the steady-state symmetrical fault current, which the faulty circuit may almost achieve in about three or four cycles from commencement of the short-circuit condition at point Oi (Figure 14.5) and which the interrupting device should be able to break successfully. 

These may be used to feed the solenoid or the motor of an interrupting device (such as an electrically operated breaker), indicating lights and circuits, auxiliary contactors or relays, electrical or electronic timers, hooters or buzzers, and all such auxiliary components and devices mounted on a controlgear or a switchgear assembly requiring a specified control voltage. 

This is the time required by the quenching medium of an interrupting device to regain its original dielectric strength after the final current zero. 

In all the above cases, whether the load is inductive or capacitive, the voltage and the current phasors are more than 70° apart. At a current zero, the system voltage will reappear almost in full (around 95% or so) across the pasting contacts of the interrupting device (Figure 17.11(c)). This voltage is detrimental to the successful interruption of the circuit at the current zero, unlike in the previous case. 

In the above one-half of a cycle of the natural frequency, the moving contact would separate by 1/100 s. In fast-operating interrupting devices, this time is normally adequate to achieve a sufficient contaet gap and to restore adequate dieleetric strength to interrupt the eircuit by the next current zero and allow no further restrikes of the arc. Otherwise the arc may be reestablished and the process may continue until the TRV itself attenuates or the eontact moves farther... 

As a result of a better insulation and insulating medium of an LT interrupting device, compared to its voltage rating, there are no restrikes of the contact gap after a current zero. The TRV is insufficient to establish an arc. 

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