Page 390 - Industrial Power Engineering and Applications Handbook

Switchgear and controlgear assemblies 13/365
before the actual tripping. When the tripping is electrical I,, = system fault level
it may involve a trip coil and a motor mechanism adding t,, = pre-arcing time of the HRC fuses or tripping time
10 the mechanical tripping time, actuating time of relays, of MCCBs or MCBs, which would be much less
minimum time of tripping of interrupter itself (Table than S ms (less than one quarter of a cycle for a50
19. I and some safety margins. While the actual tripping Hz system; see Figure 12.18 for more clarity)
time may still be quite low, it is customary to design a
system for one or three seconds and for which is designed then the let-out energy of the current limiter,
all the equipment. devices and components, protected I,:
by such a device. ' t,,
The value of short-time rating . t,') of the system If I, is the equivalent I -second fault current
may now exceed, the thermal rating of some of the
equipment, devices and components, i.e. I then I,' . 1 = 13 . t,,
thermal rating. This condition may be more I
smaller ratings, particularly 600 A and less (such as for or I, = I,, vFi
busbars), than larger onec. At higher currents, the natural
thermal rating itself, due to the higher cross-sectional Since tSc will be too low (< 5 ms for a 50 Hz system)
area. will become higher than the required short-time I, will be much less than 7% of I,, in all situations. To
rating. The short-time rating will remain the same for a assign a short-time rating to the protected devices and
particular fault level of a system, irrespective of the current components in such cases is therefore of little relevance.
rating of the circuit. For more details refer to Section As noted above, current is the cause of heat, for which is
28.4.1. Whenever the short-time rating exceeds the thermal assigned the thermal duty of a current-carrying device.
rating. a larger area of cross-section of the main busbar component or part. Also note the following:
system and the other current-carrying components will
become necessary. Current limiting devices need not be protected. since
Figure 13.29 illustrates a simple distribution system they are already very fast acting and, hence, self-
and location of the main buses, devices and components protected.
to define the current ratings of all such devices and But they are also rated for the same fault level for
components under different operating conditions. The which the system is designed as they are connected
ideal current ratings of these components are given in directly to the system. This is a safety requirement.
Table 13.13 for an easy illustration. Similarly, in a draw-out switchgear assembly. the I/C
The fault currents also develop electrodynamic forces, and O/G power contacts of a module and its mounts
F,,,. as in equation (28.4) due to the sub-transient d.c. (insulators and supports) being already protected may
component. These forces play an important role in the be suitable only for the thermal rating of their feeders.
mechanical design of the interrupting device, the load- When such devices are used at more than one location
bearing and mounting structures for the interrupter and in a circuit, their ratings must be meticulously
the bus system, and the hardware used in a switchgear coordinated to ensure isolation of the faulty circuit
assembly. All such mechanical parts, supports and alone. Refer to Section 12.4.2 and Figure 12.1 1 for
hardware should be adequate to withstand such forces more clarity.
when they arise. A procedure to arrive at the ideal size of A current-carrying device or component in a
the current-carrying components, mounting structure, type distribution network may be subjected to varying
of supports and hardware etc. is discussed in detail in degree of electrodynamic stresses, depending upon
Example 28.12. its location with reference to the network. Referring
If the short-time rating of the interrupting device is to Figure 13.29, the circuits away from the source of
higher than the fault level of the system, which is the supply are subjected to lower stresses than the circuits
case with modern interrupting devices, the fault level of nearer to it. Accordingly, the coordination is done
the system alone will prevail for the busbars, components between the protective devices, used in the upper and
and hardware. For example. for a system fault level of lower streams of circuits, to ensure that only the faulty
50 kA, if the interrupter used is of 65 kA short-time circuit is isolated, rather than isolating other circuits
rating, the bus system and all associated components in the upper stream (Section 12.4.1).
will be designed for SO kA only.

Current limiting type Notes on Table 13.13
An isolator, such as at locations C, and C2 in Figure
Examples are HRC fuses (both LT and HT) and MCCBs 13.29, may sometimes be used to isolate the circuits
and MCBs (LT only). which are available with current it is feeding, say, for maintenance or repair\. This
limiting features and are in extensive use. The tripping isolator is simply a switch and provides no protection
time of these devices is extremely low and much less to the circuits. For a fault on the outgoing side, the
than one half of a cycle of the current wave. They therefore individual outgoing feeders must have their own
do not allow the fault current to rise to its prospective protection. For a severe fault elsewhere in the system,
peak. The protected devices and components can thus be there must be a protective feeder closeby, in the upper
selected based on the let-out energy of such devices on stream.
fault. which is extremely low, than the fault level of the Role of an OCR: this is only an overcurrent protection
systcm. If device and does not provide short-circuit protection.