Page 389 - Industrial Power Engineering and Applications Handbook

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Note cross-sectional area of the current-carrying components.
One may observe that all the above special service conditions are To determine the cross-sectional area for this short-
generally the same as for the rotating machines discussed in Section time rating and decide the mounting structure, supports
1.6. and hardware etc. for the busbars, a brief procedure is
described in Example 28.12.
Both ratings, thermal and short-time, will define the
13.5 Deciding the ratings of current- cross-sectional area of the current-carrying components.
carrying equipment, devices The more severe of the two will then prevail. In the
and components following text we explain the procedure to assign the
short-time current rating to equipment, a device or a
component.
The rating of current-carrying equipment (switchgear
assemblies, such as for the main bus system), devices
(breakers, switches and contactors) and components 13.5.1 Assigning a short-time rating
(connecting links and wiring etc.) is defined by two The bus system of a switchgear assembly, its interconnec-
parameters: ting links and wires are the protected type components,
whereas an interrupter (breaker, switch or a fuse) may
Thermal rating or continuous current rating This be a protecting or protected type, depending upon their
takes account of heat losses ( = I; . t, t being the time application and location in the circuit. A contactor and
to reach thermal equilibrium). as generated during a an OCR are therefore protected devices in the same
long continuous operation under rated conditions. In context. for they provide no short-time protection. A
breakers, switches and contactors, it will also define protecting device may become protected when it is also
their duty cycle, i.e. the type of duty they may have to provided with a back-up protection.
perform (Section 12.10 and Table 12.5). A breaker, usually an MCCB or an MCB on an LT
To the basic current requirement is applied the system, can be provided with backup HRC fuses to
derating factors for various service conditions, as noted enhance their short-time rating. This may be done when the
in Section 13.4.2. The equipment, devices and available MCCBs or MCBs possess a lower short-time
components may then be chosen to be as close (nearest rating than the fault level of the circuit they are required
higher) to this rating as possible from the available to protect, and make them suitable for the fault level of
standard ratings. Based on these ratings, the minimum the circuit. But this is not a preferred practice and is
cross-sectional areas of the other current-carrying parts seldom used. As a rule of thumb, the device that is
used in the circuit. such as interconnecting links and protecting must be suitable to withstand electrically and
the cables-are calculated. endure mechanically the system fault current for a duration
of one or three seconds, according to the system design.
Short-time rating This will define The duration, however, is no criterion for a current
limiting type protecting device, and a protected equipment,
The electrical rating on a fault or short-term high current device or component can have a short-time rating
thermal effects, expressed by a I& . tsc , i.e., short-circuit commensurate with the tripping characteristics of the
current (Isc) and its duration (tSC- 1 or 3 seconds). protecting interrupter. Accordingly these two types of
The mechanical endurance of the current-carrying parts tripping characteristics are explained below.
of all the equipment, bus system, devices and
components, used in a particular circuit as well as the Delay tripping
load-bearing members and supports on which they are
mounted. The electrical parts of a device (breakers The breakers (OCBs, MOCBs, ABCBs, SF6 and VCBs)
and switches, etc.) are the responsibility of the for HT and ACBs and MCCBs* for LT systems are devices
component manufacturers. The manufacturer of the that have a tripping time of more than a cycle on fault.
switchgear assembly is responsible for the busbar The tripping time can vary from 1 cycle (20 ms) to 5-6
systems, metallic links and wires. cycles (100-1 20 ms for 50 Hz systems), depending upon
The mechanical requirements of the load-bearing the type of interrupter as discussed in Chapter 19 and
members, supports, busbars and metallic links in the noted in Table 19. I.
incoming circuit will depend upon the electrodynamic This time allows the fault current to reach its peak and
forces arising out of the first major peak of the fault therefore all the equipment, devices and components
current, as discussed in Section 28.4.2. This is not, protected by such a device must be suitable for the full
however, applicable to devices that are current limiting fault level of the system. While the tripping time is usually
as well as the circuits and its components that are in milliseconds the duration of fault, tSC. is considered
protected by it. These devices isolate the faulty circuit as one or three seconds. The longer duration than necessary
long before the fault current reaches its first highest is to account for the various time lapses that may occur
peak. Yet the short-time rating for the incoming circuit
(as the outgoing circuits may not experience these
faults), should be selected from the standard short- *MCCBs
(i) When they are normal duty as considered here.
time ratings only, as defined in Tables 13.7 and 13.10. (ii) They are also available in the current limiting type. When
For this short-time rating is then calculated the minimum current limiting. they will not fall into this category.