Page 338 - Industrial Power Engineering and Applications Handbook

Protection of electric motors 12/317
Table 12.5 Duty cycles for a contactor or a switch

Serial no. AC duty Application
For contactors For switches
1 AC 1 AC21 Nearly resistive switching such as heaters, resistance furnaces and lighting loads etc.
2 AC2 AC22 High-resistance and low-inductance switching such as a slip-ring motor switching
3 AC3 AC23 High-inductive switching such as the switching of squirrel cage motors and inductors.
Occasional inching and plugging operations, such as during start-up period
4 AC4* -* Stringent inductive switching, such as the switching of a squirrel cage motor with
inching and plugging operations (Section 6.20.1(B))
5 AC6b AC23 High capacitive switching such as capacitor banks

*Applicable only for contactors. A switch is neither required nor suitable to perform a duty such as inching or plugging.

tend to destabilize the distribution system and influence
other feeders connected on the same system.
A heavy drive, requiring a prolonged starting time,
may require larger fuses. In which case the cable size
may also be increased accordingly.
In some cases, where the nearest rating of the fuse
itself is too high for the rated current, a larger cable is
recommended. The thermal (Z2 - t) characteristics of all
such components will vary from one manufacturer to
another and may not be readily available with a design
or a field engineer, while making the selection. The
manufacturers of such components therefore as standard
practice, perform this coordination for their products and
make such data readily available for the user to make a
quick selection. It may be noted that OCR and fuses at
least, of different brands, will require a new coordination.

12.1 1 Fuse-free system

Fuses are prone to cause a single phasing by not operating
all three of them simultaneously. They may also require
Figure 12.57 Air break contactor (Courtesy: L&T) a longer downtime to replace. Therefore, the new concept

.- Moulded case circuit breaker
Moulded case circuit breaker (MCCB) (MCCB)
(Courtesy GE Power Controls) Controls)
Air circuit breaker (ACB) (Courtesy Siemens) (Courtesy: GE Power
Figure 12.58