Page 172 - Handbook of Electrical Engineering

SWITCHGEAR AND MOTOR CONTROL CENTRES 155

for incoming and outgoing circuits, and these can be fitted with a variety of auxiliary devices such as
motor operators for remote control, padlocks for safe isolation and shunt trip coils for rapid opening
under some fault conditions.

7.3.2 Outgoing Switching Device for Motor Control Centres

Motor control centres and some switchboards use contactors as the frequently operated switching
device for individual outgoing loads up to about 400 amps. The figure of 400 amps is about the limit
of fuse and contactor design capability. See Chapter 8 for a discussion on fuses. Contactors and their
accompanying fuses should be used where ever possible because:-
• Much less expensive than a circuit breaker.
• Much smaller and simpler in the construction.
• Heavy faults are cleared faster due to the fast action of the fuses.
• Enables the outgoing cable sizes to be significantly smaller due to the reduced fault clearing time
provided by the fuses. Cables are sized for rated running current and fault current withstand when
a major fault occurs at the load terminals. See Chapter 9.

Contactors differ from circuit breakers in that they are designed to handle rated running current
and very short-term low fault level situations. Contactors cannot withstand the high fault currents. A
fuse must be placed in series to interrupt fault currents and sustained overcurrents. This means that the
device is physically much more compact than a circuit breaker and hence much less expensive. The
fuses and the contactor must be carefully coordinated for fault current let-through capability. European
practice often refers to IEC60158 part 1, 60292 part 1, 60947 part 2 and 60632 part 1. IEC60947
part 4, clause 7.2.5.1, applies to low voltage equipment and the coordination should be at least
‘Type 2’. IEC60632 applies similarly to high voltage equipment where the coordination is referred
to as ‘Type C’, in clause B4.1 therein. The concept of this coordination is that the contactor may
suffer permanent damage if it passes the fault current for too long a period. The less stringent Type
1 for low voltage switchgear requires the contactor or starter to be repaired or replaced after a short
circuit has been cleared. Type 2 on the other hand, and Type C for high voltage switchgear, is more
stringent and requires these devices to be suitable for further service after passing the short-circuit
current. The more stringent situation has the risk of the contacts in the contactor becoming welded
together by the heat produced by the short circuit, but this is recognised and deemed acceptable.

Low voltage contactors are simple air-break electromagnetic devices. High voltage contactors
are air-break, vacuum or SF6 devices, although air-break is becoming obsolete. Most contactors
are closed and held closed by the action of a powerful fast acting electromagnet. Occasionally a
mechanically held arrangement is required to safeguard against a loss of supply and the need to
maintain power to the load once the supply is restored. This practice often applies to feeders for
distribution transformers, where restoration of the secondary supply must not be delayed by manual
intervention. In all cases the opening of the contactor is carried out by a powerful spring. With a
mechanically held arrangement an auxiliary solenoid is fitted to unlatch the holding mechanism.
Low voltage contactors are usually fitted with purpose-made protection devices for guarding
against overloading and single-phase operation. These devices are used individually or in combination
and operate on magnetic, thermal or electronic principles. Electronic static devices offer the widest
range of time-current characteristics.