The equipments intended to connect and disconnect power circuits are known collectively as switchgear (please—not switchgears and not switch-gear). Switchgear units range from the small, molded-case circuit breakers in a household panel board to the huge, air break switches on 750-kV transmission lines. They are generally divided into the four groups of disconnect or isolator switches, load break switches, circuit breakers, and contactors .
Disconnect or isolator switches are used to connect or disconnect circuits at no load or very light loads. They have minimum arc-quenching capability and are intended to interrupt only transmission line charging currents or transformer exciting currents at most.
They are usually the least expensive type of switch. Mechanically, they are designed to provide suffi cient contact pressure to remain closed through fault currents despite the high mechanical forces these currents may cause. Simple knife switches rely on multiple leaves for contact and frictional forces to maintain contact.
Others types have over-center latches, while still others have clamping locks that toggle toward the end of the closing cycle. All operate in air and have visible contacts as a safety provision, although low-voltage safety switches rely on handle position. All have provisions for lockout.
Medium- and high-voltage disconnect switches are available as indoor designs that are typically mounted in metal switchgear enclosures or as outdoor switches incorporated into elevated structures. Both horizontally and vertically operating switches are available in outdoor designs, and most are available with motor operators. Some have optional
Load break switches generally follow the basic design arrangements of disconnect switches except that they are equipped with arc chutes that enable them to interrupt the current they are designed to carry. They are not designed to interrupt fault currents; they must remain closed through faults.
Again, motor operators are available in most designs. Motor-operated load break switches can be a lower cost alternative to circuit breakers in some applications where remote control is required.
Circuit breakers are the heavy-duty members of the switchgear family. They are rated thermally for a given continuous load current, as well as a maximum fault current that they can interrupt. The arcing contacts are in air with small breakers, but the larger types have contacts in a vacuum or in oil.
High-voltage utility breakers may utilize sulfur hexafl uoride (SF 6 ) gas. Most breakers have a stored energy operating mechanism in which a heavy spring is wound up by a motor and maintained in a charged state. The spring energy then swiftly parts the contacts on a trip operation.
Typically, the circuit iscleared in 3 to 5 cycles, since rapid interruption is essential to minimize arc heating and contact erosion. Indoor breakers are usually in metal cabinets as part of a switchgear lineup, whereas outdoor breakers may be stand-alone units.
Some caution should be used when specifying vacuum circuit breakers. When these breakers interrupt an arc, the voltage across the contacts is initially quite low. As the current drops to a low value, however, it is suddenly extinguished with a very high di/dt .
This current is termed the chop current, and it can be as high as 3 to 5 A. If the breaker is ahead of a transformer, the high di/dt level can generate a high voltage through the exciting inductance of the transformer, and this can be passed on to secondary circuits.
The required voltage control can be obtained with arresters on the primary or metal oxide varistors (MOVs) on the secondary of the transformer. The MOV should be rated to dissipate the transformed chop current at the clamping voltage rating of the MOV. It also must be rated for repeated operations while dissipating the 1/2 LI 2 energy of the primary inductance where I is the chop current.
Molded case breakers are equipped with thermal and magnetic overload elements that are self-contained. They are rated by maximum load current and interrupt capacity. Thermal types employ selectable heaters to match the load current for overload protection. Larger breakers are operated from external protective relays that can provide both overload and short circuit protection through time overcurrent elements and instantaneous elements.
Nearly all relays are operated from current transformers and most are now solid-state. Because of their heavy operating mechanisms, circuit breakers are not rated for frequent operation. Most carry a maximum number of recommended operations before being inspected and repaired if necessary. Also, after clearing a fault, breakers should be inspected for arc damage or any mechanical problems.
The real workhorses of switchgear are the contactors. These are electromagnetically operated switches that can be used for motor starting and general-purpose control. They are rated for many thousands of operations.
Contactors can employ air breaks at low voltages or vacuum contacts at medium voltages. Most have continuously energized operating coils and open when control power is removed. Motor starters can handle overloads of five times rated or more, and lighting contactors also have overload ratings for incandescent lamps.
The operating coils often have a magnetic circuit with a large air gap when open and a very small gap when closed. The operating coils may have a high inrush current when energized, and the control power source must be able to supply this current without excessive voltage drop. Some types have optional DC coils that use a contact to insert a current reducing resistor into the control circuit as the contactor closes.
Any piece of electrically operated switchgear, whether breaker or contactor, has inductive control circuits that can develop high voltages in control circuits when interrupted. Good design practice calls for R/C transient suppressors on operating coils or motors.