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 are gang-operated. They are either manually operated by means of an operating handle or
electrically operated by means of a motor-operated mechanism. They are used to perform
various switching assignments such as isolating transformers, bypassing circuit breakers, and
for line sectionalizing (where small amounts of magnetizing or charging currents are to be
interrupted).
(c) An interrupter switch can use either air or oil as its interrupting medium.
Load-interrupter switches are used to interrupt transformer-magnetizing current, line-charging
current, capacitor current to isolated banks, and load current within the limits of their rating.
They are normally used where the cost of a circuit breaker with fault-interrupting ability cannot
be justified and where the use of air-break switches is hazardous because of the danger of
inconvenient and uncontrolled arcs. oad-interrupter switches differ from circuit breakers and
fault interrupter switches in that they cannot interrupt overload or short-circuit currents.
Interrupter switches may be either single-pole hook-stick operated or gang-operated, depending
upon their location and application.
(d) A grounding switch is used to connect a circuit or a piece of apparatus to ground.
Grounding switches are normally subdivided into two separate groups: manually operated and
high-speed. Where a manually operated grounding switch is installed, it is normally connected to
an air-break or gang-operated disconnect switch. It is used to effectively ground a line after the
air-break or disconnect switch has isolated it. The manually operated grounding switch is
generally interlocked with its associated switch so that the grounding switch cannot be closed
until the disconnect is open. A high-speed grounding switch has a stored-energy mechanism
capable of closing the switch automatically, within a specified rated closing time. The switch is
opened either manually or by a power-operated mechanism. High-speed grounding switches are
used to provide protection to a differential relayed area in coordination with a remote circuit
breaker. Normally, the arrangements are such that the differential relay detects the fault and
initiates the closing of the high-speed grounding switch and results in tripping the remote circuit
breaker to clear the fault.
3.4.3.2 Oil Switches. An oil switch has its main contacts submerged in oil. Oil acts as an
insulator to help quench the arc between the contacts. In addition, since the tank is airtight, the
vaporized oil caused by the arc develops pressure which assists in breaking the arc. If the voltage
is not very high, a three-pole switch can be placed in a single tank. At higher voltages, three
separate tanks are used to make it impossible for a phase-to-phase fault to occur. Oil switches
will normally open only load current. A separate trip coil is necessary to interrupt overload or
fault currents. Oil switches are generally used in capacitor switching, distribution sectionalizing,
and transformer primary switching.
3.4.3.3 Vacuum Switches. Vacuum switches (See Figure 3-11 for an example of a
three-phase vacuum loadbreak switch) are interrupters which use vacuum chambers for contact
separation. They are generally used to interrupt load, capacitor, or transformer magnetizing
3-24
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