Quantcast Types of Switches and Their Application

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the impulse crest value of withstand voltage of a specified full impulse wave (see subparagraph
3.2.3.5).
3.4.2.3 Continuous Current Rating. The continuous current rating is the designated limit
of current in amperes that the switch will carry continuously without exceeding a maximum
temperature rise of 30 above ambient. For switches having copper-to-copper contacts or the
equivalent, this ambient may be 40. For switches having all contacts silver-surfaced or the
equivalent, this ambient may be 55C. The limited heat storage capacity of a switch precludes
any overload rating. Switches, therefore, should not be operated in excess of their continuous
current rating, except under emergency conditions.
3.4.2.4 Interrupting Rating. The interrupting rating indicates the maximum short-circuit
current that a switch can interrupt without sustaining damage. No switch should ever be operated
(open or close) on an energized power system unless it has sufficient interrupting capacity. For
personnel safety, this precaution cannot be overemphasized.
3.4.2.5 Short-Time Rating. All switches have a short-time current rating corresponding to
the switch's ability to carry short-circuit current in the closed position. This rating incorporates
the limitations imposed by both thermal and electromagnetic effects. The short-time current
rating of a switch is the highest current that the switch is designed to carry without damage for
specified short-time intervals. The short-time current rating consists of two values:
(a) That which the switch can carry for one cycle, referred to as its momentary rating.
(b) That which it can carry for four seconds.
Numerically, the momentary rating is 1.6 times the four-second rating.
3.4.3 Types of Switches and Their Application. There are a variety of switches used in the
transmission and distribution of electric power. The switches are designed for specific purposes
and operational conditions. In general, the switches are distinguished by the current handling
capability; i.e., continuous, loadbreak or non-loadbreak, and fault current.
3.4.3.1 Air Switches.
(a) A disconnect switch is one used for; closing, opening, or changing the connections
in a circuit or system, or for isolating purposes. It has no interrupting rating and is intended to
be operated only after the circuit has been deenergized by some other means. Disconnect
switches may be either hook- or gang-operated. A series-connected circuit breaker or circuit
recloser should be open on all three phases before a disconnect switch is opened or closed.
(b) Air-break switches are normally mounted on top of their supporting structure and
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