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Difficulty: MediumSwitches and Relays

A technician is installing a master disconnect switch for a 240 V AC equipment circuit supplied by two active 120 V lines (L1L_1 and L2L_2). To completely isolate the load by opening both hot conductors simultaneously using a single mechanical toggle, which type of switch is required?

  1. A
    Single-Pole, Single-Throw (SPST) switch
  2. Double-Pole, Single-Throw (DPST) switchAnswer
  3. C
    Single-Pole, Double-Throw (SPDT) switch
  4. D
    Thermal overcurrent fuse

Answer

Double-Pole, Single-Throw (DPST) switch
A Double-Pole, Single-Throw (DPST) switch features two independent internal contact sets (poles) mechanically linked to move together under a single toggle. This enables simultaneous opening and closing of both L1L_1 and L2L_2 hot conductors to achieve complete electrical isolation of the equipment.

Step-by-Step Solution

1
Identify the number of isolated conductive paths (poles) required by the circuit.
Two distinct live conductors (L1L_1 and L2L_2) must be controlled at the same time, which requires a Double-Pole (DP) configuration.
The term 'pole' defines the number of separate, electrically isolated circuits or lines the switch can open or close concurrently.
2
Determine the number of active output positions (throws) required for each pole.
The application requires simple ON/OFF isolation, which corresponds to a Single-Throw (ST) contact arrangement.
The term 'throw' defines the number of closed contact positions available per pole; a single-throw switch has one closed state (ON) and one open state (OFF).

Key Concept

Switch Pole and Throw Configurations (DPST vs. SPST/SPDT)
Estimated Time:1m 0s
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Topics

Switches and Relays

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