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Question

Difficulty: MediumSwitches and Relays

An industrial machine uses a safety interlock limit switch on an access guard door. When the guard door is fully closed, it pushes down a spring-loaded plunger to complete the control circuit. When the door is opened, the plunger returns to its unactuated resting position, breaking the circuit and cutting power. Which switch configuration describes this device?

  1. A normally open (NO) momentary switch that completes the circuit only when mechanically actuatedAnswer
  2. B
    A normally closed (NC) momentary switch that maintains electrical continuity in its resting position
  3. C
    A double-pole, double-throw (DPDT) toggle switch configured for polarity reversal
  4. D
    A thermal bimetallic breaker switch that opens automatically when current exceeds a set limit

Answer

A normally open (NO) momentary switch that completes the circuit only when mechanically actuated
The 'normal' state of any electrical switch or relay contact is defined as its resting condition when no external mechanical force or coil energization is applied. In this access guard configuration, opening the door releases the plunger to its resting position, opening the circuit (infinite resistance, stopping current flow). Closing the door applies the mechanical force necessary to hold the contacts closed. Therefore, the device is a normally open (NO) momentary switch.

Step-by-Step Solution

1
Identify the unactuated (resting) electrical state of the switch contacts.
When the guard door opens, the plunger is released back to its resting state, opening the electrical circuit.
By industry definition, the designation of a switch (Normally Open vs. Normally Closed) is determined by its state when no external force or actuation is present.
2
Determine the switch classification corresponding to this resting state and action.
Because the switch is open in its resting position and only conducts when held down by the door, it is a Normally Open (NO) momentary switch.
Momentary NO contacts require continuous physical force to maintain continuity and spring back to an open circuit when that force is removed.

Key Concept

Switch Contact Normal States (NO vs. NC) and Momentary Interlock Principles
Estimated Time:45s
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Topics

Switches and Relays

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