Red Seal Electrician Practice
Motor controlsQuestion 20 of 1506

A fire alarm uses normally open switches wired in parallel to a siren. A wire to one switch breaks. What happens?

  1. That switch no longer sounds the siren, and nobody knows until it is needed
  2. The siren sounds at once as a false alarm, which shows that the wire is broken
  3. The siren sounds continuously until the broken wire is found and repaired
  4. All the switches in the system stop working at the same time as the fault
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Correct answer: That switch no longer sounds the siren, and nobody knows until it is needed

An open failure in this design is silent. It is only found by testing or in an emergency.

Source: Lessons in Electric Circuits IV: Digital (Kuphaldt), 6.5: Fail-safe Design

Practice motor controls

Questions last reviewed: 18 October 2026