NEIEP Mechanics Practice Exam

If full line voltage is read across a door close relay coil that won't pull in, what is the likely issue?

1. Shorted Coil

2. Open Coil

3. Faulty Relay

4. Bad Power Supply

The correct answer is: Open Coil

When a full line voltage is present across a relay coil that is not engaging, this indicates that the coil itself is receiving the proper voltage necessary to operate. The most likely issue in this scenario is an open coil, meaning there is a break in the winding of the coil that prevents current from flowing through it, which in turn stops the relay from pulling in. In a functioning relay, when voltage is applied to the coil, it creates a magnetic field that pulls the armature down against the force of a spring, allowing the relay to operate. If the coil is open, there is no complete circuit for the current to flow through, which prevents the magnetic field from being generated, even though the voltage is present. The other options suggest different types of failures: a shorted coil would cause excessive current flow, potentially tripping protective devices, while a faulty relay could indicate other internal issues that might prevent operation despite full voltage. A bad power supply generally would not provide full line voltage, which contradicts the scenario presented. Therefore, an open coil is clearly the most appropriate explanation when full voltage is being read but the relay is not functioning.