Troubleshooting & Maintenance Techniques Flashcards

A variable frequency drive (VFD) display shows an 'overcurrent' fault immediately at startup. The motor has been recently rewound. What should be checked FIRST?

Answer: The acceleration ramp time setting in the VFD

A too-short acceleration ramp time causes the VFD to demand high current rapidly during startup; increasing the acceleration time is the first adjustment to make before assuming a motor or drive fault.

When troubleshooting a three-way switch circuit where the light stays ON regardless of switch position, what is the MOST likely fault?

Answer: One of the two travelers is shorted to the hot conductor in the box

A traveler shorted to the hot conductor creates a permanent hot path to the fixture regardless of switch positions, causing the light to remain on continuously.

Which of the following is the CORRECT procedure when performing a torque test on previously tightened electrical connections during preventive maintenance?

Answer: Apply torque in the tightening direction and mark if the fastener moves

The correct procedure is to apply torque in the tightening direction and note if the fastener rotates — movement indicates it was under-torqued and should be fully loosened and re-torqued; no movement confirms it was adequately tightened.

A power quality meter shows significant 3rd harmonic current (180 Hz) on a 60 Hz system. What type of load is MOST likely causing this?

Answer: Single-phase switch-mode power supplies (computers, electronics)

Single-phase switch-mode power supplies draw current in pulses rather than sinusoidally, generating high levels of 3rd harmonic and other odd-order harmonics on the system.

A ground fault on an ungrounded delta system will cause which of the following measurable symptoms?

Answer: Zero voltage to ground on the faulted phase, with the other two phases showing line-to-line voltage to ground

On an ungrounded delta system, a ground fault on one phase establishes that phase as the ground reference (0V to ground) while the other two phases rise to line-to-line voltage to ground, without tripping any protective device.

During conduit installation troubleshooting, fish tape repeatedly gets stuck at the same location. A borescope inspection is unavailable. What is the BEST non-destructive next step?

Answer: Use a mandrel sized to the conduit ID to verify conduit continuity

Pulling a mandrel confirms whether the conduit interior is clear and undamaged without risking damage to conductors or excessive force that could worsen a deformation.

An electric heating element resistance measured with an ohmmeter reads infinity (open). The element visually shows no breaks. What is the MOST likely explanation?

Answer: An internal break exists that is not visible from the outside

Resistance heating elements can develop internal breaks at connection points or within the sheathing that are invisible externally but create a complete open circuit detected by an ohmmeter.