Licensed Electrical Contractor Electrical Contractor Troubleshooting & Maintenance 3

Question 1

A three-phase motor is drawing unbalanced current across phases but voltage at the motor terminals is balanced. What is the most likely cause?

Accepted Answer

Damaged motor winding

Answer

Balanced voltage with unbalanced current draw indicates a winding fault within the motor itself, such as shorted turns on one phase.

Question 2

After replacing a VFD, the motor runs but produces excessive noise and vibration. What should the contractor check first?

Accepted Answer

VFD carrier frequency setting

Answer

A VFD carrier frequency set too low produces audible noise and vibration in the motor; adjusting it to 4-8 kHz typically resolves the issue.

Question 3

When performing preventive maintenance on a 1,200A switchboard, what must be done before retorquing bus bar connections?

Accepted Answer

Perform a lockout/tagout and verify zero energy

Answer

LOTO and zero-energy verification is mandatory before any direct contact with bus bar connections to prevent arc flash and electrocution.

Question 4

A ground fault indicator on a 480V ungrounded delta system is showing a ground on Phase B. What is the correct troubleshooting approach?

Accepted Answer

Isolate circuits one at a time while monitoring the indicator

Answer

Systematically isolating circuits while monitoring the ground fault indicator identifies which branch circuit contains the fault without unnecessary shutdowns.

Question 5

What tool is most appropriate for locating an open conductor inside a finished wall without damaging the surface?

Accepted Answer

Tone generator and probe

Answer

A tone generator and inductive probe can trace and locate the break point in a conductor hidden behind finished walls without any demolition.

Licensed Electrical Contractor Practice Test

Licensed Electrical Contractor Practice Test

Licensed Electrical Contractor Electrical Contractor Troubleshooting & Maintenance 3