Licensed Electrical Contractor Electrical Contractor Troubleshooting & Maintenance 2

Question 1

A 240V circuit is reading 120V at the outlet. What is the most likely cause?

Accepted Answer

Open neutral on the circuit

Answer

An open neutral on a 240V circuit causes each leg to read only 120V to ground instead of the full 240V across both legs.

Question 2

When troubleshooting a motor that hums but does not start, which component should be checked first?

Accepted Answer

Start capacitor

Answer

A humming but non-starting motor typically indicates a failed start capacitor that cannot provide the phase shift needed to generate starting torque.

Question 3

What does a megohmmeter reading of 0.5 MΩ on a 480V motor winding indicate?

Accepted Answer

Failed insulation requiring immediate attention

Answer

The minimum acceptable insulation resistance for a 480V motor is 1 MΩ plus 1 MΩ per kV, so 0.5 MΩ indicates dangerously degraded insulation.

Question 4

During thermal imaging of a panelboard, one breaker shows significantly higher temperature than adjacent breakers carrying similar loads. What is the most probable cause?

Accepted Answer

Loose connection at the breaker terminal

Answer

A loose connection creates high-resistance contact points that generate excess heat, which thermal imaging easily detects.

Question 5

A commercial building experiences intermittent voltage sags. The utility feed tests normal. Where should the contractor investigate next?

Accepted Answer

Main distribution panel connections

Answer

Intermittent voltage sags with a normal utility feed most commonly originate from loose or corroded connections at the main distribution panel.

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Licensed Electrical Contractor Electrical Contractor Troubleshooting & Maintenance 2