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Difficulty: HardPower Tool Safety and Power Sources

A technician is preparing corded electric equipment for maintenance work in a damp shop environment. A pre-check reveals that a plastic-cased drill displays a double-square symbol with a polarized two-prong plug, while a heavy-duty rotary grinder has a metal housing and a three-prong plug with the round grounding pin broken off. Which of the following describes the correct and safe procedure for operating these tools?

  1. A
    Install a three-to-two prong adapter on the metal-housed grinder to bypass the missing pin and connect both tools directly to any standard outlet.
  2. B
    Operate the metal-housed grinder as is because a standard 15-amp branch circuit breaker automatically protects the operator against personal electrical shock.
  3. Remove the metal-housed grinder from service until its three-prong plug is properly repaired, and operate the double-insulated drill on a GFCI-protected circuit.Answer
  4. D
    Tag both tools out of service immediately because safety regulations prohibit the use of any two-prong power tool in damp shop conditions.

Answer

Remove the metal-housed grinder from service until its three-prong plug is properly repaired, and operate the double-insulated drill on a GFCI-protected circuit.
The correct response identifies that tools with exposed metal housings require a dedicated, intact grounding conductor to safely carry fault current to earth, while tools marked with the concentric double-square symbol utilize reinforced double insulation and are safely designed for two-prong cords when paired with GFCI protection in damp areas.

Step-by-Step Solution

1
Evaluate the insulation rating and plug design of the plastic-cased drill.
The double-square symbol confirms the tool is double-insulated, meaning non-conductive barriers prevent internal live components from energizing user-contact surfaces; a grounding pin is not required.
Double-insulated tools are engineered and certified to operate safely with two-prong polarized plugs.
2
Analyze the grounding and chassis safety of the metal-housed grinder.
A metal housing is conductive and requires a functional three-prong ground path so any internal short circuit trips the breaker rather than energizing the tool body.
Operating a metal-cased tool with a broken ground pin removes the low-resistance fault path to earth, exposing the user to severe electrical shock.
3
Apply environmental safety requirements for damp work areas.
Damp conditions significantly lower skin contact resistance, necessitating a Ground Fault Circuit Interrupter (GFCI) to instantly cut power upon current leakage.
GFCI protection is mandatory in damp or wet environments for all portable electric power tools.

Key Concept

Equipment Grounding, Double Insulation, and GFCI Protection in Power Tool Safety
Estimated Time:1m 30s
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Topics

Power Tool Safety and Power Sources

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