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Difficulty: Very hardFire Safety and Classification

An electrical short in a live 240-volt shop air compressor ignites nearby cardboard packaging and wooden pallets. A technician correctly identifies the electrical hazard and discharges a Carbon Dioxide (CO2) extinguisher, completely knocking down all visible flames. However, minutes after the CO2 gas dissipates, the cardboard and wood reignite. Which of the following best explains the primary cause of this reignition and identifies the correct follow-up action?

  1. A
    CO2 is primarily rated for combustible metals and fails to displace oxygen effectively; a Class D dry powder extinguisher should be applied to completely seal the wood and cardboard.
  2. B
    The initial CO2 discharge only suppressed the electrical arc; a pressurized water extinguisher must be applied immediately to the burning materials while the compressor remains energized.
  3. CO2 displaces oxygen but provides insufficient cooling to remove deep-seated heat from Class A materials; after verifying the equipment is de-energized, a multi-purpose (ABC) dry chemical or water extinguisher should be applied.Answer
  4. D
    CO2 interrupts the chemical chain reaction but does not displace oxygen; a Halon or clean agent extinguisher should be applied immediately to the live circuit to permanently cool the embers.

Answer

CO2 displaces oxygen but provides insufficient cooling to remove deep-seated heat from Class A materials; after verifying the equipment is de-energized, a multi-purpose (ABC) dry chemical or water extinguisher should be applied.
The correct response accurately synthesizes the physics of the fire triangle and shop safety protocols. While CO2 is safe for the initial live electrical (Class C) hazard, its lack of cooling capacity makes it ineffective at permanently extinguishing the deep-seated embers of ordinary combustibles (Class A). The proper procedure is to knock down the flames, eliminate the shock hazard by de-energizing the circuit, and then completely extinguish the remaining Class A fire using an appropriate agent with cooling or crust-forming capabilities, such as water or a multi-purpose ABC dry chemical.

Step-by-Step Solution

1
Analyze the fire classes involved in the scenario.
The scenario involves energized electrical equipment (Class C) and ordinary combustibles like wood and cardboard (Class A).
Identifying all fire classes present is necessary to evaluate the effectiveness of the chosen extinguishing agent.
2
Evaluate the properties and limitations of the Carbon Dioxide (CO2) extinguisher used.
CO2 safely extinguishes Class B and C fires by displacing oxygen. However, it provides very little cooling effect and dissipates quickly into the atmosphere.
Understanding these limitations explains why the visible flames were knocked down but the underlying heat remained.
3
Determine the cause of the reignition.
Because CO2 did not cool the materials, the deep-seated heat in the Class A embers remained above their ignition temperature. Once the CO2 dissipated and oxygen returned, the fire reignited.
Class A fires require either significant cooling (water) or a crust-forming smothering agent (multi-purpose dry chemical) to fully extinguish deep embers.
4
Identify the correct follow-up procedure.
The compressor must first be de-energized to remove the Class C electrical shock hazard. Once de-energized, a water extinguisher or an ABC dry chemical extinguisher can be safely applied to soak and extinguish the Class A materials.
Safety protocols dictate that conductive agents like water must never be used until the electrical power source is confirmed to be off.

Key Concept

Fire extinguisher classifications, agent mechanisms (cooling vs. oxygen displacement), and sequential emergency response protocols for mixed-class fires.
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Topics

Fire Safety and Classification

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