The power to a burning electric motor is shut off and locked out. How should the fire now be treated?
- As a Class C fire until the motor has cooled to room temperature
- As a Class A or B fire, depending on what is burning
- As a Class D fire because metal parts are hot
- As a Class K fire if lubricating oil is involved
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B. As a Class A or B fire, depending on what is burning
Class C exists only because of the shock hazard. Once the equipment is de-energized the fire is whatever the burning material is, usually Class A insulation or Class B oil. It does not stay Class C just because the equipment is electrical.
Reference: NFPA 10: classification and rating of fires and extinguishers
The fire tetrahedron adds which fourth element to the fire triangle of fuel, heat and oxygen?
- An uninhibited chemical chain reaction
- A continuous source of ignition
- A minimum fuel vapor pressure
- A confined space to hold the heat
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A. An uninhibited chemical chain reaction
Flaming combustion needs fuel, heat, an oxidizer and a self-sustaining chemical chain reaction. Dry chemical and halon work mainly by breaking that chain. An ignition source starts a fire but is not needed to keep it going.
Reference: Fire chemistry: fire tetrahedron
In the rating 4-A:60-B:C, what does the number 60 tell you?
- The discharge time of the extinguisher in seconds
- The number of pounds of agent in the extinguisher
- The relative size of flammable liquid fire the unit can handle
- The maximum travel distance allowed to reach the unit, measured in feet
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C. The relative size of flammable liquid fire the unit can handle
The Class B number indicates the approximate square feet of flammable liquid fire a non-expert operator can be expected to put out. It is not a time, a weight or a distance, though bigger numbers usually do come with bigger units.
Reference: NFPA 10: rating system; UL 711