something about travel distance, floor area & numerical rating of fire extinguisher

In fire extinguisher regulations, the terms you've asked about—numerical rating, floor area per unit of A, and travel distance—work together to determine the correct size, number, and placement of extinguishers for a given space. There isn't a standard concept called "floor area unit time"; it is likely a reference to the "floor area per unit of A" metric. Here is a breakdown of each.

🔢 Numerical Rating

The numerical rating on a fire extinguisher indicates its relative extinguishing potential. These ratings are assigned based on standardized tests (typically by Underwriters Laboratories) and are primarily used for Class A (ordinary combustibles) and Class B (flammable liquids) fires.

• Class A Rating: The number represents the extinguisher's water equivalency. A higher number means a greater capacity to fight Class A fires. For example, a 2A rating means the extinguisher has the extinguishing capacity equivalent to 2.5 gallons of water (each unit of "A" equals 1.25 gallons).
• Class B Rating: The number indicates the approximate square footage of a flammable liquid fire that a non-expert can expect to extinguish. For instance, a 10B rating means the extinguisher is effective on a fire involving an area of about 10 square feet.
• Class C Rating: This is a letter, not a number. It simply indicates that the extinguishing agent is non-conductive and safe for use on energized electrical equipment.

📐 Floor Area per Unit of "A"

This concept, found in NFPA 10, defines the maximum floor area that a single "unit of A" rating can protect. It acts as a cap to prevent the installation of an extinguisher with a very high rating to cover an excessively large area, even if travel distance requirements are met.

The allowable floor area depends on the hazard classification of the occupancy:

Hazard Classification	Max. Floor Area per Unit of A
Light (Low) Hazard	3,000 sq ft
Ordinary (Moderate) Hazard	1,500 sq ft
Extra (High) Hazard	1,000 sq ft

How it works: In a light hazard occupancy, one unit of "A" covers 3,000 sq ft. A 4-A extinguisher provides 4 units of "A" and can therefore protect up to 12,000 sq ft (4 × 3,000 sq ft). Separately, NFPA 10 also sets a maximum floor area per extinguisher of 11,250 sq ft, which is the area of a square that fits within the 75-foot travel distance radius.

🚶‍♂️ Travel Distance

Travel distance is the maximum distance a person must walk from any point in the protected area to reach the nearest extinguisher. This is a critical factor for ensuring extinguishers are accessible before a small fire grows out of control.

The maximum travel distance is determined by the hazard class of the fire risk:

Fire Class	Maximum Travel Distance
Class A (Ordinary Combustibles)	75 feet (22.9 m)
Class B (Flammable Liquids)	30 to 50 feet, depending on hazard level
Class C (Electrical)	Follows the Class A or B requirement, depending on the underlying fuel
Class K (Kitchen)	30 feet

Travel distance is measured along the path of travel an occupant would actually take, not in a straight line.

💎 Summary

In practice, these three concepts are applied together. When placing extinguishers, you must ensure that:

1. The numerical rating is sufficient for the hazard (e.g., a minimum rating is often specified).
2. The travel distance from any point to an extinguisher does not exceed the limit for that hazard class (e.g., 75 ft for Class A).
3. The floor area per unit of A limit is not exceeded, ensuring a single extinguisher is not stretched to cover an area beyond its protective capacity.