توضیحات :
NFPA 328 – Recommended Practice for the Control of Flammable and Combustible Liquids and Gases in Manholes, Sewers, and Similar Underground Structures
۱۹۹۲ Edition
Chapter 1 The Problem
۱ -۱ General . With increasing congestion and for aesthetic considerations , the trend in civic planning is toward the installation of all types of utility services beneath the street surface. This results in a continuous pro gram of excavation and construction, with frequent damage to existing structures .
۱-۲ Sources o f Ignition.
۱ -۲-۱ The possibility of ignition of flammable gases or” vapors that might collect in underground areas is limited by certain fundamental conditions . The vapor must be mixed with sufficient air to make a flammable mixture or it must he escaping into air at a point where a flammable mixture can be created. Heat of sufficient intensity to ignite the particular air- vapor mixture involved must he present at the place where a flammable mixture exists. Such heat might be caused by an open flame, an electric arc or spark, an incandescent heated particle, or a hot surface.
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