B.  Purging and Fuel Tank Entry - Definitions
(1) 
 APPROVED PERSONS:

(a)  Persons who are trained and understand the dangers and procedures for fuel tank entry and are responsible to make sure the airplane, equipment and the environment is safe for maintenance operations.

(2)  
APPROVED PERSONS FOR FUEL TANK ENTRY:

(a)  Persons who are trained and understand the dangers and procedures for fuel tank entry.

(3)  
CLASS I, DIVISION 1, HAZARDOUS LOCATIONS (or equivalent standard):

(a)  
Locations where ignitable concentrations of flammable gases or vapors can exist under standard operational conditions.

(b)  
Locations where ignitable concentrations of flammable gases or vapors may exist frequently because of repair or maintenance operations.

(c)  
Locations where ignitable concentrations of flammable gases or vapors can exist because of leakage.

(d)  
Locations where equipment problems or incorrect operation of equipment or processes can release ignitable concentrations of flammable gases or vapor, and can also cause failure of electrical equipment at the same time.

(4)  
CLASS I, DIVISION 2, HAZARDOUS LOCATIONS (or equivalent standard):

(a)  Locations where flammable liquids or gases are handled, processed or used, but where the liquid, vapors, or gases will usually be in closed containers or closed systems. The containers or systems will not allow the release of liquid, gas or vapor in sufficient quantity to produce an ignitable fuel and air mixture unless the container or system fails or is damaged.

(5)  
EXPLOSION-PROOF EQUIPMENT:

(a)  
Equipment contained in a case that will not be damaged by an internal explosion caused by explosive vapors inside the unit and,

(b)  
equipment which will not cause explosive vapors around the unit to ignite even when sparks, flashes or an explosion of vapor occurs inside the unit and,

(c)  
equipment which operates at an external temperature which will not cause explosive vapors around the unit to ignite and,

(d)  
equipment which has been approved by an independent testing Laboratory such as Underwriters Laboratories (UL) or Factory Mutual, for use in Class I Division 1 hazardous locations (or an equivalent standard).

(6)  
FIRE-SAFE CONDITION:

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Hazardous Locations - Open Fuel Tank(s)  500 
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500  Fuel Tank Entry Precautions 
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(a)  10% or Less of the Lower explosive Limit (LEL).
FIRE-SAFE CONDITION
A FIRE-SAFE CONDITION OCCURS WHEN THE VAPOR CONCENTRATION IS LESS THAN 10 PERCENT OF THE LOWER EXPLOSIVE LIMIT (LEL).
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(7)  HEALTH-SAFE CONDITION:
(a)  An atmosphere where oxygen content is a minimum of 19.5% to a maximum of 23.5% by volume at sea level, and the vapor concentrations are below the permissible exposure limits (PELS).
HEALTH-SAFE CONDITION NOTE: A HEALTH-SAFE CONDITION OCCURS WHEN THE OXYGEN CONTENT IS A MINIMUM OF 19.5% TO A MAXIMUM OF 23.5% BY VOLUME AT SEAL LEVEL, AND THE VAPOR CONCENTRATIONS ARE BELOW THESE PERMISSIBLE EXPOSURE LIMITS: 
FUEL  Permissible Exposure Level Total Hydrocarbons TWA* (ppm)  Lower Explosive Level  (percent) 
Aviation Gasoline  300  1.0 
Jet A Jet A-1 JP-5 JP-8  160  0.7 
Jet B  200  0.8 

WARNING:  THERE IS NOT A HEALTH-SAFE LIMIT FOR JP-4 FUEL WHICH CAN CONTAIN BENZENE. IT IS POSSIBLE THAT BENZENE CAUSES CANCER.
*TWA Time Weighted Average
(8)  
Because kerosene has a low vapor pressure, the concentrations are usually within the limits needed for a Health-Safe condition. Thus, you usually get very low (safe) values at usual temperatures (Less than approximately 70°F). At tank temperatures of approximately 90°F, it is possible to get gas concentrations more than the Health-Safe value with kerosene. At tank temperatures of more than 90°F, it is possible to get gas concentrations more than the Fire-Safe value. It is also possible that a different type of fuel was kept in a tank that usually contains kerosene. If the purging procedure was not done, this causes high concentration values when you use the combustible gas indicator.
　
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