(c)  
Equipment which operates at an external temperature which will not cause explosive vapors around the unit to ignite.

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

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747-400 MAINTENANCE MANUAL
 S 912-378
 (6)
Fire-Safe Condition:

 (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).  |
 |  |
 |  |

 2..............................................................................1
 S 912-379
 (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 (PEL).

 (b)  
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.

 WARNING: THERE IS NO HEALTH-SAFE LIMIT FOR JP-4/JET-B FUEL WHICH
_______ CAN CONTAIN BENZENE. IT IS POSSIBLE THAT BENZENE CAUSES CANCER.
 (c)  Before you go into a fuel tank that contained JP-4 or JET B, wear a full-mask respirator with an attached breathing-air supply.
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747-400
MAINTENANCE MANUAL

 ~............................................................................. | HEALTH-SAFE CONDITION | | | | A HEALTH-SAFE CONDITION OCCURS WHEN THE 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 THESE PERMISSIBLE EXPOSURE | | LIMITS: | | | ............................................................................... | | Permissible Exposure Level | | | | Total Hydrocarbons TWA *[1]| Lower Explosive Level | | Fuel | (ppm) | (percent) | ............................................................................... |Aviation Gasoline | 300 | 1.0 | ...............................................................................
 |Jet A  |  |  |
 |Jet A-1  |  160  |  0.7  |
 |JP-5  |  |  |
 |JP-8  |  |  |

 2..................…..............................…...........................1
 *[1] TWA - Time Weighted Average
 S 912-380
 (8)  
Lower Explosive Limits (LEL):

 (a)  
The minimum concentration of flammable vapors in air below which propagation of flame does not occur on contact with a source of ignition.

 S 912-381
 (9)  
Permissible Exposure Level (PEL):

 (a)  
The time weighted average airborne concentrations of substances at which it is believed that nearly all workers may be repeatedly exposed 8 hours a day, 40 hours a week, without adverse health effects.

 NOTE: The PEL limits used in this procedure are the PEL limits
　
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