The purpose of this appendix is to provide fire safety requirements for the details of the tenable environment.

Some factors that shall be considered in maintaining a tenable environment for periods of short duration are defined as follows:

Exposure to heat can lead to life threat three basic ways:

a. Hyperthermia

b. Body surface burns

c. Respiratory tract burns

For use in the modelling of life threat due to heat exposure in fires, it is necessary to consider only two criteria - the threshold of burning of the skin and the exposure at which hyperthermia is sufficient to cause mental deterioration and thereby threaten survival.

Note that thermal burns to the respiratory tract from inhalation of air containing less than 10% by volume of water vapor do not occur in the absence of burns to the skin or the face; thus, tenability limits with regard to skin burns normally are lower than for burns to the respiratory tract. However, thermal burns to the respiratory tract can occur upon inhalation of air above 60°C that is saturated with water vapor.

The tenability limit for exposure of skin to radiant heat is approximately 2.5 kW/m². Below this incident heat flux level, exposure can be tolerated for 30 minutes or longer without significantly affecting the time available for escape. Above this threshold value, the time to burning of skin due to radiant heat decreases rapidly according to equation (1).

(1) t_Irad = 4q^-1.35

where: t_Irad = time in minutes

q = radiant heat flux in kW/m²

As with toxic gases, an exposed occupant can be considered to accumulate a dose of radiant heat over a period of time. The fraction equivalent dose (FED) of radiant heat accumulated per minute is the reciprocal of t_Irad.

Radiant heat tends to be directional, producing localised heating of particular areas of skin even though the air temperature in contact with other parts of the body might be relatively low. Skin temperature depends on the balance between the rate of heat applied to the skin surface and the removal of heat subcutaneously by the blood. Thus, there is a threshold radiant flux below which significant heating of the skin is prevented but above which rapid heating occurs.

Based on the preceding information, it is estimated that the uncertainty associated with the use of equation (1) is ±25%. Moreover, an irradiance of 2.5 kW/m² would correspond to a source surface temperature of approximately 200°C, which is most likely to be exceeded near the fire, where conditions are changing rapidly.

Calculation of the time to incapacitation under conditions of exposure to convected heat from air containing less than 10 percent by volume of water vapor can be made using either equation (2) or equation (3).

As with toxic gases, an exposed occupant can be considered to accumulate a dose of convected heat over a period of time. The FED of convected heat accumulated per minute is the reciprocal of t_Iconv.

Convected heat accumulated per minute depends on the extent to which an exposed occupant is clothed and the nature of the clothing. For fully clothed subjects, equation (2) is suggested:

(2) t_Iconv = (4.1 x 108)T ^-3.61

where: t_Iconv = time in minutes

T = temperature in °C

For unclothed or lightly clothed subjects, it might be more appropriate to use equation (3):

(3) t_Iconv = (5 x 10⁷)T ^-3.4

where: t_Iconv = time in minutes

T = temperature in °C

Equations (2) and (3) are empirical fits to human data. It is estimated that the uncertainty is ±25%.

Thermal tolerance data for unprotected human skin suggest a limit of about 120°C for convected heat, above which there is, within minutes, onset of considerable pain along with the production of burns. Depending on the length of exposure, convective heat below this temperature can also cause hyperthermia.

The body of an exposed occupant can be regarded as acquiring a “dose” of heat over a period of time. A short exposure to a high radiant heat flux or temperature generally is less tolerable than a longer exposure to a lower temperature or heat flux. A methodology based on additive FEDs similar to that used with toxic gases can be applied. Providing that the temperature in the fire is stable or increasing, the total fractional effective dose of heat acquired during an exposure can be calculated using equation (4):

(4)

Note 1: In areas within an occupancy where the radiant flux to the skin is under 2.5 kW/m², the first term in equation (4) is to be set at zero.

Note 2: The uncertainty associated with the use of this last equation would be dependent on the uncertainties with the use of the three earlier equations.

The time at which the FED accumulated sum exceeds an incapacitating threshold value of 0.3 represents the time available for escape for the chosen radiant and convective heat exposures.

Maximum of 2000ppm (parts per million) for a few seconds, averaging 1500ppm or less for the first 6 mins of the exposure, averaging 800ppm or less for the first 15 mins of the exposure, averaging 50ppm or less for the remainder of the exposure.

Smoke obscuration levels shall be continuously maintained below the point at which a sign internally illuminated at 80 lux is discernible at 30m and doors and walls are discernible at 10m. This is equivalent to a light attenuation coefficient of 0.267 per m.

Air velocities in the enclosed trainway shall be greater than or equal to 0.76m/s and less than or equal to 11.18m/s.

Maximum of 115dBA for a few secs, maximum of 92dBA for the remainder of the exposure.

Some factors that shall be considered in establishing a tenable environment in stations are as follows:

The evacuation path requires a height clear of smoke of at least 2m. The current precision of modelling methods is within 25%. Therefore, in modelling methods a height of at least 2.5m shall be maintained above any point along the surface of the evacuation pathway.

The application of tenability criteria at the perimeter of a fire is impractical. The zone of tenability shall be defined to apply outside a boundary away from the perimeter of the fire. This distance will be dependent on the fire heat release rate and could be as much as 30m.