Page 317 - Offshore Electrical Engineering Manual

304 CHAPTER 4 Hazardous Area Installation

TEMPERATURE CONSIDERATIONS

IGNITION TEMPERATURE
The minimum temperature at which a gas, vapour or mist ignites spontaneously
at atmospheric pressure is known as the ignition temperature. To avoid the risk of
explosion, the surface temperature of the equipment must always remain below the
ignition temperature of the explosive mixture. Maximum permissible surface tem-
peratures are classified as in Table 5.4.1. Classification applies to the equipment, not
the gas; the actual classification designation can be taken as the next below the igni-
tion temperature of the gas.
Motors to European standards conforming to BS EN 60079-0 and BS EN 60079-1
have temperatures 10°C lower than that shown in the table for classes Tl and T2, and
5°C lower for class T3 and below.

FLASHPOINT TEMPERATURE
The flashpoint of a liquid or solid is the minimum temperature at which the vapour
above the material just ignites by application of an external flame or spark, in stan-
dard test conditions.
The flashpoint gives a very useful indication as to how hazardous a material is,
and is used when drawing up a schedule of hazardous sources for a particular instal-
lation. As discussed later in this chapter, equipment designed for use in hazardous
areas should never allow sparks or flames to come in contact with the external envi-
ronment, although in the case of Ex ‘d’ equipment flames or sparks may occur within
the enclosure. This is inherently the case with Ex ‘d’ switchgear.
If the flashpoint of a substance is higher than 38°C, it is not normally regarded as
a source of explosive hazard on North Sea installations.
However, it should be noted that in hotter climates the storage area for the avia-
tion kerosene (flashpoint 38°C) required for helicopters will have to be classified
as hazardous, since there is a higher risk that ambient temperatures will exceed this
temperature.

Table 5.4.1 Surface Temperature Classes
International (°C) IEC/BS EN 60079 US (°C) NFPA 70
T1 – 450 T1 – 450 T3A – 180
T2 – 300 T2 – 300 T3B – 165
T3 – 200 T2A – 280 T3C – 160
T4 – 135 T2B - 260 T4 – 135
T5 – 100 T2C – 230 T4A – 120
T6 – 85 T2D – 215 T5 – 100
T3 – 200 T6 – 85