Page 319 - Offshore Electrical Engineering Manual

306 CHAPTER 4 Hazardous Area Installation

Table 5.4.2 Gas Group Examples
Temperature
Material Flashpoint (°C) Ignition Temperature (°C) Class Gas Group

Ethylene Gas 425 T2 IIB
Hydrogen Gas 560 T1 IIC
Methane Gas 538 T1 IIA
Kerosene 38 210 T3 IIA
Propane −104 466 T1 IIA

EXPLOSION-PROOF EQUIPMENT GROUPS

The maximum dimensions for flamepaths in Ex ‘d’ equipment (see later), obtained
from experiment, are used by the responsible standards authorities to subdivide flam-
mable gases and vapours into three groups, A, B and C. The flamepath dimension is
associated with the molecular size of the gas, the largest being in group A. Group C,
which contains only hydrogen, is the most onerous as it requires the smallest flame-
path cross-section and hence the finest machine tolerances.
Some examples of ignition temperatures and flashpoints are given in Table 5.4.2.

INCREASED SAFETY EX ‘E’ EQUIPMENT
This type of protection relies on the reduction or to make negligible the risk of an
explosion by careful design of the motor and its control gear in order to eliminate any
potential sparking which may come in contact with an air/gas flammable mixture, or
an excessive temperature anywhere within or on the surface of the machine. Design
criteria for this type of motor are given in BS 5501 and BS 5000.
This type of protection is not used in North American equipment, and the stan-
dards have therefore been written with European voltages only in mind, quoting a
maximum nominal system voltage of 11 kV. Strictly speaking, equipment made for
the North American standard voltage of 13.8 kV cannot comply with these stan-
dards. However, it is usual under these circumstances for such motors to be tested
and then issued with a certificate of inspection rather than a certificate of author-
ity. Acceptance of this method of protection must therefore be obtained from the
platform certifying body and underwriters before any decision to use such a motor
is made.
Ex ‘e’ motors offer increasing savings in weight and bulk as ratings increase.
Beyond the 315 frame size, the only economic alternative is the Ex ‘p’ type of protec-
tion, as Ex ‘d’ would be too heavy or the castings too difficult to manufacture.
The following criteria should be considered by the installer to ensure certification
is not invalidated and/or the risk of explosion increased.