Temperature Class and Types of Explosion Protection

(Adapted from:D.T. Hall:Practical Marine Electrical Knowledge)
 The
flammable gases in which explosion
protected electrical equipment may have
to operate are grouped according to the
amount of electrical energy, in the form of
an arc, which is needed to ignite the gas.
 This
defines the maximum surface temperature
of the components in the electrical equipment
under normal and fault conditions. This
maximum surface temperature must not
exceed the gas ignition temperature.
 The temperature class is stated with reference
to a maximum ambient temperature of 40° C,
should any other reference temperature be
adopted, regulations require that this
temperature be shown on the equipment.
 It
is important to note that the apparatus
gas grouping and temperature class are
not related. For instance, hydrogen
requires very little spark energy to ignite,
but the surface temperature necessary for
ignition is very high (560 °C).
 The
following table relates the
temperature class to the maximum
surface temperature under fault
Temperature Class / Maximum surface temperature
 T2
 T3
 T4
 T5
 T6
450 °C
300 °C
200 °C
135 °C
100° C
85° C
For example, an electric motor may have a maximum
surface temperature of 120 °C and would be classed as T4.
 There
are a number of different
constructional techniques employed in
preventing electrical equipment causing
explosions in hazardous areas. Some
techniques, such as flameproof enclosures,
have long been established but others, such
as intrinsic safety and increased safety, are
the result of developments in material and
electrical/electronic circuit design.
 It has been internationally agreed that
explosion protected equipment be identified
by the symbol "Ex" followed by a letter
indicating the type of protection employed.
The following table lists the types of protection:
Type of Protection
 Exd
flameproof enclosure
 Exi
intrinsic safety
 Exe
increased safety
 Exn
 Exq
powder filled (not applicable to
 Exo
oil immersed (not applicable to
 Exp
 Exs
special protection
 Some
equipment may use more than one
of these types of protection in its
construction. In this case, the primary
type of protection is quoted first. For
example, an increased safety motor with a
flameproof terminal box would be marked
Exe d. Equipment may also be marked
with a prefix "E" which denotes
compliance with European Standards e.g.
EExe d.
Type 'd' protection, code EExd, uses a flameproof
enclosure to contain the electrical apparatus. The
internal apparatus may include parts which arc and
surfaces which become hot. Gas may be inside the
enclosure so it must fulfill three conditions:
1) The enclosure must prevent the flame and hot gases
from being transmitted to the external flammable
2) The external surface temperature of the enclosure
must remain below the ignition temperature of the
surrounding gas under all operating conditions.
3)The transmission of flame and hot gases from a
flameproof enclosure is prevented because all joints,
such as flanges, shafts and bearings are closely
machined to achieve a small gap which is less than a
defined maximum. The pressure of an internal
explosion is then released through the small gap
between machined faces which cools the gas
sufficiently to prevent it from igniting any external
flammable atmosphere.
 The maximum permitted gap depends upon three
1)The type of gas with which the apparatus is safe for
use. This is indicated by Apparatus Group.
2)The width of the joint (L).
3)The volume of the enclosure (V).
These are circuits in which no spark nor any thermal
effect produced under prescribed test conditions (which
include normal operation and specified fault conditions)
is capable of causing ignition of a given explosive
atmosphere. Generally, this means limiting the circuit
conditions to less than 30 V and 50 mA. Naturally, this
restricts the use of Exi ' protection to low power
instrumentation, alarm and communication circuits.
The design of the circuit will depend on the type of gas
present (gas grouping).
In the UK, two grades of intrinsic safety are recognised
based on the safety factor of the equipment involved:
# Exia
the highest category based on a safety factor of 1.5 with
two faults on the circuit.
# Exib
based on a safety factor of 1.5 with one fault on the
 In
addition to apparatus in the hazardous
area being rated as intrinsically safe, an
electrical safety barrier may also be fitted to
the circuit.
 The purpose of such a barrier is to limit
voltages and currents in the hazardous area
when faults occur on the circuit.
 A separate barrier is required for each Exi
circuit and they must be fitted outside the
hazardous area.
A safety (or zener) barrier comprises:
1)A fuse to limit the maximum current through the
shunt (zener) diodes.
2)A set of resistors to limit the maximum current
into the hazardous area.
3)A set of shunt connected zener diodes to limit the
maximum voltage appearing on the circuit within
the hazardous area.
4)All components are sealed into a compact
package with clearly marked terminals at each
end of the barrier.
In the event of a short-circuit on the hazardous
area wiring or equipment, the in-line resistors
within the barrier will limit the size of fault current
while the fuse blows. Two or three zener-resistor
combinations are used within a barrier to provide
back-up voltage anchors while the fuse is blowing.
After clearing a fault, the complete zener barrier
must be replaced with an identical unit. No
alterations to the original is allowed — remember
this is a certified Ex safety device.
Cables for intrinsically safe circuits aboard ships
should be separated from power cables and the
crossing over of such cables should be at 90°. This
is to minimise electromagnetic interference from
the power cables affecting the intrinsically safe
 The metallic cable screens of intrinsically safe
circuits should be earthed at the power supply
end only to prevent circulating currents within
the sheath.
 Power and intrinsically safe cable runs should be
separately identified, i.e. by labels or by using
cables with a distinctive colour (typically blue for
(Adapted from:D.T. Hall:Practical Marine Electrical Knowledge)

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