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APPARATUS GAS GROUPS,
TEMPERATURE CLASS AND TYPES OF
EXPLOSION PROTECTION
(Adapted from:D.T. Hall:Practical Marine Electrical Knowledge)
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APPARATUS GAS GROUPS
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 ofan arc, which is needed to ignite the gas.
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TEMPERATURE CLASS
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 beadopted, regulations require that this
temperature be shown on the equipment.
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It is important to note that the apparatus
gas grouping and temperature class arenot 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 maximumsurface temperature under fault
conditions.
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Temperature Class / Maximum surface temperatureT1 450 °C
T2 300 °C
T3 200 °C
T4 135 °CT5 100° C
T6 85° C
For example, an electric motor may have a maximum
surface temperature of 120 °C and would be classed as T4.
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TYPES OF EXPLOSION PROTECTION
There are a number of different constructionaltechniques 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.
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The following table lists the types of protection:
Symbol Type of Protection
Exd flameproof enclosure
Exi intrinsic safety
Exe increased safety
Exn non-sparkingExq powder filled(not applicable to
ships)
Exo oil immersed(not applicable to
ships)Exp pressurisation
Exs special protection
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Some equipment may use more than one
of these types of protection in itsconstruction. 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.
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EXD FLAMEPROOF ENCLOSURE
Type 'd' protection, code EExd, uses a flameproof
enclosure to contain the electrical apparatus. Theinternal 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
atmosphere.
2)The external surface temperature of the enclosure
must remain below the ignition temperature of the
surrounding gas under all operating conditions.
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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 externalflammable atmosphere.
The maximum permitted gap depends upon three
factors:
1)The type of gas with which the apparatus is safe foruse. This is indicated by Apparatus Group.
2)The width of the joint (L).
3)The volume of the enclosure (V).
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EXI INTRINSIC SAFETYThese 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 safetyare 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 withone fault on the circuit.
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In addition to apparatus in the hazardous
area being rated as intrinsically safe, an
electrical safety barrier may also be fitted tothe circuit.
The purpose of such a barrier is to limit
voltages and currents in the hazardous areawhen faults occur on the circuit.
A separate barrier is required for each Exi
circuit and they must be fitted outsidethe
hazardous area.
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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 withinthe hazardous area.
4)All components are sealed into a compact
package with clearly marked terminals at each
end of the barrier.
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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 acertified Ex safety device.
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Cables for intrinsically safe circuits aboard ships
should be separated from power cables and the
crossing over of such cables should be at 90°. Thisis to minimise electromagnetic interference from
the power cables affecting the intrinsically safe
circuits.
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
Exi). (Adapted from:D.T. Hall:Practical Marine Electrical Knowledge)
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