Apparatus Gas Groups Temperature Class Types of Explosion Protection U6

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APPARATUS GAS GROUPS,

TEMPERATURE CLASS AND TYPES OF

EXPLOSION PROTECTION

(Adapted from:D.T. Hall:Practical Marine Electrical Knowledge)



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.



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.



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.



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.



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.



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



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.



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.



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).



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.



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.



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.



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.



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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Apparatus Gas Groups Temperature Class Types of Explosion Protection U6

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