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Direct / Indirect Entry
The selection of cable glands for flameproof apparatus is influenced by several factors, one of which is the method of entry into the apparatus. There are two entry methods, namely direct and indirect, examples of which are shown below.
Direct entry comprises a single flameproof chamber within which components such as switches, relays or contactors may be installed. Flameproof apparatus with indirect entry has two separate chambers, one of which contains only terminals for connection of the conductors of incoming cables or conduit. Connection to the arcing components in the second compartment is made via these flameproof terminals which pass through the flameproof interface between the two compartments.
Direct entry Indirect entry
EEx d Enclosure EEx d
Enclosure
Flamepaths
Bushings
EEx d cable
glands
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Electrical Protection
Flameproof enclosures are tested for their ability to withstand internal gas explosions only;
they are not capable of withstanding the energy which may be released as a result of an internal short-circuit. In order to avoid invalidation of the certification, it is important that properly rated/calibrated electrical protection, e.g. fuses and/or circuit breakers, are utilised.
Cover bolt (fastener) requirements
Should the requirement arise where it is necessary to replace the cover bolts etc of a flameproof enclosure, only steel bolts having the correct length, type of thread, type of head and tensile strength should be used.
Regarding cover bolt tightness the torque values specified by the manufacturer should be observed. In the absence of manufacturer’s torque values the minimum requirement is spanner tight, however, care must be exercised to avoid under-tightening as this can allow an increase in the flamepath gap. Also, over-tightening of the bolts can result in them stretching and hence reducing their strength with the consequence that an internal explosion may not be contained within the enclosure.
It is important that all cover bolts are in place and correctly tightened prior to energising a flameproof enclosure.
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Modification of Flameproof Enclosures
Flameproof enclosures are normally supplied complete with all internal components fitted and certified as a single entity by a recognised test authority. The testing procedure will take into consideration the free internal volume after all the components have been fitted, the temperature rise (determined by the maximum power dissipation), creepage and clearance distances, and the rise in pressure as a result of an internal explosion using a gas/air mixture in its most explosive proportions.
The certification, therefore, “seals” the design of the apparatus so that any unauthorised modifications will effectively invalidate the approval/certification. Modifications will modify the original test results recorded by the test/certification authority and, consequently, the following points should be observed.
a. Replacement components should always be exactly the same as the original specified components in order to avoid infringement of the certification. For example, a component larger or smaller than the original will affect the internal geometry of the enclosure. Pressure piling is a possibility if a larger component is fitted, and increased volume will result if a smaller component is fitted.
Note: Illustrations are for demonstration only and must not be carried out
Original arrangement
Replacement of ‘A’
with a larger item
Replacement of ‘A’
with a smaller item
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b. Adding components is also forbidden because of the possibility of increased explosion pressure as a result of pressure piling.
Addition of component ‘C’
c. The removal of components should also be avoided since an increase in the free internal volume will result. The original test results, prior to certification, would be compromised as a result of a modification such as this.
Removal of component ‘B’
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Note: Illustrations are for demonstration only and must not be carried out
d. Drilling and tapping of cable gland/conduit entries should only be carried out by the manufacturer of the enclosure, or his approved agent. The threads of the entries are required to be compatible with those of cable glands or conduit in terms of type of thread, thread pitch and clearance tolerance since flamepaths exist at these points.
Correct alignment of the threaded entry is also important since the flamepath length at one side will be reduced if the cable gland or conduit is not fitted perpendicular to the face of the enclosure.
The strength of a flameproof enclosure may be impaired if the number and size of entries exceeds that permitted in the original design certified by the test authority.
Compliance with the original design is paramount with regard to number, size and location of entries to ensure the enclosure will contain an internal explosion.
e. Gaskets can only be replaced; they must not be added retrospectively if not included as part of the original design.
The use of unauthorised sealants should also be avoided when it is required to maintain or improve the IP rating.
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BS EN 60079-17 Table 1: Inspection Schedule for Ex’d’, Ex’e’, and Ex ‘n’
Installations (D = Detailed, C = Close, V = Visual)
6 Enclosure, glass parts and glass-to-metal sealing gaskets
and/or compounds are satisfactory * * * * * * * * *
7 There are no unauthorised modifications * * *
8 There are no visible unauthorised modifications * * * * * *
9 Bolts, cable entry devices (direct and indirect) and blanking elements are of the correct type and are complete and tight - Physical check 10 Flange faces are clean and undamaged and gaskets, if any,
are satisfactory *
11 Flange gap dimensions are within maximal permitted values * *
12 Lamp rating, type and position are correct * * *
13 Electrical connections are tight * *
14 Condition of enclosure gaskets is satisfactory * *
15 Enclosed-break and hermetically sealed devices are undamaged *
16 Restricted breathing enclosure is satisfactory *
17 Motor fans have sufficient clearance to enclosure and/or covers * * * 4 Stopping boxes and cable boxes are correctly filled *
5 Integrity of conduit system and interface with mixed system is
maintained * * *
6 Earthing connections, including any supplementary earthing bonding connections are satisfactory (e.g. connections are tight and conductors are of sufficient cross section)
- Physical check 7 Fault loop impedance (TN system) or earthing resistance
(IT systems is satisfactory) * * *
8 Insulation resistance is satisfactory * * *
9 Automatic electrical protective devices operate within permitted
limits * * *
10 Automatic electrical protective devices are set correctly (auto reset
not possible) * * *
11 Special conditions of use (if applicable) are complied with * * *
12 Cables not in use are correctly terminated * * *
13 Obstructions adjacent to flameproof flanged joints are in
accordance with IEC 60079-14 * * *
14 Variable voltage/frequency installation in accordance with
documentation * * * * * *
C ENVIRONMENT
1 Apparatus is adequately protected against corrosion, weather,
vibration and other adverse factors * * * * * * * * *
2 No undue accumulation of dust and dirt * * * * * * * * *
3 Electrical insulation is clean and dry * *
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25 Note 1
Apparatus using a combination of both ‘d’ and ‘e’ types of protection will require reference to both columns during inspection.
Note 2
The use of electrical test equipment, in accordance with items B7 and B8, should only be undertaken after appropriate steps are taken to ensure the surrounding area is free of a flammable gas or vapour