4.1 Flameproof enclosures (Ex 'd')

Question 1

Define the ‘d’ type of protection.

Answer 1

A type of protection in which parts that can ignite an explosive atmosphere are placed in an enclosure that can withstand the pressure developed during an internal explosion of an explosive mixture and which prevent the transmission of that explosion to the explosive atmosphere surrounding the enclosure.

Question 2

Gaps in bearings, cover flanges, cable inlets (glands) and at any other place where gas could enter or leave the enclosure are not to exceed..

Answer 2

The specific values which are based on the MESG (maximum experimental safe gap).

Question 3

The threaded part of a cylindrical thread should be at least..

Answer 3

8mm in length and 6 full threads.

Question 4

If a cylindrical thread has an undercut, what should be used to ensure the required length of thread engagement?

Answer 4

A non-detachable and compressible washer or equivalent device.

Question 5

Why are 6 threads required for a cylindrical thread?

Answer 5

To ensure at least five full threads are in engagement when the parts are assembled. This ensures adequate strength and minimises any gas passage via ill-fitting threads.

Question 6

What is the maximum flange gap for a Group IIC?

Answer 6

0.1mm with enclosure volumes less than 500cm3.

Question 7

What is the maximum flange gap for a Group IIA?

Answer 7

0.15mm

Question 8

What is the maximum flange gap for a Group IIB?

Answer 8

0.15mm

Question 9

When my larger flange gaps be permissible?

Answer 9

ONLY after consultation with the appropriate superior technical authority.

Question 10

What is the minimum distance to an obstruction for a Group IIA?

Answer 10

10mm.

Question 11

What is the minimum distance to an obstruction for a Group IIB?

Answer 11

30mm.

Question 12

What is the minimum distance to an obstruction for a Group IIC?

Answer 12

40mm.

Question 13

What should be done with unused cable entry holes?

Answer 13

They are to be sealed by approved sealing plugs which can only be removed by use of special tools.

Question 14

Where gaskets are incorporated into the design, they would be made of what?

Answer 14

A metallic or non-flammable compressed material contained in a metallic sheath.

Question 15

What happens if you remove or change a gasket for home-made gaskets?

Answer 15

It would invalidate the certificate.

Question 16

When can you paint over an Exd enclosure’s seals/gaps and what should you be cautious of?

Answer 16

Seals and gaps should never be painted over prior to assembly. It is permitted to paint after assembly. Note that any paint particles left in the gap after breaking and remaking may result in an excessive gap. Aluminium paint should never be used. Aluminium powder with rust can burn very readily.

Question 17

Can grease be used?

Answer 17

Gaps can be protected with non-hardening grease to flange faces prior to tightening down. Grease can also be used to protect exposed screw threads from corrosion and thereby facilitate their easier removal.

Question 18

What types of grease can be used?

Answer 18

Silicone based greases are suitable but should not be used on seals of gas detector heads.

Question 19

When can non-hardening grease-bearing textile tape be used on the outside of a straight flanged joint?

Answer 19

1. Where the enclosure is used in conjunction with gases allocated to Group IIA. The tape should be restricted to one layer surrounding all parts of the flange joint with a short overlap. New tape should be applied whenever existing tape is disturbed.
2. Where the enclosure is used in conjunction with gases allocated to Group IIB, the gap between the joint surfaces should not exceed 0.1mm, irrespective of flange width. The tape should be restricted to one layer surrounding all parts of the flange joint with a short overlap. New tape should be applied whenever existing tape is disturbed.
3. where the enclosure is used in conjunction with gases allocated to Group IIC, tape should not be applied.

Note: the manufacturer may provide gaskets or O-rings for weatherproofing purposes.

Question 20

How should cover fixing bolts be fastened?

Answer 20

Spanner tight at all times whenever the equipment could possibly be live, even during live testing.

Fixing bolt holes are not to be drilled out or break through into the enclosure.

Question 21

What modifications are allowed?

Answer 21

Certified equipment can not be modified in any way, other than as indicated on the certificate. Modification may invalidate the certificate.

Unauthorised modifications include:

• Use of wrong size or type of hole sealing plug;
• Use of wrong type of cable entry gland;
• Use of any constructional screw for any purpose other than that original intended;
• Attaching cable brackets using cover fixing bolts;
• Drilling additional holes in any part of certified EEx ‘d’ equipment for whatever the purpose;
• Drilling holes to attach cable brackets;
• Use of any gasket other than specified;
• Use of hardening sealants or tape;
• Changing layout of components in enclosure or adding additional items not included in the EEx ‘d’ certificate.

Note: unauthorised alterations invalidate the certificate.

Question 22

What types of glands should be used?

Answer 22

For a direct entry into an Ex ‘d’ enclosure, Ex ‘d’ or Ex ‘de’ glands should be used. A barrier gland may be a requirement.

However, some Ex ‘d’ equipment may be certified with an indirect entry Ex ‘e’ terminal box. In these cases Ex ‘e’ or Ex ‘de’ glands may be used.

Question 23

What types of glands should be used for cable entry into an Ex ‘d’ enclosure containing Ex ‘i’ associated equipment?

Answer 23

All glands for direct entry into an Ex ‘d’ enclosure have to be Ex ‘d’ or Ex ‘de’ certified. For indirect entry, glands suitable for the terminal box should be used.

Question 24

When would cable entry methods not requiring Ex ‘d’ glands be used?

Answer 24

1) Cable entry devices included in the certification;
2) Plugs and sockets where the sockets are certified as parts of the equipment;
3) other special certified terminations.

Question 25

What markings should be on Ex ‘d’ equipment?

Answer 25

Marked to indicate conformity to the ATEX directives. It is also marked with the requirements of the standard (i.e. IEC 60079-1).

For more information refer to relevant standards, EEMUA 186 - A practitioners handbook or the appropriate superior technical authority.

Question 26

What is the flame path dimensions based on?

Answer 26

A function of gas group and volume of enclosure. Are to be tightly controlled during production and maintenance. If they are not exceed probability of flame reaching surrounding hazardous areas is extremely small.

Question 27

What should flame path gaps be measured with?

Answer 27

A feeler gauge.

Question 28

Is live maintenance permitted?

Answer 28

Not permitted for Ex d.

Question 29

Are unauthorised modifications permitted?

Answer 29

Not permitted for Ex d.

Question 30

Are components allowed to be removed or modified within flameproof equipment?

Answer 30

No.

Question 31

How tight should all bolts be.

Answer 31

Spanner tight.

Question 32

Can you add components to Ex d enclosure?

Answer 32

No. Because of possibly increased explosion pressure as a result of pressure piling. Volume changes are not permitted.

Question 33

How many thread adaptors per entry?

Answer 33

Only 1.