Intrinsic Safety (Ex 'i')

Question 1

What is Ex ‘i’?

Answer 1

A type of protection in which the energy available in hazardous area is limited to a level below that which could ignite a flammable atmosphere.

Question 2

What is intrinsically safe equipment and associated equipment?

Answer 2

Electrical equipment in which all circuits are intrinsically safe circuits.

Associated equipment is electrical equipment which contains both intrinsically safe circuits and non-intrinsically safe circuits and is constructed so that the non-intrinsically safe circuits cannot adversely affect the intrinsically safe circuits.

Question 3

What two basic types of safety barrier are available?

Answer 3

A shunt diode barrier (Zener barrier) and the galvanically isolated barrier.

Question 4

Explain a shunt diode safety barrier.

Answer 4

A basic shunt diode barrier, more often called a Zener barrier, includes as an absolute minimum the following components:

• Resistors
• Fuse
• At least two Zener diodes
• Intrinsically safe earth.

The requirement of intrinsic safety is met by providing current and voltage limitation into the hazardous area circuit.

Question 5

Explain a galvanically isolated barrier.

Answer 5

Galvanically isolated barriers allow transference of energy without any directly wired interconnection. An intrinsically safe earth connection is not normally required.

Question 6

What are the minimum clearances between terminals of intrinsically safe and non intrinsically safe equipment?

Answer 6

The clearance should be not less than 50mm.

Question 7

What are the minimum clearances between bare conducting parts of terminals of separate intrinsically safe circuits?

Answer 7

The clearance distance should be not less than 6mm.

Question 8

What are the minimum clearances to earth?

Answer 8

The clearance distance from bare conducting parts of external conductors connected to a terminal, to any earthed metal (or other conducting parts) should not be less than 3.0mm.

Question 9

Where should surge suppression units be fitted?

Answer 9

As closed as practical to the equipment to be protected.

Question 10

What should any system requiring different types of intrinsically safe equipment have?

Answer 10

An overall policy on earthing.

Question 11

Can you mix diode and galvanic barriers on the same mounting rail?

Answer 11

It is good practice not to mixed them on the same mounting rail.

Question 12

What is the intrinsic safety earth and why is it used?

Answer 12

Is is a high integrity dedicated earth. It is used to provide a direction connection between the earth terminal of shunt diode safety barriers and the mains power supply system earth.

Question 13

What must intrinsic safety earths comply with?

Answer 13

• A minimum conductor size of 4mm2. This may need to be increased to maintain the required low impedance.
• Insulation is required to prevent invasion by fault currents which might flow in metallic parts with which an un-insulated earth conductor (if used) could come into contact.
• Mechanical protection should be provided where there is a danger of damage (e.g. from passing traffic etc).
• Current practice suggests that a value of 0.1ohm for the earth impedance is desirable. However, standard permits an impedance of up to 1.0ohm.

Question 14

How should screens be earthed?

Answer 14

Should always be earthed in accordance with the ‘hook up’ or installation diagrams.

Question 15

What should be done with unused cores?

Answer 15

All unused cores should be terminated at both ends, through connected at any intermediate junction boxes and earthed in accordance with the diagram.

Question 16

What should you be aware of when measuring insulation resistance?

Answer 16

Insulations tests on cables should only be undertaken using appropriate measuring equipment.

Use of incorrect or unsuitable measuring equipment may damage sensitive electronic equipment. Inspection schedules provide appropriate details.

Care needs to be taken when testing cables from the non-hazardous area, that incendive sparking does not occur in hazardous area without appropriate precautions.

Question 17

What glands should be used for Ex i equipment?

Answer 17

Glands should be appropriate to the equipment protection type. If connecting cables for intrinsically safe equipment enter into and Ex d box, then Ex d or de glands of a type suitable for the cable and type of entry are to be used.

Similarly, if an Ex e box is used, the gland should meet the requirements of an Ex e enclosure unless the cables are marked intrinsically safe, in which case any suitable gland is acceptable provided it meets any IP requirements.

Where mineral insulated cable may be used, in order to maintain the required electrical clearance, Ex e certified terminating pots are to be used.

Question 18

What are the minimum conductor size for Ex i?

Answer 18

A diameter of individual wire strands should not be less than 0.1mm

Question 19

What should the outer insulated colour be for Ex i?

Answer 19

The colour of outer protective insulating covering is, if colour is used as part of the identification, light blue.

Question 20

Comment on the separation from non-intrinsically safe cables.

Answer 20

Proper routing, layout and earthing guard against the intrusion, transference or invasion of parasitic voltages from non-intrinsically safe circuits into the intrinsically safe circuits.

Cables in both hazardous and non-hazardous areas should be installed so as to ensure that intrinsically safe circuit cables cannot be inadvertently connected to circuit cables which are not intrinsically safe. This may be achieved by:

• Separating the different types of circuit cables; or
• Placing cables so as to protect against the risk of mechanical damage; or
• Using cables which are armoured, metal sheathed or screened for specific types of circuits (e.g. all circuits which are not intrinsically safe are run in armoured cable or all intrinsically safe circuits are armoured.

The armour should be earthed. Screens on intrinsically safe cables should be earthed as shown on the hookup diagram.

A visible spacing of intrinsically safe cables from high current power cables is an adjunct to minimising the effect of parasitic interference and also aids inspection and maintenance.

Question 21

Multi-core cables are commonly used in connection with intrinsically safe circuits. However, what important rules apply?

Answer 21

• non-intrinsically safe circuit cores are not to be run in the same multicore as intrinsically safe circuit cores;
• the cores for each intrinsically safe circuit should be adjacent to each other throughout the cable length;
• if groups of cores are individually screened, all the cores of any intrinsically safe circuit should (preferably) be run in the same group. If this is not possible, refer to the appropriate superior technical authority for further consideration on specific requirements.

Question 22

Comment on maintenance of intrinsically safe equipment.

Answer 22

Procedures for maintaining intrinsically safe equipment differ significantly from procedures required for equipment with other types of protection.

Some work may be carried out on live equipment provided that equipment approved for the connection to intrinsically safe circuits is used and that care is taken to prevent inadvertent damage through careless use of tools such as screwdrivers.

Question 23

What should work in the hazardous area be restricted to?

Answer 23

• disconnection of and removal or replacement of items and cabling which are part of the intrinsically safe circuit;
• adjustment of any control which is necessary for the calibration of the electrical equipment or system;
• taking electrical measurements using the specific type of test instrument stated in the relevant documentation.
• any other maintenance activity permitted by the relevant documentation.

Question 24

When can work be carried out on associated equipment of an IS circuit and which is located in a safe area?

Answer 24

• a gas free area has been declared where the remote items are located; or
• the remote area equipment has been disconnected.

Question 25

What markings should be on Ex i equipment?

Answer 25

Intrinically safe equipment and associated equipment conforming to the requirements of the ATEX directive is marked to indicate conformity with the directive. It is also marked with the requirements of the standard (IEC 60079-11).

For more information, refer to relevant standards, EEMUA 186 - Practitioners Handbook or the appropriate superior technical authority.