Is intrinsic safety needed in Zone 2, or is Ex ic enough?

Intrinsic safety is not required in Zone 2, and where it is used, Ex ic is enough. Zone 2 requires equipment protection level (EPL) Gc by default, and IEC 60079-14 accepts intrinsically safe apparatus and associated apparatus of at least level of protection "ic" for EPL Gc. Levels ia and ib may be used in Zone 2 but are not required, and Zone 2 instruments need not be intrinsically safe at all if they carry another Gc type of protection, such as Ex ec.

Short answer

• Zone 2 needs EPL Gc (ATEX category 3G); Gb and Ga equipment is also acceptable.
• For intrinsic safety this means level ic or better, for both the field device and the energy-limiting interface.
• Ex ic is still intrinsic safety: the energy must be limited, the loop parameters compared and the installation rules for intrinsically safe circuits followed.
• Other Gc concepts (Ex ec, Ex nA, Ex nR, Ex mc, Ex pzc, or any Gb concept such as Ex db) are equally valid in Zone 2.
• A higher EPL than Gc is needed only where the area classification, the explosion protection document or a documented risk assessment says so.

What the standards require in Zone 2

Zone 2 is an area where an explosive gas atmosphere is not expected in normal operation and, if it occurs, persists only for a short time. Equipment therefore needs to be safe in normal operation only, which is what EPL Gc describes.

IEC 60079-14 (in the 2007 text, which is publicly readable) states that in installations meeting EPL Gc, the intrinsically safe apparatus and the intrinsically safe parts of associated apparatus must comply with IEC 60079-11 at least to level ic. In the EU, Annex II of Directive 1999/92/EC allows category 1, 2 or 3 equipment in Zone 2 unless the explosion protection document says otherwise.

What Ex ic is, and what it still demands

Level ic replaced the old energy-limited type of protection "nL". Under IEC 60079-11 an ic circuit must not ignite the gas in normal operation and under the conditions the standard specifies, assessed with a spark ignition safety factor of 1.0. Level ib must also stay safe with one countable fault and ia with two; ib and ia use a safety factor of 1.5, except that ia with two faults uses 1.0.

That makes ic simpler, not optional:

• Energy is still limited. The field device is supplied from associated apparatus rated at least [Ex ic], or from an ia or ib interface. Interpretation sheet 3 (2016) of IEC 60079-11:2011 notes that the ic circuit starts only after the last protective component that limits voltage and current.
• The loop is still checked. Uo, Io and Po of the source are compared with Ui, Ii and Pi of the device, and cable capacitance and inductance with Co and Lo. IEC 60079-25 (2010 edition) notes that an ia or ib source used in an ic circuit may have its permitted Co and Lo derived with a unity safety factor instead of 1.5, which usually removes cable length concerns.
• The installation rules still apply. IEC 60079-14 applies the requirements for intrinsically safe circuits to all levels unless stated otherwise: segregation from other circuits, at least 50 mm or a partition between ic and non-intrinsically safe terminals, identification and earthing rules.

Choosing between ic, ia/ib and other Gc concepts

Practical case. A tank farm has 40 level and pressure transmitters, all in Zone 2, gas group IIA. The plant already stocks Ex ia transmitters for its Zone 1 units. Using the same transmitters on ic or ia interfaces is acceptable; so is buying Ex ic transmitters, provided the stores system keeps them out of Zone 1 service. Either way the loop documents must show the actual level reached: an ia device on an ic interface is an ic loop.

Common mistakes

• Assuming "ic" means no barrier is needed. Unless the device limits its own energy and is supplied as its certificate states, an ic loop needs an energy-limiting interface.
• Wiring ic circuits like ordinary circuits. The terminal separation, cable segregation and earthing rules for intrinsically safe circuits apply to ic too.
• Reusing ic instruments in Zone 1 after a plant change, or reclassifying an area to Zone 1 without checking the loops.
• Specifying ia everywhere by habit without a reason; it is acceptable, but it is a choice, not a Zone 2 requirement.

Related on excodex

• Intrinsic safety (Ex i): ia, ib and ic and the energy limitation principle
• Ex ec, Ex nA and Ex nR: protection for Zone 2
• Equipment Protection Levels (EPL): Ga, Gb, Gc, Da, Db, Dc and how they match zones
• Has Ex nL been replaced by Ex ic?
• Which Ex protection concepts are allowed in Zone 1 and which only in Zone 2?

Standards and further reading

• IEC 60079-14, design, selection and installation of electrical installations (minimum intrinsic safety level per EPL; rules for intrinsically safe circuits).
• IEC 60079-11, equipment protection by intrinsic safety "i" (levels ia, ib and ic).
• IEC 60079-25, intrinsically safe electrical systems (loop assessment, ic output parameters).
• IEC 60079-17, inspection and maintenance of electrical installations (live maintenance on intrinsically safe installations).
• Directive 1999/92/EC, Annex II Part B, equipment categories per zone: legislation.gov.uk
• IEC 60079-14:2007 text as AS/NZS 60079.14:2009: archive.org
• IEC 60079-11:2011 with interpretation sheet 3 on level ic: law.resource.org

This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.