Can Ex ib equipment be used in Zone 0?

No. Zone 0 requires equipment with equipment protection level (EPL) Ga, which corresponds to ATEX category 1G, and intrinsic safety level ib only reaches EPL Gb. An ib circuit is safe in normal operation and with one fault; Zone 0 needs protection that stays safe with two independent faults, which is what level ia provides.

Short answer

• Zone 0 needs EPL Ga by default (IEC 60079-14; category 1 equipment under Directive 1999/92/EC).
• Ex ia is safe with up to two countable faults and gives EPL Ga. Ex ib is safe with one countable fault and gives EPL Gb, so it suits Zone 1 and Zone 2 only.
• The whole loop counts: an ia field device connected to an associated apparatus marked [Ex ib] forms an ib circuit, which is not acceptable in Zone 0.
• Devices mounted in a tank wall can be marked Ga/Gb: the part inside Zone 0 is Ga and the head outside may be Gb, so the head's circuit may be ib only if the certificate says so.
• A lower EPL than the zone default would need a documented risk assessment that justifies it; for Zone 0 that is exceptional and not a route to save cost.

Why one fault is not enough in Zone 0

Intrinsic safety limits the energy in a circuit so that no spark or hot surface can ignite the gas. The levels differ in how many faults the circuit must survive:

The zones describe how long an explosive atmosphere is present. In Zone 0 it is there continuously, for long periods or frequently, so any failure of the equipment is likely to happen while gas is present. A fault that is rare on any given day will, over years, still coincide with the gas. EPL Ga therefore covers rare malfunctions, and the scope of IEC 60079-26 extends this to two malfunctions occurring independently of each other, while EPL Gb covers normal operation and expected malfunctions only.

In Zone 1 the gas is present only occasionally, so an ib circuit that becomes unsafe only after a second, independent fault is accepted: a double fault and the gas would have to coincide.

Check the whole loop, not just the field device

An intrinsically safe system has a level, and that level is set by its weakest part. For a Zone 0 measurement all of these must be at level ia:

1. The field device in Zone 0, marked for example Ex ia IIC T4 Ga (with the ambient range required by current marking rules).
2. The associated apparatus (zener barrier or isolator), marked [Ex ia Ga] IIC or similar.
3. Any simple apparatus such as a thermocouple or RTD, assessed as part of the ia system.
4. The loop parameters: the cable capacitance and inductance plus the device values must stay within the Co and Lo of the ia source.

A typical error is to buy a Zone 0 transmitter and connect it to a spare channel of an existing [Ex ib] isolator. The transmitter is fine, but the circuit is ib.

Practical case: instruments on a solvent tank

A fixed-roof tank holds toluene. The area classification shows Zone 0 in the vapour space and Zone 1 around the roof nozzles.

• Radar level transmitter with its antenna in the vapour space and electronics on the roof: choose a device marked Ga/Gb. The process connection acts as the separation between Zone 0 inside and Zone 1 outside, under IEC 60079-26. The head may be ib or even another Gb concept if the certificate allows.
• Temperature sensor inside the tank: must be Ga, typically an ia sensor or a simple apparatus RTD on an [Ex ia] interface.
• Pressure transmitter on a nozzle in Zone 1, not exposed to the vapour space: ib or another Gb concept is acceptable.

Common mistakes

• Reading "intrinsically safe" as "suitable for Zone 0". Only level ia is.
• An ia device on an ib barrier. The loop is ib.
• Ignoring Ga/Gb markings and installing a Gb device fully inside the tank.
• Using a risk assessment to justify ib in Zone 0 without a written, reviewed argument accepted by the operator.

Related on excodex

• Intrinsic safety (Ex i): ia, ib and ic and the energy limitation principle
• Equipment Protection Levels (EPL): Ga, Gb, Gc, Da, Db, Dc and how they match zones
• Which protection concepts can be used in Zone 0?
• What does Ex ia in square brackets mean, and can associated apparatus be installed in Zone 1?
• Tools: Ex marking check; standards spaces IEC 60079-11, IEC 60079-26

Standards and further reading

• IEC 60079-11, equipment protection by intrinsic safety "i" (levels ia, ib, ic).
• IEC 60079-14, design, selection and installation (EPL selection by zone, risk-assessment route).
• IEC 60079-26 (current edition 2021), equipment with EPL Ga (including equipment mounted across a boundary).
• Directive 1999/92/EC, Annex II B, selection of equipment by zone: legislation.gov.uk
• SafeWork NSW TestSafe, intrinsic safety levels ia, ib and ic: nsw.gov.au
• US Coast Guard OCSNCOE, equipment protection levels: dco.uscg.mil
• Standards Council of Canada, notice for the national adoption of IEC 60079-26 (scope: rare malfunctions, two independent malfunctions, equipment mounted across a boundary): scc-ccn.ca
• Health and Safety Authority (Ireland), instrumentation and control apparatus in explosive atmospheres (an [Ex ib] barrier reduces an ia field device to an ib system): hsa.ie

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.