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IEC 60079-11 Explained: Intrinsic Safety "i"

Understand IEC 60079-11 intrinsic safety, including Ex ia, Ex ib, Ex ic, entity parameters, associated apparatus, and circuit compatibility.
Engineer checking an intrinsically safe pressure transmitter and associated safety barrier in an instrumentation laboratory

Intrinsic safety assessment involves the field instrument, associated apparatus, and the electrical characteristics of the connected circuit.

Explosion protection does not always require a flameproof enclosure or another physical method of containing an explosion. Intrinsic Safety (Ex i) uses a different approach: the electrical circuit is designed so that its available electrical and thermal energy cannot cause ignition under the conditions specified by the protection concept.

The technical requirements for this protection method are covered by IEC 60079-11, Explosive atmospheres – Part 11: Equipment protection by intrinsic safety "i". The standard addresses the construction and testing of intrinsically safe apparatus and associated apparatus connected to intrinsically safe circuits entering explosive atmospheres.

Intrinsic safety is widely used in process instrumentation and control, including pressure transmitters, temperature sensors, gas detectors, level instruments, and other field devices installed in hazardous areas.

This guide explains the operating principle of Ex i, the differences between Ex ia, Ex ib, and Ex ic, and the checks required to establish compatibility between field instruments and associated apparatus.

1. How Intrinsic Safety Works

Intrinsic safety aims to prevent ignition by limiting the energy available in an electrical circuit. The assessment considers electrical sparks and thermal effects that could ignite the surrounding explosive atmosphere.

Relevant characteristics can include:

  • Voltage, current, and power available in the circuit.
  • Stored energy associated with capacitance and inductance.
  • Component temperatures and maximum surface temperature.
  • Normal operation and the applicable fault conditions.
  • Construction, insulation, and separation of electrical circuits.

The required fault assessment depends on the protection level. A circuit is not intrinsically safe simply because it operates at low voltage or uses a current-limited power supply. Its design and configuration must meet the applicable requirements and be assessed accordingly.

2. What IEC 60079-11 Covers

IEC 60079-11 specifies construction and testing requirements for intrinsically safe apparatus and associated apparatus whose design can affect the intrinsic safety of circuits entering explosive atmospheres.

Its technical subjects include:

  • Intrinsic safety protection levels and fault requirements.
  • Electrical and thermal ignition hazards.
  • Component selection and circuit construction.
  • Insulation, separation, and other protective measures.
  • Electrical parameters and assessment of associated apparatus.
  • Testing and verification of equipment protection.

IEC 60079-11 supplements the applicable general equipment requirements in IEC 60079-0. It should not be treated as the only standard needed for an intrinsically safe installation. IEC 60079-25 addresses intrinsically safe electrical systems, while IEC 60079-14 covers the design, selection, and installation of electrical installations in explosive atmospheres.

The applicable edition and any amendments or interpretation sheets should be checked against the project's regulatory requirements, certification basis, and specified standards.

3. Ex ia, Ex ib, and Ex ic Explained

IEC 60079-11 distinguishes three commonly used intrinsic safety protection levels. They differ in the conditions under which the equipment must maintain protection against ignition.

Protection level Associated EPL General fault basis Typical gas-zone application
Ex ia Ga Assessed for normal operation and the specified two-countable-fault conditions Zone 0, and also suitable for Zones 1 and 2 when the complete marking permits
Ex ib Gb Assessed for normal operation and the specified one-countable-fault conditions Zone 1 and Zone 2, subject to the equipment marking and application
Ex ic Gc Assessed under normal operation Zone 2, subject to the equipment marking and application

These are general relationships for gas-atmosphere applications, not a substitute for checking the complete equipment marking. Equipment suitability also depends on factors such as the gas group, temperature classification, ambient conditions, and any special conditions of use.

For a more detailed comparison, see Ex ia vs Ex ib vs Ex ic Explained.

4. Entity Parameters and Circuit Compatibility

Intrinsic safety must be assessed at circuit level when separate items of apparatus are interconnected. A field instrument and a barrier cannot be assumed compatible merely because both carry an Ex i marking.

For equipment assessed using the entity concept, the manufacturer's documentation provides electrical parameters used to check compatibility.

Field instrument parameters

Parameter Meaning
Ui Maximum input voltage permitted at the intrinsically safe apparatus.
Ii Maximum input current permitted at the apparatus.
Pi Maximum input power permitted at the apparatus.
Ci Internal capacitance relevant to the circuit assessment.
Li Internal inductance relevant to the circuit assessment.

Associated apparatus output parameters

Parameter Meaning
Uo Maximum output voltage available from the associated apparatus.
Io Maximum output current available from the associated apparatus.
Po Maximum output power available from the associated apparatus.

For a conventional entity assessment, the field instrument's input limits are compared with the associated apparatus output characteristics. The assessment also considers permitted capacitance and inductance, including the cable contribution, as well as any additional restrictions stated in the certificates or control drawings.

Checking only Ui against Uo is not enough. Current, power, capacitance, inductance, cable characteristics, and the manufacturer's specified conditions must also be considered. The exact assessment method depends on the equipment documentation and applicable requirements.

5. Associated Apparatus: Barriers and Isolators

Associated apparatus connects to an intrinsically safe circuit and can affect its safety. Common examples include:

  • Zener barriers.
  • Galvanic isolators.
  • Isolated signal repeaters.
  • Intrinsically safe interface modules and power supplies.

A Zener barrier typically uses components such as voltage-limiting elements and a specified earthing arrangement to limit energy entering the hazardous-area circuit. A galvanic isolator provides electrical isolation between circuit sections and limits energy transferred to the intrinsically safe side according to its design.

These devices are not automatically interchangeable. The selection must account for the instrument's signal type, supply requirements, entity parameters, isolation requirements, certification, and installation conditions. Where a barrier requires a specific earthing arrangement, the installation must comply with its documentation.

6. Wiring, Cable Parameters, and Circuit Separation

The electrical characteristics and installation of interconnecting wiring can affect intrinsic safety. Cable capacitance and inductance may contribute to the total values permitted for the circuit.

Engineering reviews should consider:

  • Cable capacitance and inductance per unit length.
  • Total cable length and the resulting circuit values.
  • Separation between intrinsically safe and non-intrinsically safe circuits.
  • Terminal arrangements, segregation, and insulation requirements.
  • Earthing and bonding requirements where applicable.
  • The manufacturer's control drawings and installation instructions.

Do not assume that a cable is acceptable simply because its conductor size or voltage rating appears suitable. Its electrical parameters and installation arrangement must be compatible with the approved intrinsic safety design.

Detailed system and installation requirements should be checked against the applicable editions of IEC 60079-25 and IEC 60079-14, in addition to the equipment certification documentation.

7. Temperature and Equipment Construction

Intrinsic safety primarily limits the energy capable of producing an ignition source, but thermal effects remain important. Components can become hot during normal operation or under relevant fault conditions.

The equipment assessment can therefore include component temperature, maximum surface temperature, the effects of faults, and the applicable temperature classification. The final suitability check must also consider the explosive atmosphere, ambient temperature range, gas group, and complete equipment marking.

Construction and testing are performed according to the applicable requirements of IEC 60079-11 and the relevant general requirements in IEC 60079-0. Engineers should not infer certification compliance from a simplified list of tests or from a product's low operating voltage.

8. Practical Example: Reviewing an Intrinsically Safe Instrument Loop

Consider a pressure transmitter installed in a hazardous area and connected to a control system through an associated apparatus located in a safe area. The field instrument, associated apparatus, and cable form an interconnected circuit that must be evaluated as a whole.

A practical engineering review should follow these steps:

  1. Confirm the hazardous-area classification and the required equipment protection level.
  2. Check the transmitter's complete Ex marking, certificate, and conditions of use.
  3. Identify the associated apparatus and verify its certification and output parameters.
  4. Compare the applicable entity parameters of the connected equipment.
  5. Calculate or verify the permitted cable capacitance and inductance, including the cable contribution.
  6. Check circuit separation, terminals, earthing, and other installation requirements.
  7. Confirm temperature classification, ambient limits, and the suitability of the equipment for the process environment.
  8. Verify that the final arrangement follows the manufacturer's control drawings and the applicable system and installation requirements.

The purpose is to confirm that the actual installed configuration remains within the assessed and permitted arrangement. Substituting a transmitter, barrier, or cable can require the compatibility assessment to be repeated.

9. What to Check During Procurement

For technical bid evaluation, the product description alone does not establish whether an intrinsically safe instrument is suitable for the intended circuit. Request and review the documentation needed to verify both equipment suitability and circuit compatibility.

Procurement check Purpose
Complete Ex marking and certificate Confirm the certified protection concept, EPL, gas group, temperature classification, and any special conditions.
Required protection level Confirm suitability for the classified hazardous area and project requirements.
Field apparatus parameters Review Ui, Ii, Pi, Ci, and Li where applicable.
Associated apparatus parameters Review Uo, Io, Po, and the permitted external capacitance and inductance stated in the documentation.
Cable and loop data Verify that the proposed cable and connected circuit remain within the permitted limits.
Control drawings and installation instructions Confirm wiring, segregation, earthing, and other installation conditions.
Environmental and process requirements Check ambient temperature, ingress protection where required, process compatibility, and other specified conditions.

If the vendor proposes an alternative instrument or associated apparatus, evaluate the revised combination rather than accepting the substitution based only on matching signal type or nominal voltage.

10. IEC 60079-11 vs IEC 60079-25

These standards address different but related parts of intrinsic safety engineering.

Standard Primary focus
IEC 60079-11 Construction and testing of intrinsically safe apparatus and relevant associated apparatus.
IEC 60079-25 Design and assessment of intrinsically safe electrical systems.
IEC 60079-14 Design, selection, and installation of electrical installations in explosive atmospheres.
IEC 60079-17 Inspection and maintenance of electrical installations in explosive atmospheres.

Equipment certification does not remove the need to verify the completed system and installation. The equipment, circuit design, installation, and subsequent inspection each have distinct requirements.

11. Ex i Compared with Ex d and Ex e

Ex i, Ex d, and Ex e use different approaches to controlling ignition risk.

Protection concept Primary principle Common application
Ex i Limit electrical and thermal energy so the circuit cannot cause ignition under the specified conditions. Instrumentation, sensors, and control circuits.
Ex d Use a flameproof enclosure designed to withstand an internal explosion and prevent flame transmission to the surrounding atmosphere. Equipment designed and certified with flameproof enclosure protection.
Ex e Use construction and protective measures that prevent unacceptable temperatures, arcs, and sparks under the applicable requirements. Equipment designed and certified for increased safety.

For further reading, see IEC 60079-1 Explained: Flameproof Enclosures "d" and IEC 60079-7 Explained: Increased Safety "e".

Common Intrinsic Safety Selection Mistakes

Assuming Low Voltage Automatically Means Ex i

Low voltage alone does not establish intrinsic safety. The equipment and circuit must meet the applicable construction, energy-limitation, fault, and assessment requirements.

Matching Only the Ex Marking

Two devices with intrinsic safety markings may still be incompatible when interconnected. The complete marking, entity parameters, certificates, and conditions of use must be checked.

Ignoring Cable Capacitance and Inductance

The cable can contribute to the total circuit capacitance and inductance. These values must be included in the assessment where required.

Selecting a Barrier Independently of the Instrument

The associated apparatus must be compatible with the field device, cable, signal, and intended circuit arrangement. A barrier should not be selected solely because its nominal voltage appears suitable.

Changing a Certified Circuit Without Reassessment

Replacing a field device, associated apparatus, or cable can affect the original compatibility assessment. Check the certification documentation and applicable system requirements before implementing changes.

Conclusion

IEC 60079-11 defines requirements for equipment protection by Intrinsic Safety "i". Its application relies on limiting ignition-capable electrical and thermal energy under the relevant operating and fault conditions.

For engineering and procurement, the key task is to verify the complete arrangement: equipment protection level, certification, entity parameters, associated apparatus, cable characteristics, and installation conditions. IEC 60079-25 and IEC 60079-14 provide additional requirements for system design and installation.

Always verify the applicable edition of the standards, the equipment's certification status, and the manufacturer's documentation before approving an intrinsically safe circuit.

Related Reading

References

Note: This article is an engineering overview, not a substitute for the applicable standards, equipment certificates, control drawings, or project-specific hazardous-area requirements.