IEC 60079-25 Explained: Intrinsically Safe Electrical Systems

System-level verification is essential when designing intrinsically safe instrumentation loops.

Selecting equipment marked Ex i does not, by itself, establish that an entire instrumentation loop is intrinsically safe. The field instrument, associated apparatus, interconnecting cable, and installation must be evaluated as a system.

For example, a certified pressure transmitter and a certified galvanic isolator may each meet their individual requirements, but their combination still needs to be checked for compatibility. Cable capacitance and inductance, equipment parameters, and the applicable certification conditions can all affect the verification.

IEC 60079-25 – Explosive Atmospheres – Intrinsically Safe Electrical Systems addresses the design and assessment of complete intrinsically safe electrical systems. It complements the equipment-level requirements of IEC 60079-11 and the installation requirements of IEC 60079-14.

1. Where IEC 60079-25 Fits

IEC 60079-25 focuses on the system level of intrinsic safety engineering. It addresses how equipment and associated apparatus are interconnected and how the complete system is documented and verified.

Standard	Main focus
IEC 60079-11	Equipment protection by intrinsic safety, including applicable requirements for associated apparatus.
IEC 60079-25	Design and assessment of complete 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.

The practical distinction is that equipment certification and system verification serve different purposes. A certified component is not automatically proof that every possible combination of components will form a compliant intrinsically safe system.

2. What Is an Intrinsically Safe Electrical System?

An intrinsically safe electrical system is an interconnected arrangement designed so that electrical energy and thermal effects cannot cause ignition under the applicable specified conditions.

A typical instrumentation loop may include:

• An intrinsically safe field transmitter or switch.
• A Zener barrier or galvanic isolator.
• An interconnecting field cable.
• A control system or DCS interface.
• Associated power supplies or signal-conditioning equipment, where applicable.

The electrical characteristics of these components must be considered together. The assessment should use the actual equipment documentation and circuit configuration rather than relying only on product descriptions or Ex markings.

3. Why System Verification Is Necessary

Intrinsic safety relies on limiting the energy that can be transferred into a hazardous-area circuit. Problems can arise when equipment is selected independently without checking the complete loop.

Typical issues include:

• Incompatible entity parameters between a field device and associated apparatus.
• Excessive cable capacitance or inductance.
• Incorrect barrier or isolator selection.
• Unassessed interconnections between multiple devices.
• Inadequate circuit segregation.
• Missing or outdated system verification documents.

System verification provides the engineering evidence that the selected equipment and interconnections meet the applicable intrinsic safety requirements.

4. The Entity Concept

The Entity Concept is commonly used to assess compatibility between associated apparatus and intrinsically safe field equipment. It compares relevant output parameters of the associated apparatus with the input parameters of the field device, while accounting for the cable and any applicable system conditions.

The commonly used entity relationships include:

• Voltage: U_o ≤ U_i
• Current: I_o ≤ I_i
• Power: P_o ≤ P_i
• Capacitance: C_o ≥ C_i + C_cable
• Inductance: L_o ≥ L_i + L_cable

In these relationships, the subscript o generally refers to the output parameters of the associated apparatus, while i refers to the input parameters of the field device. Cable capacitance and inductance must be included where applicable.

Important: These inequalities are not a substitute for the complete verification procedure. The applicable certificate conditions, equipment parameters, combined capacitance and inductance restrictions, protection level, and system configuration must also be reviewed. Some certified apparatus may impose additional restrictions on permitted combinations of stored capacitance and inductance.

5. Cable Parameters and Their Effect

The cable is part of the intrinsically safe circuit, not merely an installation detail. Its electrical parameters can affect the amount of energy that may be stored in the system.

Relevant information may include:

• Cable capacitance per unit length.
• Cable inductance per unit length.
• Total cable length.
• Cable construction and relevant manufacturer data.
• Shielding and grounding arrangements.

For a straightforward cable calculation, the total capacitance and inductance are generally derived from the relevant cable parameters and installed length. The resulting values must be checked against the permitted limits for the specific system.

Longer cable runs can reduce the available capacitance or inductance margin. Where the cable or circuit configuration changes, the existing verification should be reviewed rather than assumed to remain valid.

6. A Typical Ex i Instrumentation Loop

A typical loop may consist of an intrinsically safe field transmitter connected through a field cable to a barrier or galvanic isolator in the safe area, with the interface connected to a control system.

System verification normally requires the engineer to establish:

1. The field device's applicable certification and input parameters.
2. The associated apparatus's certification and output parameters.
3. The cable's capacitance, inductance, and installed length.
4. Compatibility of the interconnected components.
5. Any applicable earthing, segregation, and installation requirements.

The verification must reflect the actual proposed loop. Substituting a transmitter, isolator, or cable may affect the original calculation and require reassessment.

7. Associated Apparatus: Barriers and Isolators

Associated apparatus provides an interface between intrinsically safe circuits and other circuits. Depending on the design, it may limit or control the energy that can enter the hazardous-area circuit.

Common examples include:

• Zener barriers.
• Galvanic isolators.
• Intrinsically safe signal interfaces.
• Suitable intrinsically safe power supplies and repeater units.

The choice between a Zener barrier and a galvanic isolator affects the interface design and installation requirements. The selected device must be suitable for the loop and verified using its certified parameters and instructions.

Zener barriers and earthing

Zener barrier installations rely on a suitable grounding arrangement to operate as intended under fault conditions. The design should address the barrier manufacturer's grounding requirements, the integrity of the earth connection, fault-current paths, and equipotential bonding where applicable.

Grounding requirements should not be assumed to be identical for all associated apparatus. Galvanic isolators and other designs may have different requirements, so the applicable documentation must be followed.

8. IS Circuit Segregation and Identification

Intrinsically safe circuits must be protected against accidental connection to circuits that could introduce excessive energy. Installation design should address the applicable requirements for:

• Separation between IS and non-IS circuits.
• Wiring and terminal identification.
• Terminal arrangements and enclosure layout.
• Protection against wiring errors and unintended interconnections.
• Screening, earthing, and bonding where required.

The precise arrangements depend on the installation and applicable requirements of IEC 60079-14 and the equipment manufacturer's instructions. System verification does not remove the need to check the physical installation.

9. System Documentation

System documentation provides the basis for reviewing the design, checking the installation, and assessing future modifications.

Depending on the system, the documentation package may include:

• Loop drawings and wiring diagrams.
• Equipment certificates and relevant schedules.
• Entity parameter comparisons.
• Cable parameter calculations.
• System verification records.
• Earthing and segregation details, where applicable.
• Relevant equipment instructions and certification conditions.

Documentation should identify the actual equipment and cable used in the loop. If components are replaced or wiring is changed, the responsible engineer should determine whether the system verification and drawings need to be updated.

10. FISCO and FNICO

FISCO stands for Fieldbus Intrinsically Safe Concept. It provides a defined approach for applicable intrinsically safe fieldbus systems using specified system parameters and conditions.

FNICO stands for Fieldbus Non-Incendive Concept. It is associated with certain non-incendive fieldbus applications, traditionally including suitable Zone 2 installations.

These concepts are not interchangeable with a general entity calculation, nor can they be applied to any fieldbus system without checking eligibility. The applicable standard edition, equipment certification, system parameters, and installation conditions must be confirmed before using either approach.

11. Practical System Verification Workflow

For engineering and project execution, the verification process can be organized as follows:

1. Identify the field device. Confirm the equipment marking, certification, and relevant input parameters.
2. Identify the associated apparatus. Confirm the selected barrier, isolator, or other interface and its applicable output parameters.
3. Review certification conditions. Check any special conditions or restrictions relevant to the intended combination.
4. Establish cable data. Obtain the relevant capacitance and inductance values and determine the installed length.
5. Perform the system assessment. Evaluate entity compatibility and any additional restrictions required by the equipment documentation.
6. Check installation requirements. Review grounding, segregation, wiring, and other applicable conditions.
7. Document the result. Retain the calculations, equipment details, drawings, and verification records.

This workflow should be adapted to the actual circuit and applicable standard requirements. It is not a replacement for the full design and verification procedure.

12. Ex i Technical Bid Evaluation Checklist

For procurement engineers and technical bid evaluation teams, the main question is whether the proposed components can form a compatible system, not just whether each item has an Ex certificate.

Review item	What to verify
Field instrument	Applicable Ex i certification, marking, and input parameters.
Barrier or isolator	Certification, output parameters, and applicable restrictions.
Cable	Capacitance, inductance, length, and relevant cable data.
System compatibility	Entity parameter checks and any additional conditions affecting the combination.
Earthing	Grounding requirements for the selected associated apparatus.
Segregation	Separation and identification of IS and non-IS circuits.
Documentation	Loop drawings, certificates, calculations, and verification records.

Where a vendor proposes an alternative transmitter, barrier, isolator, or cable, the proposed replacement should be checked against the existing system design. An alternative component should not be accepted solely because it has a similar function or its own certificate.

13. Common Ex i System Design Mistakes

• Assuming individual equipment certificates prove the safety of the complete loop.
• Ignoring cable capacitance or inductance.
• Checking entity parameters without reviewing additional certificate conditions.
• Selecting an incompatible barrier or isolator.
• Ignoring the grounding requirements of Zener barriers.
• Failing to maintain appropriate IS circuit segregation.
• Changing components without reassessing the loop.
• Leaving system calculations and drawings outdated after modifications.

These issues can undermine the design verification and create uncertainty during commissioning, inspection, and future maintenance.

14. IEC 60079-25 vs IEC 60079-11 and IEC 60079-14

The standards address different aspects of an intrinsically safe installation.

Standard	Engineering purpose
IEC 60079-11	Equipment-level requirements for protection by intrinsic safety.
IEC 60079-25	System-level design and verification of intrinsically safe electrical systems.
IEC 60079-14	Design, selection, and installation of the electrical installation.
IEC 60079-17	Inspection and maintenance of the installed electrical system.

A loop may have compatible equipment parameters but still require a separate check of its installation. Conversely, correct wiring and installation do not replace the need for system verification. Each stage addresses a different part of the overall safety case.

Frequently Asked Questions

What is IEC 60079-25?

It addresses the design and assessment of complete intrinsically safe electrical systems, including the interconnection of equipment, associated apparatus, and cables.

What is the difference between IEC 60079-11 and IEC 60079-25?

IEC 60079-11 addresses equipment-level requirements, while IEC 60079-25 addresses the complete intrinsically safe system and its verification.

Why are cable parameters important?

Cable capacitance and inductance can contribute stored energy to a circuit. They must be considered against the limits and conditions applicable to the selected system.

Does an IECEx certificate for a field instrument prove that the complete loop is intrinsically safe?

No. The complete loop still needs to be assessed using the relevant equipment parameters, associated apparatus, cable data, interconnections, and applicable installation requirements.

Can a certified barrier be used with any Ex i instrument?

No. The proposed combination must be verified using the relevant parameters and certification conditions. Individual certification does not establish compatibility with every other device.

What is FISCO?

FISCO is the Fieldbus Intrinsically Safe Concept, a defined approach for eligible intrinsically safe fieldbus systems. Its applicability and system requirements must be confirmed for the specific installation.

Conclusion

IEC 60079-25 focuses on the integrity of the complete intrinsically safe electrical system. Equipment certification is essential, but system compatibility also depends on associated apparatus, cable parameters, interconnections, installation conditions, and supporting documentation.

For engineering, EPC, and procurement teams, the practical requirement is to verify the proposed loop as a system and reassess it when relevant components or wiring change. This complements equipment-level assessment under IEC 60079-11 and installation requirements under IEC 60079-14.

References

• IEC 60079-25 – Explosive Atmospheres – Intrinsically Safe Electrical Systems.
• IEC 60079-11 – Explosive Atmospheres – Equipment Protection by Intrinsic Safety "i".
• IEC 60079-14 – Explosive Atmospheres – Electrical Installations Design, Selection and Installation.
• IEC 60079-17 – Explosive Atmospheres – Electrical Installations Inspection and Maintenance.
• IECEx System – International Certification System for Equipment Used in Explosive Atmospheres.