Intrinsically Safe Systems in Hazardous Locations: Requirements and Best Practices

Intrinsically safe (IS) systems are a key protection technique for electrical and instrumentation circuits in hazardous (classified) locations. By limiting energy in the circuit to a level below that required to ignite an explosive atmosphere, intrinsically safe design allows safe operation even under fault conditions.

This article explains the main concepts of intrinsically safe systems in hazardous locations, including basic principles, equipment markings, barriers and isolators, wiring practices, documentation, and maintenance. The content is written to be SEO-friendly and compatible with Yoast SEO for WordPress.

Table of Contents

1. Introduction and Objectives of Intrinsic Safety
2. Codes, Standards, and Terminology
3. Basic Principle of Intrinsic Safety
4. Levels of Protection: Ex ia, Ex ib, and Ex ic
5. IS Barriers, Galvanic Isolators, and Associated Apparatus
6. Field Devices and Simple Apparatus
7. Wiring Methods, Separation, and Grounding
8. Entity Parameters, System Design, and Compatibility
9. Documentation, Drawings, and Labeling
10. Inspection, Testing, and Maintenance
11. Summary Tables for Intrinsically Safe Systems
12. Final Summary and Good Practice Notes

1. Introduction and Objectives of Intrinsic Safety

In hazardous locations, electrical and instrumentation circuits can provide ignition sources through sparks, arcs, or hot surfaces. Intrinsically safe systems address this risk by limiting voltage, current, and stored energy in the circuit so that ignition is prevented, even in the presence of faults.

The main objectives of intrinsic safety are:

• Prevent ignition of explosive atmospheres by controlling circuit energy.
• Enable live maintenance and troubleshooting without gas-freeing the area in many applications.
• Provide a predictable and verifiable design method based on equipment parameters and standards.
• Support flexible instrumentation architectures with relatively simple wiring methods.

2. Codes, Standards, and Terminology

Intrinsic safety is defined and standardized in several national and international documents, such as:

• IEC 60079-11 – Explosive atmospheres – Equipment protection by intrinsic safety “i”.
• NEC (NFPA 70) Articles covering intrinsically safe systems – Recognize intrinsic safety as a permitted protection technique for Class I, II, and III locations and zones.
• Certification standards from organizations such as UL, FM, ATEX, and others – Provide listing and marking rules for intrinsically safe equipment.

Common terms include:

• Intrinsically safe equipment – Equipment and circuits designed so that no spark or thermal effect produced under normal operation or specified fault conditions can ignite a given explosive atmosphere.
• Associated apparatus – Equipment that contains circuits not necessarily intrinsically safe by themselves but which include protective components to ensure that connected IS circuits remain intrinsically safe.
• Simple apparatus – Devices such as passive switches, thermocouples, or resistive sensors that, by their nature, cannot store or generate significant energy and can be used in IS circuits under defined conditions.

3. Basic Principle of Intrinsic Safety

The principle of intrinsic safety is to limit the energy available in a circuit so that it cannot ignite a hazardous gas or dust atmosphere. This includes:

• Limiting voltage and current through resistors, zener diodes, or galvanic isolation.
• Controlling stored energy in inductances and capacitances (both intentional and parasitic).
• Considering both normal operation and specified fault conditions (such as short circuits and open circuits).

The circuit is analyzed to ensure that, even under worst-case combinations of faults and component tolerances, the energy available at the field device remains below the ignition threshold for the specified gas group or dust group.

4. Levels of Protection: Ex ia, Ex ib, and Ex ic

Intrinsic safety equipment is classified according to levels of protection, which define the fault conditions considered in the design:

• Ex ia – High level of protection; equipment remains safe with two independent faults considered. Suitable for Zone 0 (continuous presence of explosive atmosphere) and often applied for higher EPL (e.g., Ga).
• Ex ib – Medium level of protection; equipment remains safe with one fault considered. Suitable for Zone 1 applications (likely occasional presence of explosive atmosphere) and EPL Gb.
• Ex ic – Basic level of protection; typically considered for Zone 2 (infrequent and short duration presence of explosive atmosphere) and EPL Gc.

The selected level of protection must match the zone classification and the required equipment protection level for the area.

5. IS Barriers, Galvanic Isolators, and Associated Apparatus

Intrinsically safe circuits are usually implemented using IS barriers or galvanic isolators located in a safe area or less hazardous area. These devices limit the energy delivered to the hazardous location.

5.1 Zener Barriers

• Use zener diodes, resistors, and fuses to clamp voltage and limit current.
• Require a reliable earthing (ground) connection to divert fault energy safely.
• Often installed on DIN rails in marshalling panels located in safe or non-hazardous areas.

5.2 Galvanic Isolators

• Provide galvanic isolation between safe-area and hazardous-area circuits using transformers or optocouplers.
• Do not rely on a direct earth reference to provide intrinsic safety (although grounding is still required for general safety and noise control).
• Can offer improved noise immunity and simpler earthing arrangements compared to zener barriers.

5.3 Associated Apparatus

Both zener barriers and galvanic isolators are forms of associated apparatus. They are certified as part of intrinsically safe systems and marked with:

• Permitted voltage, current, and power output to the IS circuit.
• Maximum external capacitance and inductance that may be safely connected.
• Gas group and temperature classification.

6. Field Devices and Simple Apparatus

Field instruments connected to IS circuits must also be certified or evaluated for intrinsic safety:

• Intrinsically safe field devices – such as transmitters, positioners, and indicators that are certified Ex ia, Ex ib, or Ex ic for the required zone and gas group.
• Simple apparatus – devices that do not generate or store significant energy (for example, mechanical contacts, resistive sensors, thermocouples), which can be part of IS loops if they meet specified criteria.

For each device, the entity parameters such as maximum input voltage, current, capacitance, and inductance must be compatible with those of the associated apparatus.

7. Wiring Methods, Separation, and Grounding

Wiring for intrinsically safe systems must preserve the integrity of the protection concept:

• IS cables should be mechanically protected and routed to minimize risk of damage.
• IS circuits must be segregated from non-IS circuits to prevent faults that could introduce excessive energy into IS wiring.
• Separation may be achieved by using dedicated raceways, barriers in terminal boxes, or specific insulation and spacing rules.
• Where zener barriers are used, provide a low-impedance earth connection according to manufacturer and code requirements.

In many installations, IS wiring is identified by color coding, labels, or specific cable types to simplify inspection and maintenance.

8. Entity Parameters, System Design, and Compatibility

Proper IS system design requires checking compatibility between associated apparatus and field devices. Key parameters include:

• Uo (Voc) – Maximum open-circuit voltage from the associated apparatus.
• Io (Isc) – Maximum short-circuit current.
• Po – Maximum output power.
• Ui, Ii, Pi – Maximum input parameters for the field device.
• Co (Ca) and Lo (La) – Maximum permitted external capacitance and inductance for the associated apparatus.
• Ci and Li – Internal capacitance and inductance of the field device and wiring.

The design must demonstrate that:

• Uo ≤ Ui, Io ≤ Ii, and Po ≤ Pi.
• Total capacitive and inductive load (wiring plus devices) does not exceed Co and Lo of the associated apparatus.
• Gas group and temperature class of all components are appropriate for the hazardous location.

These checks are typically documented in an intrinsic safety loop calculation sheet or system design record.

9. Documentation, Drawings, and Labeling

Clear documentation is essential for safe installation, inspection, and modification of IS systems.

• Prepare loop drawings showing associated apparatus, field devices, cable types, and terminations.
• Include entity parameter calculations or compatibility tables for each intrinsically safe loop.
• Mark junction boxes, terminal strips, and raceways that contain IS circuits with appropriate labels.
• Keep certification documents, datasheets, and IS control drawings accessible for maintenance and audit purposes.

10. Inspection, Testing, and Maintenance

Intrinsically safe systems require ongoing inspection and maintenance to ensure that protection is not compromised.

• Perform periodic visual and detailed inspections of cables, terminations, barriers, and field devices.
• Verify that separation between IS and non-IS circuits is maintained and that no unauthorized connections exist.
• Check earthing of zener barrier systems and integrity of bonding conductors.
• Confirm that any replacements or modifications use equipment with the same or compatible IS ratings.
• Record test results, inspections, and corrective actions in maintenance logs.

11. Summary Tables for Intrinsically Safe Systems

11.1 Levels of Protection and Typical Applications

Level	Description	Typical Zone
Ex ia	Two-fault tolerance; highest level of intrinsic safety	Zone 0 and Zone 1
Ex ib	One-fault tolerance; medium level of protection	Zone 1
Ex ic	Basic level of intrinsic safety under normal operation	Zone 2

11.2 Key Design Considerations

Design Aspect	Key Requirement (Conceptual)
Energy limitation	Limit voltage, current, capacitance, and inductance so ignition cannot occur, even under fault conditions.
Associated apparatus	Use certified barriers or isolators with appropriate output parameters for the field devices.
Entity parameter matching	Ensure Uo ≤ Ui, Io ≤ Ii, Po ≤ Pi, and that total capacitance and inductance do not exceed Co and Lo.
Wiring separation	Segregate IS circuits from non-IS circuits using dedicated raceways, barriers, or spacing rules.
Grounding	Provide low-impedance earth for zener barriers; follow manufacturer instructions for isolators.
Documentation	Maintain loop drawings, calculation sheets, and certification records for all IS circuits.
Inspection	Perform regular inspections to detect damage, unauthorized modifications, or loss of separation.

12. Final Summary and Good Practice Notes

Intrinsically safe systems provide an effective and flexible method of protecting circuits in hazardous locations by limiting energy to non-ignition levels. When properly designed and maintained, IS circuits allow safe operation of instrumentation and control systems even in high-risk environments.

To implement a robust and compliant intrinsically safe system:

• Confirm the hazardous area classification and required level of protection (Ex ia, ib, or ic).
• Select certified associated apparatus and field devices with compatible entity parameters.
• Design wiring with proper separation, mechanical protection, and grounding.
• Prepare and maintain accurate documentation for all IS loops.
• Train engineering, installation, and maintenance personnel in intrinsic safety principles and standards.

By applying these requirements and best practices, facilities can significantly reduce ignition risks in hazardous atmospheres while maintaining reliable instrumentation and control.