Industrial Automation

Why Choose Intrinsic Safety (IS) for Hazardous Area Instrumentation?

Intrinsic safety (IS) is a signaling technique designed to prevent explosions by limiting the energy transferred to hazardous areas. The energy levels used in IS systems are kept well below the threshold required to ignite an explosion, while still remaining usable for most instrumentation systems. 

The primary concerns in hazardous areas are the potential for explosions caused by:

  • A spark
  • A hot surface
Why Choose Intrinsic Safety (IS) for Hazardous Area Instrumentation? 2

Intrinsic safety is the ideal choice for low-voltage instrumentation, offering reliable solutions for various gas and area classifications. It is a preventive technique, ensuring safety by avoiding explosions rather than containing them. The ability to perform live maintenance without gas clearance certificates further enhances its suitability for instrumentation systems, making it a preferred choice in hazardous environments.

Intrinsic safety offers a comprehensive solution for hazardous area instrumentation, particularly for equipment with limited power requirements.

Intrinsic safety system has key advantages include:

  • The IS technique is recognized worldwide. International certificates under the IEC Ex scheme are increasingly accepted, though progress is ongoing. 
  • Intrinsic safety complies with local regulations such as the ATEX Directives and OSHA. The associated standards and codes of practice provide detailed guidance unmatched by other protection methods.
  • Most IS equipment is suitable for both dust and gas hazards, making it versatile for a variety of hazardous environments.
  • Intrinsic safety is the only technique with a proven track record of safety in Zone 0 instrumentation. It can be applied across all zones, using protection levels like ‘ia’ for Zone 0, ‘ib’ for Zone 1, and ‘ic’ for Zone 2, to match the level of risk.
  • Intrinsically safe apparatus typically meets Group IIC gas classification standards, making it compatible with a wide range of gas mixtures. IIB systems, while allowing higher power levels, are less compatible with gasses like acetylene, hydrogen, and carbon disulfide.
  • IS systems usually achieve a temperature classification of T4 (135°C), sufficient for most industrial gasses except for carbon disulfide (CS2), which is rarely used.
  • Many IS systems can be classified as ‘ia IIC T4’ at a reasonable cost, ensuring compatibility with almost all area classifications, gas groups, and temperature classes.
  • The IS technique allows for the use of uncertified simple apparatus like switches, thermocouples, RTDs, and junction boxes within intrinsically safe systems, adding flexibility to equipment choices.
  • Intrinsic safety is the only method that allows live maintenance in hazardous areas without requiring gas clearance certificates, a significant advantage for instrumentation fault-finding tasks.
  • Installation and maintenance procedures for IS equipment are well-documented and consistent across protection levels, minimizing the training required and reducing the chance of errors.
  • Intrinsic safety enables the use of standard instrumentation cables, which reduces overall installation costs. 
  • Cable capacitance and inductance are generally only concerns in systems with cables over 400 meters in length and in areas where IIC gasses (like hydrogen) are present.
Why Choose Intrinsic Safety (IS) for Hazardous Area Instrumentation? 2

Intrinsic safety is a low-energy technique, with restricted voltage, current, and power levels. The available power is constrained by factors such as cable capacitance and inductance, especially in circuits using Group IIC gases. Figure illustrates the power limits in intrinsically safe circuits, showing design curves that help avoid spark ignition in resistive circuits.

Key factors include:

Cable capacitance limits voltage. For example, 400 meters of cable (80nF) has a maximum permissible voltage of 29V in ‘IIC ia’ circuits.

Cable inductance limits current. For example, 400 meters of cable (400µH) allows for a maximum current of 300 mA in ‘IIC ia’ circuits.

A commonly used power limit of 1.3W ensures a T4 (135°C) temperature classification, sufficient for most industrial applications.

In practical terms, if equipment can operate within the blue-hatched area shown in Figure (typically representing ‘IIC ia T4’), it is suitable for use in nearly all circumstances. For applications that exceed these limits slightly, ‘IIB’ or ‘ic’ classification may still allow the equipment to be intrinsically safe.

For most low-voltage instrumentation, an ‘IIB ic T4’ classification (24V, 500mA) is sufficient, although it limits the equipment to Zone 2 areas and excludes gases like hydrogen, acetylene, and carbon disulfide.

The definition of intrinsic safety used in the relevant IEC apparatus standard IEC 60079-11 is a “type of protection based on the restriction of electrical energy within apparatus and of interconnecting wiring exposed to the potentially explosive atmosphere to a level below that which can cause ignition by either sparking or heating effects.” This is a concise statement introducing a multi-faceted subject.

Why Choose Intrinsic Safety (IS) for Hazardous Area Instrumentation? 1
  • A typical intrinsically safe (IS) system is illustrated in Figure, where the safe performance of each piece of apparatus depends on the integrity of all the equipment in the system. 
  • For example, the safety of the Temperature Transmitter (Tx) depends on the energy supplied by the IS Interface. In most process control applications, each apparatus in the system is certified individually. 
  • A document confirming the system’s overall safety is produced using individual apparatus certificates, in accordance with the system standard IEC 60079-25. This system document also includes cable type and simple apparatus details used in the system.
  • In interconnected IS apparatus, the safety of the system as a whole must be established. 
  • However, some apparatus, such as mobile radios and portable gas detectors, may not follow the system approach and operate as standalone devices.

Click here for Difference Between Intrinsically Safe and Explosion-Proof

Intrinsic safety employs three levels of protection ‘ia,’ ‘ib,’ and ‘ic’ to balance the probability of an explosive atmosphere being present with the likelihood of an ignition situation occurring.

  • ‘ia’: This offers the highest level of protection and is suitable for use in the most hazardous areas (Zone 0). It accounts for the possibility of two faults and includes a safety factor of 1.5.
  • ‘ib’: This level provides safety with one fault and a safety factor of 1.5, making it suitable for less frequently hazardous areas (Zone 1).
  • ‘ic’: This level considers only normal operation, with a unity safety factor, making it appropriate for infrequently hazardous areas (Zone 2). This concept was introduced in 2005, replacing the ‘energy-limited’ (nL) concept in the IEC 60079-15 standard and potentially the ‘non-incendive’ concept in North American standards.

Different parts of a system may have different levels of protection if appropriate segregation exists, and this must be clearly documented in the system documentation.

A countable fault is a fault that can affect the safety of the equipment. Specially designed components may be considered infallible, while some inadequately designed features may fail in normal operation. 

In IS systems, faults are categorized as:

  • Faults that do not occur,
  • Countable faults,
  • Imposed but not counted faults.

One significant advantage of intrinsic safety is that live maintenance on equipment is permitted without requiring gas clearance certificates. 

During safety analysis, open and short circuits in field wiring are regarded as normal operation. 

Designers and certifying authorities handle fault assessments, while system design typically does not require consideration of faults beyond field wiring issues, as apparatus certificates address these concerns.

Intrinsically safe apparatus is typically certified by an independent body like an Accredited Certification Body (ACB) under the IEC Ex scheme. However, ‘ic’ equipment may be self-certified by manufacturers. 

An exception is ‘simple apparatus,’ which does not significantly impact system safety and is exempt from certification. Examples include switches, thermocouples, RTDs, and junction boxes.

Click here for Exploring Explosion-Proof: Safety in Hazardous Environment

Cables with inductance and capacitance can store energy and affect system safety. System designs impose restrictions on cable parameters. While cable type is not closely specified in the system standard, reliable operation is the key criterion. When combining IS systems in multi-core cables, additional faults must be considered.

The energy required to ignite a gas/air mixture varies depending on the gas. Industrial gasses are classified into three groups based on their ignition energy:

  • IIA (e.g., methane),
  • IIB (e.g., ethylene),
  • IIC (e.g., hydrogen).

Table below summarizes typical gases, their classifications, and their ignition energies:

GasGas GroupIgnition Energy
MethaneIIA160 µJ
EthyleneIIB80 µJ
HydrogenIIC20 µJ

Apparatus designed for IIC is safe for all gas atmospheres, while apparatus designed for IIB allows for slightly higher power. Designing apparatus for IIA is rare due to its limited application.

Apparatus may also ignite gases by becoming hot surfaces. Apparatus is classified into temperature classes based on its maximum surface temperature, depending on the ambient temperature of the environment.

T-ClassMaximum Surface Temperature
T1450°C
T2300°C
T3200°C
T4135°C
T5100°C
T685°C

Most intrinsically safe field apparatus meets the T4 classification (135°C), suitable for use in industrial atmospheres except those with carbon disulfide (CS2), which require T6 classification.

Click here for What is a Safety Barrier? & how does Safety Barrier work?

The European Directive (ATEX 94/9/EC) for apparatus used in hazardous areas introduced the concept of categories to clarify the Zones where equipment can be used safely. 

The IEC later adopted Equipment Protection Levels (EPLs) to reflect risk analysis and zone allocation. EPLs align with levels of protection ‘ia,’ ‘ib,’ and ‘ic.’

Level of ProtectionCountable FaultsATEX CategoryIEC EPLNormal Zone of Use
ia210Zone 0
ib121Zone 1
ic032Zone 2
  • The system must operate correctly.
  • Apparatus must be certified or classified as ‘simple.’
  • Apparatus compatibility must be ensured.
  • System protection levels must be established.
  • Apparatus temperature and ambient ratings must be defined.
  • Cable parameters must be determined.

Click here for What is intrinsically safe system and what is its importance?

Intrinsic safety (IS) is a protection method used in hazardous areas to prevent explosions. It limits the electrical energy in circuits to a level below what is required to ignite flammable gases, vapors, or dust, ensuring that the equipment operates safely even in hazardous environments.

The intrinsic safety feature ensures that electrical equipment can operate safely in potentially explosive environments by restricting the energy within the circuits. This prevents sparks or hot surfaces that could ignite flammable substances.

Intrinsic safety earth refers to the grounding system used to prevent any build-up of excess energy in intrinsically safe systems. Proper grounding ensures that stray electrical currents do not accumulate, maintaining safety in hazardous areas.

In the context of hazardous areas, safety can be both intrinsic and extrinsic. Intrinsic safety refers to internal protective measures (such as limiting energy within circuits), while extrinsic safety involves external protective methods, like explosion-proof enclosures.

Intrinsically safe equipment typically falls under T-class temperature classifications, such as T4 (135°C). The classification depends on the maximum surface temperature the device can reach without igniting surrounding hazardous gasses.

An intrinsically safe case is a protective enclosure designed to ensure that the device housed within it cannot generate enough energy to cause an explosion in a hazardous area.

Another term often used for intrinsically safe is “energy-limited.” This refers to the design approach that restricts the electrical energy in circuits to safe levels.

Intrinsic safety of instruments refers to the design and implementation of electrical and electronic devices that limit their energy output, ensuring they do not cause ignition in hazardous environments.

Intrinsically safe equipment is used in hazardous areas, such as oil and gas refineries, chemical plants, mining operations, and other locations where flammable gasses, vapors, or dust may be present.

Intrinsic safety is suitable for all area classifications, including Zone 0, Zone 1, and Zone 2. It is one of the few techniques certified for use in Zone 0, the most hazardous area where explosive atmospheres are continuously present.

An intrinsic safety barrier limits the electrical energy sent to hazardous areas. It prevents any potential sparks or faults from reaching levels that could ignite flammable gases, ensuring the safety of the entire system.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

Related Articles

Back to top button

Adblock Detected

We Noticed You're Using an Ad Blocker Hi there! We understand that ads can be annoying, but they help support our website and allow us to continue providing you with high-quality content. Please consider whitelisting our site or disabling your ad blocker while you visit. Your support means a lot to us! Thank you for understanding!