Why Choose Intrinsic Safety (IS) for Hazardous Area Instrumentation?
- Advantages of Intrinsic Safety
- Available Power in Intrinsically Safe Circuits
- Definition of Intrinsic Safety
- Typical Intrinsically Safe System
- Levels of Protection in Intrinsic safety
- Countable Faults in Intrinsic safety
- Simple Apparatus
- Cables in Intrinsic safety
- Gas Classification
- Temperature Classification
- Categories and Equipment Safety Levels
- Key Requirements for an Intrinsically Safe System
- What is meant by intrinsic safety?
- What is the intrinsic safety feature?
- What is intrinsic safety earth?
- Is safety intrinsic or extrinsic?
- What class is intrinsically safe?
- What is an intrinsically safe case?
- What is another word for intrinsically safe?
- What is intrinsic safety of instruments?
- Where are intrinsically safe used?
- What area classification is intrinsic safety?
- What is the purpose of an intrinsic safety barrier?
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
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.
Advantages of Intrinsic Safety
Intrinsic safety offers a comprehensive solution for hazardous area instrumentation, particularly for equipment with limited power requirements.
Intrinsic safety system has key advantages include:
Global Acceptance of the IS Technique
- 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.
Flexibility for Both Dust and Gas Hazards
- Most IS equipment is suitable for both dust and gas hazards, making it versatile for a variety of hazardous environments.
Suitable for All Zones
- 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.
Compatibility with Various Gas Classifications
- 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.
Temperature Classification
- 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.
Cost-Effective Solution for Most Applications
- 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.
Concept of Simple Apparatus
- 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.
Live Maintenance Without Gas Clearance Certificates
- 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.
Consistent Installation and Maintenance Requirements
- 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.
Use of Conventional Cables
- 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.
Available Power in Intrinsically Safe Circuits
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:
Voltage Limitation:
Cable capacitance limits voltage. For example, 400 meters of cable (80nF) has a maximum permissible voltage of 29V in ‘IIC ia’ circuits.
Current Limitation:
Cable inductance limits current. For example, 400 meters of cable (400µH) allows for a maximum current of 300 mA in ‘IIC ia’ circuits.
Power Limitation:
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.
Definition of Intrinsic Safety
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.
Typical Intrinsically Safe System
- 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
Levels of Protection in Intrinsic safety
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.
Countable Faults in Intrinsic safety
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.
Simple Apparatus
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 in Intrinsic safety
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.
Gas Classification
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:
Gas | Gas Group | Ignition Energy |
Methane | IIA | 160 µJ |
Ethylene | IIB | 80 µJ |
Hydrogen | IIC | 20 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.
Temperature Classification
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-Class | Maximum Surface Temperature |
T1 | 450°C |
T2 | 300°C |
T3 | 200°C |
T4 | 135°C |
T5 | 100°C |
T6 | 85°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?
Categories and Equipment Safety Levels
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 Protection | Countable Faults | ATEX Category | IEC EPL | Normal Zone of Use |
ia | 2 | 1 | 0 | Zone 0 |
ib | 1 | 2 | 1 | Zone 1 |
ic | 0 | 3 | 2 | Zone 2 |
Key Requirements for an Intrinsically Safe System
- 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?
FAQ on Intrinsically safety
What is meant by intrinsic safety?
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.
What is the intrinsic safety feature?
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.
What is intrinsic safety earth?
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.
Is safety intrinsic or extrinsic?
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.
What class is intrinsically safe?
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.
What is an intrinsically safe case?
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.
What is another word for intrinsically safe?
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.
What is intrinsic safety of instruments?
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.
Where are intrinsically safe used?
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.
What area classification is intrinsic safety?
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.
What is the purpose of an intrinsic safety barrier?
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.