Intrinsic Safety, Barriers, and Isolators
Intrinsic Safety (IS) is a protection method that limits energy in hazardous areas to prevent ignition. Intrinsically safe devices are designed according to certification standards and are categorized as follows:
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Exia (Zones 0, 1, and 2)
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Exib (Zones 1 and 2)
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Exic (Zone 2)
According to ISA-RP12-6, intrinsically safe equipment and wiring must be incapable of releasing sufficient electrical or thermal energy to ignite the most easily ignitable hazardous atmospheric mixture. Most IS instruments are classified as Gas Group IIC, suitable for all Gas/Air mixtures, and achieve a T4 temperature rating (135°C), covering all industrial gases except Carbon Disulphide.
One of IS's key benefits is cost-effectiveness; standard instrument cables can be used without the need for conduits or bulky enclosures. Additionally, IS systems can be worked on live without requiring a hot work permit, although test equipment must also be IS certified.
Some engineers prefer Exd (Flameproof) or Exn (Non-Incendive) equipment due to perceived complexity in IS system design, installation, and maintenance. However, IS is the most effective method of ensuring electrical safety in hazardous areas, requiring less maintenance than alternative protection methods such as Exd or Exn.
Each IS equipment piece carries a certification label specifying the Ex category, Gas Group, and Temperature Rating. Exd or Exn equipment must not be used in an IS circuit unless designated as a "simple device" (e.g., switches) that does not require certification.
Personnel Competency in Hazardous Areas
A crucial aspect of IS compliance is personnel competency. Engineers must be trained and accredited in the design, installation, and maintenance of electrical equipment in hazardous areas. Competency should be reassessed every three years at a minimum.
Associated Apparatus: Barriers and Isolators
Associated Apparatus interfaces field equipment with control room equipment. Commonly used devices include:
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Zener Barriers: Voltage-limiting devices with Zener diodes, resistors, and fuses that ensure energy limitations in hazardous areas.
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Isolation Barriers: Provide galvanic isolation for signal integrity and protection.
Zener barriers require an intrinsically safe ground connection, whereas IS isolators use galvanic isolation components such as transformers and capacitors for enhanced performance. The choice between these devices depends on the application.
Earthing and Bonding in Hazardous Areas
Proper earthing and bonding are critical for IS installations. Electrical apparatus in hazardous areas must comply with national and international codes of practice. The installation should include an earth connection for Zener barriers and appropriate bonding techniques to mitigate risks associated with electrical faults.
Intrinsic Safety Design and Certification
IS system design involves detailed calculations to ensure compliance with IEC 60079-11 standards. Critical parameters include:
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Voltage, Current, and Power: The associated apparatus must not exceed the field device’s certified maximum values.
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Capacitance and Inductance: The total capacitance (Co) and inductance (Lo) in the circuit must remain within the acceptable limits for intrinsic safety.
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Entity Parameters: Manufacturers specify IS entity parameters to guide system designers in creating compliant circuits.
Before installation, a Hazardous Area Verification Dossier must be prepared, documenting compliance with relevant standards (e.g., AS/NZS 2381.1:2005). This dossier serves as a reference for audits and maintenance.
Junction Boxes and Cabling
Junction boxes used in IS circuits are classified as "simple apparatus" and must not store energy beyond the permissible limits. Intrinsically safe cables must consider capacitance and inductance characteristics to maintain compliance. Cable selection should align with IEC60079-25 standards, ensuring safe operation over long distances.
Evaluation and Proof of Intrinsic Safety
Intrinsic safety verification involves detailed calculations assessing:
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Voltage, current, and power constraints
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Capacitive and inductive energy storage limits
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Cable parameters influencing system safety
The final circuit design must ensure that voltage and current values remain below the ignition limit curve for the respective hazardous atmosphere, considering a safety factor of 1.5 for Zones 1 or 0.
Conclusion
Intrinsic Safety is a crucial method for protecting electrical equipment in hazardous areas. By limiting available energy, IS systems prevent ignition sources, offering a cost-effective and safe solution. Proper system design, personnel competency, and adherence to certification requirements ensure the effectiveness and reliability of IS installations.