Intrinsically Safe Isolators: Applications, Functions and Selection Guide

Intrinsically safe isolators are apparatus used to interface field devices installed in hazardous areas with control and safety systems in safe areas. They help implement the intrinsic safety protection concept by limiting the electrical energy that can be transferred to the hazardous-area circuit under specified fault conditions.

This protection concept aims to prevent a source of energy from igniting the surrounding explosive atmosphere under the conditions defined by the applicable requirements. Its verification goes beyond the isolator itself: the field device, associated apparatus, cable parameters and their compatibility must all be considered when assessing the complete circuit. The design, construction and assessment of intrinsically safe apparatus and associated apparatus are addressed by the IEC 60079 series, particularly IEC 60079-11. System-level assessment and installation requirements are covered by other applicable parts of the series.

• Galvanic Isolators vs. Zener Barriers: Key Differences
• Main Applications of Intrinsically Safe Isolators
• Key functions and features of intrinsically safe isolators
• How to Select the Right Intrinsically Safe Isolator
• Conclusions

Galvanic Isolators vs. Zener Barriers: Key Differences

Galvanic isolators and Zener barriers are two common technologies used to interface field circuits in hazardous areas with control and safety systems in safe areas while implementing intrinsic safety. Zener barriers use voltage-limiting components and current-limiting resistors to restrict the electrical energy that can reach the hazardous-area circuit. Their protection concept relies on a suitable high-integrity earth connection. Galvanic isolators, by contrast, use electrical isolation to transfer signals between circuits while limiting the energy available to the hazardous-area side. They generally do not require the dedicated high-integrity earth connection associated with Zener barriers. The choice between the two technologies depends on the application, system architecture, earthing arrangements, signal requirements and installation constraints. In both cases, the equipment must be appropriately certified, and the complete intrinsically safe circuit must be assessed against the applicable requirements.

Main Applications of Intrinsically Safe Isolators

Intrinsically safe isolators interface field instruments installed in hazardous areas with control and safety systems in safe areas. The appropriate solution depends on the field device, signal direction, electrical characteristics and functions required by the application.

Analogue Inputs: Connecting Field Transmitters

Analogue input interfaces connect field transmitters measuring process variables such as pressure, temperature, flow and level to a PLC, DCS or safety instrumented system (SIS). Depending on the transmitter and interface, the isolator may also supply power to the field device, provide galvanic isolation, preserve HART® communication or duplicate the measurement signal.

Analogue Outputs: Controlling Field Devices

Analogue output interfaces transfer control signals from a PLC or DCS to field devices such as control valves and valve positioners. The interface must be compatible with the receiving device's electrical requirements and provide the appropriate signal configuration for the application.

Digital Inputs: Monitoring Field Conditions

Digital input interfaces connect discrete field devices, such as proximity switches and limit switches, to control or safety systems. They allow the system to monitor conditions such as equipment position, machine status or the presence of an object.

Digital Outputs: Operating Field Devices

Digital output interfaces transfer switching commands to field devices such as solenoid valves and other actuators. The interface must match the load characteristics and the control system's output requirements.

Specialized Measurement Applications

Other interfaces are designed for specific measurement technologies, including resistance temperature detectors (RTDs), thermocouples, frequency sensors and load cells. These applications require interfaces suited to the sensor type, measurement range and required signal conditioning.

Key functions and features of intrinsically safe isolators

Intrinsically safe isolators vary in their signal-handling capabilities, configuration options and installation requirements. When selecting an interface, consider the following parameters in relation to the field instrument, control system and application.

Signal compatibility: Check the supported input and output signal types, measurement ranges and electrical configurations, including active or passive current signals where applicable.

Transmitter power supply: Determine whether the interface must power a 2-wire transmitter or accept a signal from an externally powered field device.

HART® communication: Where smart transmitters are used, verify that the interface preserves transparent HART® communication.

Signal accuracy: Consider measurement accuracy and the additional error introduced by the interface, particularly where precise process monitoring is required.

Fault detection and diagnostics: Check whether the interface detects conditions such as open circuits, short circuits or out-of-range signals, and how detected faults are indicated to the control system.

Galvanic isolation: Verify which circuits are isolated from one another, such as input, output and power supply, according to the requirements of the application and the device specification.

Channel configuration and signal processing: Determine whether the application requires single or multichannel interfaces, signal duplication, scaling or configurable input/output behavior.

Functional safety: Where the interface forms part of a safety-related function, verify its suitability for the required Safety Integrity Level (SIL), including the relevant certification, safety parameters and system architecture.

Installation and operating conditions: Check the permitted ambient temperature, mounting arrangement, wiring requirements and compatibility with the intended cabinet or termination board.

How to Select the Right Intrinsically Safe Isolator

Selecting an intrinsically safe isolator involves more than matching the signal type. The interface must be compatible with the field instrument and control system, meet the hazardous-area protection requirements and support the functions required by the application.

Before choosing a device, consider the following:

1. Hazardous-area classification and certification: Verify the applicable zone, gas or dust group, temperature classification where relevant, and equipment certification against the conditions of use.
2. Field-device characteristics: Identify the instrument type, signal range, power supply requirements and electrical configuration, including active or passive current signals where applicable.
3. Control-system compatibility: Check the electrical and signal requirements of the PLC, DCS or safety instrumented system (SIS), including source/sink configurations where relevant.
4. Communication requirements: Determine whether HART® communication or another communication protocol -such as Modbus RS485- must be supported.
5. Signal processing and isolation: Establish whether the application requires galvanic isolation, signal duplication, scaling, fault detection or configurable input/output behavior.
6. Functional safety: If the interface forms part of a safety instrumented function, verify that its certification, safety parameters and integration are suitable for the required Safety Integrity Level (SIL).
7. Installation and operating conditions: Consider available space, mounting options, wiring architecture, ambient temperature and maintenance requirements.

The final selection must be supported by the device's certification and technical documentation, as well as verification of the complete intrinsically safe circuit and compliance with the applicable installation requirements.

Conclusion

Intrinsically safe isolators connect hazardous-area instrumentation with industrial control and safety systems, helping implement the required protection concept while meeting application-specific signal and system requirements. Selecting the right interface means considering the field device, signal characteristics, hazardous-area certification, installation conditions and, where applicable, functional safety requirements. Evaluating these factors together helps engineers identify a solution suited to their application.

Explore G.M. International's range of intrinsically safe isolators to find interfaces for your instrumentation and control requirements.