Potentially explosive hazardous environments demand intrinsically safe electronics
Part 1 provided an overview of intrinsically safe systems; this part looks at some of the electronics and implementations.
Q: The high-level insight is useful, but what about the electronics that are required? How is the IS function implemented?
A: There are two ways: using a Zener-diode barrier or an intrinsically safe isolator.
Q: What is the operating principle of the Zener-diode barrier?
A: Zener-diode barriers are simple devices made up of an arrangement of Zener diodes, resistors, and fuses that limit the voltage, current, and power to connected devices in the hazardous area, as seen in Figure 1.

The operating principle of a Zener-diode barrier is relatively simple. Resistors restrict the current entering the hazardous area, whilst Zener diodes conduct in the event of a fault condition, clamping the voltage and diverting excess current to earth. A fuse is present to protect the Zener diodes in the event of an overload condition.
Q: What’s a galvanic isolator?
A: This scheme restricts the energy available to the hazardous area by using components such as transformers and optoisolators to provide galvanic (ohmic) isolation between the input, output, and supply with no actual current path between sides shown in Figure 2.

It is also called three-port galvanic isolation and is functionally very similar to the isolation used in non-IS situations to eliminate ground loops, enhance signal integrity, and prevent stray currents and voltages from damaging systems and putting users at risk during normal or fault conditions.
Q: Which approach is “better”?
A: As usual, it depends. Intrinsically safe galvanic isolators have largely superseded zener-diode barriers. As these devices incorporate galvanic isolation, a dedicated IS ground is not required as it is for Zener-diode designs, which simplifies installation and ongoing maintenance. Further, intrinsically safe galvanic isolators offer lower loading of the loop they isolate, the ability to convert and amplify signals, and improved noise and surge immunity.
As Zener-diode barriers are pass-through devices, the control system can have trouble measuring signals with high common mode voltage, e.g. grounded thermocouples connected to, or located close to, voltage sources and fields. Using an isolated barrier eliminates the common mode voltage problem.
The Zener barrier does not perform signal conversion. Thus, low-level thermocouple and RTD temperature signals connected to a Zener-diode barrier are subject to attenuation and error due to electromagnetic interference.
An isolated barrier/signal conditioner isolates and converts low-level temperature signals to a 4 to 20 mA current, inherently more immune to electromagnetic interference and attenuation than millivolt signals. However, Zener-diode barriers can be smaller than isolation barriers, do not require external power, and can be cheaper.
In summary, although a Zener-diode barrier is the most cost-effective solution, using isolated barriers is the superior, long-term performance solution due to the galvanic isolation and the additional electronics that can be used within the isolated, protected areas.
Q: How do you test systems in IS environments?
A: Special, more costly instrumentation is available, certified to be intrinsically safe. For example, Fluke Corp offers digital multimeters, pressure calibrators, infrared thermometers, and even flashlights, which are IS-rated (yes, even flashlights). To give you a sense of their cost, their Model 28 II Ex Intrinsically Safe True RMS Digital Multimeter, shown in Figure 3, retails for around $5000; compare that to a similar, non-IS DMM, and you’ll see the difference.

Q: Are there any other IS-focused hardware items available?
A: Yes, since the “simple apparatus” concept allows many components, such as switches, thermocouples, RTDs, and junction boxes, to be used in intrinsically safe systems without the need for higher-level certification. The simple apparatus standard imposes limits of 1.5 V, 100 mA, and 25 mW, so simple apparatus can be added to an intrinsically safe system without recalculating the system’s safety within certain constraints.
Many transducer vendors offer intrinsically safe sensors, and their use simplifies the IS-installation and set-up processes. A wide range of industrial equipment, such as cameras, gas detectors, and even radios, are available in intrinsically safe forms.
Q: Can I use my smartphone in an IS setting? After all, it only has a small battery
A: You cannot use it directly, sorry. There are still potential hazard sources present. However, you can get IS-certified cases for phones and tablets that allow their use in such environments; they range in price from a little under $1000 to several thousand dollars (phone/tablet not included).
Q: How is intrinsically safe field equipment marked?
A: Units are marked with a series of letters and numbers that fully characterize the levels of IS protection the unit provides, as defined by the types and levels of hazards, including temperature. Certified intrinsically safe field equipment for mounting in the hazardous area will be marked with the types of protection classes using a length code such as:
Ex ia IIC T6 Ex ib IIC T5 Ex ic IIB T4
Or
Ex d [ib] IIC T5 or Ex px [ia] IIB T4
(We will not decode these strings here, as it’s too complicated and would require showing all possible markings and meanings for each pair or triplet of characters.) Note that simple apparatus does not need to be marked.
Conclusion
Intrinsic safety is an important consideration in our modern industrial world, as there is an abundance of flammable or explosive gases and dust in so many industrial processes. Based on analysis, experiments, and experience, a full suite of standards and regulations have been devised and implemented to minimize the admittedly serious risks. The entire subject is extremely complicated and usually demands both a top-down and bottom-up analysis to produce an installation that meets the immediate objectives and can be approved by the regulatory authorities.
Related EE World content
Intrinsically safe meters simplify mechanical, electrical bond testing
Intrinsically-safe LVDTs for hazardous and explosive environments
Fast-acting fuses protect barrier circuits in intrinsic safety applications
Magnetic target switch designed for ready-to-install position sensing
Varistors give intrinsically safe overvoltage protection
Humidity/temperature transmitters handle hazardous environments
Avoiding electrical hazards in the lab and repair shop
20 mA current loops, Part 2: Advanced features
Gas-discharge tubes protect circuit from transients to 600 V
External references
Omega Engineering, “What does intrinsically mean?
Wikipedia, “Intrinsic Safety”
RealPars B.V., “What is Intrinsically Safe?”
EICS Technology, “Marking”
Matric Group, “What Is Intrinsically Safe Equipment?”
Matric Group, “ ‘Intrinsically Safe’ Design Guidelines for Electrical Devices”
Vaisala, “Q&A about intrinsically safe measurement devices”
Eaton/MTL, “AN9003 – A Users Guide to Intrinsic Safety” (lengthy but very informative)
Pruftechnik, “Intrinsically Safe vs. Explosion Proof: What’s the Difference?”
Industrial-Scientific, “What is Intrinsic Safety?”
PR Electronics, “Understanding intrinsic safety”
PR Electronics, “Isolated barriers vs. Zener barriers”
PR Electronics, “Difference between a Zener barrier and an intrinsically safe isolator”
Fluke Corp., “Intrinsically safe test tools for hazardous areas”
Fluke Corp, “Fluke 28 II Ex Intrinsically Safe True RMS Digital Multimeter”
Intrinsically Safe Store, “Intrinsically Safe Phone Cases”
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