Electronics Guide

Zones for Gas and Dust

Areas where flammable gas or vapor may be present are divided into three zones according to the frequency and duration of the hazard. Zone 0 describes a location where an explosive gas atmosphere is present continuously, for long periods, or frequently, such as the vapor space inside a fuel tank. Zone 1 describes a location where such an atmosphere is likely to occur in normal operation, and Zone 2 describes a location where it is unlikely and, if it does occur, will persist only briefly. IEC 60079-10-1 sets out the procedure for deriving these zones from release sources, ventilation, and fluid properties.

Combustible dust hazards follow a parallel scheme, defined in IEC 60079-10-2. Zone 20 corresponds to the continuous or frequent presence of a dust cloud, Zone 21 to likely occurrence in normal operation, and Zone 22 to unlikely and short-lived occurrence. Dust hazards introduce the additional concern of settled layers, which insulate hot surfaces, can smolder, and may be lofted into a secondary explosion by a primary event. Correct zoning is the foundation of every subsequent design decision, because the zone determines the equipment protection level a harvesting node must achieve, and that in turn drives its cost, size, and certification effort.

Gas Groups and Temperature Classes

Not all flammable substances ignite with equal ease. Gases and vapors are sorted into groups according to the energy required to ignite them and the gap through which a flame can propagate. Group IIA includes substances such as propane, Group IIB includes ethylene, and Group IIC includes the most easily ignited substances, hydrogen and acetylene. Equipment certified for Group IIC is suitable for the lower groups as well, so a Group IIC harvester may be specified once and installed across a site with mixed process fluids. Equipment for underground mines susceptible to firedamp forms a separate Group I, and combustible dusts form Group III, subdivided into IIIA for combustible flyings, IIIB for non-conductive dust, and IIIC for conductive dust such as metal powder.

Temperature classes constrain the maximum surface temperature that equipment may reach, so that it remains below the autoignition temperature of the surrounding atmosphere. Six classes apply to gas atmospheres: T1 permits surfaces up to 450 degrees Celsius, T2 up to 300, T3 up to 200, T4 up to 135, T5 up to 100, and T6 up to 85 degrees Celsius. Most process instrumentation is certified T4, which accommodates the common hydrocarbon solvents. Dust atmospheres are treated differently: rather than a class, the equipment carries an actual maximum surface temperature, which must remain below two-thirds of the ignition temperature of the dust cloud and at least 75 kelvins below the ignition temperature of a five-millimeter settled layer. A harvesting device must satisfy the relevant group and the applicable temperature limit for its installation.

Ignition Sources and Minimum Ignition Energy

An explosion requires fuel, oxygen, and an ignition source. In an established hazardous area, fuel and oxygen are assumed to be present, so protection focuses on eliminating ignition sources. The principal triggers are electrical sparks from making or breaking circuits, hot surfaces that exceed the autoignition temperature, frictional sparks, and electrostatic discharge. Each must be addressed in the design of a harvesting node.

The concept of minimum ignition energy quantifies how little energy a spark must deliver to ignite a given mixture. Hydrogen and acetylene ignite from discharges of roughly two hundredths of a millijoule, while propane and methane need on the order of a quarter of a millijoule, more than ten times as much. Combustible dust clouds are far less sensitive again, typically requiring millijoules to hundreds of millijoules, which is why dust protection concentrates on excluding dust from enclosures and limiting surface temperature rather than on spark energy alone. Intrinsic safety works by guaranteeing that no spark or thermal event within the equipment can exceed the relevant ignition threshold under any credible fault condition.

Electrostatic discharge deserves particular attention in a harvesting node, because the plastic housings and coatings that resist corrosion also accumulate charge. Standards limit the exposed area of non-conductive external surfaces and require that metallic parts be bonded and grounded, so that charge cannot build to a brush discharge. Similar reasoning applies to light-alloy enclosures, whose friction against rust can produce an incendive thermite spark, and to the antenna and its radome on a wireless node.

Intrinsic Safety: Ex ia, ib, and ic

Intrinsic safety, designated Ex i, prevents ignition by limiting the electrical and thermal energy available in a circuit to a level that cannot ignite the surrounding atmosphere. The technique constrains voltage, current, and the stored energy in any capacitance or inductance within the circuit. Because the available energy never reaches the ignition threshold, even a spark caused by a short circuit or a broken wire remains harmless.

The concept is subdivided by the degree of fault tolerance. Level ia remains safe with two independent countable faults applied and is suitable for Zone 0. Level ib remains safe with a single fault and is suitable for Zone 1. Level ic provides protection in normal operation, without applied faults, and is suitable for Zone 2. Harvesting nodes intended for the most demanding locations are designed to the ia level. Levels ia and ib additionally carry a safety factor of 1.5 applied during spark-ignition assessment, whereas ic is assessed without that margin, which is one reason a Zone 2 node can be substantially cheaper and more capable than a Zone 0 node of otherwise identical function.

Certification expresses the result as a set of entity parameters. An intrinsically safe apparatus is characterized by the maximum voltage, current, and power it may receive, denoted Ui, Ii, and Pi, together with the internal capacitance Ci and inductance Li that it contributes to the circuit. An associated apparatus, such as a barrier, is characterized by the corresponding output values Uo, Io, and Po and the maximum external capacitance Co and inductance Lo it can safely drive. A combination is valid when the source values do not exceed the apparatus ratings and when the apparatus reactances plus the cable reactances stay within Co and Lo. IEC 60079-25 extends this bookkeeping from a single apparatus to a complete intrinsically safe system, which is the level at which a harvester, its storage element, and its sensor electronics are usually assessed together.

Enclosure-Based Concepts

Flameproof protection, designated Ex d, contains an internal explosion within a robust enclosure whose joints quench any escaping flame before it can ignite the external atmosphere. This concept permits higher internal power than intrinsic safety but relies on heavy, precisely machined housings. Increased safety, designated Ex e, takes the opposite approach by preventing sparks and excessive temperatures through enhanced construction of terminals and connections.

Encapsulation, designated Ex m, embeds potentially sparking components in a solid compound that excludes the surrounding atmosphere, and it is a common choice for a supercapacitor or cell that would otherwise be difficult to certify. Pressurization, designated Ex p, maintains a protective inner pressure of clean air or inert gas to keep the flammable atmosphere out of the enclosure, but its need for a purge supply makes it impractical for a self-powered node. For dust atmospheres the analogous concept is protection by enclosure, designated Ex t, which relies on dust-tight construction and a controlled surface temperature.

Zone 2 has its own lighter-weight options. The former Ex nA marking for non-sparking equipment was withdrawn when IEC 60079-15 was revised, and its requirements now appear as Ex ec within IEC 60079-7; equipment previously marked Ex nA is re-marked Ex ec on recertification. Optical sources warrant a separate mention, because a focused beam can heat a particle to ignition: IEC 60079-28 limits optical power and irradiance and defines markings such as Ex op is for inherently safe optical radiation, which matters when light is used to carry power or data into a classified zone. A practical harvesting installation typically combines concepts, using intrinsic safety for the electronics and an Ex e, Ex m, or Ex t enclosure for the harvester and storage element.

Why Harvesting Suits Intrinsic Safety

Energy harvesting and intrinsic safety share a common principle: both operate at inherently low power levels. A thermoelectric or vibration harvester typically produces microwatts to tens of milliwatts, which sits far below the power an intrinsically safe circuit may handle. For perspective, IEC 60079-11 allows a small component to dissipate up to 1.3 watts and still be assessed as temperature class T4, and practical barriers deliver on the order of a watt; a harvester operates two or three orders of magnitude below that. This natural alignment simplifies certification, because the designer is not fighting against a high-power source that must be artificially constrained. It also removes the barrier or isolator that a conventionally powered instrument would need, since no energy crosses the boundary from a safe area at all.

The challenge shifts from limiting the source to managing the energy that is stored. A supercapacitor or small rechargeable cell accumulates charge over hours, and that reservoir could in principle deliver an igniting spark in microseconds. The intrinsically safe design therefore focuses on bounding the stored energy and controlling its release, ensuring that even a fully charged storage element cannot exceed the permitted limits. Current-limiting resistors, redundant series switches, and physical separation of the storage bank from any accessible terminal are the usual means.

Where the harvested power exceeds what conventional intrinsic safety permits, a further option exists. IEC TS 60079-39 describes Power-i, a technique that monitors the circuit electronically and truncates the duration of any spark, allowing more power to be delivered while preserving an equivalent level of safety. The technique applies to direct-current circuits of no more than 40 volts and does not extend to level ia, so it suits Zone 1 and Zone 2 installations that need to power an actuator or a higher-duty radio rather than a Zone 0 sensor.

Thermoelectric Harvesting

Thermoelectric generators exploit the temperature difference between a hot surface and the cooler surroundings to produce electricity through the Seebeck effect. Refineries and chemical plants abound in hot process pipes, reactor vessels, and steam lines whose surfaces are substantially warmer than the ambient air. A thermoelectric module clamped to such a surface can deliver a steady, predictable supply with no moving parts, which makes it attractive for long-term reliability.

Output scales with the square of the temperature difference actually developed across the module, which is always smaller than the difference between the pipe and the air because the thermal resistances of the clamp, the module, and the heat sink divide it. A compact unit relying on natural convection typically yields a few milliwatts to a few tens of milliwatts, while a larger assembly with an extended heat sink and a difference of many tens of kelvins can reach hundreds of milliwatts. Because most process lines run well above ambient for months at a time, the resulting supply is remarkably steady, and the harvester's output rises exactly when the plant is running and monitoring matters most.

The absence of moving parts is a meaningful safety advantage, because it removes a potential source of frictional sparking and mechanical wear. The designer must, however, ensure that the hot side of the module and its mounting hardware do not themselves create an accessible surface that exceeds the applicable temperature class. The usual remedy is to keep the hot interface entirely within the pipe insulation and to expose only the cool side, so that every touchable surface stays near ambient.

Vibration and Kinetic Harvesting

Rotating equipment such as pumps, compressors, fans, and electric motors generates continuous vibration that can be converted to electricity. Piezoelectric harvesters produce charge when a crystal or ceramic element is flexed, while electromagnetic harvesters generate current as a magnet moves relative to a coil. Both approaches suit the steady, periodic vibration that machinery produces during normal operation.

Because process machinery often runs continuously, vibration harvesting can supply a dependable base load. Resonant designs tuned to the dominant operating frequency of the host machine maximize energy capture, while broadband designs trade peak output for tolerance of varying conditions. Tuning is a real constraint rather than a detail: a resonant harvester matched to the 50 or 60 hertz line frequency, or to twice it, loses much of its output if the machine runs on a variable-speed drive, so variable-speed installations favor broadband or frequency-tunable designs. Commercially available electromagnetic harvesters certified to ATEX deliver on the order of tens of milliwatts from the casing vibration of a running pump or motor, which is ample for a condition-monitoring node.

Sealed construction protects the moving elements from the corrosive and dusty atmospheres common in these settings, and it also confines the moving mass so that it cannot become a frictional ignition source. Piezoelectric elements deserve a specific caution: a flexing ceramic can develop open-circuit voltages of tens or even hundreds of volts, so the certification analysis must bound that voltage and the charge stored on the element's own capacitance rather than assume the low voltages typical of the rest of the circuit.

Photovoltaic, Radio-Frequency, and Flow Sources

Photovoltaic cells convert ambient light to electricity and serve outdoor installations as well as indoor locations with sufficient artificial illumination. A silicon panel in full sun yields on the order of ten milliwatts per square centimeter, but the few hundred lux typical of an indoor plant area leave only tens of microwatts per square centimeter, a difference of two to three orders of magnitude that dictates whether a panel the size of a business card is sufficient. Output also varies with weather, season, and the film of dust or salt that accumulates on the cover glass, so photovoltaic nodes are always paired with storage sized for the longest expected dark period. Radio-frequency harvesting captures energy from ambient or dedicated electromagnetic fields; ambient fields in a plant rarely provide more than a few microwatts, so this approach is generally reserved for interrogated tags and for deliberately illuminated nodes rather than for continuous operation.

Fluid systems offer additional opportunities. A pressure differential across a valve or orifice, or the flow of liquid or gas through a pipe, can drive a small turbine or a pressure-actuated generator, and pipelines in particular carry enormous pressure energy that is otherwise dissipated at regulating stations. These flow-based harvesters tap the hydraulic and pneumatic energy already circulating through the plant, converting a small fraction to useful electrical power for instrumentation. They reintroduce a rotating element, however, so bearing wear, sealing, and the possibility of frictional heating return as certification concerns, and any turbine in a fuel stream must be assessed for ignition risk on its wetted side as well as its external surfaces.

Energy Storage Within Safe Limits

The storage element is the most safety-critical part of a harvesting node, because it accumulates energy over time. Supercapacitors and small rechargeable cells must be sized so that their stored energy remains within the limits permitted for the relevant gas group and protection level. The certification analysis treats the storage element as a potential spark source and verifies that its discharge cannot ignite the atmosphere.

The arithmetic is unforgiving. The energy in a capacitor is half its capacitance times the square of its voltage, so a single farad charged to two and a half volts holds roughly three joules, which is more than a hundred thousand times the minimum ignition energy of hydrogen. No credible certification treats such a reservoir as inherently harmless. Instead the design either encapsulates the storage element so that the atmosphere cannot reach it, or interposes current-limiting and redundant switching elements so that no accessible terminal can ever see the full discharge, or does both. Rechargeable cells attract further scrutiny because their failure modes include internal short circuit, venting, and thermal runaway, which is why many certified nodes prefer supercapacitors or hybrid layer capacitors despite their higher leakage.

Designers therefore partition storage, keeping the energy reservoir in a protected section and exposing only a strictly limited amount of energy to any field wiring. Low-leakage capacitors and cells reduce the standby drain that would otherwise waste hard-won harvested energy, which matters greatly when the average input is only microwatts: a supercapacitor leaking ten microamperes at two volts consumes twenty microwatts continuously, potentially more than the harvester supplies. Temperature compounds the problem, since leakage and self-discharge rise steeply with the elevated ambient temperatures common near process equipment, and cell life shortens accordingly.

Barriers, Isolation, and Transient Suppression

Where a harvesting circuit connects to wiring that leaves the intrinsically safe region, an interface device limits the energy that can cross the boundary. Zener barriers clamp voltage and limit current through a network of diodes, resistors, and a fuse, while galvanic isolators achieve the same result with transformer or optical coupling that breaks the direct electrical path. Both ensure that a fault on one side cannot inject dangerous energy into the hazardous area.

Transient and surge suppression protect the sensitive electronics from electrostatic discharge, switching transients, and induced surges. Careful suppression also prevents these events from becoming ignition sources in their own right. Galvanic isolation between the harvester, the storage element, and the sensor electronics further contains faults within defined sections of the circuit.

Power Management and Conditioning

Ambient sources rarely deliver power at the voltage or current the load requires, so a conditioning stage is essential. Ultra-low-power boost converters raise the low output of a thermoelectric or photovoltaic source to a usable level, and maximum power point tracking adjusts the operating point to extract the greatest available power as conditions change. Cold-start capability is the decisive specification: a thermoelectric module working across a modest temperature difference may open-circuit at well under a hundred millivolts, and only converters designed to start from a few tens of millivolts, typically with a step-up transformer to bootstrap the oscillator, will bring such a node to life from a fully discharged state. These converters must also achieve high efficiency at microwatt input, where a conventional regulator's own quiescent current would consume more than the source delivers.

The power management circuitry must be spark-free and must respect the energy limits at every node. Components are derated so that they operate well within their ratings even under fault, in line with the derating factors that IEC 60079-11 prescribes for infallible components and connections, and conformal coating protects the assembled circuit from moisture, dust, and corrosive vapors. Creepage and clearance distances follow the standard's tables rather than ordinary printed-circuit practice, and separation between intrinsically safe and non-intrinsically-safe tracks is enforced physically. Together these practices preserve both safety and longevity in an unforgiving environment.

Energy Budgets and Duty Cycling

A hazardous-area harvesting node lives or dies by its energy budget, and the budget is dominated by the radio. Transmitting a short measurement burst may draw tens of milliamperes for a few milliseconds, a peak power hundreds of times greater than the harvester's average output. The storage element exists chiefly to bridge that mismatch: it accumulates energy slowly and releases it in brief, high-current bursts. Sizing follows directly from this role, since the reservoir must cover the peak demand of a transmission plus the longest interval during which the source may be unavailable.

Duty cycling reconciles the two scales. A node that sleeps at a microampere and wakes once a minute to measure and report averages tens to a few hundred microwatts, which a modest harvester can sustain indefinitely. Lengthening the reporting interval reduces average power roughly in proportion, so the specified update rate is in practice a negotiation between the process engineer, who wants frequent data, and the power budget, which grows with every report. Adaptive schemes help, reporting slowly while a measurement stays within limits and accelerating when it moves, and event-driven modes transmit only on an alarm.

Because ambient sources fail in predictable ways, many certified nodes are hybrids, pairing a harvester with a primary cell that carries the load through outages and extends service life from a few years to the life of the asset. The hybrid arrangement complicates certification, since the primary cell brings its own stored energy and its own chemistry-specific requirements, but it removes the fragility of a purely harvested supply. Whichever path is chosen, the design must define its behavior on depletion: a node that browns out mid-transmission and restarts in a loop can drain its reservoir faster than the harvester refills it, so a hysteretic enable threshold that holds the load off until sufficient charge has accumulated is essential.

Ingress Protection and Sealing

Ingress protection ratings describe how well an enclosure resists the entry of solid objects and water, expressed by the familiar IP code of IEC 60529. Outdoor and washdown installations demand high ratings to exclude wind-driven rain and cleaning sprays, and IP66 or IP67 is the usual specification for a field node. In dust-hazard areas, ingress protection stops being a matter of convenience and becomes the protection concept itself: IEC 60079-31 requires dust-tight IP6X construction where conductive dust or the highest equipment protection level is involved, since any dust that reaches the interior can both ignite and blanket the electronics. Robust seals and gaskets maintain this protection across temperature cycles and over long service life, and the enclosure must be sealed against the pressure changes that daily heating and cooling drive, which otherwise breathe moisture in through the smallest imperfection.

For flameproof enclosures, the mechanical design centers on the flame paths, the precisely dimensioned gaps at joints and shaft penetrations that cool and quench escaping gases. These joints must be manufactured to tight tolerances and kept free of corrosion or damage, because their geometry is what prevents an internal explosion from propagating outward.

Surface Temperature Management

A thermoelectric harvester deliberately couples to a hot source, which creates a tension with the temperature-class requirement that limits external surface temperature. The mechanical design must route heat in a way that powers the generator without allowing any accessible surface to exceed the autoignition margin of the surrounding atmosphere. Thermal barriers, heat spreaders, and careful placement of the hot interface help reconcile these competing demands.

Thermal management must also account for the heat generated internally by the electronics and storage element, modest though it is. Settled dust layers complicate the picture, because they reduce heat dissipation and can themselves ignite if a surface stays hot. The design therefore verifies surface temperatures under worst-case conditions, including fouling and reduced airflow.

Corrosion Resistance and Durability

Process environments expose equipment to salt spray, hydrogen sulfide, acidic vapors, and other aggressive species that attack metals and polymers. Enclosure materials such as stainless steel, marine-grade aluminum, or engineered plastics resist this attack, and protective coatings extend service life further. Corrosion of a flameproof joint or a sealing surface would compromise protection, so material selection is a safety matter, not merely a maintenance one.

Long-term reliability ties directly to the central promise of harvesting, which is freedom from maintenance. A node that must be opened for repair forfeits much of its advantage, so the mechanical design favors sealed, solid-state, and wear-free construction wherever possible. The goal is an installation that operates untouched for the life of the host asset.

The IEC 60079 Series

The IEC 60079 series provides the technical foundation for explosion-protected equipment worldwide. The general requirements appear in IEC 60079-0, while individual protection concepts have their own parts: flameproof enclosures in IEC 60079-1, pressurization in IEC 60079-2, increased safety in IEC 60079-7, intrinsic safety in IEC 60079-11, encapsulation in IEC 60079-18, and dust protection by enclosure in IEC 60079-31. Related parts cover area classification in IEC 60079-10-1 and 60079-10-2, intrinsically safe systems in IEC 60079-25, and the additional requirements for equipment protection level Ga in IEC 60079-26. A harvesting node is assessed against the general part together with the parts for each protection concept it employs.

These standards specify the tests, the design constraints, and the documentation required to substantiate a claim of safety. For an intrinsically safe harvester, the assessment scrutinizes the energy stored in capacitances and inductances, the spark-ignition behavior of the circuit as measured on the standardized spark test apparatus, and the surface temperatures reached under fault. The assessment is performed on the circuit as built, including its printed-circuit layout, component derating, and separation distances, which is why an apparently minor design revision can require a fresh evaluation. Certificates whose numbers carry an X suffix impose specific conditions of use, such as a restricted ambient temperature range or a requirement to install the device where it is protected from impact, and an installer must read those conditions rather than trust the marking alone.

ATEX and the IECEx Scheme

Within the European Union, equipment for explosive atmospheres falls under the ATEX directive 2014/34/EU, applicable since April 2016, which sets the legal requirements for products placed on the market. Equipment Group I covers underground mining and is divided into categories M1 and M2; Equipment Group II covers surface industry and is divided into categories 1, 2, and 3, each carrying a G or D suffix for gas or dust. Category 1 equipment may be used in Zone 0 or 20, category 2 in Zone 1 or 21, and category 3 in Zone 2 or 22. Conformity for the higher categories requires the involvement of a notified body, and the directive draws its detailed technical requirements from harmonized standards that track the IEC 60079 series. A companion directive, 1999/92/EC, places the corresponding duties on the employer who operates the installation.

The IECEx scheme provides an international certification system that allows a single assessment, documented in a certificate of conformity and its supporting test report, to be recognized across participating countries. Closely related is the concept of the equipment protection level, which expresses the assurance of protection directly rather than by inference from the protection concept. The levels Ma and Mb apply to mining, Ga, Gb, and Gc to gas atmospheres, and Da, Db, and Dc to dust atmospheres, mapping in each case to the zones in which the equipment may be installed. The equipment protection level appears in the marking as a suffix on the protection concept, so a node marked Ex ia IIC T4 Ga announces at a glance that it is intrinsically safe to the highest level, suitable for hydrogen and acetylene, limited to 135 degrees Celsius, and permitted in Zone 0.

North American Class and Division Systems

North American practice has traditionally followed the Class and Division system defined in Article 500 of the National Electrical Code, which classifies areas by the type of hazard and the likelihood of its presence. Class I covers flammable gases and vapors, Class II combustible dusts, and Class III ignitible fibers and flyings. Division 1 corresponds broadly to the continuous or likely presence of a hazard, and Division 2 to its occasional presence. Material groups identify the specific substances, and their lettering runs opposite to the IEC convention: Class I Group A is acetylene, Group B hydrogen, Group C ethylene, and Group D propane, so the most hazardous group is A rather than IIC. Class II dusts fall into Groups E, F, and G for metal, carbonaceous, and other dusts respectively.

North American codes have also adopted the zone system to align with international practice. Article 505 provides Class I Zone 0, 1, and 2 classification for gases, and Article 506 provides Zone 20, 21, and 22 classification for combustible dusts and ignitible fibers. Either framework may be used, but the two are not mixed within a single classified area, and the Canadian Electrical Code is built primarily around the zone system. Equipment certified to the North American adoptions of the IEC 60079 standards can therefore be installed in either scheme, provided the marking and the documented conditions of use match the area classification actually assigned.

A designer serving a global market must therefore understand both frameworks and ensure that equipment carries the appropriate certification and marking for each jurisdiction, which in practice means seeking IECEx, ATEX, and North American approvals for the same design. The marking on the nameplate communicates the protection concept, the gas or dust group, the temperature class, and the equipment protection level, allowing an installer to verify suitability for a given location at a glance.

Installation and System Documentation

IEC 60079-14 sets the rules for electrical installations in explosive atmospheres, covering the selection of equipment against the assigned zone and group, cable entries, bonding, and the verification that a designer must document. For an intrinsically safe system, the central artifact is the descriptive system document, which records every apparatus in the loop, its entity parameters, the cable parameters, and the calculation showing that the combination remains safe. A harvesting node reduces the scope of this document considerably, because no wiring crosses from the safe area, but it does not eliminate it: the harvester, the storage element, the sensor, and any temporary connection made for commissioning still form a system that must be shown to be compatible.

Practical installation details matter more than their modest appearance suggests. A thermoelectric harvester must be clamped so that the intended thermal path is achieved and the hot interface stays enclosed; a resonant vibration harvester must be mounted rigidly and in the correct axis, since a compliant mount detunes it and squanders most of its output. Grounding and bonding must follow the certificate's conditions, and any non-conductive external surface must be protected from the charge-generating processes, such as high-pressure steam cleaning, that plant maintenance routinely involves.

Inspection, Maintenance, and Repair

IEC 60079-17 defines the regime of initial, periodic, and sample inspections that keep an installation compliant, graded as visual, close, or detailed according to the scrutiny applied. A harvesting node is inspected for the integrity of its enclosure and seals, the condition of its gaskets and cable entries, the absence of corrosion or accumulated dust, and the continued validity of its marking against the area classification. Because these nodes are deployed in large numbers and in awkward places, sample-based inspection regimes are common, with the fleet's own diagnostics flagging outliers for closer attention.

Repair and overhaul fall under IEC 60079-19, which constrains what may be done to certified equipment without invalidating its approval. Substituting a supercapacitor for one of different rating, replacing a gasket with a non-approved part, or opening a potted assembly can all void certification, so the practical policy for a sealed harvesting node is replacement rather than repair. This aligns neatly with the technology's own logic: a node designed to run untouched for a decade is not meant to be serviced, and the diagnostics that report its state of health exist so that a failing unit can be swapped during a planned shutdown rather than investigated in the field.

Oil, Gas, and Petrochemical Facilities

Upstream wellheads, refineries, liquefied natural gas terminals, and petrochemical plants present extensive Zone 1 and Zone 2 areas. Self-powered wireless sensors monitor pressure, temperature, flow, and equipment vibration across these sites without the prohibitive cost of running conduit and cable to every measurement point. Harvested power lets operators instrument remote and previously inaccessible locations, improving both safety and process visibility.

Condition monitoring of rotating equipment is a leading use case, because pumps and compressors generate the very vibration that powers the sensors observing them. Leak and gas detection forms another important application, providing early warning of escaping hydrocarbons before a hazardous accumulation can form.

Pipelines, Storage, and Dust Hazards

Gas pipelines extend across remote terrain where grid power is unavailable, making harvested energy attractive for the sensors that monitor pressure, flow, and corrosion along their length. Storage tanks and terminals present similar challenges. In these settings, thermoelectric and flow-based harvesters frequently supply the modest power that wireless corrosion and pressure monitors require.

Combustible-dust environments add their own demands. Grain elevators, silos, flour mills, and paint or solvent handling areas form dust or vapor hazards that require Zone 20, 21, or 22 protection. Harvesting nodes deployed here monitor temperature, gas concentration, and equipment condition while satisfying the dust-tightness and surface-temperature constraints those zones impose.

Wireless Networks and Mining

Industrial wireless protocols were designed for the process plant and integrate naturally with harvested power. WirelessHART, standardized as IEC 62591, and ISA100 Wireless, standardized as IEC 62734, both operate in the 2.4 gigahertz band using time-synchronized, channel-hopping mesh networks, an approach chosen precisely because it tolerates the multipath and interference of a steel-framed plant while keeping radios asleep for all but a scheduled fraction of each second. A self-powered node joins the mesh, reports its measurements, and relays the traffic of neighboring nodes, all on energy drawn from its surroundings. The relaying duty is worth noting, because a node that routes for its neighbors spends materially more energy than a leaf node, so network design must place the better-supplied harvesters at the routing positions.

Underground mining presents a distinct but related hazard in the form of firedamp, the methane that accumulates in coal workings, which is classified as Group I with its own equipment protection levels Ma and Mb. Self-powered gas detection and asset tracking improve safety in these environments, where conventional power is both expensive to distribute and hazardous to service, and where coal dust adds a second hazard on top of the methane. Across all of these settings, the recurring theme is the same: harvested energy supports continuous monitoring precisely where wiring and battery maintenance are most difficult.

Limitations and Trade-offs

The case for harvesting in classified zones is strong but not universal, and honest design begins by acknowledging where it fails. Ambient energy must actually be present at the measurement point, and many of the most valuable measurements sit precisely where it is not: a corrosion probe on an unheated pipe in a dark, still corner offers neither heat, nor vibration, nor light. Site surveys that measure the available temperature difference or vibration amplitude before specification, rather than after installation, prevent the common disappointment of a node that reports intermittently because its source was assumed rather than verified.

Certification cost is the second constraint. Approving a design to ATEX, IECEx, and North American schemes takes months and substantial expense, and it must be repeated in part whenever the circuit changes, which discourages the incremental refinement that low-power design usually rewards. The commercial consequence is a narrower catalog of certified harvesting products than the underlying technology would suggest, and a strong incentive to reuse a certified power module across a family of sensors rather than optimize each one.

Finally, harvested power constrains what the node can do. Bandwidth-hungry measurements such as continuous high-resolution vibration spectra, and actuation of any kind, exceed what microwatts can sustain; harvesting suits periodic scalar measurements and event reporting far better. Where more capability is required, the honest options are a hybrid supply, a Power-i design under IEC TS 60079-39, or conventional wiring with a barrier. Choosing among them on the basis of the measurement's actual energy demand, rather than on the appeal of a self-powered device, is what separates installations that run for a decade from those that are quietly abandoned.