Surge Protection for Oil, Gas and Petrochemical Plants: Hazardous Area and Corrosive Environment Selection

Oil, gas and petrochemical plants combine two problems that rarely appear together elsewhere: explosive atmospheres that tightly constrain what may be installed and how, and long outdoor cable routes that couple lightning and switching surges deep into the control system. Petrochemical surge protection is therefore a compliance exercise as much as an electrical one — the right device in the wrong enclosure, or with the wrong certificate, is simply not an acceptable installation.

This article explains how surge protective device (SPD) selection is typically approached in these plants: how hazardous and non-hazardous areas are treated differently, why an SPD module is not automatically Ex-certified equipment, and how corrosive atmospheres, outdoor cabinets, long 4–20 mA instrument loops and the plant grounding grid shape the final solution.

Key takeaways

• Area classification (Zone 0/1/2, 20/21/22) defines the installation rules; it does not change the physics of the surge, but it changes where and how SPDs may be mounted.
• An SPD module is not Ex-certified by default — the complete assembly of enclosure, glands, wiring and bonding is what carries the Ex marking.
• Long 4–20 mA loops and the plant grounding grid usually drive signal-side protection more than any other factor.
• Certificate scope is confirmed per model documentation and per project specification, rather than assumed from a product family name.

Contents

1. Hazardous vs Non-Hazardous Areas: Two Different Rulebooks
2. The SPD Is Not the Ex Certificate
3. Power Distribution: From Substation to Field Cabinets
4. Instrument Signals, 4–20 mA Loops and the Grounding Grid
5. Corrosive Atmospheres and Outdoor Cabinets
6. Risk Distribution Across the Plant
7. Selection Checklist
8. FAQ

Hazardous vs Non-Hazardous Areas: Two Different Rulebooks

Every plant starts from its area classification drawings. Locations where flammable gas or vapour may be present are classified as Zone 0, 1 or 2, and combustible dust locations as Zone 20, 21 or 22, following the IEC 60079 series of standards for explosive atmospheres. Substations, motor control centres and control buildings are typically outside these zones, in what is loosely called the safe or non-hazardous area.

For surge protection this split has a practical consequence: most SPD hardware lives in the non-hazardous area. Main distribution boards, MCCs and marshalling cabinets are standard industrial installations, and the usual staged approach applies — Type 1 devices at service entrances where partial lightning current is expected, Type 2 devices downstream. Power SPDs for low-voltage AC systems are covered by IEC 61643-11, and selection follows the same voltage and coordination logic as any industrial site.

Inside the hazardous area, the rules change. Nothing is installed unless its construction, certification and installation method are permitted for that zone. A hazardous area surge protector is therefore not a catalogue category but a solution: an SPD module placed inside a certified Ex enclosure or junction box, with certified cable glands, correct bonding and documentation that matches the zone, gas group and temperature class on the area classification drawing.

The zoning exercise usually runs alongside the plant's lightning risk assessment. Where structures fall under an external lightning protection system designed to IEC 62305, the lightning protection level and the resulting lightning protection zones define how much partial lightning current each boundary may carry — which in turn decides whether Type 1, Type 2 or combined devices are appropriate at each intake. Plants without a formal risk assessment typically commission one before the SPD concept is frozen, because retrofitting coordination later is far more disruptive than designing it in.

The SPD Is Not the Ex Certificate

This point deserves emphasis because it is the most common misunderstanding in procurement. An SPD module tested to IEC 61643 standards has demonstrated surge performance; it has not, by default, demonstrated suitability for explosive atmospheres. Unless a specific model carries its own Ex certificate and marking, the module itself is ordinary industrial equipment.

In practice, Ex compliance is achieved at the assembly level. The enclosure or junction box carries the certification, the glands and blanking elements preserve the protection concept, the wiring and bonding follow the installation rules, and the whole installation is executed and inspected under the project's Ex framework — for example the IEC 60079 series internationally, or the ATEX Directive 2014/34/EU within the EU. The SPD inside is one component of that certified solution, not the certificate holder.

It also means "explosion-proof" is not treated as a default attribute of any surge protector. Whether a given Ex area SPD solution is acceptable depends on the certificate scope of the enclosure and components, the zone and gas group on the drawing, and the project's own specifications. Where a project requires certified equipment, the applicable certificate and its conditions of use are confirmed against the model documentation before ordering — our certificates page lists the documentation available per product family.

Power Distribution: From Substation to Field Cabinets

Plant power distribution typically runs from medium-voltage intake through unit substations to low-voltage MCCs, and from there to field distribution boards, analyser houses and outdoor cabinets. Each transition is a candidate SPD location, and coordination between stages is what keeps residual levels manageable at the loads.

At the main low-voltage boards, Type 1 or combined Type 1+2 devices are common where the plant has external lightning protection or overhead lines. Downstream boards and field cabinets then use Type 2 devices matched to the local supply voltage and earthing arrangement. Product families such as AC power SPDs cover these positions; the final ratings come from the project single-line diagrams rather than from generic assumptions.

Two site-specific checks are worth the effort. First, plants with co-generation or large drives see frequent internal switching surges, so SPD energy coordination with upstream protection and the actual prospective fault levels should be verified, not assumed. Second, remote field boards fed by long cable runs behave electrically like separate structures: local ground potential rise during lightning can stress equipment even when the substation protection works correctly, which is why field-level Type 2 protection is so common in petrochemical surge protection designs.

Instrument Signals, 4–20 mA Loops and the Grounding Grid

Instrumentation is where surge damage in process plants most often shows up. A plant may have thousands of 4–20 mA loops running from field transmitters through multicore cables to the control system, with runs of several hundred metres crossing pipe racks and open areas. These cables are efficient antennas for lightning-induced surges, and the I/O cards they terminate on are among the most sensitive electronics on site.

Signal SPDs for these loops are selected from families addressed by IEC 61643-21, which covers SPDs for telecommunication and signalling networks. For 4–20 mA circuits the practical checks are the SPD's nominal voltage relative to the loop supply, its series resistance and leakage current (so loop accuracy is unaffected), and its surge current rating relative to the exposure of the route. Families such as signal and data SPDs are matched to each interface by datasheet, including digital I/O, fieldbus and communication lines where present.

Placement follows the cable route. A signal SPD at the control-room cabinet entry is standard practice; for long runs across open plant, a second SPD at the field end — typically inside the junction box — is frequently added, since ground potential differences between the two ends are exactly what stresses the loop. Inside hazardous areas that field-end SPD sits within the certified enclosure solution described earlier.

The grounding grid ties all of this together. Plants bond structural steel, cable trays, instrument earth and protective earth into a common grid, and every SPD diverts its surge current into that grid. Screen grounding philosophy, separation of instrument and protective earth bars, and bonding conductor routing all follow the project design — an SPD connected to a long, inductive earth lead will not deliver its datasheet residual level, whatever its ratings say.

Beyond analogue loops, plants increasingly rely on digital infrastructure in the field: fieldbus segments, serial links to analyser systems, CCTV over fibre and copper, and industrial Ethernet to remote I/O. Each of these interfaces has its own voltage levels and bandwidth constraints, so the SPD checks shift toward insertion loss, return loss and connector format rather than loop resistance. The placement logic, however, stays the same — protect where lines cross zone boundaries and at the ends of long exposed runs.

Corrosive Atmospheres and Outdoor Cabinets

Beyond the Ex question, chemical plants and coastal facilities attack hardware continuously. Hydrogen sulphide, chlorine, ammonia and salt-laden air corrode terminals, housings and coating systems, and many SPD locations — tank farms, loading racks, jetty areas — are fully outdoor. The corrosion category for each location is defined by the project specification, often referencing the ISO 12944 classifications.

For surge protection this shifts attention from the module to the packaging: enclosure material (coated steel, stainless steel or GRP as specified), ingress protection rating, gland material, and terminal construction. The SPD module inside is generally standard; what changes is the enclosure system and the installation detail that keep the atmosphere out over a 15–25 year service life.

Outdoor cabinets also concentrate several stresses at once — temperature cycling, condensation, vibration near rotating equipment, and the long cable runs already discussed. Selection therefore typically confirms the operating temperature range, the enclosure and coating against the corrosion category, and maintainability: pluggable SPD modules with status indication allow replacement during routine inspections without rewiring, which matters when the cabinet sits in a classified area where every intervention requires a permit.

Inspection regimes complete the picture. SPD status indicators and remote signalling contacts are commonly integrated into the plant's maintenance rounds and DCS alarm lists, so a degraded module is found during routine checks rather than after the next storm season. In classified areas this is particularly valuable, because each physical intervention involves gas testing, permits and sometimes isolation of adjacent equipment — remote status reduces the number of times the enclosure needs to be opened at all.

Risk Distribution Across the Plant

The table below summarises how surge risk and protection focus typically distribute across a plant's main areas. It is a planning aid; the area classification drawings and project specification remain authoritative.

Note: risk levels and protection stages are indicative. Actual zoning, exposure and SPD selection are determined by the project's lightning risk assessment and area classification documentation.

Selection Checklist

The checklist below organises the questions typically answered before an SPD solution is specified for an oil, gas or petrochemical site. Values are project-dependent and confirmed against datasheets and certificates.

Note: this checklist supports engineering review and procurement discussion. It does not replace the project specification, the hazardous area verification dossier, or certificate conditions of use.

Engineering review note: Standard references (IEC 61643-11/21, IEC 60079 series, ATEX 2014/34/EU, ISO 12944) and circuit descriptions have been checked for consistency. No product in this article is described as Ex-certified or "explosion-proof" by default; all Ex-related requirements are stated as matters to be confirmed against project specifications and the certificate documentation of the specific model. Ratings are expressed as selection rules because final values are project-dependent.

Frequently Asked Questions

Is a standard SPD suitable for direct installation in a Zone 1 hazardous area?

A standard SPD module is not, by itself, Ex-certified equipment. In hazardous areas it is typically installed inside a certified Ex enclosure or junction box, with certified glands and bonding, as part of a complete installation designed to the project Ex rules. Any Ex marking applies only to the certified assembly or model as stated in its certificate.

Does the SPD change the Ex certification of my instruments or loops?

The loop and its instruments remain governed by the instrument vendor's documentation and the project's Ex design. Adding surge protection is an engineering change that should be reviewed within the project's approval process, and the certificate scope of every component should be confirmed against its own model documentation.

Where should surge protection for a 4–20 mA loop be installed?

Common practice places a signal SPD at the control-room cabinet entry and, for long cable runs crossing open plant areas, another SPD at the field end, typically inside a junction box. SPD leakage, series resistance and voltage rating are matched to the loop so measurement performance is not affected.

How does a corrosive atmosphere affect SPD selection?

Corrosive gases and coastal atmospheres mainly affect enclosures, terminals and coatings rather than the SPD module's electrical ratings. Outdoor cabinets are typically specified with suitable materials, ingress protection and coating systems, and the project specification defines the applicable corrosion category.

What information is needed to select an Ex area SPD solution?

A practical starting package is the area classification (zone, gas group, temperature class), the instrument signal types and loop counts, the system voltages and earthing arrangement, plus the environmental conditions and the certificates required by the project. You can share these details through our contact page for selection support.