Yes, PLCs can be used in hazardous or explosive environments, but only when they are specifically certified and configured for that purpose. Standard off-the-shelf PLCs are not suitable for use in areas where flammable gases, vapors, dusts, or explosive atmospheres may be present. The right combination of certification, protection method, and installation practice determines whether a PLC can operate safely in these conditions. This article walks through the key questions engineers and plant managers face when specifying PLCs for hazardous locations.
What makes an environment hazardous or explosive for electrical equipment?
A hazardous or explosive environment is any location where flammable substances such as gases, vapors, mists, or combustible dusts are present in concentrations that could ignite if exposed to a spark or a sufficiently hot source. Electrical equipment, including PLCs, can generate sparks during normal operation or fault conditions, making them a potential ignition source in these settings.
Hazardous areas are classified into zones based on the likelihood and duration of an explosive atmosphere being present. For gases and vapors, the classification works as follows:
- Zone 0: An explosive atmosphere is present continuously or for long periods
- Zone 1: An explosive atmosphere is likely to occur during normal operation
- Zone 2: An explosive atmosphere is not likely during normal operation, but may occur occasionally
For combustible dusts, equivalent classifications apply under Zone 20, 21, and 22. The zone classification directly determines which protection methods and equipment certifications are required. Any electrical device installed in or near these zones must be evaluated against the specific hazard present, whether that is a flammable solvent in a chemical plant, natural gas in an oil and gas facility, or flour dust in a food processing environment.
What certifications do PLCs need for hazardous area use?
PLCs intended for use in hazardous areas must carry recognized explosion protection certifications that confirm the equipment has been independently tested and approved for use in specific zone classifications. In Europe, the primary certification framework is ATEX (Atmosphères Explosibles), governed by EU directives. In international markets, the IECEx scheme provides equivalent globally recognized certification. In North America, UL and FM approvals under NEC classifications are standard.
An ATEX-certified PLC will carry a marking that indicates the equipment group, category, and the type of explosive atmosphere it is suitable for. For example, a marking of II 2G indicates the device is suitable for surface industries (Group II) in Zone 1 (Category 2) where gas hazards are present (G). The temperature class (T1 through T6) indicates the maximum surface temperature the device can reach, which must remain below the ignition temperature of the substances present.
Certification alone is not sufficient. The complete installation, including barriers, enclosures, cabling, and field instruments, must also meet the applicable standards. A PLC with ATEX certification must be installed as part of a fully compliant Ex system to maintain its protection integrity.
How do intrinsic safety and explosion-proof enclosures differ for PLCs?
Intrinsic safety and explosion-proof enclosures are two fundamentally different approaches to protecting PLCs and associated equipment in hazardous areas. Intrinsic safety limits the energy available in a circuit so that any spark or heat generated is too weak to ignite an explosive atmosphere. Explosion-proof enclosures contain any ignition that does occur, preventing it from reaching the surrounding atmosphere.
Intrinsic safety (Ex i)
Intrinsically safe systems use barriers or isolators to restrict voltage and current to levels that cannot produce a spark capable of igniting the hazardous substance. This approach is particularly suited for sensor and signal circuits connected to PLCs. The PLC itself is typically located in a safe area, while only the intrinsically safe field circuits extend into the hazardous zone. Intrinsic safety is the preferred method for Zone 0 and Zone 1 applications because it eliminates the ignition risk rather than containing it.
Explosion-proof enclosures (Ex d)
An explosion-proof or flameproof enclosure is designed to withstand an internal explosion and cool any escaping gases before they can ignite the surrounding atmosphere. PLCs housed in Ex d enclosures can be placed in Zone 1 or Zone 2 areas, but the enclosure must be opened using specific procedures, and any maintenance work typically requires the area to be declared gas-free first. This method is more common where the PLC must be physically located within the hazardous area rather than in a remote safe area cabinet.
Which Siemens PLCs are approved for use in hazardous areas?
Siemens offers several PLC and process control products that carry ATEX and IECEx approvals for use in hazardous environments. The SIMATIC ET 200 range of distributed I/O systems includes variants specifically designed for Ex Zone 1 and Zone 2 applications, allowing field-level signal processing to take place close to the process while the main controller remains in a safe area. The SIMATIC ET 200iSP is an intrinsically safe distributed I/O station certified for Zone 1 and Zone 2 (gas) as well as Zone 21 and Zone 22 (dust).
For process automation, the SIMATIC PCS 7 distributed control system supports Ex-rated field devices and I/O through its open architecture, allowing engineers to integrate certified intrinsic safety barriers and Ex field instruments within a unified control strategy. The system’s modular design means that Ex-rated remote I/O can be placed in hazardous areas while the process controller and engineering station remain in a non-hazardous control room.
Siemens also provides SIMATIC S7-300 and S7-1500 modules that can be configured for use in Zone 2 environments when housed in appropriate certified enclosures. The specific approval scope, temperature class, and gas group compatibility should always be verified against the current product documentation and the site’s zone classification before specifying any hardware.
What installation and wiring rules apply to PLCs in Ex zones?
Installing PLCs and associated wiring in hazardous zones requires strict adherence to standards such as the IEC 60079 series, which covers everything from equipment selection to installation, inspection, and maintenance. The installation rules vary depending on the protection concept used, but several principles apply across all Ex installations.
- Cable selection: Cables must be suitable for the protection concept in use. Intrinsically safe circuits require dedicated cables that are physically separated from non-intrinsically safe circuits to prevent energy transfer.
- Earthing and bonding: All metallic components must be properly earthed and bonded to prevent static charge buildup, which itself can be an ignition source.
- Cable entry points: Enclosures must use certified cable glands that maintain the integrity of the Ex protection method. An improperly sealed gland can compromise an explosion-proof enclosure.
- Segregation: Intrinsically safe wiring must be routed separately from power and non-IS signal cables, using dedicated cable trays or conduits where necessary.
- Documentation: Every Ex installation must be supported by an Ex documentation package that records the equipment used, the zone classification, the protection concepts applied, and the inspection schedule.
Ongoing inspection and maintenance are also regulated. Initial inspections must be carried out before commissioning, and periodic inspections are required throughout the operational life of the installation. Only personnel with recognized Ex competency qualifications should carry out work on Ex-rated installations.
Can a standard PLC be used near — but not inside — a hazardous zone?
Yes, a standard PLC without Ex certification can be used outside a hazardous zone, provided it is installed beyond the defined zone boundary. In practice, this means placing the PLC in a control room, a pressurized enclosure, or a designated safe area that is physically separated from the classified zone. The PLC then communicates with Ex-rated field devices or distributed I/O through intrinsically safe barriers or other certified interface devices.
This approach is widely used in process industries because it allows engineers to use standard PLC hardware and software environments while keeping only the necessary Ex-certified components in the hazardous area. It also simplifies maintenance, since work on the PLC itself can be carried out without the restrictions that apply inside an Ex zone.
The critical requirement is that the zone boundary is clearly defined and that all cables and equipment crossing that boundary are properly certified and installed. A standard PLC connected to an uncertified cable running into a Zone 1 area would not meet the requirements, regardless of where the PLC itself is physically located. The integrity of the entire circuit, not just the PLC, must be maintained.
How CoNet helps with PLC safety in hazardous environments
Specifying, engineering, and commissioning PLCs for hazardous environments requires both deep knowledge of Ex standards and hands-on experience with the hardware involved. As a Siemens specialist with decades of experience in process automation, we support industrial clients across the full project lifecycle when it comes to PLC process safety and Ex-rated installations.
We work with clients in the chemical, oil and gas, food and beverage, and energy sectors to deliver compliant, reliable automation solutions for hazardous areas. Our work in this area includes:
- Zone classification review and Ex documentation preparation
- Selection and specification of ATEX-certified Siemens hardware including SIMATIC ET 200iSP and PCS 7 distributed I/O
- Engineering of intrinsically safe field circuits and barrier specification
- Full installation, commissioning, and inspection support for Ex zones
- Ongoing maintenance and periodic inspection programs for Ex installations
- Integration of Ex field devices into plant automation systems built on the SIMATIC PCS 7 platform
We are the only company in the Netherlands certified as both a PCS 7 Process Safety Specialist and a Siemens COMOS partner, which means our clients have a single point of contact for both process safety engineering and automation. If you are planning a project that involves PLCs in hazardous or explosive environments, get in touch with us to discuss how we can support your specific requirements.
