Signaling in hazardous locations

Hazardous locations are classified using the Class and Division system, as defined by the National Electrical Code® (NEC).

Classes – Type of hazard present

• Class I: Locations where flammable gases or vapors may be present in sufficient quantities to produce an ignitable or explosive mixture.
• Class II: Locations made hazardous by the presence of combustible dusts that may be electrically conductive or capable of ignition.
• Class III: Locations where easily ignitable fibers or flyings are present, but not likely to be suspended in the air in quantities sufficient to produce ignitable mixtures.

Divisions – Likelihood of the hazard

• Division 1: A location in which a hazardous atmosphere exists under normal operating conditions, or where it may exist frequently due to maintenance, repair, or leakage.
• Division 2: A location in which hazardous materials are normally contained, but may become hazardous under abnormal conditions, such as equipment failure or accidental release.

Signaling devices in hazardous locations

The increasing use of visual and audible signaling devices to improve workplace safety is a positive trend, particularly in petrochemical, oil, and gas facilities, where the consequences of an incident are significantly greater than in most industrial environments.

Hazardous locations are defined as areas where flammable gases, vapors, or combustible dusts may be present:

• Continuously or for long periods
• During normal operating conditions
• Only under abnormal or infrequent conditions

Additional requirements apply based on temperature class (T-code) and the auto-ignition temperature of the gas or dust present, ensuring that installed equipment cannot become an ignition source.

Products designed for hazardous locations must comply with increasingly stringent regulations:

• North America: UL standards and NEC compliance
• Europe: ATEX
• International / Asia-Pacific: IECEx
• International marine: DNV

In addition to global certifications, many installations also require regional and local fire and safety approvals, such as the California State Fire Marshal (CSFM) and FM approval.

Types of hazardous location protection

Hazardous location signaling devices generally fall into one of two protection concepts:

• Intrinsically Safe (IS)
• Explosion-Proof (XP)

Both approaches are designed to provide reliable operation in harsh industrial environments while minimizing ignition risk.

This document does not attempt to explain hazardous location standards in detail, but instead provides practical guidance on selecting signaling devices for these applications.

Intrinsically Safe (IS) devices

Intrinsically safe devices are designed to limit electrical and thermal energy to levels below those capable of igniting a hazardous atmosphere. They are suitable for the most hazardous locations and can be applied across multiple classifications.

IS devices typically use standard industrial enclosures, with safety achieved through specially designed electronics. When used with a Zener barrier or galvanic isolator, they provide a highly reliable and installation-friendly solution.

Due to strict energy limitations, IS devices deliver lower output performance than equivalent devices installed in non-hazardous areas. Typical performance includes:

• Alarm horns: 90–105 dB at 1 meter
• Strobes: Low-power LED indicators rather than high-intensity xenon strobes

Because of these output limitations, IS alarm horns are often not suitable for high-noise outdoor environments. They are best applied in indoor locations such as control rooms, storage facilities, pharmaceutical plants, and indoor fire alarm systems serving hazardous areas.

Explore The Signal Source’s range of Intrinsically Safe (IS) products: IS products

Explosion-Proof (XP) devices

Explosion-proof devices use conventional electronics housed in heavy-duty enclosures such as aluminium or stainless steel, designed to contain an internal explosion and prevent ignition of the surrounding atmosphere.

Although heavier and more demanding to install than IS devices, explosion-proof equipment offers substantially higher output performance, including:

• Horns: Up to 120 dB at 1 meter
• Strobes: High-output xenon strobes with effective light output up to 500 candela

For example, an intrinsically safe LED strobe may only provide warning within a few feet, while a 21-joule explosion-proof xenon strobe can deliver an effective warning distance of up to 115 feet (35 meters).

Explosion-proof signaling devices are widely used in fire alarm, gas detection, and public address (PA) systems at large petrochemical installations worldwide. Increasingly, they are also deployed in combustible dust environments, such as grain elevators, sugar processing facilities, and bulk material handling operations.

All equipment installed in these locations must be appropriately certified for combustible dust hazards.

Explore The Signal Source’s range of explosion-proof products: XP products

Increased safety (Lower-risk areas)

Many hazardous locations fall into lower-risk classifications (e.g., Division 2 or Zone 2), allowing the use of signaling devices specifically designed for these environments.

These products offer:

• Higher performance than IS devices
• Easier installation
• Lower total installed cost

Despite these advantages, many lower-risk areas continue to be equipped with higher-rated products, as designers often adopt a conservative approach when specifying equipment.

Selecting an effective alarm system

Strobes and status lights

Emergency signaling can be achieved using several light-generation technologies, each with distinct advantages and limitations.

Rotating mirror strobes provide excellent visibility and remain common on vehicles and mobile equipment. However, their mechanical drive systems and short lamp life (typically ~200 hours) result in higher maintenance requirements, making them generally unsuitable for hazardous locations.

Xenon strobes, particularly higher-energy models, offer service lives exceeding 2,000 hours and provide warning effectiveness comparable to rotating strobes. As a result, xenon strobes are the preferred choice for critical fire, gas, and PA systems.

Alarm horns

Alarm horns form the backbone of most emergency warning systems. Tone selection is critical in high-noise industrial environments.

Continuous tones can blend into ambient noise from motors, compressors, and process equipment. Frequency-modulated tones, by contrast, are far more effective at attracting attention and maintaining audibility over distance.

Modern electronic horns can generate dozens of selectable alarm tones and support multiple alarm stages, allowing a single device to indicate different emergency conditions such as fire, toxic gas, or evacuation, reducing installation and wiring costs.

Typical hazardous location horns produce 110–120 dB at 1 meter. For effective warning, the alarm level should be at least 5 dB above ambient noise.

When multiple alarms are used, tones must be clearly distinguishable to ensure rapid recognition and response by plant personnel.

Traditional electromechanical alarms including bells, buzzers, and sirens are highly recognizable but rely on moving parts. Modern digital electronics can now replicate these tones with greater reliability, higher output, and availability in weatherproof and hazardous-location-rated enclosures.

Disaster sirens

Large industrial facilities increasingly extend warning systems beyond process areas to cover parking lots, storage yards, and surrounding communities.

Coverage requirements vary from 650–1,300 feet (200–400 meters) to 1.2 miles (2 km) or more.

Modern disaster siren systems typically include:

• Battery backup
• Silent testing capability
• Multiple communication options (TCP/IP, radio, cellular, RS-485)

Due to high power requirements, disaster sirens are typically installed in non-hazardous areas. In some cases, control electronics may be housed in explosion-proof enclosures while speakers are mounted at elevated heights classified as safe areas.

High-quality voice messaging can also be integrated, allowing extension of site-wide PA systems.

Temporary alarm systems

Temporary alarm systems are increasingly used during construction, expansion, and maintenance activities.

Wireless and radio-controlled solutions allow rapid deployment with minimal infrastructure. Systems may be powered by standard AC supplies or solar panels and relocated once work is complete.

This approach enables construction teams to implement full emergency warning capability without fixed cabling, providing safety performance comparable to permanent installations.