When an abnormal condition (such as a capacitor failure, IC short, etc.) occurs in an electronic circuit, it can create an electrical fault that, if not controlled quickly, can quickly cause thermal runaway, which is very hazardous, particularly in environments filled with potentially explosive gases, fuels, or dust. To address the risks of operating electrical and electronic equipment in hazardous locations, electrical codes and standards are used to identify and define the necessary device requirements for industrial and occupational safety in these locations. This article will provide you with an introduction to both these codes and standards, as well as a background on intrinsically safe fuses that are used to limit the spark energy and surface temperature in a faulty circuit in a hazardous location.
What is a Hazardous Location?
Based on the properties of flammable gases, vapors, and combustible dust that are present in an installation, a location can be designated as a hazardous location. The Occupational Safety and Health Administration (OSHA) defines a hazardous location as an “area where flammable liquids, gases, vapors, or combustible dusts exist in sufficient quantities to produce an explosion or fire. In hazardous locations, specially designed equipment and special installation techniques must be used to protect against the explosive and flammable potential of these substances" (OSHA Publication 3073).
The U.S. National Electrical Code (NEC) and the Canadian Electrical Code (CEC) describe a hazardous location as an area where the presence of flammable gases or vapors, combustible dusts, or ignitable fibers may exist that can cause a potential hazard of fire or explosion. NEC Articles 500 to 504 and CEC Section 18 also specify the requirements for electrical and electronic equipment that are used in hazardous locations.
OSHA, NEC, and CEC, along with the National Fire Protection Association® (NFPA®), have formulated regulations on hazardous locations and have defined them by means of a Class/Division System, which we will cover in the next section.
Hazardous Location: Class/Division System
NFPA® Publication 70, NEC Article 500.5, and CEC Articles 18-004, 006, 008, and 010 identify and designate hazardous locations by means of Classes and Divisions. Class I, Class II, and Class III are the three broad classifications of hazardous locations, and are sub-divided into Divisions 1 and 2.
A Class I Location is alternatively grouped into Zones 0, 1, and 2. Class I locations exist where flammable gases and vapors are present in the atmosphere and can cause an explosion. Class I Zones 0 and 1 Locations exist where flammable materials are present in normal operating conditions. Typical locations include fuel storage locations, gas generator rooms, and cleaning and dyeing plants where flammable liquids are used.
Class I Division 2 locations exist where flammable materials are handled and confined within closed containers or systems. Typical locations include flammable liquid or compressed gas in sealed containers in storage areas, and locations where flammable material can escape in the event of an accident.
Class II locations can be found where combustible dust material is present. Examples include flour mills, feed mills, coal preparation plants, and sugar plants.
Class III locations exist where ignitable fibers are handled or manufactured. Examples include textile mills or wood cutting and processing plants.
|Classes||Division 1||Division 2||Groups|
|I Gases, vapors, and liquids||Normally explosive and hazardous||Not normally present in an explosive concentration (but may accidentally exist)|
c. Ether, etc.
d. Hydrocarbons, fuels, solvents, etc.
|II Dusts||Ignitable quantities of dust normally are or may be in suspension, or conductive dust may be present||Dust not normally suspended in an ignitable concentration (but may accidentally exist). Dust layers are present.|
e. Metal dusts (conductive and explosive)
f. Carbon dusts (some are conductive) and all are explosive
g. Flour, starch, grain, combustible plastic or chemical dusts (explosive)
|III Fibers and flyings||Handled or used in manufacturing||Stored or handled in storage (exclusive to manufacturing)||Textiles, wood-workings, etc. (easily ignitable, but not likely to be explosive)|
Table 1: Class I, II, and III Hazardous Locations
International Standards: IECEx and ATEX
IECEx, the IEC System for Certification to Standards Relating to Equipment for Use in Explosive Atmospheres, is an international conformity assessment system for hazardous areas. ATEX is the European Directive 2014/34/EC, concerning explosion-proof electrical and mechanical equipment, components, and protective systems. Let's briefly discuss these standards.
Explosion protection is an essential part of the overall risk management for industrial verticals such as mining, oil-and-gas, and chemical industries. One must ensure safety in industrial processes using or producing hazardous materials such as combustible gas, dusts, or vapors. In these operating environments, the risks to life and property loss when using the equipment which operates in the presence of explosive atmospheres are high.
Most national regulatory frameworks require that conformity assessment be conducted by independent and third-party inspection bodies.The IECEx Certification System offers a global framework for equipment, repair facilities, or personnel with the objective of facilitating international trade in equipment and services for use in explosive atmospheres, while maintaining the required level of safety. "Ex" areas can be known by different names such as “Hazardous Locations,” “Hazardous Areas,” “Explosive Atmospheres,” and the like, and relate to areas where flammable liquids, vapors, gases, or combustible dusts are likely to occur in quantities sufficient to cause a fire or explosion.
The IECEx system aims to have a high safety level and interaction with national officials for regulation. One of the unique and key features of the IECEx is the IECEx “On-Line” Certificate System with full public display of all IECEx Certificates, which provides a one-stop location that provides instant verification and checking of claims of holding IECEx Certification for either Ex Products, Ex related Service Organisations, and Persons certified as Competent in the Ex field.
ATEX is the name commonly given to the two European Directives for controlling explosive atmospheres:
1) Directive 99/92/EC (also known as 'ATEX 137' or the 'ATEX Workplace Directive'): the minimum requirements for improving the health and safety protection of workers potentially at risk from explosive atmospheres.
2) Directive 94/9/EC (also known as 'ATEX 95' or 'the ATEX Equipment Directive'): the approximation of the laws of Members' States concerning equipment and protective systems intended for use in potentially explosive atmospheres.
ATEX stands for "atmosphères explosibles." It’s a European Union directive from the European Committee for Standardization that covers “equipment and protective systems intended for use in potentially explosive atmospheres.” An atmosphere can be explosive for several reasons, including flammable gases, mists or vapors, or combustible dust. All equipment and protective systems intended for this type of use in the EU must meet ATEX health and safety requirements. It is generally similar to an OSHA or NEC standard in the United States.
Equipment manufacturers whose products are intended to be used in Europe are responsible for making sure their equipment complies with ATEX standards. This process involves conformity assessment procedures and certification by a third party called a “Notified Body.” When electronic or electrical equipment of any type is intended for use in a hazardous area, the equipment must be ATEX certified as required by the EU directive 94/9/EC. Within Europe, ATEX certification of products for use in potentially hazardous atmospheres is compulsory.
Underwriters Laboratories: UL 913
Combustible gases, vapors, and airborne dusts tend, by their very nature, to be explosive if present in the right concentrations along with sources of sparks or excessive heat. Over the years, these hazards have led to some catastrophic losses of life and property. In response to this hazardous potential, regulatory bodies around the world, including Underwriters Laboratories, Inc., have worked to establish and refine a standard that minimizes the hazards associated with electrical equipment for use in hazardous locations.
UL 913 is the Underwriters Laboratories (UL) standard for Intrinsically Safe Apparatus and Associated Apparatus for Use in Class I, II, III, Division 1, Hazardous (Classified) Locations. The standard applies to:
- Apparatus or parts of apparatus in Class I, II, or III, Division 1 locations
- Those parts of apparatus located outside of the Class I, II, or III, Division 1 location having a design and construction that may influence the intrinsic safety of an electrical circuit within the Class I, II, or III, Division 1 location
- Apparatus or parts of an apparatus in Class I, Zone 0 or 1, Groups IIA, IIB, and IIC locations
- Those parts of apparatus located outside of the Class I, Zone 0 and 1, Groups IIA, IIB, or IIC location having a design and construction that may influence the intrinsic safety of an electrical circuit within the Class I, Zone 0 and 1, Group IIA, IIB, or IIC location
The purpose of this standard is to specify requirements for the construction and testing of electrical apparatus, or parts of an apparatus having circuits that are not capable of causing ignition in Division 1 Hazardous (Classified) Locations as defined in Article 500 of the National Electrical Code, ANSI/ NFPA 70. Limiting sources of electrical spark energy and high surface temperatures and maintaining separation distances are key aspects of the UL 913 standard.
These requirements are based on consideration of ignition in locations made hazardous by the presence of flammable or combustible material under normal atmospheric conditions. This standard does not cover mechanisms of ignition from external sources, such as static electricity or lightning, which are not related to the electrical characteristics of the apparatus. This standard does not cover apparatus based on high voltage electrostatic principles, such as electrostatic paint spraying apparatus.
Intrinsically safe apparatus
Intrinsically safe apparatus is defined as equipment and wiring which is incapable of releasing enough electrical or thermal energy under normal or abnormal conditions to cause ignition of a hazardous atmospheric mixture. This is achieved by preventing electrical equipment from becoming sources of ignition of explosive atmospheres in two ways:
- Energy limitation – To limit the spark energy
- Temperature limitation – To limit the surface temperature
For Class I hazardous locations, flammable gases or vapors can be ignited by sparks caused by electronic switching, arcing, or by the high surface temperature of parts exposed to the explosive atmospheres. For Class II and III hazardous locations, the main concern is that the apparatus's surface temperature may trigger an explosion if the temperature is higher than the ignition point of (for example) combustible dusts. Intrinsic Safety (IS) is a practical method of achieving such protection. Figures 1 and 2 show examples of common intrinsic safety systems.
Figure 1: Intrinsic safety system using zener barrier
Figure 2: Intrinsic safety system using galvanic isolator
Intrinsically Safe Fuses for Hazardous Locations
Within a hazardous area, a variety of electronic devices are often used, including:
- Motor controllers
- Communication handsets
- Flow meters
- Process control and automation
To use this type of apparatus safely in a hazardous location, the available energy must be limited to avoid igniting explosive materials in the environment. An intrinsically safe certified fuse is useful in limiting the current under abnormal conditions to ensure that the circuit will open without generating a spark capable of causing ignition. Arcing can occur when the fuse opens, which must be contained within the intrinsically safe fuse’s encapsulation. The surface temperature of the fuse also must be kept below the temperature that could ignite explosive gases or dust.
The PICO® 259-UL913 Intrinsically Safe Fuses from Littelfuse are encapsulated fuses approved under the UL 913 standard for Intrinsic Safety of Electrical Equipment to operate in hazardous locations. They are rated at 125V. These sealed fuses, available with ratings ranging from 62mA to 5A. They are ideal for applications in the oil, gas, mining, chemical, and pharmaceutical industries because they are designed to operate within environments where there is danger of explosion from faulty circuits. In addition to UL 913 certification, these fuses meet ATEX (EN 60079-0 & EN 60079-11) and IECEx (IEC 60079-0 & IEC60079-11) requirements.
|Littelfuse PICO® 259-UL913 Series||Littelfuse PICO® 259-UL913 Cross-sectional View|
Figure 4A: Intrinsically Safe Fuse is certified under the UL 913 Standard.
Figure 4B: The >1mm-thick encapsulant surrounding 259-UL913 Series fuses keeps them sealed, limiting the energy, and temperature exposed to the explosive atmosphere. It also prevents gases, dusts, and fibers from entering the fuse body.
Figure 4: PICO® 259-UL913 Intrinsically Safe Fuse
Other Examples of Intrinsically Safe Fuses
|259 Series - Hazardous Area Safe-T-Plus Fuse||PICO® 308 Series - 30V Intrinsically Safe Fuse|