It is probable that engineers will encounter equipment and control locations both inside and outside the United States where a question exists if there are any hazard protection requirements.
The various locations encountered during the progress of a job must be recognized, identified and categorized after consulting with the facility owner and the appropriate authority having jurisdiction.
A hazardous location is any area where the atmosphere contains or may contain flammable or explosive gases, dust or vapors in sufficient quantities that could cause a fire or explosion to occur. In such an atmosphere, three basic conditions must be present — fuel, gas and an ignition source. In order to protect people and property, an appropriate protection method is required.
There are many approval and certification strategies used for manufacturing equipment throughout the world. Table 1 (see link to pdf below) lists the codes and standards used in this article. ATEX is a European directive used for controlling explosive atmospheres and the standards of equipment and protective systems used in them. If the manufacturer and supplier of equipment is located outside the European Union, it must have its equipment tested and certified by a third-party certification body, which is known as a notified body.
Once certified, it is marked by an “EX” label. ICEEx is an international strategy intended to facilitate international trade in electrical equipment used in explosive atmospheres. An “EX” area is a hazardous location and equipment used in these locations is known as “EX” equipment.
Hazardous areas and location classifications
The selection and placement of equipment to reduce the possible occurrence of fires and explosions requires an understanding of the various locations that must be considered. The area classifications vary between the United States and other countries. The hazardous location environment categories for the United States are classified by organizations such as NEMA, UL and NFPA. The codes cited for the United States often are adapted by foreign countries. The European hazardous area classifications follow that of the IECEx/ATEX, which is affiliated with the International Organization for Standardization.
The entire gamut of classifications, including zones, groups, divisions, classes and methods of explosion protection, is outlined here. These definitions may vary with the area where AHJ of the project are located. The NEC uses the terms “type, condition and nature,” which correspond to “class, divisions and groups.” These classifications define the level of safety required for all equipment installed in these areas.
It is important to remember all the information must be verified and accepted by the local authorities.
Zones: The “Ex” zone hazardous atmosphere definitions are used by both Europe and United States and are shown in Table 2 (see link to pdf below). The European hazardous areas generally follow IECEx/ATEX classification.
Classes: Classes are used only by the NEC of the United States and are intended to define the general nature of hazardous material in the surrounding atmosphere and the level of safety required for the equipment installed in these locations. The classes are defined in Table 3 (see link to pdf below).
Divisions: Divisions are used only by the NEC of the United States and intended to define the probability of hazardous material being present in an ignitable concentration in the surrounding atmosphere. The divisions are defined in Table 4 (see link to pdf below).
Groups: Groups are used only by the NEC of the United States and are intended to define the nature of the hazardous material in the surrounding atmosphere. The groups are defined in Table 5 (see link to pdf below). This table includes gas groups E, F and G, which are concerned about hazards associated with dust.
Gas grouping: Gas grouping, only for gases, is used both by the United States, Europe and other countries. Table 6 (see link to pdf below) gives the ignition and combustion properties only for gases and are classified to assist an engineer in the selection of the hazardous area devices. This group takes into account the maximum amount of energy that could be released under operating or fault conditions. The same table applies to both flame proofing and intrinsically safe equipment. Additional tables are available from manufacturers showing individual equipment classification.
Temperature classes: Air-gas mixtures can be ignited by contact with a hot surface. Equipment often is located in areas where flammable gas, mist or vapor exists. Where the temperature of the installed equipment is a concern, they are classified by its maximum surface temperature. Table 7 (see link to pdf below) shows both the United States (using NEC) and Europe (using EEC) for the gases listed in Table 6.
All temperature classifications use an ambient surface temperature of 104° F (40° C). If the equipment proposed to be used has a higher surface temperature or classification, the manufacturer should be consulted.
Methods for safe control
The presence of the correct proportions of fuel, oxygen and a source of ignition is necessary for combustion to occur. One approach for the creation of a safe environment is confinement. Isolating the area of installation creates a safe installation and reduces the possibility of accidents. Another is the removal or confining of any element that could produce a spark and ignite an explosion. There are three commonly used methods of providing safe control within a hazardous location categorized by the technology used:
An intrinsically safe system.
A pneumatic system is powered by air. The length of control circuits is limited. An explosion-proof housing provides a simple means to adapt controls to hazardous locations. The housings are designed so the explosion of a mixture inside the enclosure prevents the spread of the flame to the outside. This method lacks the flexibility in the use of sensing techniques and increases the size of the enclosure.
An intrinsically safe system is based on the NEC where equipment and wiring shall not be capable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific flammable or combustible atmospheric mixture in its most easily ignitable concentration.
There is a fundamental difference between an intrinsically safe system and the other equipment application techniques. None of the other methods aim at preventing the release of energy at a dangerous level. Instead, they avoid possible effects in a number of ways:
Ensuring when a spark occurs, it is prevented from reaching an explosive mixture.
Ensuring any explosion is contained.
Reducing the hazard by diluting the gas mixture.
Protecting against excessive temperature or spark.
Intrinsic safety deals with the root cause and ensures there is insufficient energy available, no matter what happens, to cause an explosion. It is considered by many to be the safest and technically best method. It has a number of practical advantages — among them a compact design, reliability, simple installation and low cost.
NEMA classifications: NEMA classifications are unique to the United States and provides the equipment-enclosure design for devices used in general-purpose applications up to and including hazardous locations. NEMA classifications are given in Table 8 (see link to pdf below).
Ingress Protection: Ingress Protection (IP) ratings specify the extent of all environmental protections that an enclosure provides and gives a system for classifying nonhazardous-area devices. This is not a hazardous-area classification. Rating information for nonhazardous areas comparing both IP and NEMA standards is given in Table 9 (see link to pdf below).
Explosion-proof housings: Explosion-proof housings provide a simple means to adapt electric, electromechanical and electro-pneumatic controls to a hazardous location. Explosion-proof housings are designed to withstand the explosion of a mixture inside the enclosure and to prevent the spread of flames to the outside. These enclosures are effective, especially at interrupting high currents to motors using limit switches. This method, however, lacks the flexibility in the use of sensing techniques because of the size of the device. In addition to the space required for the explosion-proof devices, material and labor costs for installation and service may be high.
Explosion-proof is the overall term used in Europe covering all methods of protection. The symbol used is Ex. In the United States, engineers tend to use the term explosion-proof as synonymous with the term flameproof, which causes confusion. The methods of explosion protection are given in Table 10 (see link to pdf below). The United States NEMA classifications for explosion-proof housings are given in Table 8.
Discussion of intrinsically safe elements: The primary concept behind intrinsic safety is the design of equipment intended to be installed in a hazardous area so ignition of a hazardous atmosphere (explosive vapor, gas or dust) cannot occur. This is achieved by ensuring only voltages and currents that enter the hazardous area are low enough to prevent a spark. No significant energy storage is possible.
Referring to Figure 1 (see link to pdf below), when conditions are abnormal the intrinsic safety barrier is designed to divert the excess current to the ground. Should the current continue to increase, the fuse would open the circuit, completely arresting the current flow.
All other methods of protection, such as pressurization, use of explosion-proof enclosures or oil filling — rely on the maintenance of a physical barrier between the explosive atmosphere and the electric circuit. Physical barriers, if breached at a single point, no longer provide the appropriate level of protection. In contrast, intrinsically safe elements provide inherent protection by restricting the energy at its source and as a result offer some advantages as follows:
Economy: Enclosures are lighter and non-energy storing field equipment could use ordinary specifications.
Maintenance: Cutting power is not necessary when adjusting field equipment.
Reliability: The system remains safe if seals fail or covers of enclosures are improperly replaced.
Safety: Personnel cannot be harmed by the low voltages in intrinsically safe circuits.
It is important to consider that a component with an intrinsically safe approval only will be safe when installed within the terms of the approval documents and of the required relevant codes of practice or when the system configuration has been given an appropriate prior approval.
Typically, the cost of an intrinsically safe component is comparable to or only slightly higher than a standard explosion-proof device. When one considers explosion-proof fittings and conduits are no longer needed, on the average the installed equipment may be less costly for an intrinsically safe device. Operating costs also are lower since power consumption is negligible. An intrinsically safe system can be serviced in hazardous areas when the power is on.
Some information for the article came from the following sources:“Where Hazards Lie,” by Wayne Ulanski in Flow Control Magazine. Aug. 8th, 2013; “Facility Piping Systems Handbook-3rd ed.,” by Mike Frankel, McGraw Hill; “Hazardous Locations 101,” in Control Engineering Magazine. “Hazardous Locations,” by Allen Bradley, Bulletin; Emerson Process Management, Product Bulletin 9.2.001.