Intrinsic safety in hazardous locations

Since the dawn of the industrial age, using electricity in potentially explosive areas has been problematic. Electrical equipment may generate arcs, sparks, or hot surfaces, which could cause an explosion.

Hazardous materials such as gases, vapors, dusts, liquids, and fibers are present in many industrial plants. Oxidation, combustion, and explosion are exothermic reactions that require fuel, oxidizer, and ignition energy simultaneously. Fuel could be flammable vapors, liquids, gases, combustible dusts or fibers; oxidizer is obviously oxygen (O₂); and ignition energy could be electrical or thermal. Eliminating one or more of these factors reduces the risk of an explosion.

Many times we hear terminology such as "Class 1, Div 1" and the like. But what does this mean and why is it important? Class refers to types of hazardous atmosphere. Div or Division refers to the likelihood of a hazardous atmosphere being present. The importance of knowing this resides in keeping you and your plant safe.

Hazardous area classifications, according to NFPA standards, are:

• Class I — Gas or vapor
• Class II — Dust
• Class III — Fibers or filings.

North American standards identify hazardous areas by class, division, and group, or optionally by class, zone, and gas group (see "Gas and dust groups"). Europe and other regions that follow IEC or CENELEC standards designate these areas by class, zone, and group (see "Hazardous area designations"). As for the Class 1, Div 1 terminology example, refer to the "Class 1, Div 1 group designations" table for specific group information.

A hazardous atmosphere may be present because of:

• Repair or maintenance operations
• Leakage
• Breakdown or faulty operation of equipment
• Faulty operation of process which causes simultaneous failure of electrical equipment.

The primary safety goal is to avoid having a source of ignition occur in the presence of a hazardous atmosphere. To accomplish this, the three recognized methods of preventing injury or damage from an explosion are:

• Containment — allows an explosion to occur, but it is confined in an enclosure built to resist the excess pressure created by the internal explosion
• Segregation — does not allow the dangerous air/gas mixture to penetrate the electrical equipment enclosure. The enclosure is pressurized with an inert gas, such as nitrogen (N2)
• Limitation — reduces the amount of energy contained in the equipment or circuitry that could cause an ignition of the dangerous air/gas mixture.

There are national and international standards as well as codes of practice for equipment design and installation for these three techniques. For instrumentation applications, the simplest and most cost effective technique is intrinsic safety.

Intrinsic safety is a protection method based on the principle of limiting the energy in an electrical circuit under normal or fault conditions such that any spark or thermal effect produced is incapable of causing an ignition of a hazardous material.

In the United States installation of intrinsically safe and associated apparatus devices must conform to Article 504 of the National Electrical Code and ANSI/ISA-RP- 12.6. For Canada the Canadian Electrical Code, Part I, C22. 1 applies. These standards require that intrinsically safe wiring be separated from nonintrinsically safe wiring, and that intrinsically safe wiring, terminals, and raceways be clearly labeled. Other considerations include grounding and shielding requirements.

In an intrinsically safe system, electrical equipment in the hazardous area and the interconnected instrumentation in the safe area should be designed to reduce energy. An intrinsically safe apparatus is designed to intrinsic safety standards and approved by third-party testing laboratories. This means that the open-circuit voltage and short-circuit current are reduced to values that will not cause an ignition by opening, closing, or grounding the circuit or any parts of the circuit.

Intrinsically safe standards apply to all equipment that can create one or more of these potential explosion sources:

• Electrical sparks
• Electrical arcs
• Flames
• Hot surfaces
• Static electricity
• Electromagnetic radiation
• Chemical reactions
• Mechanical impact
• Mechanical friction
• Compression ignition
• Acoustic energy
• Ionizing radiation.

Simple apparatus devices are low energy and do not need certification. A simple apparatus is a device in which none of the following values are exceeded:

• 1.2 V
• 0.1 A
• 20 µJ (microjoules)
• 25 mW.

Simple apparatus examples include passive sensors such as resistance temperature detectors (RTDs), light-emitting diodes (LEDs), thermocouples, and photocells. These devices can be directly mounted in the hazardous location and do not require certification or labeling, but they must be connected to an intrinsically safe barrier either directly or indirectly.

Generally, only sensors are located in the hazardous area. These sensors may be certified as intrinsically safe. Energy-storing devices such as indicators and transmitters must be certified. Intrinsically safe certification is done at the unit level or by loop.

An associated apparatus is a device that is located in the safe area, but connected to components in the hazardous location (Fig. 1). An associated apparatus is designed to limit the energy to which components in the hazardous location would be exposed and is either a passive barrier or an isolated barrier. Associated electrical apparatus devices must be placed in the safe area unless protected by another protection method, and must be certified to be intrinsically safe.

Intrinsically safe barriers are used to interface between electrical devices in a hazardous location, and electrical devices located in the safe area (associated apparatus). The two types of barriers are passive barriers and galvanically isolated barriers.

In the passive intrinsically safe barrier, zener diodes are connected in parallel with the input, while a resistor and a fuse are connected in series with it (Fig. 2). Under normal operating conditions, the barrier passes electrical signals in both directions. It is designed to withstand a fault voltage of up to 250-Vac. When the terminals on the safe side are exposed to voltage levels that are too high, the resulting increased current blows the fuse to prevent the failure of the zener diodes. The zener diodes limit the maximum voltage, while the resistor limits the maximum current to the circuitry within the hazardous area. The maximum allowable capacitance and inductance values must not be exceeded, thereby keeping stored energy levels below the ignition limits.

The efficiency of any passive barrier depends on the barrier ground connection. For passive intrinsically safe barrier applications, a dedicated ground must be run separately from any other structural ground. The resistance from the furthest barrier in the system to the intrinsically safe ground point should be less than 1 ohm.

The advantage of using passive intrinsically safe barriers is low component cost. However the component cost is usually offset by the installed cost, which could be high due to the requirement and maintenance of the high-integrity ground.

The disadvantages of using passive intrinsically safe barriers include:

• The high-integrity ground connection is required and must be maintained
• Improper connection or voltage surges could blow the fuse and may result in permanent damage
• The voltage drop across the barrier limits its use in some applications
• The system could have ground loops due to the lack of isolation and poor common mode rejection
• Measurement errors are introduced by the limiting resistor when used in RTD applications.

Isolated barriers provide isolation between hazardous and nonhazardous location circuits by using components such as transformers, relays, optical couplers, and isolation amplifiers. An advantage to isolation amplifiers is that they are frequently associated with or built into signal conditioning circuitry.

The input power, signal inputs from the hazardous location, and the output to the controller are isolated from each other. This circuit does not allow dangerous voltages on the nonhazardous side of the circuit to be transferred to the equipment and circuitry on the hazardous side. Since the entire circuit is floating with respect to ground, there is no possibility of the fault current flowing through the energy-limiting circuit, eliminating the need to ground it. The fuse is present in an isolated barrier circuit to prevent failure of the energy-limiting components. In this case, the fuse is not replaceable and will blow only if the isolation circuitry fails.

The advantages of using isolated barriers include:

• The high integrity ground is not required
• Grounded sensors can be used
• Isolation prevents ground loop problems and provides high common-mode rejection
• Signal conditioning and intrinsic safety typically are combined in one package.

The disadvantage of using an isolated barrier is the slightly higher component cost. The installed cost is comparable to the installed cost of passive zener barriers.

The choice between using passive barriers and isolated barriers depends on application requirements, equipment location, power supply availability, and user preference.

Because intrinsic safety involves low power, it has inherent advantages over some of the other protection types. Intrinsic Safety is the only protection method accepted for Zone 0, which is the most hazardous area. No special protection of field wiring, such as seals, glands, or airtight conduit, is required. Also, low voltages and currents enable maintenance and calibration to be carried out without shutting down the plant.

The low voltage and current in intrinsically safe circuits allow the use of ordinary instrumentation cables as long as the inductance and capacitance are taken into account. Cable parameters are seldom a problem and long distances are easily achieved.

Intrinsic safety is a safe and proven technology commonly used in Europe and is rapidly gaining popularity in North America. One of the reasons for its acceptance is because the installed cost of intrinsic safety is significantly lower when compared to other protection methods such as explosion-proof or purged systems.

PLANT ENGINEERING magazine extends its appreciation to Fluke Corp; MTS Systems Corp., Sensors Division; Pepperl+Fuchs, Inc.; Phoenix Contact Inc.; and Rockwell Automation for the use of their materials in the preparation of this article.