Explosion Protection: A Primer
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Work in hazardous locations must be undertaken with the utmost care by persons who are thoroughly familiar with technical issues, insurance coverage, and code requirements-national, local, and, if appropriate, international.
Accordingly, this primer is intended only to provide an overview for the unfamiliar or a reminder for the experienced. Before undertaking work in hazardous locations, consult with engineering experts, either those within your own company or consultants.
When you're ready to buy, remember that Honeywell Sensotec carries thousands
of sensors appropriate for use in hazardous locations. We also offer engineering
support to help you design your installation and select the proper equipment.
To go directly to our catalog, click here: catpages.shtml
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Here's a quick guide to the abbreviations that you'll encounter
ATEX: Derived from the French "ATmosphere EXplosible" (explosive atmosphere). Refers to Atex Directive 04/9/EC, the European regulation governing equipment and protective systems intended for use in potentially explosive atmospheres. >>More Covers all mechanical and electrical equipment and protection systems used under ground, on the surface, and in fixed offshore installations (excluding maritime usage). In force since July 1, 2003.
CE: Derived from the French "Conformité Européene" (European Conformance). Official marking required by the European Economic Community EEC) for all electric and electronic equipment that will be sold, or put into service for the first time, in any of the EEC nations. The CE mark indicates that a product fulfills all essential safety and environmental requirements as defined in the European Directives. Note, however, that those directives do not address use in hazardous locations; thus, CE-marked products are not necessarily safe for use in such locations [true?]. >>More The CE marking directive (93/68/EEC) was adopted on 07-22-1993. Note that manufacturers who affix the CE marking are solely responsible for ensuring that a particular product conforms to a particular directive. By contrast, in the case of hazardous location approvals, the certifying agency reviews the product and decides if it meets certification requirements.
CSA: Canadian Standards Association. Sets standards that govern various aspects of product design and production, including eligibility for certification in hazardous locations within Canada. More>> CSA also evaluates, tests, and inspects products on an ongoing basis to ensure that they meet its standards. Through interlaboratory agreements, they also assess products for conformity to American and European standards.
DNV: Det Norske Veritas, a Norwegian foundation that provides independent third-party assessment of a product's conformity to national or international standards, including standards for safe and reliable use in hazardous locations. >>More DNV works with many industries worldwide but has particular expertise with maritime shipping, oil and gas exploration, various process industries (petrochemical plants, oil refineries, etc.), and transportation (rail and automotive). In addition to assessing products, DNV also assesses systems, projects, and management systems with respect to their effectiveness in managing risk.
Ex, XP: Markings used by manufacturers to indicate that a product has been tested and certified as explosionproof or, in Europe and elsewhere, fireproof. More>> In the United States and Canada, Underwriters Laboratories and Factory Mutual are the primary organizations [true?] that certify products as explosionproof. The primary certifying agencies in Europe and elsewhere [true?] are Cenelec (European Committee for Electrotechnical Standardization) and IEC (International Electrotechnical Commission).
FM: Factory Mutual Research Corporation. Their Approvals Division determines whether equipment and materials used in hazardous locations meet National Electrical Code standards for safety and reliability. >>More Factory Mutual considers not only product performance but also quality control standards in place during production. Like the Canadian Standards Association (CSA), Factory Mutual has interlaboratory agreements; they certify to Canadian and European as well as U.S. Standards.
XP: See "Ex, XP."
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Explosion Protection Basics: Frequently Asked Questions
We've based this section on questions most frequently asked of our inside
sales representatives. If your question isn't answered here, call us at
1-800-298-9228. Our technical support staff is ready to help.
Q: What is a hazardous location?
A: An area (building or premises) where fire
and explosion could occur because volatile (flammable, ignitable, or combustible)
dust, gas, liquids, vapors, or fibers are present. Likely candidates:
industries that manufacture, store, or distribute volatile materials like
fuels, chemicals, paints, and grain. Government, industry, and professional
and scientific organizations have jointly developed systems for classifying
hazardous locations according to the nature and intensity of the hazards
present. They've also developed techniques for providing explosion protection
in those locations.
International systems for classifying hazardous locations use zones and groups as the basis for classification.
Canadian and U.S. systems use classes, divisions, and groups.
Q: What is the "combustion triangle"?
A: The three basic ingredients necessary to cause an explosion:
- An oxidizer (oxygen in the surrounding air)
- Fuel (provided by the volatile material present in the hazardous location)
- Energy (thermal or electrical) sufficient to cause ignition in the presence of the other two elements.
Q: What is the basic principle of explosion protection?
A: Preventing the three elements of the combustion triangle from occurring in the same place at the same time under both normal operating conditions and the most extreme abnormal occurrence (or fault condition) known to be possible. This is accomplished by specifying design and operational parameters for all electrical and electronic equipment and wiring used in hazardous locations. "Most extreme abnormal occurrence" means that the oxidizer and the fuel are both present in their most easily ignitable concentrations.
Q: What are fault conditions?
A: Opening, shorting, or grounding of any field wiring for any reason; electrical equipment failure; miswiring; or application of higher voltages than intended for a particular circuit.
Q: How many ways are there to prevent explosions?
Three basic approaches are commonly used around the world:
- Explosion confinement: Containing explosions within an enclosure able to prevent
electrical or thermal energy from the explosion from reaching the volatile
atmosphere outside the enclosure. Called "explosionproofing" in the
United States and Canada and "flameproofing" in Europe and elsewhere.
Note that this is the only one of the three basic approaches in which
an explosion could actually occur. Several methods of explosionproofing
are available; examples are filling the containment enclosure with oil
or surrounding the electrical apparatus with molding (resin). See
Honeywell Sensotec explosionproof sensors at index.shtml.
- Ignition source isolation: Isolating the ignition source (the combustion triangle's
energy) from the volatile atmosphere by any of many available methods.
In purging (pressurization outside the United States and Canada), the
most common method, clean air or inert gas supplied to an enclosure
reduces flammable gas and vapor concentrations to acceptable levels
and maintains those levels by positive pressure. Other methods are restricted
breathing, encapsulation, and oil immersion (which is also a form of
explosion confinement).
- Energy release limitation: Designing electrical and electronic equipment that can be exposed to volatile atmospheres because the thermal or electrical energy it produces is insufficient to cause ignition, even under the most extreme abnormal fault conditions. Most widely used techniques: designing equipment that is nonincendive, intrinsically safe, or both.
Q: How does "nonincendive" differ from "intrinsically safe"?
A: Nonincendive devices, circuits, and components are incapable of generating thermal or electrical energy sufficient to ignite a volatile atmosphere under normal operating conditions-although sufficient energy for ignition could be generated under fault conditions.
Intrinsically safe equipment, circuits, and components are incapable of generating
thermal or electrical energy sufficient to ignite a volatile atmosphere
under either normal or abnormal operating conditions. Consequently, intrinsically
safe systems have much wider application than their nonincendive counterparts.
Nonincendive systems are generally less costly and easier to maintain
than either explosionproof or intrinsically safe systems.
See Honeywell Sensotec intrinsically safe and nonincendive sensors at index.shtml.
Q: What's the difference between intrinsically safe and explosionproof?
A: It all boils down to the difference between preventing and containing. In an
intrinsically safe system or component, explosions are prevented-they
cannot possibly occur under any known fault conditions. In an explosionproof
system or component, an explosion can indeed occur, but a specially designed
enclosure would keep the resulting flames, sparks, or hot gases from reaching
the volatile atmosphere outside the enclosure. To distinguish these two
approaches, just remember that the "proof" in "explosionproof" doesn't
mean "will prevent from happening." Instead, it means "will contain what
does happen."
Q: So what's the safest approach to explosion protection?
A: Intrinsic safety is generally regarded as the safest. No known explosion has occurred in an intrinsically safe system.
Q: Then shouldn't I just use intrinsic safety techniques instead of explosionproofing?
A: You may need one or the other or both, depending on your facility or operations.
Both methods have appropriate applications as well as benefits and drawbacks.
| Approach | Benefits | Drawbacks |
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Explosionproof
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- Eases design concerns by enabling the three basic ingredients of the combustion triangle to coexist. That is, certain components can be placed within the hazardous location that otherwise would have to be placed outside it.
- Can be used with systems and components requiring any amount of power.
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- Enclosures are bulky, heavy, hard to install, and expensive.
- All wiring into and out of the enclosure must be in hardened conduit system with special seals and fittings installed according to regulations.
- System requires frequent integrity inspections.
- Even slight failure of any part eliminates explosion protection.
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Intrinsically Safe
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- Safest approach to explosion protection.
- Compared with most other methods, inexpensive and easy to implement.
- Integrity inspections are unnecessary because energy limitations are designed into the component or wiring-no conceivable failure could cause a problem.
- The low energy involved eliminates shock hazards and enables instrument calibration and maintenance while power is flowing to the system.
- Easily adaptable to both older technologies and modern systems.
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- Only components that require less than 1 Watt of power can be made intrinsically safe.
- Low power requirement limits usage to measurement and control circuits.
- Does not protect against risk of explosion induced by mechanical or electric sparking, chemical reaction, radio waves, or lightning strikes.
- Does not guarantee safety unless used in an intrinsically safe system.
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Q: "Inherently safe" means the same thing as "intrinsically safe", right?
A: No, it does not-even though dictionaries identify these words as synonyms-and the distinction is important in the safety world. Nothing related to electricity (circuit, device, component, or wiring design) is inherently safe-that is, safe by its very nature. Rather, electricity involves energy that can cause hazards under certain conditions and is therefore inherently unsafe.
Electricity can, however, be made intrinsically safe-that is, safe because of the way it is used. Therefore, an electrical circuit, device, component, or wiring design can be intrinsically safe if (and only if) it is used in an intrinsically safe system; it cannot be intrinsically safe by itself. (Exceptions: self-contained devices such as gas detectors.)
Q: What are the parts of an intrinsically safe system?
A: The device, component, or circuit itself; an "associated apparatus"; and interconnecting wiring.
Q: How does the associated apparatus work?
A: The apparatus is a device that is
placed outside the hazardous area so that it is between the automated
input/output modules and the field device, component, or circuit located
inside the hazardous area. The apparatus acts as a barrier that limits
energy (current and heat) available to the field device. These devices
are classified by type: They can be either "simple" or "intrinsically
safe." A "simple apparatus" is one that cannot generate or store more
than 1.2 V, 100 mA, 25 mW, or 20 ?J; such an apparatus consists of a zener
diode, resistor or series of resistors, and fuse network; it is unamplified.
The passive simple apparatus requires no power supply but must be connected
to a dedicated intrinsic safety earth ground.
INTRINSIC SAFETY INSTALLATION
System Concept
The system concept states that an intrinsically safe field instrument is certified for use with a specific intrinsic safety barrier (associate apparatus). This approach simplifies the installation; however, it does not allow flexibility in the type and selection of barrier supplies.
Entity Concept
The entity concept allows the safe connection of two independently certified pieces of equipment. By comparing parameters assigned to each instrument, the intrinsic safety of the system can be guaranteed.
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Outside the United States and Canada, manufacturers commonly self-declare their device to be a simple apparatus. Within the United States and Canada, users often prefer that a device be certified by Factory Mutual (FM) as meeting all the requirements for a simple apparatus. Insurance requirements and local codes also affect how a simple apparatus must be wired and whether a system using a simple apparatus must be reviewed by an approving authority. Whether users prefer FM certification also depends on the hazard classification for their location: The more hazardous the location, the more likely that users will want FM approval.
An "intrinsically safe" apparatus is one that provides intrinsically
safe connections, has an internal amplifier, and provides a galvanic barrier
between the hazardous and nonhazardous area circuits. Because of the isolating
barrier, a dedicated earth ground is unnecessary, but this active type
of device does require its own power supply. An intrinsically safe apparatus
must be used if the electrical characteristics of the device being installed
in the hazardous area exceed those of a simple apparatus-that is, if the
device can store energy in excess of 1.2 V, 100 mA, 25 mW, or 20 ?J.
Which Honeywell Sensotec products work with simple apparatus and which work with intrinsically safe? Almost all Honeywell Sensotec sensors can be fitted for use with either type of apparatus. Those to be used with a simple apparatus are manufactured without amplification (they consist only of a series of resistors). Those to be used with an intrinsically safe apparatus are manufactured with internal amplifiers [and what else? hardened circuits? pressure ports?].
In certain hazardous locations, it is acceptable to use a sensor or other device that is self-declared by the manufacturer (as opposed to certified by an approving agency that has tested the sensor or device) as appropriate for use with an intrinsically safe apparatus. The sensor or device will carry an ATEX marking (see Certifying Agencies and Certification Indicators certificates.html applied by that manufacturer and be shipped to the user with the manufacturer's document of conformity. Honeywell Sensotec will self-declare certain sensors as intrinsically safe if requested and if appropriate for the particular sensor, but you are ultimately responsible for making sure that such a sensor is appropriate for use in your operation.
For a complete listing of models that can be made intrinsically safe, click here: pdf/intrinsicallysaferated.pdf
Which Sensotec products are certified as explosionproof? The 811 (for gauge or absolute pressure) and the 911 (for differential pressure) are enclosed in housings that are certified as explosionproof by Factory Mutual (FM). These housings are fitted with conduit fittings for attachment to the required hard conduit to carry wiring back to your system and with pressure ports for connection to the device you're monitoring. [Insert diagram from catalog page PR-16]
Honeywell Sensotec does sell components that can be assembled by system integrators into explosionproof housings that can be kept intrinsically safe by purging (see Ignition source isolation [make this a link]). But such housings add considerable expense to system installation, and for the 811 and 911, they are completely unnecessary.
What do the terms "loop" and "entity" mean? These terms refer to the two different methods for approving intrinsic safety equipment. >>More Under a loop (or system) approval, every component is specified and the approving body certifies the entire system. Any change to any component voids the approval.
Under an entity (or parametric) approval, the approving body evaluates each device separately and assigns it a separate set of entity (or safety) parameters. With this type of approval, you can, for example, connect a field device to any associated apparatus (barrier) with compatible safety parameters.
Does the method for connecting sensors depend on the kind of explosion protection I'm using? Whether you're using explosion confinement (explosionproofing), ignition source isolation (purging), or energy release limitation (intrinsic safety) as a safety approach, the principal for connecting sensors is the same: You must make keep the sensor from generating energy in quantities sufficient to cause ignition.
To limit energy generation within the sensor, you must match the capacitance of the sensor to the capacitance of your system's associated apparatus (barrier). In other words, the capacitance parameters for the sensor must be less than or equal to the capacitance parameters of the barrier. Here's the formula:
sensor capacitance + sensor inductance ? barrier capacitance + cable capacitance + cable inductance
Every sensor Honeywell Sensotec sells is shipped with an installation drawing and that provides the necessary parameters and a manual that provides information about sensor connections under all major codes, anywhere in the world. In the United States, you should also refer to the National Electrical Code before beginning your connections.
Following is a sample installation drawing.
[Insert drawing 001-0799-01 from Web site; or see Steve Vicars]
How do I know what barrier to choose for a particular explosion protection sensor? This is simply the converse of the preceding question. As with the connection of sensors, you must limit the amount of energy that can be generated in the sensor by matching the energy parameters of the barrier with the energy parameters of the sensor. You can use any barrier for which the capacitance parameters are greater than or equal to the energy parameters of the sensor. Here's the formula from the barrier point of view:
barrier capacitance + cable capacitance + cable inductance ? sensor capacitance + sensor inductance
Barrier capacitance consists of open voltage, cross circuit voltage, allowable inductance, and allowable capacitance. [Is this right? Shouldn't it be barrier energy?]
How do I design the installation for an intrinsically safe sensor? Our 2N intrinsically safe amplifier (which can be used with almost any Honeywell Sensotec sensor) has been certified by Factory Mutual (FM) and the Canadian Standards Association (CSA) with entity parameters (see What do the terms "loop" and "entity" mean? [make this a link]. Therefore, to design your installation to accommodate our intrinsically safe sensors, you simply need to make sure that your system matches the parameters shown on the installation drawing that accompanies the sensor when it is shipped to you-or you can check the catalog [give link] or call us at [give number] to check the parameters in advance,
Certifying Agencies and Certification Indicators
Various certifying agencies issue standards and directives stating what requirements equipment and wiring must meet to be considered safe for use in hazardous locations. In addition, manufacturers affix markings to their products to indicate that those products are in conformity with certain standards.
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