This article is intended to provide general information about hazardous areas and how to determine a the proper control hardware for the area The recommendations listed within this article are based on research, definitions, and guidelines established by the North American Electrical Codes (NEC), International Electrotechnical Commission (IEC), European, and other agencies throughout the world. By no means is this article going to cover all types of installations, it is meant to provide general information and guidelines.
The hazardous area classification and the category of equipment to be placed in the area is dependent upon site and application specifics. A careful examination must be made of the equipment and the area.
Note: The final authority on what is and is not a safe equipment installation rests with the “Safety Enforcement Authority” for that geographical area.
Non-hazardous (Safe) Area
An area classified as “non-hazardous” has a small probability of a flammable mixture being present. It is also referred to as a “safe area”, which includes most control rooms which house the control panels.
An area classified as “hazardous” is an area in which the atmosphere contains, or may contain, flammable or explosive gases, dusts, or fibers. In this type of area, a fire or explosion could possibly occur when all three (3) of the following basic conditions are fulfilled:
- Flammable gas, vapor, dust, or fibers are present.
- The combustible material is mixed with air in a proportion that produces a flammable mixture.
- A source of ignition acts to ignite the mixture. The source under consideration is any portion of the control system, which could release sufficient energy to cause ignition. A spark or hot surface may release the incendiary energy.
In the United States, the National Electrical Code, Article 500 for both zones and divisions, Canada, Canadian Electrical Code, Section 18 for zones, defines a hazardous area by applying the three-part classification. In Europe the classification is in type of industry and degree of hazard.
Definitions of zones
Class 1, Zone 0
A class I, zone 0 location is a location in which:
- Ignitable concentrations of flammable gases or vapors are always present.
- Ignitable concentrations of flammable gases or vapors are present for long periods of time.
Class 1, Zone 1
A class I, zone 1 location is a location in which:
- Ignitable concentrations of flammable gases or vapors are likely to exist during normal operating conditions.
- Ignitable concentrations of flammable gases or vapors may exist frequently due to repair or maintenanceoperations, or because of leakage.
- Equipment is operated or processes are carried out in such a way that equipment breakdown or faulty operationscould result in the release of ignitable concentrations of flammable gases or vapors. This may also cause simultaneousfailure of electrical equipment in a mode to cause the electrical equipment to become the ignition source.
- Ignitable concentrations of vapors could move to an adjacent class I, zone 0 location, unlessmovement is prevented by adequate positive-pressure ventilation from a source of clean air, and safeguards against ventilation failure are present.
Class 1, Zone 2
A class I, zone 2 location is a location in which:
- Ignitable concentrations of flammable gases or vapors are not likely to occur during normal operation and, if they do occur, they will only exist for a short time period.
- Volatile flammable liquids, flammable gases, or flammable vapors are handled, processed, or used, but the liquids, gases, or vapors are normally confined within closed containers or closed systems from which they can only escape o as a result of accidental rupture or system breakdown, or as the result of abnormal equipment operation.
- Ignitable concentrations of flammable gases or vapors are normally prevented by positive mechanical ventilation, but may become hazardous as the result of the ventilation equipment failing or experiencing abnormal operation.
- Ignitable concentrations of flammable gases or vapors could move to an adjacent class I, zone 1 location, unless such movement is prevented by positive-pressure.
The Instrumentation System
An electrical system generally contains potential sources of ignition that are of concern in a hazardous area installation. Therefore, the types of ignition sources and applicable methods of preventing ignition must be considered by the control system design when instrument and control hardware selections are being made.
Systems that are designed to meet certain safety criteria may receive certification from a safety standards approval agency such as Canadian Standards Association (CSA). ATEX (European) certification can be obtained from some of the same agencies.
Sources of Ignition
Generally, a potential source of ignition from an electrical system is any spark or hot component that can release enough energy to ignite the combustible mixture surrounding it. The ignition source may occur in any of the following mechanisms:
- Discharge of capacitive circuits
- Interrupting (opening) of inductive circuits
- Opening or closing of resistive circuits with slow intermittent interruption increasing the ignition capability (hazard)
- High temperature sources.
The ignition mechanisms may occur in relay contacts, switch contacts, fuses, short circuits (from damage or component failure), and arc-over between components or conductors. The components or circuits that present a potential ignition source may be designed in a variety of ways in order to prevent ignition of a hazardous atmosphere.
Methods of Preventing Ignition
Three (3) basic approaches can be used individually or in combination to prevent ignition of hazardous atmospheres:
- Intrinsically Safe/Non-incendive Equipment: Eliminates the ignition source by designing equipment that cannot cause ignition when in contact with flammable or explosive atmospheres.
- Gas Purge/Hermetic Sealing: Controls the atmosphere surrounding the potential ignition source so those components are enveloped within a non-hazardous atmosphere.
- Flame-proof/Explosion-proof Housing: Contains the explosion within housing, preventing the ignition of the surrounding atmosphere.
Designing Intrinsically Safe Circuitry
Intrinsic safety prevents instruments and other low-voltage circuits in hazardous areas from releasing sufficient energy to ignite volatile gases. Applying safety barriers requires some knowledge of intrinsic safe principles and terminology. All intrinsically safe circuits have three (3) components:
- The field device, referred to as the intrinsically safe apparatus such as;
- Switching devices
- Mechanical switches
- Proximity switches
- 2-wire transmitters
- Load cells
- Solenoid valves
- I/P transducers
- Switching devices
- The energy-limiting device, also known as a barrier or intrinsically safe associated apparatus
- The field wiring.
When designing an IS circuit, begin the analysis with the field device. This will determine the type of barrier that can be used so that the circuit functions properly under normal operating conditions but still is safe under fault conditions. Under normal operating conditions, the barrier, is passive and permits the field device to function as it was designed. However, under a fault condition, it prevents excess voltage and current from reaching the hazardous area.
More than 85% of all intrinsically safe circuits involve commonly known instruments as shown in the list above.
An intrinsically safe apparatus (field device) is classified either as a simple or non-simple device. Simple apparatus is defined in paragraph 3.12 of the ANSI/ISA-RP 12.6-1987 as any device which will neither generate nor store more than 1.2 volts, 0.1 amps, 25 mW or 20 μJ.
Examples are simple contacts, thermocouples, RTDs, LEDs, non-inductive potentiometers, and resistors. These simple devices do not need to be approved as intrinsically safe. The circuit is considered intrinsically safe if the devices are connected to an approved intrinsically safe associated device (such as a barrier or galvanic isolator). A non-simple device can create or store energy levels that exceed those listed above. Typical examples, of these devices, are:
- Solenoid valves
When these devices are approved as intrinsically safe, under the entity concept, they have the following entity parameters:
- Vmax (maximum voltage allowed)
- Imax (maximum current allowed)
- Ci (internal capacitance)
- Li (internal inductance).
The Vmax and Imax values are straightforward. Under a fault condition, excess voltage or current could be transferred to the intrinsically safe apparatus (field device). If the voltage or current exceeds the apparatus’ Vmax or Imax, the device can heat up or spark and ignite the gases in the hazardous area. The Ci and Li values describe the device‘s ability to store energy in the form of internal capacitance and internal inductance.
The barrier is comprised of three (3) components that limit current and voltage:
- A resistor
- Zener diodes (minimum of 2)
- A fuse
The resistor limits the current to a known value and the zener diode limits the voltage. There are multiple zener diodes present, wired in parallel, within each barrier to provide maximum protection. The fuse will blow if the zener diode conducts which interrupts the circuit preventing the zener diode from burning and allowing excessive voltage from reaching the hazardous area.
A galvanic isolator contains all of the elements of a zener barrier, but the interface is constructed differently. A transformer is used as the power supply and the return signal can be through an optocoupler, transformer, or a relay. Using this type of construction, the hazardous area has been isolated from the safe area circuit.
A zener barrier must be connected to earth for it to remain safe, whereas a galvanic isolator is not required to be connected to earth. The following table outlines the pros and cons of each device:
|Cannot be repaired||Can be repaired|
|Safety earth required||Safety earth not required|
|Low-power dissipation||High-power dissipation|
The use of a barrier vs. a galvanic isolator depends upon the installation and although barriers are accepted around the world, there are countries that have additional requirements.
An intrinsically safe system consists of one or more interfaces (galvanic isolators or barriers), field devices, and interconnecting wiring in which any circuits are intended for use in a hazardous atmosphere. The wiring for these type of circuits is normally light blue in color.