A hazardous location is an area where flammable gas, vapour or combustible dust may be present in quantities that could be ignited by a spark or a hot surface. Electrical equipment in these areas must be designed so it cannot ignite that atmosphere. The work starts with classifying the area, which sets how likely an explosive atmosphere is and where it extends, and then selecting and installing equipment rated for that classification.
Where hazardous locations occur
Hazardous locations are common in Canadian industry, and they also turn up in ordinary buildings:
- Oil and gas facilities, compressor stations, gas plants and refineries
- Fuel storage and dispensing, including fleet fuelling and generator fuel systems
- Paint spray booths and solvent storage rooms
- Wastewater lift stations and treatment plants, where gases can accumulate
- Grain elevators, feed mills, flour handling and woodworking shops, where dust is the hazard
- Battery rooms, in some cases, and laboratories using flammable gases
- Some parking garages and repair garages, depending on use and ventilation
How areas are classified
In Canada, Section 18 of the Canadian Electrical Code sets the rules for hazardous locations. Current editions use the zone system for both explosive gas atmospheres and explosive dust atmospheres, while older installations may have been classified under the Class and Division system, which the code still addresses for existing work. Check the edition adopted in your province.
| Zone | Atmosphere | When an explosive atmosphere is present |
|---|---|---|
| Zone 0 | Gas or vapour | Continuously or for long periods |
| Zone 1 | Gas or vapour | Likely in normal operation, from time to time |
| Zone 2 | Gas or vapour | Not likely in normal operation, and only briefly if it does occur |
| Zone 20 | Combustible dust | Continuously or for long periods, as a dust cloud |
| Zone 21 | Combustible dust | Likely in normal operation, from time to time |
| Zone 22 | Combustible dust | Not likely in normal operation, and only briefly |
The classification depends on the sources of release, how much is released and how often, ventilation, and the properties of the substance, such as whether a gas is heavier or lighter than air. Recognized methods and guidance, such as the IEC and CSA 60079 series, and industry practices such as API RP 505 for petroleum facilities, are used to determine zone types and extents.
Gas groups and temperature classes
Classification also records the properties of the hazard. Gases are placed in groups according to how easily they ignite and how readily a flame passes through a gap; hydrogen and acetylene are in the most demanding group. Each substance also has an autoignition temperature, and equipment carries a temperature class that limits its maximum surface temperature. The equipment's temperature class must keep its hottest surface below the ignition temperature of the gas or dust present.
Protection methods
Equipment for hazardous locations uses one or more protection techniques, identified by marking on the nameplate:
- Flameproof (Ex d): the enclosure can contain an internal explosion and cool escaping gases so they cannot ignite the surrounding atmosphere.
- Increased safety (Ex e): extra measures prevent arcs, sparks and hot surfaces in equipment that does not produce them in normal operation, such as terminal boxes and some motors.
- Intrinsic safety (Ex i): the energy in the circuit is limited so it cannot cause ignition even under fault conditions. Common for instruments and sensors.
- Pressurization (Ex p): the enclosure is kept at positive pressure with clean air or inert gas.
- Encapsulation, powder filling, oil immersion and non-sparking (Ex m, q, o, n) for specific uses.
- Dust protection by enclosure (Ex t) and other dust-specific methods.
Each method is suitable for some zones and not others. Equipment must be certified for hazardous locations by an accredited certification body, and its marking must match the zone, gas or dust group and temperature class of the location.
Installation details that matter
- Seals. Conduit and cable seals stop gas or flame travelling through raceways from a hazardous area into a non-hazardous one or into equipment.
- Wiring methods. The code limits which cable types and wiring methods can be used in each zone.
- Glands and entries. Cable glands and entry fittings must match the protection method of the enclosure. An unsuitable gland can defeat a flameproof enclosure.
- Bolts and gaps. Flameproof enclosures depend on every bolt being present and tight and flame paths being clean and undamaged.
- Intrinsically safe circuits need barriers or isolators and must be kept separate from other wiring.
- Location. The simplest approach is often to place equipment outside the classified area altogether.
What owners should keep on file
- The area classification drawings, showing zones, extents, gas or dust groups and temperature classes
- The basis for the classification: sources of release, ventilation assumptions and the method used
- An equipment register listing every item in classified areas, with its certification marking
- Inspection records, including detailed inspections of flameproof and increased safety equipment
- Records of modifications and repairs, and who did them
- Training records for the people who install, inspect and maintain the equipment
A typical case
A municipality is upgrading a wastewater lift station. The original drawings show the wet well as a hazardous location but do not explain why or how far the classification extends. During design, the area classification is redone using current methods, taking into account the ventilation that will be installed. The study sets the wet well and the space immediately above the access hatch as classified, with a defined envelope around the hatch, and leaves the separate dry room unclassified because of its own ventilation and physical separation. The control panel, which had been inside the classified area, is moved to the dry room, with sealed conduits to the instruments in the wet well, which use intrinsically safe circuits. The result is fewer items of expensive certified equipment and easier maintenance.
Change is a hazard. Changes in process, ventilation, stored materials or layout can change the classification. A small modification, such as a new receptacle or a replacement light fitting that is not rated for the zone, can create an ignition source. All work in hazardous locations should be reviewed by someone competent in the area and done under the site's permit system, including hot work permits where needed.
How ehZee can help
As part of our electrical design work, we design electrical systems for buildings and sites with hazardous locations, working from the area classification, and can review existing installations against the classification. Drawings for permit are sealed by a licensed professional engineer where required, and installation is done by licensed electrical contractors under permit. For general code context, see Canadian Electrical Code basics for owners.
Common questions
Who is responsible for area classification?
The owner is responsible for having the classification done and kept current, usually by an engineer with process and electrical knowledge. It should be documented on drawings and reviewed when the process or facility changes.
Can we use ordinary equipment if we improve the ventilation?
Ventilation is one of the factors in classification, and good ventilation can reduce the extent or type of a zone. Any change must be assessed and documented, and the ventilation must be reliable, sometimes monitored with alarms.
Can certified equipment be repaired?
Only following the manufacturer's instructions and recognized repair practices, by people competent to do it. Improper repairs, such as machining a flame path or replacing parts with non-certified ones, can void the protection.