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In Electrical engineering, a hazardous location is defined as a place where concentrations of flammable gases, vapors, or dusts may occur. Electrical equipment that must be installed in such locations is especially designed and tested to ensure it does not start an explosion, due to arcing contacts or high surface temperature of equipment.
For example a household light switch may emit a small, harmless visible spark when switching; in an ordinary atmosphere this arc is if no concern, but if a flammable vapor was present, the arc might start an explosion. Electrical equipment intended for use in a chemical factory or refinery is designed either not to produce any sparks, or else to safely contain the arc and make sure it cannot ignite any explosive gases, vapors or dusts that might be present around the equipment.
Many strategies exist for safety in electrical installations. The simplest strategy is to minimize the amount of electrical equipment installed in a hazardous area, either by keeping the equipment out of the area altogether or by making the area less hazardous by process improvements or ventilation with clean air. Intrinsic safety is a practice where apparatus is designed with low power levels and low stored energy, so that a fault is unlikely to set off an explosion. Equipment enclosures can be pressurized with clean air, and interlocked so that the equipment is disconnected if the air supply fails. Or, arc-producing elements of the equipment can be isolated from the surrounding atmosphere by encapsulation, immersion in oil, sand, or by sturdy enclosures that prevent propagation of an internal explosion to the surrounding atmosphere.
As in most fields of electrotechnology, different countries have approached the standardization and testing of equipment for hazardous areas in different ways. As world trade becomes more important in distribution of electrical products, international standards are slowly converging so that a wider range of acceptable techniques can be approved by national regulatory agencies.
History
Soon after the introduction of electric power into coal mines, it was discovered that lethal explosions could be touched off by electrical equipment such as lighting, signals, or motors. The hazard of fire damp or methane accumulation in mines was well known by the time electricity was introduced, along with the danger of suspended coal dust. However, at least two British mine explosions were attributed to an electric bell signal system. In this system, two bare wires were run along the length of a drift, and any miner desiring to signal the surface would momentarily touch the wires to each other or bridge the wires with a metal tool. The inductance of the signal bell coils, combined with breaking of contacts by exposed metal surfaces, resulted in sparks which could ignite methane, causing an explosion.[1]
Zones (vapours & gases)
In an industrial plant such as a refinery or chemical factory, handling of large quantities of flammable liquids and gases creates a risk of leaks. In some cases the gas, vapor or dust is present all the time or for long periods. Other areas would have a dangerous concentration of flammable substances only during process upsets, maintenance, or during an accident. Refineries and chemical complexes are then divided into areas of risk of release of gas, vapor or dust known as zones. The process of determining the type and size of these hazardous areas is called area classification.
Typical gas hazards are from hydrocarbon compounds.
- Safe area
- A domestic domain such as a house would be classed as safe area where the only risk of a release of explosive or flammable gas would be the propellant in an aerosol spray. The only explosive or flammable liquid would be paint and brush cleaner. These are classed as very low risk of causing an explosion and are more of a fire risk (although on rare occasions gas explosions in domestic property are known to occur). Safe area on chemical and other plant are present where the hazardous gas is diluted to a concentration below 25% of its lower flammability limit (or lower explosive limit (LEL)).
- Zone 2 area
- This is a step up from the safe area. In this case it has been decided that in this zone the gas, vapour or mist would only be present under abnormal conditions (most often leaks under abnormal conditions). As a general guide, unwanted substances should only be present under 10 hours/year or 0–0.1% of the time. Explosion safety compliant equipment, should be used.
- Zone 1 area
- These areas are where special or classified electrical equipment must be used. It is expected that the gas, vapour or mist will be present or expected to be present for long periods of time under normal running. As a guide this can be defined as 10–1000 hours/year or 0.1–10% of the time. Explosion safety equipment that has a higher safety level than Zone 2 equipment must be used.
- Zone 0 area
- This is the worst scenario as gas or vapour is present all of the time (over 1000 hours/year or >10% of the time). Although this is the worst case it is very rare that a zone 0 area will be in the open. Usually this would be the vapour space above the liquid in the top of a tank or drum.
Zones (dusts)
In the case of dusts there is still a chance of explosion. An old system of area classification to a British standard used a system of letters to designate the zones. This has been replaced by a European numerical system, as set out in directive 1999/92/EU implemented in the UK as the Dangerous Substances and Explosives Atmospheres Regulations 2002
The boundaries and extent of these three dimensional zones should be decided by a competent person. There must be a site plan drawn up of the factory with the zones marked on.
The zone definitions are:
- Zone 20
- A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods, or frequently.
- Zone 21
- A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally.
- Zone 22
- A place in which an explosive atmosphere in the form of a cloud of combustible dust is not likely to occur in normal operation, but if it does occur will persist for a short period only
Guidance on assessing the extent of zones is given in EN61241-10:2004. This has since been superseded by IEC 60079-10-2:2009.
Gas groups
Explosive gases, vapors and dusts have different chemical properties that affect the likelihood and severity of an explosion. Such properties include flame temperature, minimum ignition energy, upper and lower explosive limits, and molecular weight. Empirical testing is done to determine parameters such as the maximum experimental safe gap, minimum ignition current, explosion pressure and time to peak pressure, spontaneous ignition temperature, and maximum rate of pressure rise. Every substance has a differing combination of properties but it is found that they can be ranked into similar ranges, simplifying the selection of equipment for hazardous areas.[2] Each chemical gas or vapour used in industry is classified into a gas group.
| Group | Representative Gases |
|---|---|
| I | All Underground Coal Mining. Firedamp (methane) |
| IIA | Industrial methane, propane, petrol and the majority of industrial |
| IIB | Ethylene, coke oven gas and other industrial gases |
| IIC | Hydrogen, acetylene, carbon disulphide |
If a piece of equipment has just II and no A, B, or C after then it is suitable for any gas group.
A list must be drawn up of every chemical gas or vapour that is on the refinery/chemical complex and included in the site plan of the classified areas. The above groups are formed in order of how volatile the gas or vapour would be if it was ignited, IIC being the most volatile and IIA being the least. The groups also indicate how much energy is required to ignite the gas by spark ignition, Group IIA requiring the most energy and IIC the least.
Temperature classification
Another important consideration is the temperature classification of the electrical equipment. One thing that must never be allowed to happen is the surface temperature of the electrical equipment rise beyond the auto-ignition temperature of the gas or vapour that it is put into.
The temperature classification on the electrical equipment label will be one of the following (in degree Celsius):
| America °C | UK °C | Germany °C Continuous - Short Time |
|
|---|---|---|---|
| T1 - 450 | T3A - 180 | T1 - 450 | G1: 360 - 400 |
| T2 - 300 | T3B - 165 | T2 - 300 | G2: 240 - 270 |
| T2A - 280 | T3C - 160 | T3 - 200 | G3: 160 - 180 |
| T2B - 260 | T4 - 135 | T4 - 135 | G4: 110 - 125 |
| T2C - 230 | T4A - 120 | T5 - 100 | G5: 80 - 90 |
| T2D - 215 | T5 - 100 | T6 - 85 | |
| T3 - 200 | T6 - 85 | ||
The above table tells us that the surface temperature of a piece of electrical equipment with a temperature classification of T3 will not rise above 200 °C.
Auto-ignition temperatures (vapours & gases)
The auto-ignition temperature of a liquid, gas or vapour is the temperature at which the substance will ignite without any external heat source. The exact temperature value determined depends on the laboratory test conditions and apparatus. Such temperatures for common substances are:
| Gas | Temperature |
|---|---|
| Methane | 580 °C |
| Hydrogen | 560 °C |
| Propane | 493 °C |
| Ethylene | 425 °C |
| Acetylene | 305 °C |
| Naphtha | 290 °C |
| Carbon disulfide | 102 °C |
Auto-ignition temperatures (dust)
The auto-ignition temperature of a dust is usually higher than that of vapours & gases. Examples for common materials are:
| Substance | Temperature |
|---|---|
| Sugar | 460 °C |
| Wood | 340 °C |
| Flour | 340 °C |
| Grain dust | 300 °C |
| Tea | 300 °C |
Type of protection
To ensure safety in a given situation, equipment is placed into protection level categories according to manufacture method and suitability for different situations. Category 1 is the highest safety level and Category 3 the lowest. Although there are many types of protection, a few are detailed
| Ex Code | Notified Body Name | Description | Location | Use | |
|---|---|---|---|---|---|
| Flameproof | d | Will have a CENELEC Hexagon followed by apparatus group and Safety Category | Equipment is robust can stand an explosion from within, without transmitting the flame to the outside Equipment has flameproof gaps (max 0.006" propane/ethylene, 0.004" acetylene/hydrogen) |
Zone 1 if gas group & temp. class correct | Motors, lighting, junction boxes |
| Increased Safety | e | Will have a CENELEC Hexagon followed by apparatus group and Safety Category | Equipment is very robust and components are made to a high quality |
Motors, lighting, junction boxes | |
| Oil Filled | o | Will have a CENELEC Hexagon followed by apparatus group and Safety Category | Equipment components are completely covered with a layer of oil | Zone 2 or Zone 1, depending on edition of the standard used. | Heavy current equipment |
| Sand/Powder/Quartz Filled | q | Will have a CENELEC Hexagon followed by apparatus group and Safety Category | Equipment components are completely covered with a layer of Sand, powder or quartz | Zone 2 | Electronics, telephones, chokes |
| Encapsulated | m | Will have a CENELEC Hexagon followed by apparatus group and Safety Category | Equipment components of the equipment are usually encased in a resin type material | Zone 1 (Ex mb) or Zone 0 (Ex ma) | Electronics (no heat) |
| Pressurised/purged | p | Will have a CENELEC Hexagon followed by apparatus group and Safety Category | Equipment is pressurised with a positive pressure; gas cannot get in for air coming out or equipment is purged with a diluting gas such as air. If air is used, it is ducted in from outside the hazardous area. | Zone 1 | Analysers, motors, control boxes, computers |
| Intrinsically safe | i | Will have a CENELEC Hexagon followed by apparatus group and Safety Category | Any arcs or sparks in this equipment has insufficient energy (heat) to ignite a vapour Equipment can be installed in ANY housing provided to IP54 |
'ia': Zone 0 & 1 'ib': Zone 1 |
Instrumentation, measurement, control |
| Non Incendive | n | Now CENELEC recognised; so will have a hexagon followed by apparatus group and Safety Category | Equipment is non-incendive or non-sparking. | Zone 2 | Motors, lighting, junction boxes, electronic equipment |
| Special Protection | S | Has a BASEEFA Crown Not CENELEC recognised; no hexagon. |
This method, being by definition special, has no specific rules. In effect it is any method which can be shown to have the required degree of safety in use. Much early equipment having Ex s protection was designed with encapsulation and this has now been incorporated into IEC 60079-18 [Ex m]. Ex s is a coding referenced in IEC 60079-0. The use of EPL and ATEX Category directly is an alternative for “s” marking. | Zone 1 depending upon Manufacturers Certification. | As its certification states |
Multiple Protection
Many items of EEx rated equipment will employ more than one method of protection in different components of the apparatus. These would be then labeled with each of the individual methods. For example a socket outlet labeled EEx'de' might have a case made to EEx 'e' and switches that are made to EEx 'd'.
Hazardous Locations Classifications Descriptions - North American terms
Class 1, Div. 1 - Where ignitable concentrations of flammable gases, vapors or liquids are present within the atmosphere under normal operation conditions.
Class 1, Div. 2 - Where ignitable concentrations of flammable gases, vapors, or liquids are present within the atmosphere under abnormal operating conditions.
Class 2, Div. 1 - Where ignitable concentrations of combustible dusts are present within the atmosphere under normal operation conditions.
Class 2, Div. 2 - Where ignitable concentrations of combustible dust are present within the atmosphere under abnormal operating conditions.
Class 3, Div. 1 - Where easily ignitable fibers or materials producing combustible flyings are present within the atmosphere under normal operation conditions.
Class 3, Div. 2 - Where easily ignitable fibers or materials producing combustible flyings are present within the atmosphere under abnormal operating conditions.
Common Materials within Associated Class & Group Ratings, such as "Class I, Division 1, Group A":
Class I Areas: Group A: Acetelyne / Group B: Hydrogen / Group C: Propane and Ethylene / Group D: Benzene, Butane & Propane
Class II Areas: Group E: Metal Dust / Group F: Carbon & Charcoal / Group G: Flour, Starch, Wood & Plastic
Class III Areas: NO GROUP: Cotton & Sawdust
For more information see Article 500 of NFPA 70 - The National Electric Code, as published by the National Fire Protection Association.
Equipment category
The equipment category indicates the level of protection offered by the equipment.
Category 1 equipment may be used in zone 0, zone 1 or zone 2 areas.
Category 2 equipment may be used in zone 1 or zone 2 areas.
Category 3 equipment may only be used in zone 2 areas.
Labelling
All equipment certified for use in hazardous areas must be labelled to show the type and level of protection applied. In Europe the label must show: -
a) The CE mark.
b) The code number of the certifying body for the ‘quality certificate’.
c) CE marking is complemented with the Ex mark, followed by the indication of the Group, Category and, if group II equipment, the indication relating to gases (G) or dust (D). For example: - Ex II 1 G (Explosion protected, Group 2, Category 1, Gas)
2. In addition, the normative marking will be able to establish the specific type or types of protection being used, for example: -
EEx ia IIC T4. (Type ia, Group 2C gases, Temperature category 4).
EEx nA II T3 X (Type n, non-sparking, Group 2 gases, Temperature category 3, special conditions apply).
3. Also included in the marking are at least: -
a) The manufacturers name or trademark and address.
b) The apparatus type, name and serial number.
c) Year of manufacture.
d) Any special conditions of use.
The IP code may also be indicated if appropriate.
British standards
Industrial electrical equipment for hazardous area has to conform to standard BS 60079 and in some cases, certified as meeting that standard. Independent test houses (known as Notified Bodies)are established in most European countries, and a certificate from any of these will be accepted across the EU. The DTI appoint and maintain a list of Notified Bodies within the UK, of which Sira and Baseefa are the most well known. All equipment certified for use in hazardous areas must be labelled to show the level of protection offered.
See also
- Electrical conduit
- Intrinsic safety
- ATEX directive
- Pressure piling
- Mineral-insulated copper-clad cable
- Grounding kit
References
- ^ Bossert 86 page 17
- ^ John Bossert and Randolph Hurst, Hazardous Locations A Guide for the Design, Construction and Installation of Electrical Equipment, Canadian Standards Association, Toronto 1986 ISBN 0-9690124-5-4, Chapter 9
External links
Further reading
- Alan McMillan, Electrical Installations in Hazardous Areas, Butterworth-Heineman 1998, ISBN 0750637684, preview available at Google Books at http://books.google.ca/books?id=Ojke07cRCzQC&printsec=frontcover&dq=electrical+installations+in+hazardous+areas+mcmillan#v=onepage&q=&f=false
- Peter Schram Electrical Installations in Hazardous Locations, Jones and Bartlett, 1997, ISBN 0877654239, preview available at Google Books at http://books.google.ca/books?id=WbKRE9FfhOQC&printsec=frontcover&dq=electrical+installations+in+hazardous+areas&lr=&source=gbs_book_other_versions_r&cad=5#v=onepage&q=electrical%20installations%20in%20hazardous%20areas&f=false
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