- Electrical Equipment in Hazardous Areas
Domestic electrical equipment in British homes is made and tested to a
British Standard. The aim of the testing is to ensure that the equipment is fairly robust and has no exposed parts that can cause harm to the user. No other protection is usually required in this case.
However, for example, in some domestic light switches you can see the arc inside when the switch is being operated, especially in the case of fluorescent lighting. In a chemical factory or refinery the electrical equipment obviously must be made to a much more robust industrial standard and designed not to create an ignition risk because of any explosive gases, vapours or dusts that might be present around the equipment.
The above industrial electrical equipment that is to be put into the hazardous area has to be conform to one of a set of detailed standards, parts of 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 [http://www.berr.gov.uk/files/file44775.pdf list of Notified Bodies within the UK] , of which [http://www.siracertification.com Sira] and
Baseefaare the most well known. All equipment certified for use in hazardous areas must be labelled to show the level of protection offered.
Note also that mechanical equipment can also create heat or sparks, and must also be certified for use in hazardous areas.
Zones (vapours & gases)
If you look at the size of a
refineryor chemical factory and the amount of liquids and gases that circulate the various processes in that plant there must be a certain amount of risk of leaks and other hazards. In some cases the gas, vapour or dust is present all the time or for long periods. Refineries and chemical complexes should thus be divided into areas of risk of release of gas, vapour or dust known as zones. The type and size of these hazardous areas is determined using 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 0.25% of its lower explosive concentration limit.
;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 dust 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 dust will be present or expected 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.
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.
Each chemical gas or vapour used on the refinery or chemical works comes under a certain gas group. For this industry there are three:
Propane:Representative gas for group IIA.; Ethylene:Representative gas for group IIB.; Hydrogen/ acetylene:Representative gas for group IIC.
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'.
The 'IP' Code
The IP (Ingress Protection) Code gives an indication of the equipment's ability to keep out unwanted liquids and solids. The code is given in the form of two numbers, for example: IP54. The first numerical digit, in the previous example '5', is the degree of protection against solids and is given a value between 0 and 6. The second number is the degree of protection against liquids, and has a value between 0 and 8.
A third number which will indicate resistance to impacts is being considered by the industry, and will be based on a standard impact, measured in
joulesthat a particular enclosure can withstand.
Table of Reference
Hazardous Locations Classifications Descriptions - North American terms
Class 1, Div. 1 - Where ignitable concentrations of flammable gases, vapors or liquids can exist all of the time or some of the time under normal operation conditions.
Class 1, Div. 2 - Where ignitable concentrations of flammable gases, vapors, or liquids are not likely to exist under normal operating conditions.
Class 2, Div. 1 - Where ignitable concentrations of combustible dusts can exist all of the time or some of the time under normal operation conditions.
Class 2, Div. 2 - Where ignitable concentrations of combustible dust are not likely to exist under normal operating conditions.
Class 3, Div. 1 - Where easily ignitable fibers or materials producing combustible flyings are handled, manufactured or used.
Class 3, Div. 2 - Where easily ignitable fibers are stored and handled.
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.
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). Forexample: -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.
Equipment and protective systems intended for use in potentially explosive atmospheres
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