Electrical equipment in hazardous areas
From Wikipedia, the free encyclopedia
 |
The examples and perspective in this article may not represent a worldwide view of the subject. Please improve this article or discuss the issue on the talk page. |
 |
This article does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (February 2009) |
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 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 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.
Note also that mechanical equipment can also create heat or sparks, and must also be certified for use in hazardous areas.
Contents |
[edit] Zones (vapours & gases)
If you look at the size of a refinery or 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.
[edit] 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.
[edit] Gas groups
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.
| 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.
[edit] 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.
[edit] Auto-ignition temperatures (vapours & gases)
The auto-ignition temperature of a liquid, gas or vapour is the temperature which the substance will ignite automatically by itself without any external heat source. Such temperatures for common substances are:
- Methane
- 580 °C
- Hydrogen
- 560 °C
- Ethylene
- 425 °C
- Acetylene
- 305 °C
- Naphtha
- 290 °C
- Carbon disulfide
- 102 °C
[edit] Auto-ignition temperatures (dust)
The auto-ignition temperature of a dust is usually higher than that of vapours & gases. Examples for common materials are:
[edit] 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 below.
| 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 |
[edit] 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'.
[edit] 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 joules that a particular enclosure can withstand.
[edit] Table of Reference
| 1st Digit | Degree of Protection
(Foreign Bodies) |
2nd Digit | Degree of Protection
(Liquids) |
|---|---|---|---|
| 0 | No protection | 0 | No protection |
| 1 | Protection against ingress of large solid foreign bodies | 1 | Protection against drops of water |
| 2 | Protection against ingress of medium sized solid foreign bodies | 2 | Protection against drops of liquid falling at any angle up to 15° from vertical |
| 3 | Protection against ingress of small solid foreign bodies greater in diameter than 2.5 mm |
3 | Protection against drops of liquid falling at any angle up to 60° from the vertical |
| 4 | Protection against ingress of small solid foreign bodies greater in diameter than 1 mm |
4 | Protection against splashing. Liquid splashed from any direction shall have no harmful effect |
| 5 | Protection against the ingress of dust in an amount sufficient to interfere with satisfactory operation of the enclosed equipment |
5 | Protection against water projected by nozzle from any direction |
| 6 | Complete protection against ingress of dust | 6 | Protection against powerful water jets |
| 7 | Protection against temporary immersion in water | ||
| 8 | Protection against immersion in water for a defined pressure and time |
[edit] 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.
[edit] 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.
[edit] 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.
