Electrical equipment in hazardous areas

[ali]

In electrical engineering, a hazardous location is defined as a place where concentrations of flammable gases, vapors, or dusts occur. Electrical equipment that must be installed in such locations is especially designed and tested to ensure it does not initiate 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 of no concern, but if a flammable vapor is present, the arc might start an explosion. Electrical equipment intended for use in a chemical factory or refinery is designed either to contain any explosion within the device, or is designed not to produce sparks with sufficient energy to trigger an explosion.

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 and non-incendive equipment and wiring methods are practices where apparatus is designed with low power levels and low stored energy, so that an arc produced during normal functioning of the equipment or as the result of equipment failure has insufficient energy to initiate ignition of the explosive mixture. Equipment enclosures can be pressurized with clean air or inert gas and designed with various controls to remove power or provide notification in case of supply or pressure loss of such gases. Arc-producing elements of the equipment can also be isolated from the surrounding atmosphere by encapsulation, immersion in oil, sand, etc. Heat producing elements such as motor winding, electrical heaters, including heat tracing and lighting fixtures are often designed to limit their maximum temperature below the autoignition temperature of the material involved. Both external and internal temperatures are taken into consideration.

As in most fields of electrical installation, 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.

Area classification is required by governmental bodies, for example the U.S. Occupational Safety and Health Administration and compliance is enforced.

Documentation requirements are varied. Often an area classification plan-view is provided to identify equipment ratings and installation techniques to be used for each classified plant area. The plan may contain the list of chemicals with their group and temperature rating, and elevation details shaded to indicate Class, Division(Zone) and group combination. The area classification process would require the participation of operations, maintenance, safety, electrical and instrumentation professionals, the use of process diagrams and material flows, MSDS and any pertinent documents, information and knowledge to determine the hazards and their extent and the countermeasures to be employed. Area classification documentations are reviewed and updated to reflect process changes.

[ali]

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]

Divisions or zones (gases, vapors and dust)
In an industrial plant such as a refinery or chemical process plant, handling of large quantities of flammable liquids and gases creates a risk of leaks. In some cases the gas, ignitable 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, equipment deterioration between maintenance periods, or during an incident. Refineries and chemical plants are then divided into areas of risk of release of gas, vapor or dust known as divisions or zones. The process of determining the type and size of these hazardous areas is called area classification. Guidance on assessing the extent of the hazard is given in the NFPA 497 Standard, or API 500 and according to their adaptation by other areas gas zones is given in the current edition of IEC 60079.10. For hazardous dusts, the guiding standard is IEC 61421.10.

Typical gas hazards are from hydrocarbon compounds.

Safe area
An area such as a residence or office would be classed as safe area, where the only risk of a release of explosive or flammable gas would be such things as 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 gas explosions in residential buildings do 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)).
Division 2 or Zone 2 area
This is a step up from the safe area. In this zone the gas, vapor 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[2].
Division 1 or Zone 1 area
Gas, vapor 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[2].
Zone 0 area
Gas or vapor is present all of the time. An example of this would be the vapor space above the liquid in the top of a tank or drum. The ANSI/NEC classification method consider this environment a Division 1 area. As a guide this can be defined as over 1000 hours/year or >10% of the time[2].
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

[ali]

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.[3]

Flammability of combustible liquids are defined by their flash-point. The flash-point is the temperature at which the material will generate sufficient quantity of vapor to form an ignitable mixture. It is a critical data in determining whether the area needs to be classified or not. A material may have a relatively low autoignition temperature yet its flash-point is above the ambient temperature then the area may not need to be classified. Conversely if the same material is processed(heated) above ambient temperature and the handling temperature is above its flash-point, the area MUST be classified. (NFPA reference needed)

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

Apparatus marked IIB can also be used for IIA gases. IIC marked equipment can be used for both IIA and IIB. 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 vapor 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 vapor 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. The surface temperature or any parts of the electrical equipment that may be exposed to the hazardous atmosphere should be tested that it does not exceed 80% of the auto-ignition temperature of the specific gas or vapor in the area where the equipment is intended to be used.

The temperature classification on the electrical equipment label will be one of the following (in degree Celsius):

USA°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.

[ali]

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 - ANSI/NFPA(NEC)terms
Class I, Div. 1 - Where ignitable concentrations of flammable gases, vapors or liquids are present within the atmosphere under normal operation conditions.

Class I, Div. 2 - Where ignitable concentrations of flammable gases, vapors, or liquids are present within the atmosphere under abnormal operating conditions.

Class II, Div. 1 - Where ignitable concentrations of combustible dusts are present within the atmosphere under normal operation conditions.

Class II, Div. 2 - Where ignitable concentrations of combustible dust are present within the atmosphere under abnormal operating conditions.

Class III, Div. 1 - Where easily ignitable fibers or materials producing combustible flyings are present within the atmosphere under normal operation conditions.

Class III, 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: Acetylene / Group B: Hydrogen / Group C: Propane and Ethylene / Group D: Benzene, Butane, Methane & 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.

Labeling
All equipment certified for use in hazardous areas must be labelled to show the type and level of protection applied.

1. 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. The suitability of equipment for the specific hazardous area in the ANSI/NFPA(NEC) regulated market is required to be tested by a NRTL(Nationally Recognized Testing Laboratory). Such institutes are UL, FM or ETL, just to name a few.

a) Dependent on the specific equipment and the applicable division the label will always list the Class(es), Division(s) and may list the Group(s) and temperature Code.

b) Directly adjacent on the label one will find the mark of the listing agency.

c) Some manufacturers claim "suitability" or "built-to" hazardous areas in their technical literature, but in effect lack the testing agency's certification and thus unacceptable for the AHJ (Authority Having Jurisdiction) to permit operation of the electrical installation/system.

4. Also included in the marking are: -

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.

e) The NEMA enclosure rating or IP code may also be indicated, but it is usually independent of the Classified Area suitability.

5. Manufacturers will provide specific installation instructions that must be followed for:

a) the equipment to function properly, and

b) to provide the designed protection in the Classified Area according to the listing.

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 (

[link Point to another website Only the registered members can access]

[f81aa]

Ali:

Your posts have enough information to serve for quick consultation without hitting the books. Good effort.

Regards

[Thames Jones]

When you are going to start construction at chemical plant, the first & foremost thing is to schedule safety meeting that should be attended by all the workers as well as officials. Agenda of the meeting would be to give detail description about various kind of hazards & how to avoid accidents and what equipment should be used to avoid co-incidental accidents. You should use Personal Protection Equipment for that. Thanks.