Equipment and products operating in hazardous areas are required to meet stringent criteria. They must be ‘protected’ to avoid the possibility of them becoming a source of ignition. Weighing systems are often an integral part of a process within the hazardous area and therefore must have suitable protection.
Hazardous areas are essentially designated places where the presence of certain materials, such as gases, dust or fibres make an explosion probable if a source of ignition is introduced. (Probably the best known example of a hazardous area is the petrol station.)
Three essential elements are required for an explosion to take place:
- Gas, mist, vapour or dust in sufficient proportions.
- Sufficient supply of air or oxygen.
- Source of ignition
Hazardous area classification defines the where, when and what;
Where – Group l covers underground applications (mining) and Group ll covers surface applications
When – Group ll applications are divided into Zones (Europe) or Divisions (USA) which take into account the probability of the presence of hazardous material.
| AREA CLASSIFICATION | ||||
| HAZARD | Hazard continuously present (> 1000 hours per year) | Hazard present under normal operation (10-1000 hours per year) | Hazard only present, under abnormal conditions (< 10 hours per year) | |
| IEC/CENELEC (Europe) | GASES | ZONE 0 | ZONE1 | ZONE 2 |
| DUSTS | ZONE 20 | ZONE 21 | ZONE 22 | |
| NORTH AMERICA | GASES & DUSTS | DIVISION 1 | DIVISION 2 | |
What – Relates to the nature of the hazardous substance
INDUSTRY | REPRESENTATIVE GAS / DUST | EUROPE | USA & CANADA |
SURFACE | Acetylene Hydrogen Ethylene Propane | Group llC Group llc Group llB Group llA | Class l Group A Class l Group B Class l Group C Class l Group D |
| SURFACE | Metal Dust Carbon Dust Flour, Starch, Grain Fibres | Unclassified | Class ll Group E Class ll Group F Class ll Group G Class lll |
Temperature classification
Hazardous area apparatus is classified according to the maximum temperature produced under normal or fault conditions at an ambient temperature of 40C. It goes from T6 classification, which allows a maximum surface temperature of 85C( no known products have an ignition temperature as low as this), to T1, which allows a maximum temperature of 450C.
World Wide Certification
As with legal for trade approval, hazardous area certification requirements differ between the US and Europe. However there is growing cooperation between the US authorities and CENELEC. The1996 edition of the National Electrical Code ANSI/NFPA No 70 has introduced article 505. This allows for an alternative area classification scheme based on the IEC three zone system and identifies the areas as Class l, Zone 0,1; Group llA. B or C as applicable. Introduction of this three zone system allows the use of protection concepts already in use in Europe.
EEx ia llc T6 certification for example, is equivalent to, or exceeds Division l, Class l, Group A T6 certification in the USA.
USA
The standards for electrical equipment for use in all hazardous areas are written by the National Fire Protection Agency (NFPA) and laid down in the National Electrical Code (NEC) Handbook in Articles 500-504. Products are tested by Factory Mutual Research Corporation (FM) or Underwriters Laboratories Inc. (UL)
Europe
In Europe the requirements and legislation for apparatus in hazardous areas is covered by new style ATEX European Directive 94/9/EC which came into force on 1st March 1996 and becomes mandatory in July 2003. An overview of ATEX is given in the following PDF –Understanding ATEX (information by kind permission of IML Ppublishing Group)
Requirements are laid down by the European Committee for Electrotechnical Standardisation (CENELEC) based on the International Electrotechnical Commission( IEC) standard 79.
METHODS OF PROTECTING EQUIPMENT FOR USE IN HAZARDOUS AREAS
| Ignition protection method | Principle and typical applications |
Oil immersion “O” | The ignition source is constantly immersed in oil. Used in switch devices and transformers. |
Pressurised enclosure “P” | A protective gas contains the ignition source. Used in machines, collector motors, switch cabinets, control rooms, monitors, keyboards and pads and analysers. |
Sand filling “q” | The fine grained filling surrounds the ignition source and therefore an arc from the inside of the housing cannot ignite the surrounding combustible atmosphere. Used in capacitors, ballasts for lamps and measurement apparatus. |
Flameproof enclosure “d” | If an ignition occurs inside the enclosure, the enclosure must be able to withstand the pressure and contain the internal explosion (s-gap width) Used in heavy current applications, switches and components that arc. This method can be used for load cells and weighing terminals. |
Increased safety “e” | Measures that reduce the chance of arcing and increased temperatures. No ignition source can be present during normal operation. Used in connection systems (motors). |
Encapsulation “m” | The ignition source is embedded in an encapsulating compound in such a way that it cannot ignite a hazardous explosive atmosphere. Used in measuring apparatus and variable speed drives. |
Intrinsic Safety “i” | The energy in the circuit is limited to values which do not result in unacceptably high temperatures and/or arcs of sufficient energy to cause an explosion. Used in instrumentation and control technology. |
Intrinsic safety
Intrinsic safety (IS) is one of several techniques for preventing explosions in hazardous areas and is the most effective. Intrinsic safety operates by limiting the electrical energy in circuits and equipment to levels that are too low to ignite the most easily ignitable mixture of gas or dust that is ever likely to be present.
All other methods of protection such as oil filling or explosion proof enclosures rely on the continual maintenance of a physical barrier between the explosive atmosphere and the electrical circuit. Unfortunately any breach of the barrier renders the protection inoperative.
By contrast IS gives inherent protection by restricting the energy at its source and therefore has both commercial and technical advantages.
Circuits and equipment are designed so that safety is maintained both in normal use and under all possible fault conditions. Approvals can be given for individual components (entity approval) and complete systems.
Load cell certification
The question as to whether load cells need to be certified for use in hazardous areas is often not fully understood.
Load cells are sometimes referred to as ‘simple apparatus’ . The simple apparatus category was introduced to avoid the certification of simple passive devices such as switches, thermocouples and LEDs. Simple apparatus refers to products that cannot, on their own, generate or store energy above defined levels.
However, the classification of products as ‘simple apparatus’ is the opinion of the manufacturer based on their knowledge of their products. Although it may be technically possible to define load cells as simple apparatus, most hazardous area applications will call for certification of products such as load cells.
The technique of intrinsic safety is ideally suited to strain gauge load cells which are non-energy storing low power devices.
ZENER diode barriers are fitted to the input and output leads of the load cell to prevent dangerous levels of electrical energy reaching the hazardous area.
