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ELECTRICAL APPARATUS WITH TYPE OF PROTECTION "n" FOR USE IN POTENTIALLY EXPLOSIVE ATMOSPHERE ZONE 2 | |||||||||||||||||||||||||||||||
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However, the probability that a potentially explosive atmosphere depending on the operational situation. to achieve an adequate and also economically acceptable level of protection from explosion hazards, work places with potentially explosive atmospheres are divided into different zone. This is based on the probability of an explosive atmosphere occurring ranging from continuous or long term (zone 0,zone 20), through occasional (Zone 1, Zone 21) to abnormal (Zone 2, zone 22). Construction and testing of Electrical apparatus - zone 2 Standards concerning the construction and testing of explosion protected electrical apparatus in Europe are set by CENELEC, the European Committee for Electro-technical Standardization in electrical Engineering. For the operation of electrical equipment specified for potentially explosive atmospheres, the stipulations of the European Directive 94/9/EC (ATEX 100a) for the harmonization of legal requirements in the Member states for equipment and protection systems apply. This Ec Directive provides a classification into 3 categories for electrical apparatus Group it in potentially explosive atmospheres with regard to gas and dust, other than mining. In Zone 0 and one 1 (gas) as well as in Zone 20 and Zone 21 (dust) only explosion protected, electrical explosion protected apparatus certified for use in such areas is permitted. Since there is as yet no harmonized and detailed standard for construction of Zone 2 apparatus in Europe, such apparatus for Zone 2 is constructed in Germany according to the code of Practice DIN VDE 0165/02.91. In future, the construction of electrical apparatus for hazardous areas of Zone 2 will be subject to the regulations of the European standard EN 50 021 - electrical apparatus with Type of protection "n". European Standard EN 50 021 Apparatus with Type of protection "n" The new European Standard contains the requirements for the construction, testing and marking of electrical apparatus of Group II with type of protection "n" for use in zone 2. It refers to areas where an explosive atmosphere of gas , vapor normal operation, but may occur, even if only for a short time. Application of EN 50 021 The standard is applicable to non-sparking apparatus as well as to devices with parts or circuits which could produce arcs or sparks or have hot surfaces, which - if not protected - could be capable of igniting the surrounding explosive atmosphere. The requirements of the standard should ensure that the occurrence of a malfunction, which could cause an ignition, would remain unlikely during normal operation. One example of an abnormal operating condition would be a light fitting with a defective lamp. The common practice to date , i.e. using equipment of good industrial quality in many applications in Zone 2, which is defined by the selection criteria for Zone 2 apparatus in the appropriate regulations for installation of electrical equipment VDE 0165/02.91, is not acceptable any more. Apparatus for zone 2 are explosion protected devices and consequently have a substantially higher level of protection that standard industrial equipment. Methods of Type of Protection "n" of Zone 2 apparatus Apparatus which is approved for Zone 0 or Zone 1 may also be installed in areas with potentially explosive atmospheres Zone 2. In addition, there is now the concept of Type of Protection "n" for Zone 2. It contains a collection of protective methods, which are partially derived from the protection types for category 2, by adjusting them to the lower level of category 3. The Type of Protection "n" is defined by EN 50 021 as follows: "Type of protection applied to electrical apparatus such that, in normal operation and in certain abnormal conditions specified by this standard, it is not capable of igniting a surrounding atmosphere." The standard differentiates between apparatus, which cause arcs/sparks and /or have hot surfaces, and apparatus which do not usually cause arcs/sparks and/or have hot surfaces. to simplify the terminology, it will be described as' sparking' and 'non-sparking' apparatus in the following. The relevant methods of protection are applied on the basis of this differentiation.
Non-sparking Apparatus ex nA This describes devices, which have been designed in such a way minimize the risk of arcing, sparking or hot sports that constitute an ignition hazard during normal use. Normal use does not include the removal or installation of components, while the circuit is energized. Examples of electrical apparatus, which do not cause arcing/sparking and /or hot surfaces under normal operation are electric motors (cage motors), terminal boxes, fuses, light fittings, transformers, low energy devices (instrumentation and control technology), receptacles and plugs as well as cells and batteries. Sparking apparatus Switches, circuit breakers, relays, contact switches, electric motors (slip-ring, wound rotor) are examples for apparatus which may cause sparking/arcing and/0r have hot surfaces under normal operational conditions. Apparatus with protected contacts Ex nC The 'Spark' of a device with protected contacts is enclosed in such a way that it cannot come in direct contact with the potentially explosive atmosphere. The types of protection used for this are similar to the "flameproof enclosure" or "moulding". Type of protection "nC" covers five different protection concepts to avoid ignition and are presented in the following paragraphs. Enclosed-break device this describes devices containing electrical contacts for making and breaking a circuit, which will withstand an internal explosion resulting from the possible entry of a combustible gas or vapor, without being damaged or transferring the internal explosion to the external surrounding combustible gas or vapor. In this context, the differentiation between the types of protection - restricted breathing, energy limitation and simplified pressurization needs to be pointed out. Non-incendive components non-incendive devices are components having contacts with the ability to open and close circuits, which could be an ignition source. For those devices, either the contact mechanism or the housing containing the contacts is constructed in such a way that the ignition of a particular flammable gas or vapor will be prevented under specified operational conditions.
Hermetically-sealed device This term is used to describe a device, designed to ensure that an outer atmosphere cannot enter the interior. The sealing effect is achieved by a melting process, such as soldering, brazing, welding or fusing of glass with metal. Sealed device Sealed devices are constructed in such a way that they cannot be opened during normal service, and therefore prevent the external atmosphere from entering the inside o f the device. Encapsulated device an encapsulated device may contain voids, which are constructed in such a way that they are totally immersed in an encapsulation compound. This prevents the entry of the external atmosphere. An encapsulated device is to be considered a sealed device. Restricted breathing enclosure Ex nR This includes enclosures designed to limit the ingress of gas and vapor. This prevents explosive gases from building to a critical concentration within the enclosure, while a cloud of concentrated gas drifts past the device. Sparking devices may only be installed into restricted-breathing enclosures where it is guaranteed that the average temperature in the enclosure is not more than 10 K above the ambient temperature outside the enclosure Additional provision has to be made that restricted breathing is also guaranteed during installation and maintenance. The penetration of gas is limited, so that the gas concentration K2 is always below the lower explosion limit at moments t1 and t2. Simplified pressurization Ex nP This describes the use of protective gas for a housing to avoid the formation of a potentially explosive atmosphere inside the housing by maintaining overpressure against the surrounding atmosphere. The easier condition in comparison to Zone 1 is that no purging is required and that a reduction in overpressure which indicates a malfunction does not enforce the instant shutdown of the installation. Energy-limited apparatus Ex nL. This technology is based on the idea of intrinsic safety. This concept can be applied to circuits, which, under the test conditions described in this standard, do not produce sparking or other thermal effects capable of igniting an existing flammable gas or vapor. In the case of this apparatus, circuits and components are designed in accordance with the concept of energy limitation. Requirement for all Electrical Apparatus with Type of Protection "n". The new Standard EN 50 021 contains requirements for the construction and testing of electrical apparatus for a variety of protection concepts of Zone 2, as described above. Inspite of the various protection concepts, some requirements are common to all electrical apparatus for Zone 2. Enclosure must at least satisfy IP 54, i.e. the device must be protected against dust and splashing water. The clearances for Zone 2 apparatus are approximately 2/3rd of the values of devices to be used in Zone 1. The requirement for creep age distances amount to half the values for Zone 1. In addition, plastic enclosures have to be constructed in such a way that the danger of ignition through electrostatic charges is avoided. The temperature classes (limitation of surface temperature) for Zone 2 is the same as those for Zone 0 and 1. |