Hazardous areas motors

Except for areas with fire or explosion hazards (hazardous areas), motor enclosures are designed to provide protection to the internal working parts. The development or improved insiilating materials and finishes has affec ted the required degree of protection and consequently the design and classification of enclosures. Examples of several types of enclosures are shown in Fig. 29-4. 

In India, the Central Mining Research Institute, Dhanbad carries out this testing and provides the necessary certilication for motors used iti explosive atmospheres. But for approval of the equipment, whether it is worthy of use in a particitlar hazardous area, there are accredited agencies.. Some of these are Directorate General Mines Safety. Dhanbad, Chief Controller of Explosives. Nagpur and Directorate General of Factory Advice Service and Labour Institute. Bonibav. 

Since it is not practical to manufacture a llameproof enclosure due to its size and bulk and the number of knockouts and openings on the doors for switches, metering, indicators, and pushbuttons (PBs) etc., it is common practice to locate the.se assemblies some distance from the affected area in a separate well-ventilated room. Depending upon the location and intensity ol contamination, it may be permissible to meet the requirement by using a pressurized enclosure by maintaining a positive pressure inside the enclosure similar to that for motors (Section 7.1.3..3). When there arc many switchgear assemblies, the room itself can be pressurized, which is safer and easier. Small enclosures, however, such as a PB station, switch or a switch fuse unit or an individual starter unit etc., which can be easily made of MS plates or cast iron, as discussed in Section 7.13, can be mounted in the hazardous area while the main MCC can be installed in the control room, away from the contaminated area and from where the process can be monitored. 

A motor used in standby operation mode should be equipped with a space heater to keep the motor windings dry. In classified areas these space heaters must meet the surface temperature requirement of the specific hazardous area. 

Type and specifications on fan driver, gear types, pow er voltage, phase, cycles. Motors should at least meet specifications equivalent to totally enclosed, fan cooled, or if in explosive hazardous area, TEFC Class I, Group D (except this not acceptable in hydrogen or acetylene atmosphere). Due to moisture conditions around this equipment, it should be protected against moisture penetration and corrosion. 

As you can see, it is important to plan the extent of the hazardous areas and various methods, such as fire walls, to prevent the transfer or propagation of volatile/flammahle materials from one plant area to another. When in douht, err on the side of being safe and conservative. Also keep in mind that dust can be just as hazardous as flammable liquids/ vapors and must be classified and handled accordingly. Tables 14-9 and 14-10 identify the NEMA Standard for Motors. ... 

The installation of generators in hazardous areas is focused on special applications. The specific advantages of pressurization as demonstrated for motors apply in full for generators. Two examples shall be given ... 

This usually means a physical barrier, and may also include distance. The ideal I is to place any circuits, motors, pumps, generators, batteries, etc., in another area away from the directly hazardous area. Anything that sparks, arcs or heats up should be isolated in some way from the hazardous atmosphere. 

Electric motor design for hazardous areas has been given in Chap. 4, and cost of motors can be found from Fig. 6-22. 

Often explosion-proof equipment is not available for example, it is difficult to buy explosion-proof motors greater than 250 hp as a stock item. One of the following methods can be used to provide low-cost protection in hazardous-area operations ... 

Motors are controlled by circuit breakers or contactors. With high voltage motors it is necessary to ensnre that the main terminal box and the terminals inside can withstand the effects of a major three-phase fanlt inside the box. This applies especially to motors that are to be used in a hazardous area. As a guide to the level of safeguard, Table 5.13 may be used. 

In the European and UK standards the term Zone is used for hazardous areas, whereas the term Division is used in the USA. In practice the end result of selecting appropriate equipment for a Zone or Division is usually very similar. There are a few subtle differences, especially when selecting electric motors. The zonal definitions vary in wording from one document to another but the essential elements are as follows. 

Other types of equipment than motors are often chosen with type n enclosmes, e.g. luminaries, junction boxes, terminal boxes, if the designer can be sure that they will be located in a Zone 2 or non-hazardous area. 

Whether equipment is certified for hazardous area use or not, it needs to be suitable for the daily environment in which it will be installed. The description of the environment as daily takes account of human interaction with the equipment. For example motors may be installed in a normally dry location either indoors or outdoors, but the plant personnel may regularly hose down the location with water. Equipment may be installed in a plant room that is protected against fire by water spray heads. 

Where a four-wire supply is needed in a hazardous area it is necessary to use four-pole circuit breakers and isolators so that the neutral is completely isolated when maintenance work is required to be undertaken. If the neutral is not electrically separated and a fault occurs elsewhere in the same network then the neutral in the hazardous area could have its potential elevated sufficiently above zero to cause a spark (or even an electric shock). This aspect is especially important when a switchboard or motor control centre is supplied from more than one source such as two parallel transformers. 

The application of variable speed motors in the oil and gas industries tends to be for the larger pumps and compressors in the several thousands of kilowatt range. In such cases the motor and driven machine unit would often be located in a hazardous area or zone. 

Cooling tower. The electric motors driving fans on cooling towers tend to be difficult to maintain if it is required that they also should be flameproof. Such a requirement is likely since flammable solvent from a condenser leak is liable to be returned to the cooling tower. Furthermore, the leakage is liable to reach the pond beneath the cooling tower so this should be installed in the hazardous area. 

Second, the discussions will be focused mainly on instrumentation and control parts. The specifics concerning other electrical equipment such as motors, etc. will not be covered unless called for as part of instrumentation discussions. In general, the guiding factors for EAC, hence hazardous area classification, shall include but not be limited to the following ... 

Electric motors, outlets and any other spark generating sources in hazardous areas... 

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