Flameproof installations

Steel conduits are made to a specification defined by BS 4568 and are either heavy gauge welded or solid drawn. Heavy gauge is made from a sheet of steel welded along the seam to form a tube and is used for most electrical installation work. Solid drawn conduit is a seamless tube which is much more expensive and only used for special gas-tight, explosion-proof or flameproof installations. 

Do I need to consider any special regulations such as those required by agricultural and horticultural installations, swimming pools or flameproof installations ... 

PVC armoured cables terminated into accessories with approved flameproof glands or any other wiring system which is approved by the British Standard. All certified flameproof enclosures will be marked Ex, indicating that they are suitable for potentially explosive situations, or EEx, where equipment is certified to the harmonized European Standard. All the equipment used in a flameproof installation must camy the appropriate markings, as shown in Fig. 3.72, if the integrity of the wiring system is to be maintained. 

Some installations require special consideration because of the inherent dangers listed above. Installations requiring special consideration are flameproof installations, construction sites, agricultural and horticultural buildings. All of these installations are described in detail in Part 7 of the lET Regulations and some are described in Chapter 4 of this book. 

Where a.c. supplies exist, transformer-rectifiers are the most economical source of d.c. for cathodic protection systems. In the case of pipelines, standard transformer-rectifiers, either oil or air cooled, can be employed. They range in size from 5A, 5V for small systems to 100 A, 48 V for major pipeline schemes. A typical output for a well-coated cross-country pipeline in the UK would be 5 A, 48 V. In the case of sea-water jetties where the voltage required is usually low because of the lower sea-water resistivity, a typical rectifier size for a major installation would be 500 A, 18 V. For offshore pipelines and loading platforms where a fire hazard exists, it is usual to employ certified flameproof or intrinsically safe rectifiers to overcome any possibility of fire hazard should faults develop in the unit. 

Figure 6.10 shows a typical example for a q-apparatus. It is a power supply for intrinsically safe electric circuits to be installed in zone 1 (and zone 2). The power input (at 24 V AC/DC level) is fed via flameproof plug-and-socket connectors to the apparatus. So, the power supply can be replaced under load without conflicting with the requirements of explosion protection. 

Plug-and-socket connectors complying with increased safety - e - (see Section 6.7.2) or with flameproof enclosure - d - (see Section 6.8.10) provide a simple installation and maintenance of pressurized apparatus, especially in coal mines where frequent and rapid changes of place are required. Figure 6.18 shows flameproof plug-and-socket connectors fitted at the sidewall of a Group I pressurized switchboard. 

Apart from inserting the complete gas analyser into a p-enclosure, the analyser head may be explosion protected for zone 1 application (e.g. according to flameproof enclosure - d - ) on its own, whereas the central control unit may be installed in a non-hazardous area, or alternatively, in a pressurized enclosure (Fig. 6.25) when zone 1 installation is required. 

Figure 6.69 gives an example for an optical current sensor. The light path is wound around a current-carrying conductor equidirectionally with the azimuthal magnetic field of the current. The rotation of the plane of the electric vector is not detectable on its own and is converted to light intensity variations by a polarizer/analyser combination. A photo diode is used as a light intensity detector. The optical sensor itself is installed in the - e - compartment, the electronics shall be protected in an adequate type of protection, e.g. in a small flameproof - d - enclosure or in encapsulation - m -. In the special case of an energy distribution system with combined - e - and - d - compartments, the optical fibres may enter the d-compartment to the electronics inside via bushings complying with d -standards EN 50018 or IEC 60079-1 respectively (Fig. 6.70). The evacuation of the sensors into the e-compart-ment results in additional available space in the more expensive d-compart-ment, compared with increased safety - e -. ...

Figure 6.71 Enclosure in increased safety -e - with installed circuit breaker in flameproof enclosure - d Types of protection ...

Note At the right side/on top, a flameproof enclosure - d is installed. 

A typical flameproof enclosure for Group II installation is shown in Fig. 6.101. A cover with a threaded joint enables quick access for replacement... 

Such a flameproof enclosure may be considered as a building stone of a modular energy distribution system containing extensive low voltage installations (Fig. 6.102) in a chemical plant. 

Frequency convertors for motor drives with variable speed have shown reliability and cost reduction for some time. An example for application in coal mines is shown in Fig. 6.104. A frequency convertor with impressed current in the DC link is installed in two separated flameproof enclosures ... 

Figure 6.129 shows a modular analyser system. The flameproof analyser head (left) is suitable for zone 1 installation as well as the central unit (right), ex-protected by pressurization - p The analyser head covers a wide range of analyser methods, e.g. such as described in Figs 6.124-6.128 and others based upon infrared absorption or electrochemical sensors for oxygen. 

Indeed, it is a requirement of associated apparatus even if an Ex i-isolator designated for a safe area is installed in a potentially explosive gas atmosphere and guarded by an additional type of protection, e.g. flameproof enclosure - d -. ... 

Currently, remote I/O systems of several manufacturers are available for operation in zone 1 or Division 1. To a large extent, these systems are based upon the technology of intrinsic safety. The power supply only is designed according to another type of protection, e.g. flameproof enclosure - d A remote I/O suitable for hazardous areas may be an integration of input (I) and output (O) assembly units of automation systems and of Ex i-isolators of classic design. The installation may be made in a hazardous area and supersede the classic field distribution box (terminal box). Standardized fieldbus systems may be used for data transmission from/to the automation system. These remote I/O systems for hazardous areas are characterized as follows ... 

A typical installation on location of a remote 1/O system is shown in Fig. 6.211. At the right side/top in the housing, the power supply with an integrated fieldbus interface can be seen. The module complies with flameproof enclosure - d and is designed in such a way that it can be replaced under live voltage and explosion hazard ( hot swap ). The auxiliary power lines and terminals comply with increased safety - e All I/O modules comply... 

In the example of Fig. 6.233 a power supply for installation in zone 1 is protected according to flameproof enclosure - d The outputs (max. four) to supply the other system modules comply with intrinsic safety - i - with maximum values as follows ... 

Figure 9.12 Firedamp-proof PC workstation for installation in coat mines. The main components are fitted into a flameproof- d - Group I housing. The interfaces for PC keyboard and mouse are intrinsically safe as well as interfaces for 4-20 mA analogous inputs, RS 485 and FSK-PROFIBUS. An optical-electrical interface (Ethernet 10/100 Mbit) is available additionally.

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. 

Ordinary electrical equipment cannot be installed in zone 0, even when it is flameproof protected. However, many chemical and oil-processing plants are entirely dependent upon instrumentation and data transmission for their safe operation. Therefore, very low-power instrumentation and data-transmission circuits can be used in special circumstances, but the equipment must be intrinsically safe, and used in conjunction with a safety barrier installed outside the hazardous area. Intrinsically safe equipment must be marked Ex ia or Ex s , specially certified for use in zone 0. 

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