Cables trenches

Figure 13.48(b) A typical installation of a low-rating switchgear assembly on a cable trench... 

The installation costs for a single impressed current anode of high-silicon iron can be taken as Kj = DM 975 (S550). This involves about 5 m of cable trench between anodes so that the costs for horizontal or vertical anodes or for anodes in a common continuous coke bed are almost the same. To calculate the total costs, the annuity factor for a trouble-free service life of 20 years (a = 0.11, given in Fig. 22-2) should be used. For the cost of current, an industrial power tariff of 0.188 DM/kWh should be assumed for t = 8750 hours of use per year, and for the rectifier an efficiency of w = 0.5. The annual basic charge of about DM 152 for 0.5 kW gives about 0.0174 DM/kWh for the calculated hours of use, so that the total current cost comes to... 

Anode installation near surface 100-m cable trench, including cable and laying (58 DM m" ) DM 7600... 

The need for metrics is particularly important in process safety, in no small measure because the hazards may not be readily evident. Unlike some other safety risks where dangerous situations are more apparent—such as unsafe scaffolding, unsecured cables, trenches, and other excavations—information on the status and safety of hazardous containment systems (e.g., internal corrosion, an improperly sized relief valve) is not generally visible. Without a constant and reliable flow of information on process safety performance and management systems, leaders may, in essence, be flying blind. 

Plans and elevations for cable trenching, racking and routing. 

Details of cable trench cross-sections and contents. 

Electrical cable schedule Voltage drop calculations Electrical area classification Electrical instrument interface drawings Underground electrical cable and cable trench arrangement Electrical equipment layout drawings Motor control center and power layout Load and short-circuit analysis VSDs configuration Plant lighting plan... 

Buried of soils Cables Trenches on land Magnetometer... 

Electrical cables should be away from pipelines carrying acids, hot gases, and steam. There shall be no chance for entry of strong acids or alkalis in the cable trenches. The cables shall be laid at a safe distance from such pipelines. 

Another responsibility of the plant layout designer is to establish the location of the instrument cable trays and analyzer houses. Both items are coordinated with the instrument engineer during the early phase of the job. The main instrument cable runs are located either in elevated trays, generally in the pipe rack, or below ground in cable trenches. These requirements are dis-cu.ssed in Chapters 11 (Pipe Racks) and 13 (Underground Piping). 

Where there is available ground and the specific resistivity of soil in the upper layers is low, the anodes are laid horizontally [3]. A trench 0.3 to 0.5 m wide and 1.5 to 1.8 m deep is dug with, for example, an excavator or trench digger (see Fig. 9-2). A layer of coke 0.2-m thick is laid on the bottom of the trench. The impressed current anodes are placed on this and covered with a 0.2-m layer of coke. Finally the trench is filled with the excavated soil. No. IV coke with a particle size of 5 to 15 mm and specific gravity of 0.6 t m" is backfilled at a rate of 50 kg per meter of trench. The anodes are connected in parallel and every three to four anode cables are connected to the anode header cable by a mechanical cable crimp encapsulated in an epoxy splice kit to give an economical service life at high current output. 

Installation of a Brillouin system is most cost-effective when applied to a new pipeline construction when the trenching is available for inserting the optical fiber cables. If the existing sections of buried pipeline require replacement or repair, then the Brillouin system can also be used selectively in high consequence areas. In general, total system costs will vary for each application. 

A cable may experience varions different environments along its ronte. For example it may start at a switchboard, run through the switch room in a trench with a lid or steel flooring, pass throngh a dnct in a wall and under a roadway, mn a long way directly bnried and finish on a ladder rack at the consumer. At each of these environments the thermal resistivity and ambient temperatnre will be different. The environment that canses the most derating of the rated current should be taken and nsed for the whole cable. 

Instrumentation cables frequently run in parallel along the same routes as heavy current power cables. The routing is designed in such a manner that a prespecified spacing is used between power cables and instrument cables. Table 13.1 gives typical minimum spacings between the cables that run in the same trench or set of racks. There are situations where a power cable can radiate interference, particularly in the form of mutually induced currents, for example -... 

Before beginning any subsurface work such as trenching, contact the local utility alert service to establish the location of other underground services such as natural gas, sewer, telephone, electric power, and cable television. 

There is probably no issue in soil mechanics where theory is less useful than this question of vertical bank stability. Many lives have been tragically lost because of the col lapse of vertical banks, especially those forming the sides of trenches in which workers are laying pipes or cables. Such collapses have occurred in situations where the above formula would suggest the banks would be stable. Teachers of soil mechanics should make it clear to students that the formulae above are of theoretical interest only. Statements found in text books such as for vertical cuts the best solution, and the one that is commonly used in design is He = 3.8S /y, are a recipe for tragedies. 

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