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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Section 7. Electrical equipment of special installations Electrothermal installations. Installations of direct and indirect resistance furnaces
Encyclopedia of radio electronics and electrical engineering / Rules for the installation of electrical installations (PUE) 7.5.61. Furnace step-down and control dry transformers (autotransformers), as well as transformers with non-flammable liquid and control panels (if they do not have devices sensitive to electromagnetic fields) can be installed directly on the structures of the resistance furnaces themselves or in close proximity to them. Installations of direct-acting electrothermal resistance devices should be connected to the electrical network through step-down transformers; autotransformers can be used in them only as adjusting ones, their use as step-down transformers is not allowed. 7.5.62. The width of the passages around the electric furnaces and the distance between the electric furnaces, as well as from them to the control panels and cabinets, are selected depending on the technological features of the installations. It is allowed to install two electric furnaces side by side without a passage between them, if, according to the operating conditions, it is not necessary. 7.5.63. Electrical devices of power circuits and pyrometric devices are recommended to be installed on separate boards. The devices should not be affected by vibrations and shocks during the operation of switching devices. When installing electric furnaces in industrial premises where vibrations or shocks occur, pyrometric and other measuring instruments must be mounted on special shock absorbers or panel panels with such devices must be brought into separate switchboard rooms (instrumentation and automation rooms). Panels of instrumentation and control panels for installations of resistance furnaces are recommended to be located in separate rooms also in cases where production rooms are dusty, damp or damp (see Chap. 1.1). It is not allowed to install switchboard panels with pyrometric devices (in particular, with electronic potentiometers) in places where they can be subjected to sudden temperature changes (for example, near the entrance gates of the workshop). 7.5.64. Joint laying of wires of pyrometric circuits and wires of control or power circuits in one pipe, as well as the combination of these circuits in one control cable is not allowed. 7.5.65. It is recommended to connect the wires of the pyrometric circuits directly to the devices, without leading them to the control panel terminal assemblies. The compensating wires of pyrometric circuits from thermocouples to electrical devices (including millivoltmeters) must be shielded from induction pickups and the shields must be grounded, and the shielding device along the entire length must be securely connected at the joints. 7.5.66. The termination of wires and cables connected directly to the heaters of electric furnaces should be carried out by crimping lugs, clamping contact connections, welding or brazing. 7.5.67. In installations of resistance furnaces with a power of 100 kW or more, it is recommended to install one ammeter for each heating zone. For furnaces with ceramic heaters, as a rule, ammeters should be installed on each phase. 7.5.68. For installations of resistance furnaces with a capacity of 100 kW or more, it is necessary to provide for the installation of active energy meters (one per furnace). 7.5.69. In installations of indirect resistance furnaces with manual loading of material (products) into the working space, electric furnaces should be used, the design of which excludes the possibility of accidental contact of service personnel with live parts energized above 50 V. If the probability of such a contact is not excluded in these furnaces, then either the loading doors (lids) should be blocked to prevent their opening until the voltage is removed, or other measures should be taken to guarantee electrical safety. 7.5.70. In direct heating installations operating at voltages greater than 50 V AC or greater than 110 V DC, the work area containing the plant equipment and service personnel must be isolated from earth. For continuous operation installations where winding and winding devices are energized, protective nets or walls must be placed along the boundaries of the working site isolated from the ground, excluding the possibility of the unwinding tape or wire being thrown out of the site. In addition, such installations must be equipped with an insulation monitoring device with action on the signal. 7.5.71. When using liquid contacts in direct heating installations that emit toxic or strong-smelling vapors or fumes, the tightness of contact assemblies and reliable trapping of vapors and fumes must be ensured. 7.5.72. The leakage current in direct heating installations must not exceed 0,2% of the rated current of the installation.
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