Natural grounding. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Natural grounding. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Grounding and grounding

First of all, for grounding electrical installations use natural ground electrodes: metal parts (fittings) of reinforced concrete structures (foundations of power transmission lines and substations, building foundations); metal underground communications (pipelines, armor and cable sheaths); ground communications (railways), etc. If natural ground electrodes ensure the fulfillment of the requirements for the parameters of grounding devices, then artificial ground electrodes are used if it is necessary to reduce the currents flowing through natural ground electrodes or flowing from them into the ground. This means that in some cases it is possible to limit the use of natural ground electrodes and abandon artificial ones, which reduces the cost of materials and labor during installation and facilitates the operation of grounding devices.

The use of reinforced concrete building foundations as grounding conductors is currently considered possible only in soils with a moisture content of at least 3% (due to the high electrical resistance of concrete at lower humidity) and only when exposed to non-aggressive or slightly aggressive groundwater in the absence of waterproofing or surface protection foundations with bitumen (or bitumen-latex) coating in accordance with the requirement of SNiP II-28-73.

Reinforced concrete structures (slabs, beams, trusses, columns) with prestressed reinforcement, as well as metal and reinforced concrete structures of buildings belonging to the first category in terms of lightning protection, cannot be used in grounding devices in medium or highly aggressive environments (this can increase corrosion of structures). , to protect these buildings from direct lightning strikes.

Taking into account the above limitations, the use of building structures as grounding devices made it possible at a number of facilities to completely abandon the implementation of artificial grounding conductors in the ground, to drastically reduce the length of grounding conductors inside buildings and obtain a significant economic effect.

All elements of metal and reinforced concrete structures (foundations, columns, trusses, rafters, under-rafters and crane beams) in grounding devices are connected so that there is a continuous electrical circuit for metal. In reinforced concrete columns, in addition, embedded parts are provided on each floor of the building for connecting grounded electrical and technological equipment. Welded, as well as bolted or riveted connections of metal columns, trusses and beams available in buildings are sufficient for the continuity of the electrical circuit. In places where individual elements of metal structures do not have such connections, welding of flexible jumpers with a cross section of at least 100 mm2 is provided.

Prefabricated reinforced concrete foundations are recommended to be used as ground electrodes, if it is possible to connect the reinforcement of individual blocks to each other.

The vertical reinforcement of piles in pile foundations is connected to the reinforcement of the grillage or foundation blocks by electric arc welding. Spatial metal frames of columns and foundation glasses, as well as reinforcing meshes of their soles, are welded by spot welding on contact machines.

Embedded parts (products) are recommended in the form of pieces of angle steel 63x63x5 60 mm long, welded to the reinforcement and protruding to the concrete surface; metal jumpers - in the form of rods with a diameter of at least 42 mm, welded to embedded parts.

A method has been developed for calculating the resistance of foundations used as grounding conductors and leveling conductors.

If a lightning protection mesh is being constructed on the building, it is connected by jumpers to a continuous electrical network with

columns used as down conductors, and foundations used as ground electrodes. All metal devices protruding above the roof (ventilation shafts, etc.) are attached to the grid.

Metal jumpers must be installed when using water pipes on water meters and valves as natural grounding conductors. If during repair, it is necessary to remove the jumper, another one must be installed in advance. It is necessary to connect grounding conductors from electrical equipment to the water supply line after the water meter. It is not allowed to use a sewer pipeline, since sewer pipes do not have reliable electrical contact at the joints.

Natural grounding conductors at substations can be reinforced concrete racks made of electrical concrete.

As natural ground electrodes on power lines, reinforced concrete steps and piles are used in the most common soils with a resistivity of up to 300 Ohm-m, i.e. clays, sandy loams. Observations and studies have shown that not only in such soils, but also in sandy and rocky soils, there is constant wetting of concrete due to capillary suction of moisture from the adjacent layers of the earth, as a result of which reinforced concrete foundations become natural grounding conductors with little changing during year resistance values. This gave reason to recommend their use in soils with a resistance of up to 1000 Ohm-m, which saves metal and costs.

In addition to the natural ground electrodes described, others can also serve as them, for example, metal pipelines for non-combustible liquids, casing pipes of artesian wells. In all cases of using natural grounding conductors, the currents flowing during a short circuit should not exceed the allowable for each element of the grounding conductor during the entire operation of the electrical installation.

Author: Bannikov E.A.

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