How to carry out the outside ground loop?
For safety of people carry out protective grounding of electric units. Are subject to grounding:
- metal casings and body of electric units, different units and drives to them, lamps, etc.;
- metal frameworks of distribution boards, control panels, guards and cabinets;
- metal designs and metal body of cable couplings, metal covers of cables and wires, steel pipes of the electrical wiring;
- secondary winding of measuring transformers.
Are not subject to grounding:
- armature suspended and probes of basic insulators;
- the equipment installed on the grounded metal designs as on their basic surfaces the smoothed-out, not painted over places for providing the electrical link have to be provided;
- the body of electrical measuring instruments and the relay installed on guards, boards, cabinets and also on walls of cameras of distributing devices;
- metal covers of control cables in cases which make a reservation in the project especially.
Protective grounding consists of the outside (external) device representing the natural or artificial grounding conductors laid in soil and connected among themselves in the general contour and the internal network consisting of the grounding conductors laid on walls of the room in which there is the installation and attached to the outside contour.
The metal grounding conductors put in soil, having the big area of contact with the earth, provide the small electrical resistance of the contour.
For grounding of electric units first of all natural grounding conductors — the metal pipelines laid in the earth (except pipelines with combustible, flammable and explosive liquids or gases), casing pipes have to be used; the metal and steel concrete structures of buildings and constructions which are reliably connected to the earth; lead covers of the cables laid in the earth and working wires of air-lines up to 1000 V, zero with repeated grounding conductors. Natural grounding conductors (except the last) have to join the grounding highway of the electric unit not less than in two places.
Accession of the grounding conductors to grounding conductors and also connection of the grounding conductors among themselves is made by welding, and the overlapping length (the welding seam) has to be equal to double conductor width at its rectangular section and to six diameters — at round. At Tee connection lapped of two strips length of overlapping is defined by their width.
Accession of the grounding conductors to pipelines is carried out by welding or if it is impossible, collars from input of pipelines to the building (to the hydrometer, the latch, the flange). The welding seams located in the earth after mounting for protection against corrosion become covered with bitumen.
If there are no natural grounding conductors or they do not meet rated requirements, mount the contour of outside grounding from artificial grounding conductors which can be vertical, horizontal and profound.
Vertical grounding conductors are the steel pipes hammered into the earth (unconditioned) or angular steel (with wall thickness not less than 4 mm and 2,5 long... 3 m) and also the steel rods screwed in the earth (with the diameter of 10... 16 mm and 4,5...5 m long). The steel strips not less than 4 mm thick laid in the earth or round steel with a diameter not less than 10 mm are the horizontal artificial grounding conductors playing the role of independent elements of grounding or employees for communication with each other of vertical grounding conductors.
Kind of horizontal grounding conductors are the profound grounding conductors put on the bottom of ditches at the construction of the bases of support of air-lines and buildings under construction. They are made in workshops of the mounting organization after preliminary measurement of strip steel with section 30 x 4 mm or round steel with a diameter of 12 mm. The form of grounding conductors, their number, section and placement are defined by the project.
As the grounding conductors natural conductors, i.e. metal structures of buildings can be used (farms, columns, etc.); metal designs of production assignment (subcrane ways, frameworks of distributing devices, galleries, platforms, mines of elevators, elevators, etc.); steel pipes of electroconductings; metal covers of cables (but not armor). For nulling it is enough in all cases of the aluminum cover of cables, and lead, as a rule, it is not enough.
In explosive rooms specially laid grounding conductors are applied, and natural are considered as the additional measure of protection. At grounded the neutral (networks 380/220 or 220/127 B) nulling of electroreceivers of explosive installations has to be made by separately allocated veins of conductings and cables; at the isolated neutral steel conductors can be applied to grounding.
Use of naked aluminum conductors as grounding is forbidden because of their fast destruction owing to corrosion. Mounting of the outside ground loop and laying of the internal grounding network are made according to working drawings of the project of the electric unit.
Performance of penetrative works, installation of mortgage parts, preparation of free openings, furrows and other apertures, laying of pipes through passage in walls and the bases, digging of earth trenches for laying of the outside ground loop are carried out at the first stage of preparation for electric installation work.
The external circuit of grounding is laid in earth trenches of 0,7 m in depth. Artificial grounding conductors in the form of pieces of steel pipes, round rods and corners 3 long... 5 m are buried in soil by folding or vibroimmersion so that the head of the electrode appeared at the depth of 0,5 m from the Earth's surface. The buried grounding conductors connect with each other steel strips to the section of 40x4 mm by means of welding. Places of welding of the strip to grounding conductors become covered with the warmed bitumen for protection against corrosion. The grounding conductors and the grounding conductors located in the earth should not be painted. Trenches with the grounding conductors and grounding conductors laid in them are covered with earth, not containing stones and construction debris.
Natural grounding conductors contact the grounding highways of the electric unit not less than two conductors attached in different places. Connection of the grounding conductors with extended grounding conductors (for example, pipelines) is carried out near their inputs to buildings by means of welding or collars which contact surface obluzhivatsya. Pipes in places of the pad of collars are smoothed out.
Places and ways of connection of receivers of current are chosen so that at separation of the pipeline for repair work continuous operation of the grounding device was provided. At hydrometers and latches arrange bypass connections.
The internal grounding network is carried out by open laying in rooms on construction surfaces of naked steel conductors with rectangular and round sections.
The grounding conductors which are openly laid naked are located vertically, horizontally or parallel to inclined structures of buildings. Conductors with rectangular section are established by the big plane to the basis surface. On straight sections of laying conductors should not have roughnesses and bends noticeable approximately. The grounding conductors laid on concrete or the brick in the dry rooms which are not containing caustic vapors and gases become stronger directly on walls, and in rooms crude, especially crude, with caustic couples and gases — on support at distance not less than 10 mm from surfaces of walls. In channels the grounding conductors are located at distance not less than 50 mm from the lower surface of removable overlapping. The distance between support for fastening of the grounding conductors on direct sites makes 600... 1000 mm.
The grounding conductors in places of their crossing with cables and pipelines and also in other places where mechanical damages are possible, protect pipes or different ways.
In rooms the grounding conductors have to be available to survey, but this requirement does not belong to the zero veins and metal covers of cables, pipelines of the buried electrical wiring and the metalwork which are in the earth. Through walls the grounding conductors are laid in open apertures, pipes or other rigid frames.
Each grounded element of the electric unit has to join the grounding highway by means of the separate branch. Consecutive connection to the grounding conductor of several grounded elements is forbidden.
The neutral of transformers grounded tightly or via the devices compensating the capacity current join the grounding conductor or the combined grounding tires by means of the separate grounding conductors.
The grounded conclusions of secondary winding of measuring transformers join their casings the grounding bolts. The flexible crossing points serving for grounding of metal covers and armor of cables are attached to them by the bandage from the wire and soldered, and then connect bolt contacts with cable seal (coupling) and the grounding design. Sections of flexible crossing points have to correspond to sections of the grounding conductors accepted for this electric unit. Junctions of the grounding crossing point with the aluminum cover of the cable after soldering become covered with asphalt varnish or hot bitumen.
Connection with each other of the grounding conductors and accession them to designs of installation are carried out by welding, and connection to the body of devices and machines — welding or the reliable bolt connection. For prevention of weakening of contact at concussions and vibrations lock-nuts, spring washers, etc. are established.
Contact surfaces on the grounded electric equipment in places of accession of the grounding conductors and also contact surfaces between the grounded equipment and designs on which it is established have to be smoothed out to metal gloss and become covered by the vaseline lamina.Top