What is the protective grounding?
Protective grounding is called deliberate electrical connection with the earth or its equivalent of metal not current carrying parts which can appear energized.
Protective grounding has to provide protection of people from defeat with electric current at touch to metal not current carrying parts which can appear energized as a result of insulation failure or for other reasons. It is necessary to distinguish protective grounding from grounding:
- lightning protection;
- working grounding;
- repeated grounding;
- zero protective conductor.
The principle of action of protective grounding consists in decrease to safe values of tension of touch and the step caused by fault to frame that is reached due to reduction of potential of the grounded equipment (at reduction of resistance of the grounding device) and also due to alignment of potentials of the basis on which there is the person, and the grounded equipment.
Scope of protective grounding: a) in networks up to 1 kV: alternating current three-phase three-wire with the isolated neutral; alternating current single-phase two-conductor isolated from the earth; the direct current two-conductor with the isolated average point of windings of the current source; b) in networks over 1 kV: the variable and direct current with any mode of neutral or average points of windings of the current source.
The grounding conductor is called the conductor or set metal of the connected conductors which are in contact with the earth or its equivalent. Distinguish artificial and natural grounding conductors.
Artificial eazemlitel (horizontal and vertical) are intended only for the grounding purposes. It is recommended to apply steel to production of artificial grounding conductors. At the same time they should not have colourings.
The smallest sizes of steel artificial grounding conductors are specified below.
Diameter of round bar grounding conductors, mm:
- neozinced - 10;
- zinced - 6;
- section of rectangular grounding conductors, mm ² - 48;
- thickness of rectangular grounding conductors, mm - 4;
- thickness of shelves утл copies of steel, mm - 4.
The sectional area of horizontal grounding conductors for electric units over 1 kV is chosen on thermal firmness (proceeding from admissible heating temperature of 400 °C).
At danger of corrosion of grounding conductors one of the following events has to be carried out: increase in number of grounding conductors taking into account the rated term of their service, use of galvanized grounding conductors, application of electric protection.
As artificial grounding conductors use of grounding conductors from electroconductive concrete is allowed
Natural grounding conductors — the electroconductive parts of communications, buildings and constructions of production or other assignment which are in contact with the earth used for grounding.
It is recommended to use first of all as grounding conductors of electric units natural grounding conductors.
As natural grounding conductors use: the water and other metal pipelines laid in the earth (except for pipelines of combustible liquids, combustible or explosive gases and mixes); the metal and steel concrete structures of buildings and constructions which are in contact with the earth; lead covers of the cables laid in the earth (aluminum covers of cables are not allowed to be used as natural grounding conductors); if covers of cables are the only grounding conductors, then in calculation of the grounding devices they have to be considered at quantity of cables not less than two; railway lines of the main neelektrofitsirovanny roads and access roads in the presence of the deliberate device of crossing points between rails, etc.
Metal and steel concrete designs when using them as the grounding devices have to form the continuous electric circuit on metal embedded parts for connection of electric and processing equipment have to be provided In steel concrete designs.
The Grounding Device (GD) — set of the conductors which are structurally integrated the grounding conductor and grounding.
Distinguish the planimetric and portable grounding devices.
When performing of the planimetric grounding device the grounded equipment is in close proximity to it; when performing portable — the grounded equipment is located out of the site on which the grounding conductor is located (out of the zone of spreading of current of the grounding device).
The grounding devices have to have the following resistance to spreading of current of R3:
- in electric units up to 1 kV: no more than 10 Ohms at the total power of the generators or transformers feeding this network, no more than 100 kV And, no more than 4 Ohms in all other cases;
- in electric units over 1 kV: no more than 0,5 Ohms for electric units voltage from 110 to 750 kV in network with effectively grounded neutral;
- R3=250/I3 Ohm for electric units over 1 to In in network with the isolated neutral when using grounding for electric units only over 1 kV;
- R3=125/I3 The ohm for electric units over 1 kV in network with the isolated neutral when using of the grounding device at the same time for electric units to and is higher than 1 kV.
Here I3 — the design current of ground fault, it has to be defined for that from possible in operation of the scheme of network in which the current intensity of ground fault has the greatest value.
At the unit resistance of the earth р> 500 Om*m are allowed to enter the raising coefficients depending on the ruble grounding devices on the specified values of resistance.
It is allowed to carry out calculations of the grounding devices for norms on touch tension.Top