If the equipment has damaged insulation, then parts that should not conduct electrical current may be affected by voltage. Touching the habit of handles, casing or housing, the user receives an electric shock, and becomes a conductor in the ground. A current of 0.1 A is deadly to humans. Since the body's resistance ranges from hundreds to thousands of ohms, devices with low voltage become a threat.
An effective measure of protection against electrical injury is grounding. This device is well thought-out connection of one part of the installation with the groundwhich is done using grounding elements and conductors. They are collected in groups and laid in the ground. The basic rule of protective devices is that the ground resistance is many times less than this indicator of the human body.
To determine the maximum possible protective earthing resistance, you need to sum the voltage of the equipment and the closing ground currents. In addition, it is necessary to determine the presence of an insulated or grounded neutral conductor and other important technological features that are established in the rules of EIR.
1 External ground loop
- 1.1 Be sure to ground:
- 1.2 Do not protect the ground:
- 1.3 What determines the value of grounding resistance:
- 2 Grounding working and protective device
3 Work on the manufacture and installation of the circuit
- 3.1 What to focus on:
- 3.2 Earth preparatory work
- 3.3 Contour device
- 3.4 How to measure grounding
External ground loop
The grounding device consists of external natural or artificial elements laid in the ground and collected in a common circuit. The protection device also includes internal networks of conductors on the walls that join the external contour.
Elements of metal, laid in the ground, provide a large area of contact with the ground and have little resistance. The metal tubular lines in the ground are widely used as external elements. Do not connect to the ground pipelines of explosives and flammable substances.
Casing details, metal frame in reinforced concrete structures of houses, zero wires of aerial wiring with a voltage of 1000 V with re-grounding successfully used as elements of external protection. All random metallic elements are necessarily connected in two places to the protective circuit.
All nodes are connected by welding, the length of the seam is determined depending on the conductor cross section. If it is impossible to weld the parts, then clamps are used from the point of entry of the main line into the building. Welding compounds are treated with bitumen to protect against premature corrosion.
Be sure to ground:
- electrical enclosures and housings, units and their drives, structural frameworks of control switchboards, cabinets and enclosures;
- metal housings and structures of cable joints, iron windings of wires and cables, metal pipes for laying wiring;
- transformer secondary windings.
Do not protect the ground:
- wiring support insulators;
- devices placed on grounded platforms, as they provide an unprocessed spot for contact with the plane;
- enclosures of measurement and control devices, which stand in type-plates or cabinets.
If there are no suitable natural grounding elements, the external protection circuit is made of artificially selected in accordance with the EMP. By type they are horizontal, recessed and vertical.
The horizontal elements are strips of steel with a thickness of more than 4 mm and a width of not less than 10 mm, which are laid in the horizontal direction in the ground and connect the vertical rods.
Horizontal and recessed options are related in design, they are laid at the bottom of the pit when installing power poles. Grounding is made on the project by the installation organization in the workshops. The material is steel strip or round fittings.
Vertical grounding is a pipe driven into the ground or rolled metal and steel reinforcement.
Installation of external ground loop performed according to special schemes and in accordance with the EMP. All preparatory work in the form of punching holes, installing embedded parts, digging trenches, is carried out at the first stage of work.
What determines the value of grounding resistance:
- varieties of soil on the site, its structure and condition;
- electrode depths;
- properties of materials and electrode sections.
Soil properties are determined by its ability to resist the spreading of electric current in the earth. For the contour is considered better if this figure is less.
Grounding working and protective device
A protective device saves a person from being hit by electricity, and household appliances included in the network from breakdown during a voltage breakdown on the body. The working grounding device organizes the protection and the normal functioning of electrical appliances. A permanent working ground is used only for electrical electrical equipment, and household appliances are grounded through zero sockets. But some household units should be tightly protected by grounding:
- Washer with its own high electrical capacity, operating in wet conditions, it punches a hand into the body and “stings”;
- on microwave ovens there is a special terminal at the back for additional grounding, as it is equipped with a source of ultrahigh frequencies. If there is insufficient contact in the outlet, then the device may produce unrecorded waves at a level that is hazardous to health;
- cooking surfaces electric oven and induction furnace, in which the internal wiring operates under critical conditions and the current sometimes breaks into the case;
- desktop computer a stationary type of leakage of electricity gives a large. Hull floating potentials result in slower performance and reduced performance, and grounding is attached to any suitable screw on the rear panel.
In some cases, it is impossible to count only on one ground, since the ground does not belong to the linear conductors of electricity. Its resistance is determined by the operating voltage and the area of contact with the circuit element. If you post two circuits at a distance of 1.2–1.5 meters from each otherthen the contact area is effectively increased by a hundred times. It is impossible to increase the separation distance greater than the specified size, this will entail a break in the potential field, and the area is immediately reduced.
Grounding conductors must not be brought outside and connected to unprepared contact pads. Any metal has its potential and under wet outdoor conditions, corrosion and destruction begins. The presence of grease on contact helps only in dry conditions.. If corrosion goes under the shell of the conductor, then in a critical situation the conductor will immediately burn out and the circuit will not protect the person from damage.
If electrical installations are connected in sequential order and connected not one grounding conductor per bus, but several, then the failure on one device will pull the others along with it. They will not be able to work productively, since they will be incompatible in electromagnetic terms.
Wet clays, loams and peaty soils are ideal for contouring. It is almost impossible to install a protective structure in stony ground and rocks.
Work on the manufacture and installation of the circuit
If there is no grounding in the house and on the site, arrange such a structure on the input to the dwelling, which is repeated grounding. Most often, the connection of electricity from the city power line to the house goes through the air, and the device of the secondary grounding is required by the rules of PUE.
At the first stage, the location, size and shape of the contour are chosen. Install it near the input, and the shape of the contour is triangular, rectangular or in the form of a line, which consists of any number of vertical pins, assembled with a steel strip.
What to focus on:
- when arranging horizontal contours pit depth should not be less than 0.5–0.8 m;
- bookmark depth vertical metal profiles with a trench account of not less than 3.5–3.8 m;
- on the choice of the length of the vertical elements soil type, ground freezing depth, soil relative humidity;
- for loop efficiency increasing it in diameter is not so important; it is much better to add its length;
- distance between vertical elements must be at least the length of one of them. If we take this distance less, the performance of the circuit decreases.
Earth preparatory work
For marking set pegs with stretched twine and markup perform a bayonet spade. Land digging digging to a depth of trench width of 30 cm. For the lower layer sprinkle soft soil with a layer of 25 cm in the form of chernozem without garbage and stone additions, which will directly contact with the grounding elements. Sometimes used imported soil with the addition of peat or humus. During the backfill after the contour device, the soil is periodically compacted in layers.
In the corners of the trench, vertical pins are hammered, which are preliminarily left 30 cm above the ground level, which is necessary for the convenience of welding. After that, horizontal strips are welded with a margin of length at the ends. Strip steel must not be tensioned; it must be free.
There are special requirements for welding. All seam lengths are regulated in regulatory references. depending on the different combination of strips, round timber and square among themselves. Typically, for a single-type profile, the length of the seam is assumed to be 100 mm, and various-type elements are welded to create the largest contact area and scald all joints.
After the end of the welding joint, all welding sites are painted or coated with bitumen. For vertical contour rods and horizontal elements, the presence of paint over the entire surface is not allowed.
Next, evenly hammer the entire welded structure into the ground (face down). To facilitate the place of entry into the ground pour water. Impact loads on the welding sites repeatedly check the strength of the structure. Pre-sharpening the ends of the vertical seams with a grinder or grinder will greatly facilitate blockage.
To connect the circuit to the input and to the distribution box use a strip of metal, which is rigidly fixed on these structures.
How to measure grounding
After the contour is manufactured, its reliability is verified, for which measure the resistance to spreading electric current in the ground and resistance of the welded metal circuit. To this end, there are currently a variety of electronic devices. Use and old Soviet reliable devices. A household tester is not suitable for this, as the earth is not a linear conductor of current.
I rent or lend an electronic modern device or an old Soviet manual megohmmeter of induction mode of action. Check circuit resistance is not possible with a handheld device., but with carefully and correctly made welded joint, it has been normal for decades.
The spreading resistance is tested with bare stripped electrodes, which are immersed in the ground to a depth of one meter at a distance of one and a half meters from each other. In this case, the polarity of the megger is maintained; the protection circuit must withstand a lightning strike. But the destructive power of such a natural catastrophic phenomenon equates to an explosion and grounding from it may not save.
Therefore, to measure the resistance to fluidity, twist the knob of the megger and determine the readings on the scale. To use in this case the mains voltage, milliammeter and resistor is very dangerous.
The owner of the house, self-fulfilling grounding device, can not fully assess its quality just a visual inspection and sometimes you want to invite a specialist who owns professional techniques and knowledge. This may be an employee of the electrical service of any large enterprise.
All regulatory documents impose requirements for ohmic resistance depending on numerous factors. They take into account operating conditions, climate, operating voltages electrical appliances, power supply features and wiring diagram. And depending on this, the maximum allowable limit of the soil resistance to current flow is formed, which varies in a very large range.
Based on experimental measurements, in accordance with regulatory schemes, the permissible figure for a private house is 4 ohms. This is a very real figure that will help protect a person from electric shock. Reducing the rate will be more favorable for improving the protection of electrical appliances in the home.