- Choosing where to place the outline
- Soil resistivity and calculation of electrodes
- How to quickly hammer the main ground electrodes
- Contour binding, busbar outlet
- Checking standard parameters, circuit maintenance
Your own ground loop is the hallmark of a truly thought-out and high-quality power supply system. Its device is very primitive, while its practical benefits are invaluable. Do-it-yourself installation does not take much time, and the correct execution of the circuit guarantees its long-term trouble-free operation.
Choosing where to place the outline
To determine the place suitable for driving the grounding electrodes, you need to go through a procedure called the coordination of utility lines. Since the length of the electrodes, as a rule, is greater than the depth of occurrence of power lines, communications and pipelines, the risk of their damage is absolutely real when working within the city. Therefore, first familiarize yourself with the plans for laying communications routes, a request can be left at the local city administration.
There may be little cash costs involved, but it is almost never required to obtain an earthwork order. One interesting point is associated with the approval: you disclaim responsibility for damage to the line if it is not in the register of underground utilities. Moreover, even if underground routes have already been laid in an ideal location, you can easily bypass them using the specified values of the protection zones and anchor points. It is recommended for businesses to archive certified copies of plans.
When positioning the contour, pay attention to the soil parameters. Holders of the report on the geomorphology of the area are recommended to place the main ground electrodes at the lowest possible point of the upper aquiclude saturated with moisture. Shaded areas are also preferred, near drain pits or drainage wells, in drainage ditches. Water with dissolved salt ions (in moderation) gives good conductivity to soils, even in those categories in which it is completely absent when they are dried out..
Another criterion for assessing the area is the ratio of the groundwater level to the immersion depth of the main ground electrodes. If it is possible to arrange a contour at the bottom of a basement or viewing pit, it is better to use it. The exceptions are areas saturated with aggressive liquids: septic tanks, drain and compost pits. You should also avoid proximity to trees that actively absorb water, such as birch or willow..
Soil resistivity and calculation of electrodes
The transfer of electric potential to the lithosphere occurs from the entire surface of metal electrodes through metallized soil particles and moisture contained in the soil. Everything should be taken into account: from the roughness of the metal surface to the porosity of the soil and the density of planting steel ground electrodes in it..
The geomorphological profile and the table of soil resistivity are taken as the basis for calculating the resistance to the propagation of current through the main ground electrodes. It is recommended to use the manual “Standards for the construction of grounding networks” by R.N. Karjakin, where there is comprehensive information for calculating the required parameters, and also describes the technique of using natural ground electrodes (casing of wells, piles or pipelines).
In reality, a detailed calculation is rarely performed, usually the initial data are taken as the worst possible for specific placement conditions. The required characteristics are achieved by increasing either the length of the electrodes (which is more preferable) or their number. The safety margin ensures a long service life of the circuit: covered with rust, the electrodes lose much in conductivity, so new ones are periodically finished to them.
The calculation begins with the permissible cross-section of the elements of the grounding system, their conductivity must correspond to the power of the electrical connection of the system to be grounded. In most cases, carbon steel profiles are used, their cross-section should not be less than 80 mm2. For stainless steel, this figure is 60–70 mm2. The cross section is deliberately overestimated to compensate for the corrosive effect of the soil.
The second question is the total surface area. Angle steel, T-beams or I-beams should be used as the main grounding conductors – products with an open cross-section that are in contact with the ground on all sides. The resistance of a single ground electrode or its section is defined as the specific resistance of the surrounding soil, divided by? – multiple value of the main linear dimension (for a vertical rod, this is its length).
The result must be multiplied by the dimensionless shape factor (for a vertical rod, this is half the natural logarithm of four times the length, divided by the perimeter of the section). For example, a vertical electrode 2.5 meters long made of 50×50 mm angle steel, the coefficient will be almost 1.25, the spreading resistance (when the ground electrodes are completely embedded in loam) will be 8.3 ohms.
The total resistance of vertical ground electrodes is described as the sum of their reciprocal values:
- 1 / R = 1 / R1 + 1 / R2 + … + 1 / Rn
Thus, to achieve the standard value of 4-6 ohms, at least two electrodes of 2.5 meters each will be required, by analogy, you can calculate options with another suitable number or length of ground electrodes.
How to quickly hammer the main ground electrodes
When the required calculations are completed, it is the turn of the installation. At first glance, the trivial task of driving electrodes into the ground can turn into damaged rolled metal simply because of ignorance of the mechanics of the process.
The soil at a depth of more than a meter is quite dense and under pressure. The soil compresses the steel rod tightly, while the frictional forces prevent immersion and grow with the contact area with each impact. Troubles add fragments of solid rock encountered on the way, sometimes it is wiser to pull out the electrode and drive in a new place.
Earthing switches must be properly sharpened before driving. The total bevel angle of the tip should be of the order of 30–35 ?. From the edge of the tip it is necessary to retreat about 40 mm and reduce the descent at a more obtuse angle, about 45-50 ?. Tavr, I-beam and channel can have several descents, it is recommended to sharpen rods up to 24 mm by forging with a slow tempering.
Before driving the electrodes, they must be removed from each other by at least 230 cm, more than two (N) vertical ground electrodes are placed on the tops of an equilateral N-gon. A hole 35-50 cm deep must be dug or drilled under each electrode so that the main body of the conductor is as deep as possible. Drilling holes to full depth is not recommended. The dug pits are interconnected by trenches, along which the electrodes will be hidden..
It is best to hammer in steel rods by hand, with a sledgehammer of about 7-10 kg. Yes, vibration diving works better, but the equipment is not easy to obtain and may not be used everywhere. The main problem when hammering is the deformation of the shank from frequent blows, so you need to beat through a special shaped headstock that is put on top of the electrode and does not allow it to bend or splash excessively. You can also periodically trim the edge of the electrode with the angle grinder as it flattens, or add water to the pit in small portions.
Contour binding, busbar outlet
Vertical electrodes must be completely under a soil layer of at least 20-30 cm, at the same level all horizontal ground electrodes are located. For the bundle, a steel strip of 4×40 mm or higher is used, placed on the edge. It is connected to the electrodes by arc welding, the total length of the seam must be at least half the perimeter of the section.
From the contour, the remainder of the strip is laid under the ground to the wall of the building with the ASP. In order not to destroy the foundation blind area, the strip can be laid on top of it, secured with quick-assembly dowels, or you can arrange a tunnel and pass through a crimped hole. The grounding bus must be fixed to a stationary structure at least at two points; an M10 bolt with two washers and a nut is welded to the end.
The installation of the circuit is completed by applying a protective coating to the welding places, it can be paint or ordinary bitumen. After the ground electrodes are covered with soil, carefully ramming it.
Checking standard parameters, circuit maintenance
A single-wire copper wire (PV-1) with a cross section of at least 6 mm is clamped under the bolt on the bus terminal2. It follows as the main protective conductor to the ASU and then is divided throughout the entire grounding system to each consumer of electricity who needs to equalize the potentials.
Typically, the resistance of the lines of the grounding system is considered to meet the standard when used on copper wire branches from 2.5 mm2, as well as steel bar or strip with a cross section of 50 mm2. The grounding system usually does not provide for breaks during branching, the total resistance between the ASU and the most distant point should be in the region of 4-6 ohms.
The spreading of current along the main ground electrodes is checked using a ground megohmmeter: it measures the resistance between the metal parts of the grounding system and temporary electrodes driven into the soil 50 cm 15 and 20 meters from the circuit. The measurement results serve as the basis for signing technical specifications and admission of the power grid to operation.
Grounding resistance measurement: 1 – grounding resistance meter; 2 – ground loop; 3 – temporary electrodes
The ground loop does not require maintenance as such. It is enough to exclude the conduct of earthworks in the place of its location and make sure that the soil does not dry out. You should also exclude the ingress of aggressive liquids on the soil. This remark is due to the fact that often before periodic (and standardized PUE and PEEE) measurements of resistance, the soil is watered, for example, with a solution of sodium chloride. This temporarily improves the conductivity of the soil and, as a result, the spreading resistance is reduced. But under such conditions, the circuit will physically exist for only 1.5-2 years.