- Causes of Hazardous Potential on the Case
- Basic protective measures
- Transition to a three-wire power grid
- Ground loop device
- Grounding in apartment conditions
The electrical grids of most residential real estate objects rarely boast that they are arranged in full compliance with the PUE and electrical installation standards. Because of this, an electric shock from the body of a washing machine or other kitchen equipment is a completely natural phenomenon, but at the same time it is quite easily removable.
Causes of Hazardous Potential on the Case
Washing machines, dishwashers, electric water heaters, microwave ovens and even a conventional range hood are all potential sources of danger from electrical potential on the cabinet. As a rule, the consequences of an electric shock from household appliances are limited to unpleasant sensations, however, there is still a risk of serious electrical injury, and therefore such phenomena must be excluded in every possible way..
There are four main sources of electrical potential for household appliances:
- Breakdown of the insulation of its own power supply circuit. This is typical for old household appliances, most of which were not designed with electrical safety in mind..
- Electrical contact of equipment with conductive communications: metal pipes, ventilation ducts, construction fittings (we will leave behind the scenes the reasons for the emergence of potential in the communications themselves, we will just take them for granted and we will fight the consequences ourselves).
- Voltage in the protective neutral conductor, combined with the working one without grounding the midpoint.
- Static electricity that appears as a result of the distribution of charges is an absolutely safe, albeit rather unpleasant, case of voltage formation on the housing of household appliances.
Regardless of the source of accumulated charge, the elimination of malfunctions associated with the danger of electric shock is one of the main goals of the design of electrification systems. If appropriate protective measures were not provided during the installation of the electrical network, the responsibility for ensuring safety falls entirely on the shoulders of the users..
Basic protective measures
There are two ways to protect yourself from electric shock. One of them is to de-energize equipment when electricity passes through the human body, the other is to build a bypass path along which electricity will drain into the ground. The first type of protective measures involves the installation of differential protection devices. They compare the quantitative value of the current flowing through both wires of the phase-zero loop, and disconnect the power if these values are not equivalent..
The device and principle of operation of the RCD
This method is quite effective in terms of security, but not always convenient. If the voltage on the device case is due to insulation breakdown, the protective device will simply not allow power to be supplied. Well, since control from the side of the device is carried out only within the framework of the apartment network, differential protection does not save from potential emergence from the side of communications and static electricity.
RCD connection diagram: 1 – introductory machine; 2 – counter; 3 – RCD type S; 4 – machines; 5 – zero bus; 6 – RCD to the consumer; 7 – grounding bus; 8 – three-core wire
The second way to ensure safe use is to build a grounding system to which all conductive parts of the devices are connected, on which there should be no electrical potential. The essence of the operation of this system is extremely simple: when a person touches, he closes the body of the device and the ground, that is, serves as a conductor. If there is another conductor whose resistance to ground is much lower, the electric current will already flow through it. At the same time, the very fact of the current passing through the human body is not excluded, it is just that this current takes on an extremely insignificant value and is not physically felt in any way. Of course, grounding eliminates the influence of both static electricity and third-party sources, although in the latter case it is still recommended to ensure dielectric connections of parts.
Transition to a three-wire power grid
The inclusion of a grounding system in the electrical network requires the presence of a third conductor in most sections, called a protective zero. Unlike the working zero, the ground wire does not participate directly in the operation of the power grid, it only serves to equalize the dangerous potential between the equipment frame and the ground. In this case, the leakage currents are part of the total load acting on the main network.
The ability to work using the grounding system is provided for by the design of most household appliances that have open metal parts, a power of more than 1 kW, as well as those in which the risk of contact of electrical equipment with water is implied during operation. It is easy to distinguish these devices – their plug has a third contact in addition to the two main pins. This contact is directly connected to the body of the device, respectively, the mating contact of the socket must be connected directly to the grounding system.
Power supply systems with a protective neutral conductor use cables consisting of three conductors. Power (phase and zero) are selected in accordance with the predicted load. The third core may have a smaller cross-section, it is calculated based on the length of the conductor and the permissible resistance value between the grounding system and, in fact, the Earth. It is not necessary for the protective conductor to run inside the cable. Quite often it is laid separately, for which the methods of external laying are quite suitable: in the channel of the baseboard, open on the bases, in the cavity of finishing structures, or with brickwork in a layer of plaster.
It is forbidden to use engineering communications made of metal, such as heating pipes or water supply systems, as a protective neutral conductor. The ground wire must be copper, and in the internal distribution network, a cross section of 1.5 mm is allowed2, and for the connection of power supply and grounding systems – at least 6 mm2. In the electrical network of enterprises, it is allowed to replace copper conductors with steel ones, however, their cross-section must be at least 80 mm2, the maximum length is limited, depending on the applicable voltage class.
Ground loop device
The end point of any man-made grounding system is the circuit of the main grounding conductors. It connects the protective conductor system to the nearest aquifer, in which moisture is saturated with ions and, in fact, is an excellent electrolyte.
To ensure a low electrical resistance between the top and the protective conductor, a sufficient contact area and low resistance of the conductors are required. The main earthing switches are most often represented by rolled products made of steel grade 3 or metal parts of underground utilities. In the latter case, the admissibility of using natural ground electrodes as such is determined by the PUE.
The grounding system can be mounted by a hammer-in method or arranged with the accompanying excavation work. In the first case, rolled metal products with stiffening ribs are used: angle steel, channel, Tavr. Such products can be driven vertically downward without deformation, moreover, they have a well-developed outer surface. When grounding is buried, a steel sheet, a strip and in general any metal objects that are massive enough to exist in the soil layer for several decades can be used.
Installation of the grounding system can be done independently, however, the calculation of the number, degree of immersion and cross-section of the main electrodes must be carried out by specialists. The calculation method is based both on the type and resistivity of the soil, and on the location of the main circuit and the conditions of its operation. But you can go a simpler way: start with 3–4 electrodes piercing the watershed by 50–70 cm, and then add them if, according to the measurement results, the contact resistance of the circuit is not low enough.
Grounding in apartment conditions
The question of how you can arrange a three-wire network at secondary housing facilities, where usually the power supply is carried out according to a two-wire circuit, remained unresolved. Of course, the best option is to renovate the power grid during the next repair. During this event, the two-wire wiring in the right places is changed to three-wire, in parallel, work is underway to introduce the protective conductor into the apartment. There are two options for the latter..
The first is when the presence of a common house grounding system is provided for by the construction project. With this option, the metal housings of all driveways are connected by a massive bus or steel elements of building structures. In the basement of a house, this system contacts one or more ground loops. It is enough to connect an additional conductor to the housing of the flap in the entrance, and then connect its opposite end to the branched network of protective neutral conductors in your own home. However, the presence of local grounding must be reliably known, otherwise the protective working conductor is connected to zero, which just serves as one of the prerequisites for severe electric shock.
In some houses there is no common ground loop, the only option is to install your own protection system against electric shock. One of the best ways is to set up the circuit of the main grounding conductors using a hammer-in method in the local area opposite one of the windows of your apartment. First, you need to obtain approval for carrying out land work on the selected site, so that when the electrodes are driven in, the underground utilities are not damaged. Before entering the apartment, the wire is laid along the outer wall of the building with direct fastening, while both steel and bare copper conductors of the appropriate cross-section can be used. The common ground wire does not have to be pulled to the apartment panel, it is powerfully connected to the protective conductor system at any point using a conventional wiring box.