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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Electrical safety of computers and computer networks. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Computers Currently, more and more people use personal computers, in many organizations and institutions, computers are connected to a local network. Many have heard about uninterruptible power supplies and that "for normal operation, the computer case must be grounded", but the issues of electrical safety of computer equipment, in the opinion of the author, have not received sufficient coverage in the literature and computer periodicals. At present, the main document regulating the design, installation and operation of electrical installations is the "Rules for the installation of electrical installations" [1]. Consider the means of ensuring electrical safety. P.1.7.32 PUE regulates protective measures against electric shock to people: isolation transformer, double insulation, grounding, zeroing, protective shutdown, potential equalization. Isolation transformer - this is a transformer with increased insulation, due to which the possibility of the transition of the voltage of the primary winding to the secondary is significantly reduced. Isolating transformers do not have to be step-down, however, the secondary voltage should not be more than 380 V (see clause 1.7.44 of the PUE), in addition, only one electrical receiver is allowed to be powered from an isolating transformer. The secondary winding of the isolation transformer and the electrical receiver connected to it are not grounded. In the absence of grounding, touching live parts or a housing with damaged insulation does not pose a danger, since the secondary network of an isolating transformer is usually short, and the leakage currents in it are small if the insulation is good. If at the same time insulation damage occurs in another phase of the secondary circuit (double short circuit), then a voltage with respect to the ground may appear on the body of the power receiver, which can be dangerous under adverse conditions. To reduce the likelihood of double circuits, no more than one electrical receiver can be connected to an isolation transformer in accordance with clause 1.7.42.2 of the Electrical Installation Code. In the era of the widespread use of switching power supplies and the desire to minimize the material consumption of products, the formula "one computer + one isolating transformer" is unlikely to find mass (or even wide) application. Low voltage power supply (42 V, see clause 1.7.44 of the PUE) is also associated with significant material costs - a step-down transformer of sufficient power is required, preferably with increased insulation between the primary and secondary windings; computer power supplies must be designed for a voltage of 42 V. The author is not aware of a single case of using power supplies with a mains voltage of 42 V in IBM-compatible computers (although power supplies with such a voltage were produced for Elektronika school computers), and it is hardly worth engage in their production. So this method cannot be recommended for wide application. Consider the double insulation protection method.Double insulation, according to clause 1.7.29 of the PUE, this is "a combination of working and protective (additional) insulation, in which the parts of the electrical receiver accessible to the touch do not acquire dangerous voltage if only the working or only protective (additional) insulation is damaged. The computer power supply usually has a filter at the input , which reduces interference in the network (Fig. 1).
The second contact of the network connector is connected, as a rule, to the computer case. Capacitors C2 and C3 are connected to the supply conductors and the second terminals - to the computer case. In fact, both the phase and neutral wires are connected to the computer case through capacitors. Although these capacitors (usually ceramic) are designed for increased voltage (1,5-2 kV), it cannot be said that they have "double insulation". Consequently, both the power supply and the entire computer cannot be considered double-insulated electrical appliances, so they are not subject to clause 1.7.48.5 of the PUE, which states that it is possible not to ground (zero). In practice, there have been cases when an ungrounded computer case "pinched" when touched. Apparently, most of these cases are associated with deterioration of the interlayer insulation of capacitors C2 and C3, or, in other words, with an increased leakage current of these capacitors. Grounding and grounding. According to clause 1.7.33 of the Electrical Installation Code, grounding or grounding of electrical installations must be carried out at rated voltages above 42 V, but below 380 V AC in rooms with increased danger. If, for example, the computer is on a table, the table is near a heating radiator that is not enclosed by insulating grilles, and the distance between the computer and the radiator is 1 m or less (this situation is not uncommon), then this already creates an increased danger. If the temperature of +24 ° С was maintained in the room for 1 hours 35,1 min, then it should formally be classified as premises with increased danger. Grounding - a means designed to protect against voltage shock, which, due to insulation damage, occurs on the surface of metal or other electrically conductive elements or parts of equipment that are not normally energized [2]. Electrical safety is achieved by using a grounding device system, which is understood as a set of grounding conductors. Grounding (protective grounding) is used in networks operating with an isolated neutral (for example, 6 or 10 kV). The essence of protection with the help of a grounding device is to create such a grounding that would have a resistance small enough so that the voltage drop across it (namely, it will be amazing) does not reach a value dangerous to humans; in a damaged network, it is necessary to provide such a current that would be sufficient for the reliable operation of the protective devices. Zanulenie - this is a protective measure used only in networks with a dead-earthed neutral with a voltage below 1 kV, designed to protect against voltage that occurs on metal parts of equipment that are not normally energized (but may become energized due to insulation damage), which consists in creating in a damaged circuit of the current value sufficient to trigger the protection [2]. Zeroing is a deliberate connection of parts of an electrical installation that are not normally energized with a dead-earthed neutral of a generator or transformer in three-phase current networks. Thus, zeroing, apparently, can be considered a broader concept than grounding, and including the latter (if the body of the power receiver is grounded, then it is simultaneously grounded; another thing is whether repeated grounding conductors are used in a network with a solidly grounded neutral or not). Figure 2 explains the physical essence of zeroing, where 1 is an energy source (step-down transformer 6 kV / 380 V or 10 kV / 380 V with a dead-earthed neutral); 2 - earthing of the transformer neutral (main earthing); 3 - repeated ground electrode; 4 - energy consumer (personal computer); 5 - protection device (fusible or automatic fuse, etc.).
When the phase wire is shorted to the housing, a short-circuit current Ikz flows in the "phase wire - neutral wire" circuit, which causes the protective device to operate. To reduce the contact voltage, a repeated grounding conductor 3 is used. If it is absent, in the event of a phase-to-case short circuit, the touch voltage (voltage on the case relative to ground) will be half the phase wire if the resistance of the phase wire is equal to the resistance of the neutral wire and more than half of the phase wire if the resistance of the phase wire is less resistance of the neutral wire (which happens often). The probability of failure of a correctly selected protection (when the operator touches the case at the moment the phase wire is closed to the case) is quite low, but it cannot be completely excluded, and the touch voltage may remain on the case for some time. To reduce it, a repeated ground electrode 3 is used. A circuit appears, as if bypassing the neutral wire. The resistance of this circuit is much greater than the resistance of the neutral wire, and therefore this circuit does not significantly affect the value of the current flowing through the neutral wire, but the voltage relative to the ground decreases. If the resistance of the re-ground electrode (single or system) is equal to the resistance of the neutral of the transformer, then the contact voltage relative to the earth will be equal to half the voltage drop on the neutral wire (the contact voltage, for example, 110 V, will be equally distributed between the earth electrodes connected in series). Accordingly, by changing the ratio of the secondary and main ground electrodes, it is possible to change the touch voltage on the body of the power receiver (as well as on the body of the supply transformer). In practice, however, at both ends (at the electrical receiver and at the transformer) there are a large number of natural grounding conductors (reinforcing structures, foundations, pipelines, metal sheaths of cables, etc.); the grounding resistance of these natural grounding conductors is reflected in the grounding resistance of the main and secondary grounding conductors, and it is rather difficult to take this effect into account. Uncertainty arises, which is a disadvantage of nulling. The common (and often practiced) grounding scheme of the computer case, shown in Fig. 3, should be recognized as not providing electrical safety, due to the fact that when the phase wire is closed to the case, the short-circuit current Ikz flows not through the neutral wire, but through the series-connected main (2) and repeated (3) ground electrodes (ground resistance should also be taken into account). This current may not be sufficient to trigger the protection device 5, and the touch voltage close to the phase voltage may be maintained on the computer case 4 for a long time.
Safety shutdown - high-speed protection, which provides automatic shutdown of the electrical installation in the event of a danger of electric shock in it. There is a wide variety of protective shutdown schemes, but most often they are based on the so-called zero-sequence current transformer [4]. The principle of operation of the protective shutdown is explained in Fig.4.
Zero sequence current transformer 1 is a toroidal core (usually made of ferrite) with three windings. The operation of the device is based on the principle of separating the difference in currents Ip passing through the neutral and phase wires. The windings W1 and W2 have the same number of turns and are connected so that the currents I1 (flowing in the phase wire) and I2 (flowing in the neutral wire) create oppositely directed magnetic fluxes. If the currents I1 and I2 are equal, the resulting magnetic flux is zero, and no voltage is induced in the winding W0. When the current is branched off (due to a person touching the case, on which the phase is closed), the resulting magnetic flux will no longer be equal to zero, since the currents I1 and I2 are not equal (I1 = I2 + I4), and a voltage is induced in the winding W0, causing the actuating device 2, which disconnects both power wires from the load. The installation current (at which the load is disconnected) can be chosen small enough (a few milliamps) that it does not pose a danger to humans. The residual current device has the following advantages:
Residual current devices (RCDs) were mass-produced many years ago [4]. Modern microcircuit technology makes it possible to create such small-sized devices that they can be built into a network plug. Back in the late 80s, a microcircuit containing the main blocks of an RCD was described in the Electronics magazine. A similar chip (K1182CA1) is also produced by SPC SIT (Russia, Bryansk) [5]. The author has not yet come across computer cables with an RCD built into the plug, and it is apparently quite difficult to make such a cable on your own. However, it is quite possible to build such a device into a power block - a box made of insulating material, on which 2-3 computer (with three pins) sockets are fixed and to which a conventional two-pin mains plug with a cable and a ground wire are connected. Thus, to ensure electrical safety, a single computer user can be recommended to use an RCD in conjunction with grounding; grounding also removes static potential from the computer case, which increases the reliability of the RAM and hard drive of the computer [6]. In the case of an RCD, the requirements for grounding become not so stringent (its resistance may be more than 4 ohms, more than the resistance of the main ground electrode; this will not lead to an increase in the contact voltage as in systems with zeroing). The disadvantage of using an RCD is the possible loss of data when it is triggered, but this has to be put up with. In local computer networks, electrical safety looks a little different. The wiring diagram of the local network is shown in Fig.5.
The server is powered by an uninterruptible power supply (UPS); in this UPS, the secondary circuits are galvanically isolated from the mains. From the point of view of electrical safety, the UPS (in the English transcription UPS) can be considered an "improved analogue" of an isolation transformer; none of the two output supply wires is grounded (just as none of the outputs of the secondary winding of an isolating transformer is grounded). Of course, it would be nice to equip all computers on the local network with a UPS, which would eliminate the loss of data, but this solution is quite expensive. Of course, a single user can also equip his computer with a UPS, but the cost of a UPS is at least several times higher than the cost of an RCD. In addition, there are UPSs in which the secondary circuits are not galvanically isolated from the mains; "true" UPS with galvanic isolation are more expensive. The UPS that feeds the server is powered through the RCD, but this RCD is somewhat different from the "standard" one (in Fig. 5), through which the rest of the computers on the local network are powered. "RCD standard" cuts off power from the computer if there is a leakage current to the ground. The RCD of the server does not turn off the power in case of leakage, but only turns on an audible signal, indicating that there is a touch voltage on the UPS case. You can insert the same RCD between the UPS and the server, the sound signal in this case will indicate the deterioration of the insulation in the server power supply. The cases of all computers are additionally connected by separate conductors 8 and 10 to the grounding contact of the power block 1 (or connected by a grounding conductor directly to the grounding line 5 as a server). These conductors duplicate the grounding conductor of a standard computer cord 2. As experience shows, the grounding contact of a standard computer socket does not have sufficient elasticity, the "ground" connection is sometimes broken, which is fraught with serious consequences. In principle, one can do without these redundant conductors, but then periodic monitoring of the "ground" connection is necessary, which is not always convenient. Local network computers are connected by segments of a coaxial cable with standard lugs using T connectors, terminators and resistors with a resistance equal to the cable impedance are installed at both ends of the line; one of the terminators is grounded (grounding chain 9 in Fig. 5 can be connected to the computer case). The grounding line 5 is connected by a grounding conductor 6 to the ground electrode (or ground loop) 7. As a grounding line, you can use, for example, a copper bus with a cross section of 5-62 mm, it is flexible enough, which makes it easier to lay. The connection of the ground conductors 10 with the ground line 5 must be performed by soldering. The ground conductor 6 (preferably steel) is connected to the ground electrode 7 by welding, and to the grounding line - by soldering, and the place of soldering should be in the room. If the building has other (and even more powerful) consumers of electricity that need grounding, then their grounding conductors should be connected directly to the ground loop 7. Otherwise, a powerful consumer can create voltage fluctuations on the ground conductor 6 or ground line 5, these fluctuations can lead to failures in the local network. The cable supplying the power blocks 1 and 3 is connected to the mains through standard protective equipment (fuses or electromagnetic switches). The choice of the latter is carried out in accordance with the requirements of the PUE. References:
Author: V. I. Vasilenko
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