How to Ground a Computer.
A guide to installing grounding in a private house, apartment, office, and industrial facility.
The Nature of the Phenomenon
Technologically, grounding and neutral bonding (zeroing) are no different — they are simply different names.
A separate matter is the distinction between the working neutral and the protective neutral: the working neutral is the one found in our outlets as the second slot, while the protective neutral is the same neutral but running on a separate line — and this arrangement is called the TN-C-S neutral bonding system. That is the system used in our country.
Grounding Arrangements
Practical Importance of Grounding Electrical Appliances
Every household electrical appliance generates around itself an electromagnetic field (EMF) at 50 Hz that is harmful to human health. A person in the vicinity of an operating household appliance is exposed to both the electric and the magnetic components of the EMF. Researchers consider the magnetic component to be the greater hazard. As is well known, the SI unit of magnetic flux density is the tesla, abbreviated T. The electromagnetic field near an appliance is conventionally characterized by its magnetic component and measured in millionths of a tesla — microteslas (µT). An EMF level exceeding 0.2 microteslas is considered hazardous to human health. Around many ungrounded household appliances — and computers in particular, at which people typically work for many hours at a stretch — the EMF level is several times, and sometimes an order of magnitude, above that threshold.
According to data from the Centre for Electromagnetic Safety, the systems most sensitive to EMF exposure are the nervous, immune, endocrine, and reproductive systems. The biological effect of EMF under prolonged exposure tends to accumulate. This can lead to long-term consequences, including degenerative processes of the central nervous system, blood cancer, brain tumors, and hormonal disorders.
Proper grounding of household electrical appliances can reduce the effect of EMF on people. As an example, let us focus on grounding computers. Before discussing how to ground a computer correctly, we will review the main electrical networks from which apartment wiring receives its power. When we say "apartment" wiring, we also mean wiring in an office, a study, or a business premises — in short, anywhere a computer is present.
Types of Grounding Systems
The subtype of a grounding system is an indicator characterizing the relationship to earth of the transformer neutral at a substation or the generator neutral at a power station (in rural areas with autonomous power supply), as well as the exposed conductive non-live parts of electrical appliances at the consumer's end and the neutral conductor in an electrical installation operating at voltages up to 1 kV. TN, TT, and IT grounding systems are distinguished for electrical networks (designations per GOST R 50571.2). The first two have an earthed transformer neutral at the transformer substation (or generator neutral at the power station), while the third has an isolated neutral. The TN system, in turn, is subdivided by neutral conductor arrangement into TN-S, TN-C, and TN-C-S systems. The name of an electrical network's grounding system type is often applied to the network itself — so, for example, an electrical network with a TN-S grounding system is called a TN-S network, or simply a TN-S network. Electrical networks with TT and IT grounding systems are extremely rarely used to supply apartment wiring and are not considered in this article.
In an electrical network with a TN-S grounding system, the neutral working conductor (N-conductor) and the protective earth conductor (PE-conductor) are separated from each other throughout the entire network, from the transformer or generator all the way to the electrical load at the consumer's end. The electrical diagram of such a network is shown in Fig. 1.
Unlike an electrical network with a TN-S grounding system, in an electrical network with a TN-C grounding system the neutral working conductor (N-conductor) and the protective earth conductor (PE-conductor) are combined in a single so-called PEN-conductor throughout the entire network, from the transformer or generator all the way to the electrical load. The electrical diagram of such a network is shown in Fig. 2.
Fig. 3 shows an electrical network with a TN-C-S grounding system. It combines both of the preceding systems and contains both a combined PEN-conductor and separate N- and PE-conductors. In premises whose electrical wiring receives power from the electrical networks described above — those with TN-S and TN-C-S grounding systems — three-pole outlets are generally installed (sometimes called three-contact or three-pin outlets), in which one of the poles is used for grounding household appliances. In such outlets, the grounding pole must be connected to the PE-conductor without fail.
A natural question arises. If the electrical wiring of a premises contains a European-type three-pole outlet — one of whose special connector poles is dedicated specifically for grounding purposes — does plugging the computer's three-pin plug into that outlet constitute proper grounding? This central question cannot be answered with a simple yes or no. Everything depends on whether the three-pole outlet has been wired correctly or incorrectly. The author, having participated in investigations of fatal accidents, has encountered many errors in the installation of outlets with grounding contacts. I will describe some of the most typical ones below.
Two Conductors. Option One
Two conductors run to the outlet: one is the phase conductor and the other is the neutral working conductor (designated, as noted above, by the letter N in the TN-C grounding system). The phase conductor is connected to one of the two working contacts of the outlet, while the neutral working conductor is connected to the second working contact and simultaneously to the outlet's grounding contact, as shown in Fig. 4.
In the diagram, the numeral 1 marks the network phase conductors; 2 — the combined neutral working and protective conductor; 3 — the phase conductor of the apartment wiring; 4 — the neutral working conductor of the apartment wiring; 5 — the three-pole outlet; 6 — the outlet pole to which the neutral working conductor is connected; 7 — the conductor linking the grounding pole 8 of the outlet and the outlet pole to which the neutral working conductor is connected; 9 — the outlet pole to which the phase conductor of the apartment wiring is connected. This manner of wiring an outlet is a gross violation of electrical safety regulations and turns an outlet designed to provide protection against electric shock into its direct opposite, since it creates an elevated risk of electric shock. Indeed, any break in the neutral working conductor anywhere along its length will cause a dangerous voltage of 220 V to appear at the grounding (in quotation marks) pole of the outlet and, consequently, on the casing of the electrical load "grounded" in this way. That voltage will reach the casing via the conductivity of the single-phase electrical load — for example, a computer — plugged into the outlet.
A break in the neutral working conductor can occur, as already mentioned, anywhere along its length, and there can be many causes for such breaks. First, the conductor itself may be of poor quality: microscopic inclusions of foreign metal during manufacture can lead to localized oxidation and loss of conductor integrity. Furthermore, when a building's foundation settles, a crack may form in a wall containing embedded wiring, causing one of the conductors to break — and, as ill luck would have it, not the phase conductor but the neutral working one. Finally, you may decide to hang a picture in a room and use a punch or rotary hammer to make a hole in a concrete wall, hitting the conductors embedded in it and, again as ill luck would have it, breaking precisely the neutral working conductor.
Two Conductors. Option Two
As in the previous case, two conductors run to the outlet: one phase and one neutral. The phase conductor is connected to one of the two working poles of the outlet, while the neutral conductor, at its end inside the back-box directly behind the outlet, splits into two very short conductors just 1–2 cm long each (Fig. 5).
One of these is connected to the second working pole of the outlet. This short conductor is called the neutral working conductor and designated by the letter N. The second short conductor is connected to the grounding contact of the outlet. It is called the neutral protective conductor, i.e., the PE-conductor. The conductor before the split into N and PE is called the combined neutral working and neutral protective conductor, i.e., the PEN-conductor, and the grounding system itself is called the TN-C-S system. This is the formal description. In practice, however, this outlet wiring variant is no different from the previous one and presents the same high risk of electric shock, which can occur if the integrity of the so-called PEN-conductor is compromised.
Three Conductors
The splitting of the PEN-conductor into the neutral working and neutral protective conductors is carried out not in the back-box behind the outlet, as in the two previous cases, but in the apartment distribution board located on the stairwell landing of a multi-apartment building or at the service entrance of a one- or two-apartment single-storey house. In this case, three conductors run to the outlet rather than two: a phase conductor, a neutral working conductor, and a neutral protective conductor. The phase and neutral working conductors are connected to the working poles of the outlet, and the neutral protective conductor is connected to the grounding pole. At first glance it seems that everything is now in order. Any break in the neutral working conductor will not create a dangerous situation. The electrical load — in this case the computer — will remain grounded. But what will happen if the integrity of the PEN-conductor is compromised — that is, the conductor that combines the functions of both the neutral working and neutral protective conductors? For example, the neutral wire of the overhead line supplying a single-storey residential house in a rural area or a dacha in a dacha settlement might break. Or a short circuit might occur between the phase conductor and the PEN-conductor, generating an electric arc that burns the PEN-conductor away on the supply side while the short circuit persists. In that case, a dangerous voltage of 220 V will appear not only at the grounding pole of one outlet but at the grounding poles of all outlets in the apartment that are grounded to that PEN-conductor. It would be fortunate if the short circuit occurred in the distribution board on the stairwell landing — but what if it occurred at the service entrance of the entire multi-apartment building? Then all outlets in all apartments of that building would become dangerous.
It is worth emphasizing that the risk of electric shock described here will only arise if, in addition to splitting the PEN-conductor into the N- and PE-conductors, no protective equipotential bonding has been used. Protective equipotential bonding is an electrical safety measure that consists of reducing the relative difference in electrical potential between various points on the surface of an electrically conductive floor (covering) and parts accessible to touch.
Three Conductors Plus a Limiter
If an element is installed in a three-pole outlet that limits the electrical voltage at the outlet's grounding pole to safe levels, then such an outlet can successfully replace an ordinary two-pole outlet in an apartment whose electrical wiring receives power from electrical networks with a TN-C grounding system. In this case, no additional grounding conductor (the so-called third wire) is required anywhere in the apartment. This means there is no need to chip away at concrete walls to cut chases (grooves) for laying grounding conductors in them and then filling them with concrete, alabaster, or another filler. There is no need to whitewash, paint, or re-wallpaper the areas where the conductors were run. The electrical connection diagram for such an outlet is shown in Fig. 6.
The grounding system of such a network would appropriately be designated TN-C-R, where R is the first letter of the word Restriction, since what is involved is precisely the restriction to safe levels of the electrical voltage on the grounding pins of sockets connected to the said network, and the restriction to safe levels of electromagnetic field exposure for a person working at a computer grounded in this manner. This method is applicable not only in networks with a TN-C grounding system, but also in networks with TN-C-S and TN-S grounding systems.
This method of computer grounding was developed in the electrical safety laboratory of VIESKh and is currently in the process of being patented; accordingly, the technical characteristics of element 7 shown in Fig. 6 are not disclosed in this article.
What the results show
Comparing the computer grounding methods discussed above from the standpoint of protecting people from harmful electromagnetic field exposure, it should be noted that they are not all equivalent, since they do not all provide equally reliable protection. In networks with TN-C-S and TN-S grounding systems, if the PE conductor breaks, protection is immediately lost, but this will go unnoticed because it will have no effect whatsoever on the computer's operation. Exposure will begin for everyone working at computers, affecting all apartments whose sockets are connected to the damaged PE conductor in question. This condition may persist for a very long time. If, however, the neutral working conductor is broken in the proposed new protection system, the computer will simply fail to start and the fault will be detected immediately. The probability of damage to element 7 shown in Fig. 6 is extremely low.
A drawback of the proposed method is the absence of protection for the computer in the event of a failure of the electrical insulation between its live parts and the casing. However, the probability of such a failure is low. The computer's detachable power cable has double insulation, as does the power supply unit. In the author's many years of practice, not a single case of such a failure is known. If desired, in addition to the computer's electrical insulation, a residual current device (RCD), formerly known as a ground-fault circuit interrupter, may be installed in the apartment. A drawback of traditional grounding methods is that the neutral protective conductor very often turns into its own opposite. When its continuity is broken — for example, by an open circuit — the normal operation of the electrical installation is not disrupted, which means the fault may go undetected for a long time, sometimes for months. But should a fault to casing occur in any single electrical receiver beyond the point of the break (in the direction of energy flow), the full phase voltage of 220 V — dangerous to human life — will appear on the casings of all electrical receivers connected to the PE conductor, including computers. This voltage may likewise remain on the casings of electrical equipment for a long time, at least until people begin to die. This is the most dangerous situation associated with the presence of a PE conductor, and it can cause mass electrical injuries. But even when the PE conductor is intact, any fault to casing will inevitably result in a voltage dangerous to human life appearing on it. The exact same voltage will appear on the casings of all other electrical receivers connected to the PE conductor beyond the point of the fault, and will remain there until the primary or backup protection operates.
Operational experience with electrical installations shows that the situation described above, involving conductor damage, is not at all uncommon. For example, N. N. Mironenko, head of the Technical Production Department of Volgogradskiye Elektricheskiye Seti (Volgograd Electric Networks), describes the very real lethal danger associated with electric shock resulting from the burnout of the network neutral conductor — rarely at the power source, but quite often on overhead lines, at the service entrance panel, the floor distribution board, or between the board and the apartment entry point due to leaks — with the result that the casing of the electrical receiver ends up at the network phase voltage.
Grounding of computer equipment
Grounding of computer equipment, telecommunications equipment, and uninterruptible power supplies serves to achieve so-called electromagnetic compatibility (EMC) — ensuring that equipment operates correctly in the presence of both externally introduced and self-generated electromagnetic interference. The other, most important function of grounding is to ensure the electrical safety of personnel working with infocommunication equipment.
Depending on the objectives set, as well as on national and international standards, the schemes used may differ in electrical installations operating at different AC and DC voltages. We will examine the most common case: grounding of individual computers and local-area network workstations, active network equipment, and digital private branch exchanges (PBXs) — that is, equipment plugged into a 220 V AC outlet. In practice, two extremes are encountered: either grounding is ignored entirely and ordinary household outlets are used (or grounding is made to pipes and structures), or, conversely, excessively stringent requirements are imposed for creating a "clean" ground. In both cases, electromagnetic compatibility and electrical safety standards are not met.
Terminology and standards
To begin, let us introduce several terms and definitions.
Neutral earthing (zeroing) in electrical installations with voltages up to 1 kV refers to the intentional connection of parts of an electrical installation that are not normally energized to the solidly earthed neutral of a generator or transformer in three-phase AC networks, or to the solidly earthed terminal of a single-phase current source.
A solidly earthed neutral is the neutral of a transformer or generator connected to an earthing device directly or through a low resistance (for example, through current transformers).
An earth electrode is a conductor (electrode) or a set of conductors (electrodes) connected to one another metallically and in contact with the ground.
GOST R 50571.2-94 provides, among others, for the following types of grounding systems for building electrical networks: TN-S, TN-C, TN-C-S. These are the systems used in the case under consideration. The first letter T denotes the direct connection of one point of the live parts of the power source to earth; the second letter denotes the character of the earthing of the exposed conductive parts of the electrical installation (T — direct connection of exposed conductive parts to earth, independent of the nature of the connection of the power source to earth; N — direct connection of exposed conductive parts to the earthed point of the power source, with the neutral normally earthed in AC systems). Subsequent letters denote the arrangement of the neutral working and neutral protective conductors: S — the functions of the neutral protective and neutral working conductors are provided by separate conductors; C — the functions of the neutral protective and neutral working conductors are combined in a single conductor. The graphical symbols used in the above designations of grounding system types and in the figures are given in Table 1.
Requirements for grounding systems are set out in the following standards and regulatory documents:
- Electrical Installation Rules (PUE) — Section 1.7;
- GOST 12.1.030-81 SSBT. Electrical safety. Protective earthing, neutral earthing;
- GOST 464-79. Earthing for stationary installations of wire communications, radio relay stations, cable broadcast nodes, and antennas of communal television reception systems. Resistance standards;
- GOST R 50571.10-96 (IEC 364-5-54-80). Electrical installations of buildings. Part 5. Selection and erection of electrical equipment. Chapter 54. Earthing arrangements and protective conductors;
- GOST R 50571.21-2000 (IEC 60364-5-548-96). Electrical installations of buildings. Part 5. Selection and erection of electrical equipment. Section 548. Earthing arrangements and equipotential bonding in electrical installations containing information technology equipment;
- GOST R 50571.22-2000 (IEC 60364-7-707-84). Electrical installations of buildings. Part 7. Requirements for special electrical installations. Section 707. Earthing of information technology equipment.
Grounding errors
The presence of closed loops and connections between grounding systems of different purposes can give rise to intersystem grounding interference, which cannot be eliminated by installing uninterruptible power supplies and other power conditioning devices without galvanic isolation. In some cases, a separate grounding system is installed — for example, for an enterprise digital telephone exchange, as required by GOST 464-79, which provides for a separate grounding system for telecommunications equipment.
However, when this requirement is implemented only formally, no attention is paid to the fact that the standard calls for a separate grounding system for the pole of the DC power supply system. Powering equipment from a common AC network with a solidly earthed neutral while implementing what appears to be an independent ground is precisely what leads to the formation of ground loops, causing unstable equipment operation. A ground loop — unlike what specialists colloquially call contour earthing (the method of connecting horizontal earth electrodes in the ground, which should not be confused with earthing conductors) — is undesirable and forms when there is a connection between two earth electrodes (see Figure 1).
In the resulting loop (earth electrode No. 1 — electrical connection (conductor) — earth electrode No. 2 — medium (earth)), currents may be induced by external electromagnetic fields or stray currents from extraneous loads may flow. All of this causes electromagnetic interference in equipment operation. Local computing and telecommunications networks often include communications equipment (antennas, modems, etc.) and are susceptible to interference, including from lightning discharges — meaning that high noise immunity is important for them. This is precisely why eliminating loops deserves attention during the design and operation of building electrical installations.
[[File:028_2.gif|Figure 2. Incorrect grounding to an isolated earth electrode not connected to the transformer neutral|left}}
In practice, incorrect grounding to an isolated earth electrode not connected to the transformer neutral is encountered (see Figure 2). Such a grounding scheme violates the requirement of PUE clause 1.7.39: "In electrical installations up to 1 kV with a solidly earthed neutral or solidly earthed terminal of a single-phase current source, as well as with a solidly earthed midpoint in three-wire DC networks, neutral earthing (zeroing) must be implemented. The use of casing earthing in such electrical installations without neutral earthing (zeroing) is not permitted..." This requirement arises because it is impossible to ensure electrical safety with the scheme in question. Figure 2 shows the export of potential in the event of a short circuit to the casing of an electrical receiver earthed to an isolated earth electrode.
The appearance of a potential on the casing is caused by the voltage drop in the phase conductor up to the point of the short circuit, and by the voltage drops across the resistance of earth electrode No. 2, the medium (earth and structures), and the resistance of earth electrode No. 1. The resistance of the short-circuit path is higher than the resistance of the phase-to-neutral path, based on whose parameters the protective circuit breaker is selected, and the short circuit will most likely not be cleared by the overcurrent protection. In this case, a potential close to the phase voltage is exported to the casing, creating a life-threatening hazard. Clearing of the short circuit will occur through the thermal protection of the automatic circuit breaker, but the short-circuit clearing time will exceed the normalized values of 0.4 s for voltage U₀ = 220 V and 0.2 s for U₀ = 380 V.
Thus, incorrectly implemented grounding leads to the formation of undesirable loops, causes electromagnetic interference in equipment operation, and is dangerous to people in the vicinity.
Main earthing mode
To minimize electromagnetic interference and ensure electrical safety, grounding should be implemented with the minimum number of closed loops. This condition can be met by providing a so-called main earthing terminal (MET), or busbar. The main earthing terminal should be located as close as possible to the incoming power and communications cables and connected to the earth electrode (or electrodes) by the shortest possible conductor.
Such placement of the MET ensures the best equalization of potentials and limits induced voltages from industrial interference, lightning and switching overvoltages arriving from outside via the screens of communications cables, the armouring of power cables, pipelines, and antenna entries. The following must be connected to the MET (busbar):
- earthing conductors;
- protective conductors;
- conductors of the main equipotential bonding system;
- functional earthing conductors (where required).
The main earthing terminal (busbar) must be connected to the earth electrodes of protective and functional (process, logic, etc.) earthing, lightning protection earth electrodes, and others. The detailed rules and requirements for the installation of the MET are set out in the PUE.
Grounding systems
Grounding systems differ in their connection schemes and in the number of neutral working and protective conductors.
The TN-C system (see Figure 3) covers three-phase four-wire networks (three phase conductors and a PEN conductor combining the functions of the neutral working and neutral protective conductors) and single-phase two-wire networks (a phase conductor and a neutral working conductor) found in existing older buildings.
The absence of a dedicated neutral protective (earthing) conductor in existing single-phase network wiring creates a risk of electric shock to personnel. In some cases, information technology and telecommunications equipment is installed in premises where there is no earthing and simultaneously a non-conductive floor covering is present on which static electricity accumulates. Due to the absence of earthing and the occurrence of static electricity discharges when touching a keyboard or the casing of a personal computer, malfunctions such as "freezes" and even equipment damage, software faults, and data loss occur.
Connecting modern computer equipment to outlets of a TN-C electrical network is associated with the phenomenon of voltage export to the casing, since switching power supplies have a symmetrical L-C filter at the input, whose midpoint is connected to the casing. When the computer is connected to the neutral (earthed), technological leakage occurs through the filter, which must be taken into account when using a residual current device (RCD). In the absence of a PE conductor, the 220 V voltage is divided across the "arms" of the filter, resulting in a voltage of 110 V appearing on the casing.
Current regulatory requirements prohibit the use of the TN-C system in newly constructed and reconstructed facilities. When operating a TN-C system in an older building where information technology and telecommunications equipment is to be installed, a transition from the TN-C system to the TN-S system (the TN-C-S system) should be arranged.
The TN-C-S system is characteristic of retrofitted networks in which the neutral working and protective conductors are combined only in part of the scheme. The TN-C-S system is shown in Figure 4.
When transitioning from the TN-C system to the TN-S system, the sequence of system placement relative to the power source must be maintained as shown in Figure 4. Otherwise, the return currents of TN-C electrical receivers will close through the PE protective conductors of the TN-C-S system and cause interference. If one part of a building's electrical installation — a transformer, diesel generator, uninterruptible power supply (UPS), or similar device — has a TN-C grounding system and is used primarily to power infocommunication technology equipment, the solution should be a transition to a TN-S type system.
The TN-S system (see Figure 5) is the primary working grounding system for buildings housing information technology and telecommunications equipment. In the TN-S system, the neutral working and neutral protective conductors are routed separately from the power source. This scheme ensures the absence of return currents in the PE conductor, which reduces the risk of electromagnetic interference. During operation, it is necessary to ensure that the designated functions of the PE and N conductors are maintained. From the standpoint of minimizing interference, the optimal arrangement is the presence of a built-in (adjacent) transformer substation (TS). In this way, the minimum length of the link between the incoming power cable entry and the main earthing terminal is achieved.
Compliance with this requirement also applies to the TN-C-S system. In this case as well, what matters is the distance between the point of entry from the power supply system and the main earthing terminal. For the TN-C-S system, repeated earthing of the neutral is desirable. The TN-S system, when a built-in (adjacent) substation is present, does not require repeated earthing, since a primary earth electrode is present at the TS.
Grounding Conductors
Conducted interference propagates directly through the electrical network as current flows, entering the uninterruptible power supply system (UPS system) from the general-purpose supply network. Suppressing such interference at Group A electrical receivers to the acceptable level defined by GOST 13109-97 is achieved by supplying consumers via a dedicated network and by using active-type UPS units to protect equipment from network-borne interference. A dedicated network is an electrical network intended to supply a group of electrical receivers united by their functional purpose or by shared requirements for power quality and supply reliability. An important component of the dedicated electrical network is the grounding conductor network.
In buildings where large quantities of various information-processing equipment or other interference-sensitive equipment are installed or may be installed, special attention must be paid to the use of separate protective conductors (PE conductors) and neutral working conductors (N conductors) downstream of the supply point, in order to prevent or minimize electromagnetic interference. These conductors must not be combined; otherwise the load current — and in particular the overcurrent arising during a single-phase short circuit — will flow not only through the neutral working conductor but also partly through the protective conductor, which can cause interference.
Computer network workstations must have a grounding network configured as a single-point star. Because of the large number of connections this is difficult to implement in pure form, so a hybrid scheme is used: grounding conductors are routed together along the same path as the power supply lines (see Figure 6). From the main distribution board or the main switchboard — where the main earthing terminal (busbar) is located — to the distribution panels on each floor, the scheme is a single-point star (parallel single-point); from the distribution panel to the electrical outlet, the scheme is a series single-point.
All grounding conductors are routed using insulated wires and cables. In electrical panels, the PE busbars and terminal blocks for computer network consumers are mounted insulated from the enclosure. PE lines for grounding enclosures, trunking, cable trays, and other electrical equipment and structures are routed as separate wires and cables from the same main earthing terminal.
The trunk conductor from the main earthing terminal (busbar) is also routed together with the trunk power supply lines. Grounding of process equipment must be carried out in accordance with the requirements of the technical documentation. In doing so, the enclosures (exposed conductive parts) of the equipment must be connected to the main earthing terminal and to extraneous conductive parts serving as the equipotential bonding system.
Grounding System
The combination of the earth electrode and the grounding conductors is called the grounding system (see Figure 6). In a facility housing information, telecommunications equipment, and communications infrastructure, the grounding system must be protective and must comply with the electrical safety requirements set out in GOST 12.1.030, the Electrical Installation Rules (PUE), and the GOST R 50571 (IEC 364) standards "Electrical Installations of Buildings." No additional requirements are imposed on the grounding system beyond these.
The resistance of the grounding system must comply with the PUE (see Section 1.7). If it meets the permissible value within the building, reducing the resistance further does not affect the operational stability of the equipment, and no additional requirements are imposed on earth electrode resistance.
A building may have one, two, or several earth electrodes; however, when a single earth electrode gives a grounding system resistance that satisfies the PUE requirements, increasing the number of earth electrodes has no effect on electrical safety or stable equipment operation. It is recommended that the earth electrode (or electrodes) be located within the protected (controlled) area, which is one of the conditions for ensuring information security.
In certain cases a requirement is imposed to create a separate functional (process, logical, etc.) earth electrode, isolated from the protective earthing electrodes, for the purpose of protecting information and preventing unauthorized access to it via power supply circuits and grounding conductors.
If, due to process requirements (conditions for protecting information from unauthorized access, processing of confidential information, etc.), the functional (process, etc.) earthing electrode must be isolated from the protective earthing (neutral earthing) system, then the trunk neutral protective conductors and the functional (process, etc.) earthing electrode must be connected to a separate earthing terminal that is insulated from metalwork and from electrical equipment. To ensure electrical safety and information security, the following should be used:
- an isolating transformer;
- a double-conversion UPS with an isolating transformer;
- filters (transfilters, superfilters) with an isolating transformer.
The primary requirement for using this equipment is the absence of a conductive connection to the primary side via either PE or N. Accordingly, UPS bypass operation must not violate this condition, which can be achieved by installing an isolating transformer in the bypass circuit.
The functional (process, etc.) earthing electrode must be located within the protected (controlled) zone to prevent unauthorized access to it.
Electrical Outlets
In conclusion, it is necessary to mention electrical outlets, since they provide the reliable connection of grounding conductors to equipment. When direct grounding is used, installation is made under the nut (clamp, boss) provided for in the equipment design. When connected via an outlet, grounding is accomplished through the separable contact connections of the electrical outlet and the three-conductor supply cable.
The market offers a fairly large number of electrical outlet types. At present, European-type outlets (known as "euro outlets") are widely used in Russia. Under the normative designation system adopted in European countries, they are designated E10-G: CEE 7 Schuko. The letter G denotes the German form factor. The less commonly used French/Belgian form factor, E10-F: French/Belgian, differs in the position and shape of the third grounding contact. In the E10-G: CEE 7 Schuko, the grounding contact takes the form of two metal strips positioned around the circumference of the outlet (see Figure 7a), while the grounding contact of the E10-F: French/Belgian outlet is a pin projecting above the socket contacts (see Figure 7b). Most power plug types used with infocommunication equipment can be inserted into both outlet types, though exceptions exist. When selecting electrical wiring accessories, preference should be given to the German-type E10-G: CEE 7 Schuko outlet.
"Euro outlets" differ from those previously manufactured in the USSR in the diameter of the socket contact aperture. The former have a diameter of 4.8 mm, while the latter have a diameter of 4 mm. For this reason, modern plugs with 4.8 mm pins do not fit old outlets. Furthermore, the absence of grounding in those old outlets makes them incompatible with current electrical safety requirements.
Author: Alexander Vorobyov, staff member of the Information Systems Department, VTB Bank (JSC). He can be reached at: vorobyov@vtb.ru.
FAQ: Grounding a Computer in an Apartment
1. One must be aware that there is NO GOOD solution to this problem. Most computers are designed assuming that proper grounding is present at the outlet, which in the majority of our residential (and not only residential) buildings is simply absent by definition. What one seeks is the least bad solution. Moreover, what I consider (IMHO) a widely propagated piece of idiocy is the fact that the midpoint of the mains filter, the metal of the case, and the signal ground are all permanently and inseparably tied together. In well-designed equipment, however, they are separated.
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He is wrong — take a look inside the power supply unit: the midpoint of the filter is indeed hard-wired to the case (usually a wire with a lug under a screw, sometimes soldered or welded). And around the mounting holes through which motherboards are screwed down, wide ring-shaped pads are deliberately made so that the screw makes better contact with them. If you look closely, you will find that these pads are in fact the board's signal ground. Also notice the spring-loaded tabs that contact the shells of the rear-panel connectors (USB, COM, LPT, Ethernet, ...) on the motherboard — on the board itself, those shells sit on the signal ground (soldered). | 
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2. Why I am not satisfied with the trivial solution — "don't ground it."
2.1. I don't like the fact that between two metal objects sitting a metre apart — the computer case and the heating radiator — there is constantly 110 V with a quite perceptible current. There is nowhere to move them apart. Children may like this even less. If the case is plastic and there are absolutely no exposed metal parts, this point is moot. The paint on the corners of the case stops being insulation after about six months — check for yourself.
If the radiator is far away and the floor is guaranteed to always be dry... fewer problems.
2.2. I don't like the fact that these same volts are permanently sitting on the galvanic isolation of the modem. And it is well known that impulsive (microsecond) common-mode and differential-mode voltage spikes on the mains (the lift stopped...) can reach a couple of kilovolts. Such impulses from the telephone line side are considerably less likely — the power levels there are different ;-). Furthermore, no input transformer is perfectly symmetrical or free of capacitive leakage, and at such amplitudes (110 V) common-mode interference penetrating the modem input may be far from negligible. Simply put, an extra source of noise on the line. No modem — this point is moot.
2.3. The mains filters in the power supply unit, and their midpoints, were not put there for decoration. Proper and thorough grounding (along with other measures) once helped me personally eliminate the problem of total computer crashes ("Elektronika-60" ;-) every day at 18:00 when some distant unknown monster switched over. The Polytech...
3. What must never be done.
3.1. Connecting the case neutral to the "neutral" wire of an ordinary two-pin outlet. The most important point is that both wires are the same cross-section, run through identical fuses in the apartment, and are equally poorly connected to the panel terminals. If the "fuse" on the neutral wire blows, if the wire itself burns through, or if the terminal loosens (have you ever seen an arc in the stairwell panel? A small one, like a cigarette glow) — the computer case will then carry 220 V with no current limitation. Through the light bulbs. Dark, and the radiator is right there...
Feel it? Furthermore, neutral and phase can be (and are legally entitled to be) swapped any number of times by the aforementioned electrician.
3.2. Connecting to the lightning rod busbar outside. Of course, my building is not the tallest in the neighbourhood, but... it's not the shortest either. And there you'll definitely get real big kilovolts on a strike.
3.3. Connecting to the gas pipe. No comment. Also prohibited by the Regulations.
3.4. Using two different methods simultaneously. Like neutral bonding to a three-pin outlet — and the radiator as well.
Don't take on yourself the task of equalizing potentials in those loops... let that happen somewhere else. Or not happen at all. Because right here, with welding, you can easily end up with the grounding conductors glowing red-hot. While you're out...
Everything else, it seems, is at least not lethal.
4. What is permitted by the Regulations?
4.1. Grounding to a dedicated earth. Like driving a metal rod a metre into the ground — and having a whole platoon urinate on the spot... for conductivity. But that's for professionals.
4.2. Neutral bonding to the fourth wire of a three-phase cable. But where would I find one? Only in the basement and in the lift room.
4.3. Grounding to the metal structural elements of a reinforced-concrete building. The floor beams of an old brick building do not qualify as such, IMHO.
4.4. In some versions of the Regulations, grounding to the cold water pipe was permitted. In others — not. No comment.
5. What does common sense suggest?
5.1. The heating circuit is in practice a very decent ground. We don't go overboard with thermal insulation (especially inside buildings), and as a rule there are welded or threaded connections to the water supply. The boiler is definitely grounded, the path to it is welded, and the cross-section is adequate. Though there are no guarantees... And since the circuit is a closed water loop, you would need to break it in two places to disrupt contact.
5.2. In my experience, pipe welding in the basement goes completely unnoticed by both the computer and the person. The voltage there is low, after all — 70 V open-circuit, no more. Otherwise there would be no welders left alive... And microwave interference from down there doesn't travel well over tens of metres either... otherwise no automotive electronics would have survived. But against that particular hazard, the presence or absence of direct contact makes no difference — arc welding a few metres away really can fry an unshielded computer just through the air. Especially one that is running. A well-known fact.
That is, unless you violate point 3.4... then yes.
5.3. The primary object of protection is me (the beloved), not the computer (merely the respected). So if I've properly bonded the machine to the three-phase wiring by the book, and enemies (or the reverse) have put 220 V on the nearest radiator, and I touch it — I really won't care that the regulations were followed.
Moral. (proposed ;-)
Water pipe, electric cooker, or radiator — determined by the specific physical layout. What you can touch simultaneously. But only one!
Everything else is either barely realistic or dangerous.
And if the case is entirely plastic, or all metal parts are far away and the floor is good, and there is no modem / you don't care much about it — you can ignore the whole issue. Bigger things to worry about.
Personally, I use the radiator. Reasons — layout + modem. In my department at the Polytech, in the old rooms with single-phase wiring, we've been doing the same since 1984 (Elektronika-60) right up to the present day, after long and repeated discussions with people who know their electrical engineering ;-). Though everyone understands it would be better to replace this with a welded 50 mm² flat bar running from the grounding loop three floors below... the kind that doesn't burn through. Maybe they'll do it someday.
References
- Rules for Electrical Installations. — Moscow: NTs ENAS Publishing House, 2002.
- Mironenko N.N. The Neutral Conductor Connection Requires Special Attention. Novosti Elektrotekhniki, No. 5 (17), 2002.