Grounding/earthing: Concepts and practice

Confusion about the ground/earth reference has bedeviled EMC ever since the subject came into being. It has especial relevance to systems, where the concepts of functional ground/earth, safety ground/earth, RF ground reference and the interconnection of metallic parts are all interrelated and have become the objects of entrenched and not necessarily correct practices and opinions over the years. A major part of the problem has been that different practices are appropriate for different purposes; even if the purpose is purely EMC, frequency range is an important parameter, and its relevance is only imperfectly understood. This section will discuss the principles behind the purpose of ground/earthing and will then look at how these are best implemented.

Ground/earth: concept / practice

The purposes of the ground/earth connection

First of all, it's necessary to decide what we mean by the term " ground/earth". Difficulties immediately arise through the alternative American usage of the word "ground" to mean essentially the same thing; but because the words are often used interchangeably and in different contexts, confusion easily results. It has been argued cogently that both terms should be swept aside in favor of the more accurate "reference", leaving “ ground” or “earth” purely for application to the safety function. In this guide we shall attempt to stick with “ ground” or “earth”, only because it’s the more familiar, and to indicate either explicitly or by context what function is meant.

Safety ground/earth

The purpose of the safety ground/earth is to guarantee personnel safety under fault conditions.

The IEEE Wiring Regulations define " ground/earthing" as Connection of the exposed conductive parts of an installation to the main ground/earthing terminal of that installation.

Ground/earthing ensures the provision of a low impedance path in which current may flow under fault conditions. Exposed conductive parts are those conductive parts of equipment which may be touched and which may become live in the case of a fault. The ground/earthing connection prevents such live parts from reaching a hazardous voltage.

Equipotential bonding, in this context, is a means of electrical connection intended to maintain various exposed and extraneous conductive parts at substantially the same potential under both operational and fault conditions. The protective conductor (typically color coded green-and-yellow) provides this means and also connects the conductive parts to the installation's main ground/earthing terminal. The prospective touch voltage within the installation is then the product of the impedance of the protective conductor and the ground/earth fault current.

Equipotential bonding therefore creates an equipotential zone within which exposed and extraneous conductive parts are maintained at "substantially" the same potential. It’s worth noting that although the voltages within such a zone may be safe, they are not necessarily, and not even usually, zero. Continuous currents from various sources, including equipment ground/earth leakage, are likely to be flowing, even in a "healthy" circuit. Allowable ground/earth leakage levels from individual items of equipment are covered. An "equipotential" zone may protect people but may not protect equipment or wiring. Moreover, within a large installation there may be more than one "equipotential" zone for safety purposes and the voltages existing between them may well be large and undefined; special precautions need to be taken for signal wiring (such as local area networks) that may cross such zones, to prevent them importing potentially hazardous voltages.

Protection against electric shock is typically provided by ground/earthing in conjunction with automatic disconnection of the supply. For this purpose, it’s vital to co-ordinate selection of the protective device ( e.g., fuse, circuit breaker or RCD) with the installation's ground/earth fault impedance, to ensure that disconnection occurs sufficiently rapidly to prevent the touch voltage from rising high enough to cause a shock. The sizing and hence resistance of the ground/earth protective conductor will therefore be determined largely by the prospective fault current available from the rest of the system.

In the case of protective multiple ground/earthing (PME), the protective conductors may take substantial continuous currents, even when the supply is off, as a result of circulating currents in the bonding network, and therefore may need over-rating. Since the concern is currents at low frequencies, it’s resistance rather than inductance which determines the conductor impedance; this is not the case for high-frequency ground/earths, as we see later.

Functional ground/earth

In order for an electrical circuit to interface correctly with other equipment, there must be a means both of relating voltages in one equipment to those in another, and of preventing adjacent but galvanically separate circuits from floating.

This is the purpose of the functional ground/earth and it must be distinguished from the safety protective ground/earth. Ground/earth conductors which are used for functional purposes only need have no requirements for sizing according to safety but should be colored cream to identify them. Even so, because of the threat of circulating currents and potential differences between ground/earthing zones, there may be other practical constraints on the widespread use of functional ground/earthing on large systems.

Signal circuits of equipment should normally be specified for a maximum common mode voltage, which will be the voltage that appears between different parts of a functionally- ground/earthed system. If this voltage is likely to be exceeded, implementation of a Common Bonding Network (CBN) in the system is recommended. If this is impractical or inadequate, isolated circuit interfaces are the normal solution.

Lightning protection ground/earth

In building installations, there is a further important safety-related ground/earthing function, and this is to provide a return connection for currents induced by a lightning strike. In many respects this is the only correct use of the term " ground/earth", since this function is normally provided by ensuring a low-impedance connection throughout the building fabric to the literal ground/earth on which the building sits. Since lightning potentials are built up between the cloud structure of a thunderstorm in the atmosphere and the surface of the Ground/earth, connection to ground/earth is the correct way to complete the circuit in the shortest manner.

Several standards for lightning protection have been published by the various standards bodies (e.g.,) and the reader is referred to these for detailed advice. ---the main principles.

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Standards for lightning protection:

Protection of structure | Protection of contents

Risk assessment IEC: IEC 61024-1:1990 IEC 61312-1:1995 IEC 6i662:1995 IEC 61024-1-1 guide A CENELEC ENV 61024-1:1995 BSI BS 6651:1992 BS 6651:1992 App C BS 6651:1992

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EMC ground/earth

The EMC ground/earth has the sole purpose of ensuring that interfering voltages are low enough compared to the desired signal that incorrect operation or excessive emission does not occur. It has no explicit safety or operational function. Because of this, and because of the wide frequency range over which it must work, ground/earthing for EMC usually takes advantage of distributed structural components that are part of the whole system--typically, chassis members, enclosure panels and so on. The value of an EMC ground/earth is directly related to its physical geometry. This means that design and implementation of an EMC ground/earthing system is not restricted to the electrical engineering discipline alone--it must also involve constructional aspects, that is, the mechanical designers and installers.

Definitions of the EMC ground/earth

For EMC purposes, we can distinguish three almost hierarchical definitions of the ground/earth:

++ An equipotential area or plane used as a system reference;

++ A low impedance path for currents to return to their source;

++ A low transfer impedance path to prevent common mode currents converting to differential mode.

The equipotential area

A highly desirable feature of an ground/earth is that there should be no potential difference developed between two points that are geographically separate. Thus connections to each point can be regarded as connections to the same potential. In the context of EMC, no noise voltages exist between these points and therefore no interference is injected into a system that is " ground/earthed" only to these two points.

Such a perfect equipotential structure is in fact impossible, but it can be approximated by a highly conducting plane. --- section that wires have inductive impedance which renders them anything but equipotential, whereas a plane does not exhibit inductance, only resistance. The backplate of a systems equipment cabinet is an example of such a plane, and the meshed ground/earthing structure of a computer room is an approximation to it. (Compare this description with the comments.)

The low impedance return path

--- Ground/earth as a return current path

A perfect equipotential structure is impossible as soon as currents are allowed to flow in it. Any practical structure has a non-zero impedance between any two points, and when a current passes through such an impedance a voltage must be developed across it. In practice, the EMC engineer is always trying to minimize the interference voltages developed in the ground/earth, and therefore the prime requirement for a good ground/earth design is that it should offer the minimum impedance to whatever interference currents are flowing. When a circuit is designed to use the " ground/earth" as its functional return path -- by deliberately connecting the returns of the source and load to the ground/earth structure rather than to each other- then the path between these points must have the minimum impedance. ---where Ire t flows between points A and B on the ground/earth structure. Clearly, if the circuit is to be at all controlled, the current path between A and B must be as carefully designed as that between C and D which carries the signal current, otherwise an unknown potential difference between A and B is injected directly into the circuit. Usually, the ground/earthing structure is outside the control of the circuit or equipment designer, if only because external interference currents are easily induced in it, and therefore designing the circuit to use the ground/earth as a return path is strongly discouraged.

If the ground/earth is under the designer' s control, then the method (a) can be successful, provided that the ground/earthing structure does offer a low impedance path to the return current. But the more usual technique is to be preferred, where a separate conductor is provided for the return current, which can be paired with the signal conductor thereby minimizing the total circuit inductance. If the circuit must be "referenced" to ground/earth this can be done at one point, thereby preventing signal currents polluting the ground/earth and vice versa. (The question of which point to connect to ground/earth may not be trivial.)

The low transfer impedance path

The ground/earth structure still has a function with respect to the circuit that it’s "host" to, and this is arguably its most important function. It should be so designed as to provide a low transfer impedance (Z T) to the circuit. Transfer impedance is defined as the voltage developed within the victim circuit divided by (interfering) currents flowing within the source. Low Z T means that interference currents flowing within the ground/earth - which are, for all practical purposes, unavoidable - transfer minimum interference voltages to the circuit. and vice versa, currents within the circuit don’t create significant interference voltage differentials within the ground/earth structure. Since the components of the ground/earth are passive, the coupling is reciprocal, and a structure which has low transfer impedance in one direction will also be good in the other.

The essence of transfer impedance is geometry. It’s mediated by near field (inductive and capacitive) coupling between the structure and the circuit and therefore whatever affects the coupling parameters, particularly separation, orientation and coupled area, will also affect the transfer impedance. Actually predicting the value of transfer impedance in a given situation is rarely feasible- the coaxial screened cable being the main exception to this.

It’s though, possible to rank particular structures in order of reducing transfer impedance. A circuit on its own (a) cannot be described in terms of transfer impedance; there is no structure to transfer from, so all induced interference current flows within the circuit itself. The simplest structure to visualize is an ground/earth wire in parallel with a circuit (b). This can be improved upon by mounting the circuit against a flat plate (c); the wider the plate, and the closer the proximity of the circuit to it, the lower is Z T. This will be recognized as an alternative description of a printed circuit board on a metal chassis, but the same principle applies to a cable laid against a cabinet wall, for example. The transfer impedance is dominated by coupling from the edges of the plate; the further away these are from the circuit the better, but of course this is limited by available space.

Folding the edges of the plate up to surround the sides of the circuit (d) allows the edge effects to be reduced. This describes a conduit or a U-shaped chassis. Covering the conduit or chassis with a lid reduces or (if the lid makes continuous contact) eliminates the edge effects. The minimum transfer impedance is reached when the circuit is made coaxial with a cylindrical outer structure (e). Although this is largely impractical for equipment, it’s straightforward for cables; the lowest transfer impedance for coaxial screened cable is achieved with a solid outer sheath, and braided cables compromise this to a certain degree, but the geometry of the coaxial cable is optimum.

All these drawings show the ground/earthing structure as being continuous while it’s surrounding or adjacent to the circuit. This is the crucial requirement for such a structure; if the ground/earth currents are disrupted by discontinuities across the direction of flow -- e.g., slots in a metal panel, or gaps in a cable conduit- the transfer impedance rises dramatically.

---The transfer impedance of structures

Ground/earthing techniques

As far as electrical signals and power (and their associated electromagnetic disturbances) generated and consumed within a building are concerned, the connection of the building to the local ground/earth mass ( ground/earth, soil) is immaterial. But a coherent ground/earth- bonded structure within a building is still required for safety, signal integrity, equipment reliability, and EMC within that building.

As the frequencies used by modern electrical/electronic apparatus increase, the achievement of a good quality ground/earth for signal integrity and EMC reasons becomes more difficult. and as the integrated circuits within electronics get ever-smaller and hence more vulnerable to damage from lower levels of transients, the achievement of a good quality ground/earth which will protect modern electronics from damage also gets more difficult.

When electrical signals and power entering or leaving a building, and lightning, are considered, it becomes necessary to connect the building' s ground/earth-bonding system to the local ground/earth mass with electrodes which penetrate the soil, usually (at least) around the perimeter of the building.

There will also be a lightning protection structure (LPS) required for the external surface of the building, and this will be connected to the ground/earth electrodes. The details of the construction of the LPS are considered further in a later section.

Ground/earthing is thus required for a large number of reasons, and a good ground/earth-bonding system will deal with them all.

Independent ground/earthing

Where the approach for historical reasons employed independent ground/earthing it’s now understood to create a safety hazard. When heavy ground/earth currents flow (usually due to nearby lightning strikes, or ground/earth-faults in HV or MV distribution) the independent soil electrodes can take on very different voltages -- perhaps exceeding 10,000 volts in the transient case.

This can cause electrical shock, as well as showering arcs, toxic fumes and smoke, equipment damage, personnel burns, and serious fire. This type of ground/earthing scheme is also very poor for EMC purposes.

---Independent and star ground/earth-bonding

Single-point or "star" ground/earthing

Still found these days in older buildings is the "star ground/earth" system. This eliminates the gross safety hazards of the independent ground/earthing system, but was developed in the days when electronics was in its infancy.

When carefully installed and the topology maintained, star ground/earthing systems are capable of providing safety at power frequencies. But for general use in modern installations of any size they are no longer considered adequate to control interference at the frequencies used by today's instrumentation, control, computing, and telecomm' s technologies. The traditional concepts of providing separate "clean" ground/earthing systems for communication equipment has largely been dispelled with the code of requirements of providing single point ground/earthing to buildings, and the increasing use of ground/earth as a reference potential for "single-ended" signaling in information technology and telecommunications apparatus. Although ground/earth-referenced signaling is not recommended, it seems that equipment manufacturers are increasingly using this technique to reduce the cost of their products.

Individual cables, wires, and structural steelwork of any practical length must all be considered to be effective antennae at the frequencies in common use, which severely limits the possibility that a star ground/earthing system can provide any EMC benefits whatsoever. Without a great deal of attention to detail and rigorous checking and maintenance, star ground/earthing leads to large amounts of interference with signal conductors between equipment. It’s very easy to compromise star ground/earthing systems accidentally. For example, a data cable might be connected between two items of apparatus that are on different limbs of the star, during a modification or refurbishment many years after the initial installation. Such cables may be installed on a temporary or permanent basis by personnel with no skills in ground/earthing or lightning protection, indeed they may not even realize that such issues even exist. During a surge event, however, such unapproved modifications can cause damage to the apparatus at either end, or even fires or electric shock, due to the impression of large voltages and currents across the ends of the new data cable.

A similarly hazardous situation can arise where an item of equipment is moved to a new location (maybe only a few feet away), or when walls or boundaries are changed (e.g. partitions removed to make several small offices into one large one). Touch or step voltages which had been acceptable could now be unacceptable, resulting in serious hazards to personnel.

According to voice telephony system requires a "star" functional ground/earth (FE) from the customer's main ground/earthing terminal (CMET), however a guaranteed isolation of the functional ground/earth of a generic cabling system from the protective ground/earth would tend to be unworkable and could in some cases be dangerous. Time break recall, which does not need an ground/earth reference, is used in the U.S. (ANSI/TIA/EIA-607 refers) and is becoming a standard function on most PABXs and therefore the need for dedicated FE networks in commercial building applications will gradually fall. Furthermore, recall signaling in modern digital telephone systems does not require functional ground/earthing.

Three-dimensional meshed equipotential ground/earth-bonding

---Common bonded network

The ground/earth-bonding method required for safety in buildings is now the Common-Bonded Network, or CBN.

Today's complex installations require their CBN to be enhanced to create a three- dimensionally meshed equipotential system, often referred to as a MESH-BN (for mesh Bonding Network). This bonds every piece of structural and non-structural metalwork together, including concrete reinforcement bars, girders, cable trays, ducts, deck plates, gratings, frameworks, raised-flooring stringers, conduits, lift (elevator) structures, window- and door-frames, and the metallic carriers of services such as gas, water, smoke, air, etc., to make a very highly interconnected system, which is then connected to the lightning protection system (LPS) at ground level. This highly-meshed three-dimensional system is then very highly interconnected to the screens and armoring of all electrical cables, and the frames or chassis of every piece of electronic equipment. Where existing metalwork or conductors don’t already exist, large cross- sectional-area conductors are added to complete the mesh either vertically or horizontally so that nowhere is the mesh size greater than about 3 or 4 meters. The main ground/earthing terminal (MET) for the incoming power supply to the building requires a number of bonds to the MESH-BN. The MESH-BN is used for safety, functional, and EMC ground/earthing, all at the same time - an integrated ground/earth-bonding system. By creating such a MESH-BN it’s possible to achieve the various aims of safety, signal integrity, equipment reliability, and EMC, which are often seen as being in conflict, at reasonable cost in a reasonable time, without compromises, and without any requirements for burdensome on-going management or restrictions on future modifications.

Nothing in this section should be taken to mean that safety considerations may be neglected. The requirements of the state-of-the-art for lightning protection and electrical safety for both the installation and the apparatus in it should always be followed. Due to the meshing of the ground/earth structure some of the necessary calculations will be made more difficult, and some may well be made easier, nevertheless they should all be performed and the necessary actions taken.

Screened cabling systems require full equipotential bonding systems with defined current carrying capacity. CBNs implemented as MESH-BNs are ideal, making screened cable systems straightforward to install with no additional cable management problems, anything else is less than perfect. Some IT and telecommunication system blocks will require an even higher quality ground/earth reference than can be provided by the MESH-BN described above, this is sometimes called a System Reference Potential Plane (or SRPP) and usually requires a "bonding mat". The bonding mat is ideally a seamless metal plate under the entire system block of interconnected equipment and its internal cables, but it’s more usual in commercial or industrial buildings to use a grid of copper conductors, or the metal framework of the raised flooring, or a hybrid of both. SRPPs and bonding mats are described in more detail.

Although each of the elements of the MESH-BN will have their own resonances and behave like antennas and /or poor ground/earth-bonds at those frequencies, its highly interconnected nature will ensure that there are usually alternative current paths that are not resonating and so provide a high degree of equipotentiality over a wide frequency range. One consequence of this is that very regular bonding structures should be avoided, since all their elements will exhibit similar resonances at the same frequencies. This is usually not a problem for most buildings. Although the concrete reinforcing bars ("re- bars") and the structural steelwork in a building can have a very regular structure, the use of three-dimensional ground/earth bonding between all metalwork at every possible point, and the bonding of cable screens and armor of every type at both ends and even along their length will randomize the mesh size and shape. This will ensure that there are always many possible ground/earth paths for a current, all resonating at different frequencies.

As well as providing an equipotential ground/earth, a MESH-BN with 3 to 4 meter spacing provides a degree of shielding, especially against the intense low frequency fields (<1MHz) created by nearby lightning strokes. Where the 3-D ground/earth mesh size has to be larger than 3 or 4 meters (e.g. shipping bays, display windows) electronic equipment should not be fitted unless it’s especially "hardened" to withstand higher levels of interference, particularly lightning surge. The best electrical bonds are direct metal-to-metal, preferably seam-welded, but other types of bonding may be used. Practical techniques for three-dimensional ground/earth bonding are discussed in more detail.

For safety from electrical hazards, all ground/earthing and bonding conductors need to provide sufficiently high current conducting capability and low impedance according to the relevant safety standards. This is to avoid electric shock, fire, or damage to the equipment under normal or faulty operating conditions within an equipment or within the power distribution network, or due to the impact of induced voltage and current, e.g. by lightning. The return conductor for 48Vdc telecommunications power, and similar dc supplies feeding more than one item of equipment, are recommended to be made part of the MESH-BN, by connecting to it at multiple locations. This improves fault-clearance time and personnel safety as well as reducing the possibilities for interference.

For the control of voltage differences in the ground/earth structure at higher frequencies for the same level of power, or at higher powers for the same frequency, the mesh size of the MESH-BN needs to be smaller. With the different types of apparatus having been segregated according to whether they are "noisy" or "sensitive", the building should then be partitioned into areas with different ground/earth mesh sizes, depending on the ground/earthing needs of each.

Note that each segregated apparatus area with its individual meshing or bonding is surrounded by a complete conductor. These are known as bonding ring conductors (BRCs) and are important for protecting the apparatus in an area against lightning transients, ground/earth faults, and other low-frequency surges originating outside of their area.

What do we mean by "equipotential"?

Electricians tend to refer to any ground/earth-bonding structure which achieves safe potential differences (50Vac or less) during normal use as "equipotential". But such ground/earth-bonding networks can easily produce 160V AC for several seconds during an ground/earth fault and this may destroy inter-equipment I/O electronics. During a transient surge such as caused by a lightning stroke such a weak ground/earth-bonding system could expose electronic equipment signal and power cables to voltages of several kV. A number of information technology (IT) and telecommunication standards have recommended for years (since the 8th edition of IBM' s Cable Planning and Installation Guide) that the maximum potential difference for the meshed ground/earth- bonding within a system block must not exceed 1V AC RMS. In addition to maximum voltages of 1 V, EN 50173, ISO 11801, and ANSI/TIA/EIA 568A all recommend that the ground/earthing network that cable screens are connected to has a low impedance at the frequencies of concern. The rationale behind the choice of 1V maximum for equi-potentiality is governed as much by the need to control electro-galvanic corrosion in the ground/earth-bonding system of DC powered telecommunication systems as it’s for signal integrity. A bonding network that corrodes creates unreliable operation.

It seems that some manufacturers of IT and telecommunication equipment are increasingly using single-ended signaling, using the ground/earth as the signal return. This is considered bad practice by and , but it does make for low selling prices. Their instruction manuals apparently state that their equipment must be connected to a "clean ground/earth", but it appears that "clean" in this case means equipotential to no more than 15 milli-Volts over the frequency range of the signals employed (this might not be so clearly stated in the manuals). The installer (and user) is thus faced with the difficulty and expense of achieving a remarkably high quality of SRPP, usually using a bonding mat, and one can only hope that they fully understand this onerous requirement before beginning to install the equipment, and preferably before selecting it over possibly more expensive equipment which is more forgiving of installation type.

The bogey of ground loops

A common objection to the meshed ground/earthing system just described is that it creates "ground loops" (equally known as " ground/earth loops", but the word ground is used here to distinguish between incorrect and correct practice). Historically, currents flowing in ground loops, and their associated driving potential differences across different parts of the ground/earth network, have been found to be particularly serious contributors to interference problems, and therefore a practice has developed of trying to eliminate all such loops. This practice, although often superficially successful, is unfortunately misguided.

In a situation where high ground/earth potential differences exist, closing a loop between two such ground/earth points will allow a high current to flow in the structure. If the conductors in that loop include a segment which either forms part of, or is closely coupled to, a signal or low-level power cable, then substantial interference can be induced in the circuits of that cable, if the loop is opened, the current no longer flows, and the interference disappears- although the high potential differences remain, ready to create problems again when another loop is closed somewhere else. This is the principle which is formalized in the star or single-point ground/earth regime: remove all ground loops and live with the resultant high voltages between different parts of the ground/earthing system.

Such an approach is fairly easy to implement and quite successful in simple low- frequency systems, but it represents a retreat from best practice. Now that interference frequencies are typically measured in MHz rather than Hz, it’s untenable. This is because the star ground/earthing conductors present a high impedance to these frequencies and therefore de-couple a system from ground/earth, rather than couple to it. Also unfortunately, larger star systems tend to degenerate into accidentally ground-looped systems as time passes and systems and buildings are modified and added to, requiring a heavy management and control burden if their efficacy is to be maintained and safety and equipment reliability is to be ensured. In these circumstances the only reliable ground/earthing system is a mesh. The mesh does indeed provide a multiplicity of ground loops, but they are small and controlled: voltage differences between parts of the structure are minimized, resulting currents are low and the interference consequences, if any, are negligible. The safety, reliability, EMC, and ease of use advantages of properly implemented MESH-BN systems far outweigh their possible disadvantages.

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