A large part of dealing with EMC lies in understanding and controlling interference current flows, of which ground current is a major component. Capacitively- or inductively-coupled interference will create current flow in the ground network, and will also induce currents in other conductive structures even if there is no intended circuit current in them. Because the impedance of any conducting structure is not zero, these currents will create voltage differentials between different parts of the structure. For this reason the geometry of these structures relative to the wanted signal circuits is crucial.

For EMC we are concerned only with controlling interference voltages and currents to be low compared to the desired signal. Other purposes of the ground - providing safety paths and a reference for interconnected circuits - are operationally necessary, but do not guarantee low interference, and may on occasion work against it. We then have three ways of describing a ground system:

(A) An equipotential point or plane used as a system reference

(B) A low impedance path for currents to return to their source

(C) A low transfer impedance path to prevent common mode currents converting to differential mode

(A) is the classical definition of a ground network, but this is misleading in the presence of ground current flow. Especially as the frequency increases, finite impedance of conductors causes voltage differentials across supposedly "equipotential" regions. Even where signal currents are negligible, induced ground currents due to environmental magnetic or electric fields will cause shifts in ground potential.

Definition (A) takes no account of topology or geometry of the ground network; by contrast these factors are critical in (B) and even more so in (C). Ground currents always circulate as part of a loop. The task is to design the loop in such a way that induced voltages remain low enough at critical places, by ensuring that the ground circuit is as compact and as local as possible.

The local ground return

The actual path taken by return currents is not always obvious. Consider a circuit consisting of a source and load, interconnected by a cable, each side of which is grounded. The return current in this case could flow either via the cable (I_ret1) or via the ground (I_ret2).

In practice the return currents will divide according to the relative impedances of the two paths. It might be thought that the ground path has the lower impedance, but in fact the cable path’s impedance is dominated by the coupling due to mutual inductance with the signal path, at frequencies where inductance is more important than resistance. Close coupling means a lower effective impedance, because the magnetic flux due to I_sig tends to cancel that due to I_ret1. The significance of this is that signal currents can be separated from the ground path by proper cable construction even when both ends are earthed; thus, interference pollution from one path to the other (in either direction) is minimised.

This effect is most pronounced with coaxial cable structures where the mutual coupling is almost unity, but it is readily observed even with ordinary cables. It dominates at frequencies where circuit inductive impedance is greater than resistance, that is, above a break frequency where

          2F · L  >  R

For typical small cable pairs where the mutual inductance is around 0.2µH/m and the resistance is around 0.1/m, this break frequency is about 80kHz.

As a direct consequence of this effect, if we are concerned about controlling the return current flow, then the signal circuit must be physically located as close as possible to its ground return.

Ground loops

A ground loop is formed when there are two or more routes for ground current to flow between two circuits - as shown in the example above, for instance. Typically a ground loop will appear when two items of equipment are both supplied with a protective earth and there is also a signal ground return between them; in this case the ground loop is large and ill-defined, depending on cable layout. But strictly speaking, a ground loop can be formed by any such connection, even within a circuit, and is inherent in all multipoint ground schemes.

The danger of a ground loop is that any external magnetic field will induce a circulating current in the loop which in turn will result in a noise voltage V_N in the signal return conductor, that is injected in series with the signal. The amplitude of V_N will be proportional to the frequency of the interference and the area of the loop. Cures for ground loop interference include breaking the loop by signal isolation or by using earth line chokes or resistors (in which case V_N appears across the high impedance), using balanced signal circuits to improve common mode rejection, and where possible control of loop area and orientation.

But ground loops are not by themselves inherently bad. If the loop area is controlled and predictable, the magnetic induction may be entirely manageable, even negligible, and the structure may well benefit from several ground connections. This is typically the case when a PCB ground plane is connected to a metal chassis at multiple points.

Bonding

Electrical bonding refers to the processes by which parts of an assembly, equipment, or subsystem are electrically connected by their joints or any low impedance medium. The purpose is to make the structures homogeneous with respect to the flow of EMI currents, and hence implement a controlled grounding system.

In many applications permanent bonds are not practical due to flexibility and accessibility requirements. Normally manufactured from copper or aluminium, metals of excellent conductivity, bond straps may be designed into a product to enable all the various ground returns to be connected along it. This saves bringing many thick cables back to one central bonding point.

Bond straps are made of flexible, good conductivity, copper bars, braids or thick flexible cables. Their dimensions are critical for EMC use. They must not be longer than L =  /5, where  is the wavelength of the highest frequency to be controlled, which can only be assessed on a product-by-product basis.

Once this length is known, then the width of the bonding strap should be L/5 to L/10 minimum. The thickness is not too important: if the bond is used for safety purposes, it will depend upon the safety ground current requirements. For higher frequency use, the skin depth of the material δ is more important. The objective of the bond-strap is to provide a low-inductance grounding path, so it's not good practice to use more than 2 bond-straps in series to provide such a path.

Bonding maintenance

Throughout the lifetime of the equipment, system, or facility, the bonds must be inspected, tested and maintained. A high impedance bond can be identified from its contact resistance, although the DC resistance of a joint doesn't necessarily indicate its impedance at a higher frequency. A standard of 2m (milliohms) across each bond and 25m between any two points in the earth system is accepted in military situations. For installations, the bonding should be documented as follows:

A block schematic (also called a ground map) for each sub-system in an installation which identifies equipment used and its interconnections.

A statement on the bonding policy, giving the rationale for the particular bond arrangements adopted.

The bonding implementation statement should describe features of the particular implementations used. This includes identifying bonds that require insulation, and specifying regular maintenance checks on bonding impedance.

The Parallel Earth Conductor

The concept of "transfer impedance" can be used to describe the mode conversion capability of any structure, for instance a conduit, a plate or a screened enclosure. Transfer impedance Z_T is the ratio of voltage induced within a wanted circuit by unwanted current flowing in the grounding structure. It is mentioned elsewhere with respect to cable screens. Note that, by this definition, Z_T has nothing to do with any operational circuit impedances.

For EMC purposes, grounding provides a set of interconnected current paths, designed to have a low Z_T, in order to minimize interfering voltages at sensitive interfaces which may or may not be ground-referred. For each interface, we aim to reduce the transfer impedance of an appropriate part of the interconnected grounding paths. The consequence is a grounding structure, whose three-dimensional shape is designed for low Z_T. Such a structure is typically provided by conduit or a ground plate within or along which a cable loom runs, but even a parallel wire can have some effect; whatever the structure, it is known as a Parallel Earth Conductor (PEC) – more correctly, a Parallel Bonding Conductor, but the term PEC is in widespread use.

The important quality of such a structure is that when a disturbance current flows in it, only a small interference voltage is generated differentially in the signal circuit. Z_T is the ratio of these two quantities and is determined by mutual capacitance and inductance between the circuit and the structure; for external interference currents flowing in the ground the dominant contributor is usually mutual inductance between common- and differential-mode circuits, since voltages along ground structures are (by design, at least) low. Z_T is minimized by a solid enclosing tube and is highest if the structure is a single parallel wire. Practical compromise structures are the conduit or the flat plate. The closer the differential mode circuit can be positioned against such a grounding structure, the less the Z_T, provided that the structure is unbroken in the direction of current flow.

The PEC is only as effective as its own bonding allows. That is, it must be bonded at each end to the platform structure or the enclosure that shields the cables it is carrying. The quality of this bond should be matched to the type of PEC. So:

a single wire PEC is simply terminated in a lug to the appropriate metalwork

a flat plate or a conduit PEC is terminated by one or more short, wide bond straps, or by a metal plate, to the appropriate metalwork; the metal plate can alternatively be implemented by bending out the base or side of the plate or conduit

an enclosing tube (shielding conduit) PEC is terminated by a 360° bond, typically provided by a metal gland, to the appropriate metalwork

As always, the bonding has to be metal-to-metal, with all paint or other insulating material removed from the mating surfaces, and the joint protected from subsequent corrosion.

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