Ground/earth connection: impedance

Impedance of wires

Ground/earthing wires don’t offer a low impedance end-to-end at radio frequencies. Even at quite moderate frequencies, their impedance is significant. By way of illustration the measured attenuation in a 50 -ohm system of a 60cm length of 24/0.2mm wire that is well bonded at one end to a reference plane. If the wire were a perfect short circuit, attenuation would be infinite; its actual impedance ensures finite attenuation. This measurement can be used to illustrate a number of factors.

---Attenuation due to a shorted 60cm length of wire

DC- low frequency impedance

At DC, the impedance of this length of wire is purely resistive. Standard wire tables reveal that 60cm of 24/0.2mm should have a resistance of around 15.6m -ohm. This will give an attenuation of over 60dB in the 50 -ohm system.

Low- medium frequency impedance

Any length of wire of length I and diameter d in free space has self-inductance. This can be determined from equation: k = 0.002. l. (In (41/d)- 0.75) gH for l, din cm

The inductive impedance part exceeds the resistive part at no more than a few kHz. For as the inductance dominates. The low frequency plot also section that when the wire is laid next to the reference plane, the impedance reduces- although it remains inductive, as can be seen from the slope of the plot. In this case equation does not correctly predict the value of the inductance, which is reduced by the proximity of the plane.

High frequency impedance

As the frequency rises to a point at which the wire length becomes an appreciable fraction of a wavelength - which is another way of saying that it becomes resonant- then inductance alone is not sufficient to describe the impedance characteristic. In the high frequency plot, we can see that above about 50MHz the attenuation offered by the length of wire is negligible. I.e., its impedance has exceeded 50~ by a large margin. But at some higher frequencies there are some evident notches in the attenuation. When the wire is held 3cm away from the reference plane, the first of these occurs at 240MHz. At this frequency the impedance seen looking into the end of the wire has dropped to somewhat less than 5~. This is an example of the transmission line resonance of the wire; it’s in fact the first half-wave resonance M2, that is, the end of the wire is exactly half a wavelength away from the ground/earthing point (theory would predict 250MHz; the discrepancy is due to slight imperfections in the layout). Further resonances appear at multiples of the first, the next one visible on the plot being 2Z/2 or the full wave resonance at 480MHz.

When the wire is placed next to the plane, the first resonant notch drops to 170MHz; the second one appears at 380MHz. The change in geometry has affected the resonant frequency but not the basic property of the line. Proximity to the plane and the presence the 60cm of 24/0.2 (which has a diameter of about 1.1mm) the DC resistance of 0.0156~ is equaled at 3kHz. At 2MHz, the impedance of 0.832gH is 10.5 -ohm (Z = 2~.f.L), which will give an attenuation in the 50 -ohm system of just 17dB - as we see in the plot. The impedance of any length of wire rises linearly with frequency for as long of the wire's pvc sheath which now fills the gap has increased the capacitance of the wire, hence the lower frequency; this is another way of saying that the transmission line's phase constant, and hence its electrical length, has been altered. Discontinuities at each end mean that the geometry no longer approximates an ideal line and the second resonance is not an integer multiple of the first.

When the line is exactly a quarter wavelength long, its impedance is a maximum; at this frequency (which would be around 85MHz with the wire close to the plane, or 120MHz with it held away) the supposedly " ground/earthed" connection is in fact almost completely decoupled, with impedances of thousands of ohms being typical.

Generalizations

A number of points can be drawn from this illustration, as follows:

++ any length of wire becomes predominantly inductive above a few kHz; short fat wires have a higher transition frequency than long thin ones

++ the inductive impedance of typical lengths of wire reaches ohms at around a MHz, and tens of ohms in the tens of MHz range

++ the impedance of a length of wire connected at one end to an ground/earth reference plane reaches a resonant maximum when its length is a multiple of a quarter wavelength, and falls to a resonant minimum at multiples of a half wavelength

++ the exact frequencies at which these resonant peaks and nulls occur are strongly affected by layout; if any of them coincides with a susceptible or emissive frequency of the equipment, surprising and unpredictable variations in equipment performance will be brought about simply by moving such a wire by a few centimeters.

The general rule with ground/earth wires is: short fat straps have the lowest impedance. But even short straps are not perfect. --- the attenuation of a tinned copper braid 10cm long by 9mm wide by 2mm thick, in the same system. Clearly, although it’s much better than a length of wire, the braid still has substantial impedance in the hundreds of MHz. Its merit is that those resonances which still exist are pushed much higher in frequency and exhibit a much lower Q, thus reducing their impact usually to negligible proportions.

--- Hierarchy of ground/earth conductors

---Attenuation due to a short ground/earth strap

Effective bonding of joints

When multiple chassis or structural members are joined, the RF impedance across the joint determines the effectiveness of the whole of the metalwork as an RF reference. A uniform, infinite plane has no inductance associated with current flow from one point to another on its surface. Concentration of current at points of contact between structural members generates inductance across the joint, causing voltage differentials and external fields. To minimize this inductance, the area of contact and the number of contact points must both be large. Wide short metal plates with multiple metal-to-metal bonds to each of the items to be bonded are better for high frequencies and /or high powers than either wires or braid straps. For best performance up to 200MHz or so, their fixings should be spaced at most every 100mm apart along the whole length of their mating surface by bolts or spot welds. For higher frequencies continuous conductive gaskets or seam welding is required.

--- Bonding across metal joints

A large contact area has the secondary effect of reducing contact resistance, and this parameter can easily be measured by installation technicians using proper test instrumentation. A faulty high impedance bond can be identified from its contact resistance, a standard of 2m -ohm across each bond and 25m -ohm between any two points in the ground/earth system being accepted in military situations.

Corrosion of the bond between dissimilar metals in the presence of moisture must be prevented by appropriate combination of metals with respect to the electro-chemical series, by protective coating, and /or by the use of sacrificial washers. It’s important that ground/earth bonds remain effective over the life of the apparatus, even though subjected to dirt, vibration, atmospheric pollution, temperature cycling, and even condensation and spray in some environments.

Better control of high frequencies and /or high powers may be achieved when using wires, straps, or narrow plates, by using several such bonds. If using two, they should be positioned one on each side, but even better control will be obtained if several are used, spread all along the length of the mating edges. Where high-power RF equipment (such as induction furnaces) are involved, or powerful VHF broadcast transmitters are nearby, a short fat braid every 100mm must be regarded as the minimum for electrical bonding. Where UHF and higher frequencies are concerned, only wide metal areas in continuous contact will do, wires or braids are no use at all.

Use of gaskets

Closer spacing of fixing points gives better control of higher frequencies and /or higher powers, although if more frequent bonds are required it may be easier to slip a length of a suitable EMC gasket material between the mating surfaces. The pressure of the fixings should be sufficient to squeeze the gasket over the entire distance between the fixings with a pressure in excess of its minimum specification.

Where the metalwork is less than sturdy, the pressure of the gasket material may make it bow between the fixing and look untidy. Worse than this, the metal may be bowed so much that a gap is opened up and the purpose of the gasket defeated. Additional strengthening plates may be found necessary to prevent this bowing, or else a lower- pressure gasket material could be used.

A very wide variety of gasket materials and products are now available, some of them with low cost, and some with additional chemical or environmental properties.

Beryllium-copper or stainless-steel spring fingers may be used instead of foam or mesh type gaskets, to achieve good contact without excessive pressure. Care must be taken to ensure that the gasket materials are compatible with the metal types being bonded, taking account of the environment (condensation, spray, corrosive gases, etc.) to ensure that the electrical bonds last the life of the installation.

Bonding of equipotential mesh structures

--- Bonding structural components of the equipotential ground/earth mesh

As described above, a three-dimensional ground/earth structure is required to provide equipotentiality over a wide range of frequencies.

All structural metalwork and cable supports should be RF bonded across all their joints, and RF bonded between each other whenever they are close enough, to make a 3-dimensional ground/earth mesh. Plumbing, pipework, air ducts, chimneys, re-bars, I-beams, cable trays, conduits, walkways, ladders, ceiling supports, etc., should all be RF bonded. Building steel and reinforcing rods are recommended to have welded joints and a sufficient number of access points to them for frequent bonds to the ground/earthing network to create the appropriate mesh size for the MESH-BN.

suggests that bonding between metal elements can be by short thick round wires up to 1 meter in length, although these will only be adequate for the control of the effects of DC and 50/60Hz power. Adequate control of the effects of lightning surges requires the use of heavy-gauge round bonding wires under 500mm in length, or two or more wires in parallel each of up to 1 meter in length and spaced well apart (around 500mm). Flat wide copper strip or braid 500mm long provides improved bonding, possibly at frequencies of up to 30MHz. Braid or wide flat conductors are always better for RF than round wires of the same length and csa, but increasing the diameter or width of a bonding conductor is not an acceptable substitute for shortening its length.

The length of the connection between a structural item and the CBN should not be more than 500mm, and an additional connection should be added in parallel some distance away. Connecting the ground/earthing bus of the electrical switchboard of an equipment block, or the ground/earth bonding bar of a local AC power distribution cabinet, to the bonding network, should use conductors of under 1 meter length and preferably under 500mm, or two < 1 meter cables in parallel at least 500mm apart.

---recommends that mechanical bonds of questionable continuity are bypassed by wire jumpers that are visible to inspectors. The jumpers should have low impedance at high frequencies. Achieving good signal integrity and EMC performance at frequencies of 100MHz and above requires direct metal-to-metal bonds at multiple points, preferably seam-welding, for each joint.

Note the these guidelines for the lengths of bonding conductors don’t apply to termination of the screens of screened cables (). "Pigtail" terminations of cable screens can render a screen totally ineffective at frequencies as low as 50MHz even for pigtails as short as 25mm.

Constructing a Bonding Ring Conductor (BRC)

Each segregated area is surrounded by a continuous conductor known as the Bonding Ring Conductor or BRC. This is generally a copper conductor of round or flat section with a large cross sectional area (csa). Other terms used for this include ground/earthing bus conductor and interior ring bonding-bus. At the very least any building containing any significant amount of electronic equipment in it (including remote unmanned cubicles) requires at least one BRC around its internal periphery on each floor.

Where there are segregated areas within a building (including segregated system blocks connected only by LANs and power cables), each should have its own BRC. The use of BRCs is also a requirement for the creation of lightning protection zones.

--- Application of Bonding Ring Conductors

A BRC is really an extension of the main ground/earthing terminal for the floor, and should be bonded to the building's CBN at least at its four comers, preferably at many more points. The provision of a BRC allows equipment to be bonded by the shortest length of conductor, directly to the nearest point on the BRC. Where equipment is not near to the BRC, the BRC may be bisected by an additional conductor which bonds to the BRC at both its ends and "picks up" the equipment concerned along its path. Alternatively, equipment far from a BRC may use two or more widely-separated conductors to connect its chassis to the BRC. In a metal room or building it may be possible to use the metal walls as the BRC, depending upon the construction of the walls and their equivalent csa.

Cables and metallic services entering a segregated area or zone must be ground/earth- bonded to its BRC at the point where they cross it. This includes all cable armor and screens (with the exception of LAN cables guaranteed to be galvanically isolated for their entire path). Where surge protection devices (SPDs) and /or filters are fitted to power or signal cables to help protect a zone, they should be fitted at the BRC boundary. A transient suppression plate is sometimes recommended to ease these boundary connections. This is a metal plate at least 1 meter square bonded to the BRC and to the CBN or MESH-BN, which provides an area suitable for the bonding of cable armor, cable screens, SPDs, filters. Not only does such a plate provide a handy area for fixings, it improves the high-frequency and high-current performance of the bonds made to it. Transient suppression plates may be horizontal (with cables passing across them), or vertical (with cables passing through them).

Electricians working in the hazardous atmospheres industries should be familiar with the BRC concept, as every metallic cable or service that enters a hazardous zone must be ground/earth-bonded to the BRC for that zone (often a very sturdy conductor indeed), and standard explosion-proof cabinets are made with fixing points for ring-tags fitted to the cut ends of the BRC.

Bonding cable trays and ducts

Galvanized cable trays and rectangular conduits are best jointed by seam-welding, but it’s often acceptable to use U-brackets with screw fixings every 100mm or less around the periphery of the U instead, as the top drawing. Using lengths of wire will only control low frequencies (such as 50/60Hz). Shorter wires, or short fat braid straps, or multiples of each, all help increase the frequencies (or power levels) at which interference can be controlled.

Cable trays, ducts, and conduits will be required to act as "parallel ground/earth conductors" ( PECs), as described. The bonding methods at their joints and end terminations should relate to the frequencies it’s important to control for the sake of the cables they are carrying (for both their emissions and immunity). Where a rectangular cable tray or duct terminates at the wall of an equipment cabinet (or similar) a short wire or strap may be used for bonding which is effective at controlling DC and 50/60Hz disturbances. Two or more wires or straps will give better control of higher frequencies. An alternative is to cut away a few inches from the sides of the duct or tray, bend remaining floor section over and bolt it to the cabinet wall in at least two places, just below the aperture where the cables enter the cabinet. A U- bracket may also be used (with at least one metal-to-metal fixing every 100mm) and this can give very good control of electromagnetic disturbances up to very high frequencies.

Circular conduits are best jointed (either inline or at comers or junctions), using standard screwed couplings which make a 360 degree electrical bond. Similar 360 degree bonding glands should be used wherever a round conduit is terminated at cabinet walls, other types of cable ducts, or similar metal surfaces. These will generally employ some type of EMC gasket in their internal construction.

Choice of materials for trays, ducts, cabinets and other structural metalwork Galvanized, tinned, or stainless steel are preferred, as they make it easy to achieve corrosion-free metal-to-metal bonds between surfaces when they are pressed together (e.g., by a bolt). Aluminum is used instead of steel in some chemical industries. This should either have an unfinished surface, or be alo-chromed (a conductive finish). Anodizing should be avoided if possible as it produces a very good insulating finish and is difficult to remove when preparing an electrical bond. Similar materials should be used for both sides of each bond, to reduce the risk that galvanic potentials will create corrosion which will limit the lifetime of the bond- this is most important for environments in which water or corrosive materials are present.

Where there is any paint or similar insulating protective finishes on one or both of the surfaces to be RF bonded, these should first be removed from the areas concerned, and a suitable corrosion protection will then need to be applied. (Even if the fixing pressure prevents corrosion at the actual contact, this will be no help when the material around the contact rusts completely away.) Corrosion can be a problem where metal finishes have been removed, or where holes have been drilled in galvanized steel. This should be dealt with in a way which does not degrade the quality and lifetime of the electrical bond. Petroleum jelly and copper grease are common coatings used to prevent corrosion at such places, although they can be rather messy and need to be applied with care so as to cover all exposed metal. Paint may also be used to protect a bond, but it must not have such low viscosity that it insinuates itself between the metal contacting parts and increases the bonding resistance.

--- Methods of bonding cable trays and ducts

Electro-galvanic corrosion is a particular problem, especially where the ground/earth currents contain a DC component. Protect against this at all bonds by choosing the metals to be in contact for a low-enough galvanic potential, given the physical ambient.

Where this is not possible interpose a different metal which is mid-way in the galvanic series. 300mV has been found to be adequate for maintaining a low galvanic effect in a moderately corrosive atmosphere. Zinc chromate inhibitor or paste can be very useful, For example when fixing to aluminum with steel screws. Take great care with the electro-galvanic corrosion effects on the bonding of filters, which may have to maintain about 0.1m -ohm to the enclosures of the susceptible equipment they protect over its operational life. As pointed out at the beginning of this section, the important issue for good control of high frequency and /or high power disturbances is the metal-to-metal bonding of the two mating metal surfaces, with a good flat area around the bonding point. Point- contact electrical connections are inferior to the pressing together of two flat areas.

Assembly methods for ground/earth bonds in cabinets

--- METHOD A: connecting safety ground/earths to cabinet walls

--- Bonding method for safety ground/earths in cabinets. This uses welded or screwed-in ground/earth studs fitted to the cabinet wall within 150mm of the mains cable entry point. For safety reasons, the user must connect the incoming protective ground/earth directly to a method A stud on the cabinet wall. The standard for safety of electrical machinery specifies that this connection must be the only one labeled "PE". A separate method A stud, near the PE stud, is used to connect the ground/earth to the backplate and is labeled ~ .

Doors and removable panels should be ground/earthed to the cabinet walls using their own ground/earth studs and short wide ground/earth straps or braids.

Method B is used for internal ground/earth wire bonding to a captive nut in the backplate, within cabinets. The captive nut is positioned so that the shortest length of ground/earth wire may be used. For a plastic-bodied electronic unit the ground/earth terminal should be bonded to the nearest possible backplate captive nut with a short, thick ground/earth wire.

---METHOD B: internal ground/earth wire connections

Electronic metal-housed units (especially filters) must be bonded directly to the backplate at every mounting point.

--- METHOD C: fixing metal enclosures of electronic units

Using fasteners in RF bonding

The body of a bolt (or other mechanical fixing method) is used ideally to apply pressure to mating metal surfaces to create an electrical bond. The metal mating surfaces must be conductive, clean, free from contaminants such as the remains of roughly scratched- off paint, and be protected from corrosion, all as described elsewhere. Such bonds are often acceptable for safety purposes as well as being good for EMC at RF frequencies.

Relying solely on the body of a bolt or other mechanical fixing (e.g. a rivet) to provide a bond may not be acceptable for safety purposes, and won’t be as good at RF as directly mating metal surfaces.

The pressure of a fixing should be enough to ensure a gas-tight joint between the two mating surfaces, to help prevent corrosion. Larger bolt sizes are preferred because greater torque can be applied, improving the "gas-tightness" and longevity of the bond, and also because the increased surface area improves the bond's RF performance.

Washers with multiple anti-vibration "spikes" around their circumference are often used to help ensure gas-tight joints by concentrating the available mating pressure at their spikes. These may apply so much pressure that they cold-weld to the metal surfaces. Such washers may be inserted (with some difficulty) between metal mating surfaces to help overcome the lack of a good surface preparation, although the spacing they create between the mating surfaces will reduce the effectiveness of the bond at RF. Such "spiky" washers may be used on both external sides of a bonding bolt, so that the body of the bolt also acts as a reliable bond between the two metal parts. Such external spiky washers can be very useful in remedial work when improving the bonding of existing painted or oxidized metalwork- providing that the spikes and pressure applied are both aggressive enough to guarantee biting through the insulating layers and achieving gas-tight points of contact to the underlying metalwork.

Shake-proof washers without circumferential spikes, e.g. spring washers, are not generally acceptable for bonds which depend upon the body of the bolt. Military equipment often uses wavy shake-proof washers, so as to prevent metal swarf from being created in high-vibration environments, but these only provide electrical bonds themselves when the metal surfaces they bear against have been carefully prepared and corrosion protected, as they may not generate sufficient pressures to guarantee gas-tight contacts.

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