Cable screening techniques

Cable screening: Options

Any quick glance through a cable catalogue will reveal a huge range of types of screened cable. This implies that they are all imperfect in some way (counting high cost as an imperfection), and that it is going to be quite difficult to choose the most affordable type with the fewest imperfections for the job.

Manufacturers of apparatus should provide information on the cable and connector types to be used in their installation instructions. They should have had their equipment EMC tested in some way, and they will have discovered which cable and connector types were at least adequate when they did so. Quite often they will specify a cable type as (for example) "Belden 9829 or equivalent". It goes without saying that you need to be fairly knowledgeable to determine what is an equivalent in EMC terms, so it is safest to stick with the specified type, provided of course that it is readily available.

If the manufacturer does not specify the cable type, they should be asked to. It is important to discuss cable and connector details with possible suppliers of apparatus well before making the decision to purchase from one of them. The cost of installing expensive made-to-order cable and/or exotic connectors can often swing the purchasing decision to a manufacturer whose quote is higher because his product is designed to achieve the required EMC performance with standard low-cost cables and connectors.

Where screened cables have to be chosen without the benefit of instructions from the manufacturer, a few guidelines may help:

++ Twist the send and return current conductors together for every single load.

Do not rely on any other wire or cable (or cable screen) to carry the return currents, even if they appear capable of doing so. This means using twisted pairs at least (sometimes twisted triples or quads are needed).

++ Despite their prevalence for RF signals, co-axial cables are not very good for low-to-mid frequency EMC, because the screen is used to carry both the signal return current and the interference currents, and they get mixed up.

Only use them when the manufacturer has specified them as being suitable for the intended operational electromagnetic environment.

++ The drain wire in a foil-screened cable is not there to provide a neat termination for the screen. The foil screen should be bonded 360 dgr , and the drain wire can either be bonded with it or fixed to the body of the connector by some other means.

++ Overall braid screens are electrically superior to overall spiral-wrapped foil screens.

++ Braid is generally better the greater its optical coverage (the less you can see through it).

++ Braid and foil are generally better the heavier their gauge, since this gives greater conductivity; but the skin effect limits the improvement at frequencies over a few MHz.

++ Longitudinally-wrapped foil (i.e. wrapped lengthwise) is better than spiral foil, but is hard to come by and not as flexible.

++ Metal foil, the thicker the better, gives better shielding performance than metallized plastic film in a full shielded cable. Unfortunately, it also makes the cable less flexible.

++ Overall braid and foil screen, or double braid, is much better than a single screen, even when the two screening layers are not isolated from each other.

The best configuration for braid and foil cables is when the braid lies against the conductive side of the spiral foil so as to "short out its turns".

++ Individual twisted pairs (triples, quads, etc.) in an overall screened cable may need individual screens to prevent capacitive crosstalk between signals (usually individual spiral foils are adequate) - this will be dictated by the circuit parameters.

++ Multiple isolated screens are better than non-isolated screens, in general (although resonances can make them no better than single screened wires at some frequencies).

++ Better cables for screening are generally thicker, heavier, have a larger minimum bend radius, and cost more.

++ Almost the best possible double-screened cable is one which is run in its own circular metal conduit (pipe) with 360 dgr bonding at all joints and to the equipment cabinets at both ends.

++ "Low capacitance" cables, and/or "high velocity factor" cables, will generally be better at carrying data for longer distances.

++ For high-rate data (or long distances) the characteristic impedance of the cable and its connectors must match the specification for the "physical layer" of the data standard, which may also specify the detailed build of the cable. Where the datacomms specification used is proprietary to a manufacturer, he must be able to specify cable and connectors precisely.

Where it is a published data communication standard, read the standard or obtain one of the many industry guides.

++ Screened cables subjected to tight bend radii, or to repeated flexing, can find gaps opening up in their screening layers and reducing their effectiveness.

This is a particular problem for screened cables to the moving parts of robotic systems. Careful control of bend radii and choice of flexible cable is required.

--- Transfer impedance of a single-conductor coaxial cable

Cable transfer impedance

The screening performance is formally specified in the "transfer impedance" parameter Z T of the cable. This is the same concept as the transfer impedance of structures, but specifically related to screened cables. It offers a convenient way to describe and compare the quality of different types and make of cable. So far, it is rarely found in catalogue descriptions except for high-performance types, but manufacturers can often supply details of a particular type on request.

Single coaxial cable

For a single conductor coaxial screened cable the transfer impedance is simple to define. It is the voltage V t which appears between the inner and outer of the coax at an open circuited end, divided by the interference current I S flowing on the screen which produces this voltage (V/I has dimensions of ohms). --- the equivalent circuit. The far end of the coax is shorted so that no voltage appears at this end. The transfer impedance is specified as a differential parameter with respect to the length of the cable, and is quoted per unit length in units of ohms per meter. If the screening were perfect, then no matter how much current flowed in the screen, no voltage would appear on the inner, and Z T would be zero ~/m.

The screening imperfections can be modeled as resistance in the screen conductor, and leakage inductance between the inner and outer. At very low frequencies and DC, only the resistance is significant, and the transfer impedance actually equals this resistance. This can be seen intuitively: no current flows in the inner conductor, so the voltage to ground/earth at the measurement end equals the voltage to ground/earth at the shorted end, which must be R_screen x I S. At higher frequencies the leakage inductance between inner and outer becomes dominant. This is the difference between the self-inductance of the cable as a whole, and the mutual inductance of the two conductors. It can be written as Lc(1 - k), L c being the cable inductance and k being the coupling coefficient. As k approaches unity the leakage vanishes; this is the situation for a cable with a 100% solid outer sheath, such as rigid copper coax. In this case the transfer impedance actually improves (reduces) with frequency through another cause, which is the separation of inner and outer sheath currents through the skin effect. Typical braided coax has a k value of around 0.996 as a result of apertures and other effects in the braid, so that its transfer impedance starts to rise above 0.5 -1MHz.

Measurements of transfer impedance show these effects for various types of cable. Once the frequency rises above that for which the cable can be considered electrically short (length < X/10), Z y measurement becomes complicated.

This is because the Z T definition above does not take into account standing waves on the inner conductor; it assumes that the measurement of the induced voltage is "current- free", i.e. terminated in a high impedance and not in a matched load. In reality, such a measurement set-up is untenable once the cable length approaches resonance.

Measurement at higher frequencies therefore implies either shorter lengths of cable, causing sensitivity issues and contamination of the results by artifacts of the end terminations, or the use of matched triaxial systems. This has resulted in a "perplexingly large number" of Z y measurement methods which seek to overcome the problem.

Although only one of these is quoted in IEC 96-1, which defines screened cable performance parameters, many others remain in widespread use, defying proper comparison of cable data.

Multiconductor screened cable

Although Z T is most usually quoted for single conductor coaxial cables, it is equally applicable to multi-way screened cables. In this case the voltage of interest can be developed either between individual internal circuit pairs, or between the screen and all the inner conductors connected together, as a result of the interfering screen current. Whilst the former is of more use for determining the performance of a system using that cable, it cannot be defined independently of the circuit in which it will be used. Screen currents here develop a common mode disturbance in the wanted circuit, and the overall impact depends on the common mode rejection of this circuit.

This is not usually dominated by the cable. In other words, it is more sensible to define a transfer impedance for the system rather than a per-unit-length Z y for the cable alone.

Z T in the case of multi-conductor cables is still dominated by the leakage inductance Lc(1 - k) at high frequencies. The common aluminum foil shields with drain wire are less effective than overall braid shields, partly because of their higher resistance, and partly because the drain wire distorts the magnetic coupling of the shield with the inner, giving a lower value of k. --- curve (2) for such a foil- shielded cable, configured.

--- Typical transfer impedance curves for various cable types

--- Transfer impedance of multi-conductor cables -- (a) screen to all conductors together; (b) screen to individual circuit pairs

Terminating the screen

The previous section may have implied that the performance of a screened connection between two pieces of equipment could be described by the transfer impedance of the cable alone. In reality, the cable is only part of the whole; the end terminations also contribute to the system transfer impedance, and if poorly made, will dominate it.

Cable screen RF bonding requires glands (or connector) backshells which achieve 360 deg. (full circle) electrical contact to the gland (or connector) right around the circumference of the screen. Saddle-clamps or P-clips may also be used, although they are not as good for very high frequencies or high powers. It is also important that connector backshells bond 360 deg. to the body of their mating connector, and that the bodies of glands or connectors bond 360 deg. to the metal surfaces they are mounted on.

The maintenance of the 360 deg. coverage of the cable screen and connectors, right through any joints, connectors, or glands from one electronic circuit to another, is vital.

The EMC performance of the cable type is wasted if 360 deg. screen coverage is not continuous from end to end.

360 degree screen coverage for an entire interconnection helps ensure that the currents and voltages caused by external electromagnetic disturbances remain as far as possible on the outside of the cable and connector screens, and do not get mixed up with the internal (signal) currents. It also minimizes the amount of signal voltage or current that is coupled to the outside world, helping to prevent problems with signal integrity as well as the emission of disturbances.

---Examples of cable screen termination methods

Connectors

Connectors that are properly and intentionally designed for screened cable pay considerable attention to the series impedance presented by the screen termination. A good example is the N type RF connector, which can maintain system performance up to 18GHz. In this construction the braided screen is compressed by a metal ferrule against the connector body, which is mated through a threaded cylinder to the complementary female half of the connection. Not only is 360-deg. contact maintained throughout, but the diameters of the various parts are closely controlled to minimize deviations in characteristic impedance through the connector.

General purpose screened electrical connectors do not normally need to maintain characteristic impedance, but they should ensure proper 360 deg. contact through the screen to the body of the connectors and then to the mating screen or chassis. The effect of this, in transfer impedance terms, is to maintain a high value of coupling coefficient k through the connection, which minimizes the Z T of the connection. An alternative way of seeing this is to say that a poor or inadequate 360 deg. shroud increases the inductance of the screen connection.

Commercial screened connectors vary quite widely in their form of construction and hence in their Z T. Both the method of terminating the cable screen and the method of mating the shells are important. E.g., the principal distinction between so- called "EMC" subminiature D-type connectors and their lesser brethren, is the dimples on the male connector's shell, and the tin plating on the shells of each half. These two features markedly reduce the contact impedance between the shells as compared to their non-EMC-dimpled, cadmium passivated relatives. But equally, whatever backshell is used needs a secure method of terminating to the connector shell and of clamping to the cable screen.

It should be clear that the system Z T defines the overall system screening performance, and this is made up of the sum of the cable Z T, and the connector ZTS at either end. If the connector ZTS are substantially greater than the cable, they will dominate the whole. A cable with a good Z T is entirely wasted in this situation.

Direct to metalwork

A variety of low-cost 360 deg. cable screen termination methods which tie the screen direct to the chassis are becoming available.

Some new techniques use stainless-steel cable ties, and are very fast and low-cost indeed, although not as aesthetically pleasing as more traditional connectors and glands. Alternatively, elastomeric blocks are available, which grip all around the cable screen and transfer the connection to the metalwork via a housing frame.

The conventional cable gland is entirely acceptable provided that it intentionally makes contact all around the cable screen, and all around the edge of the hole in the metalwork. "EMC" cable glands are marketed for this purpose with internal iris or ferrule arrangements that ensure good contact is made with the screen. Any paint or other coating must be removed from the chassis contact area- a partial contact to the chassis, i.e. not 360 deg. , is unacceptable.

The problem of the pigtail

"Pigtails", lengths of wire soldered to braid shields, or the drain wires of foil-shielded cables, are surprisingly ineffective at providing a good ground/earth termination even when they are as short as 25 millimeters. The pigtail introduces inductance in series with the screen-to- ground/earth connection which will dominate the transfer impedance of the complete assembly. The flux from the interference current in the pigtail wire links with the inner circuit(s), whereas the interference current on the braid generates flux which does not link with the internal circuit(s) except by leakage through the braid apertures or other inductive effects as. The mutual inductance of the pigtail section is proportional to the pigtail length; for a 25mm length the mutual inductance is a few nano-henries, which is substantially greater than the contribution from leakage inductance of a typical braided cable.

Pigtails cannot be recommended for EMC practice, except where the frequencies of concern for either emissions or immunity are very low indeed (say, less than 1MHz) and/or the signals concerned are only slightly sensitive or slightly interfering (Classes 2 or 3). In any case, keep all pigtails less than 30mm long - just long enough to assemble without too much difficulty.

Once suitable tools are available to assist with proper 360 deg. screen bonding, pigtails may be found to be more expensive and time-consuming to assemble, and less attractive to installers.

Which end to ground/earth?

This section describes why it is that cable screens should nowadays be bonded to ground/earth at both ends, in general, and how to prevent the resulting ground/earth loops from causing problems.

Single end versus both ends

Cable screens should generally be RF bonded to their local ground/earths at both ends, but to do this without suffering cable damage due to heavy circulating currents (often called " ground/earth loop" or "ground loop" currents) requires the installation of equipotential mesh ground/earthing systems . Even though cable screens have a high resistance compared to other elements of the MESH-BN, and cannot carry heavy currents, when bonded to ground/earth at both ends they improve the MESH-BN because there are many of them.

Traditionally, building ground/earth structures have only been bonded to achieve personnel safety. It has been common practice (and is still recommended by some trade associations) for the ground/earthing of a site to be improved only up to the point where the touch voltages do not exceed 50V rms. This is known as equipotential bonding, but it is clear that it is not equipotential enough to meet modern requirements for equipotentiality for EMC purposes. This traditional practice also required cable screens to be bonded at one end only, because where large potential differences exist between ground/earths, bonding cable screens at both ends has been known to cause problems with cable overheating.

Compliance with national wiring installation rules which limit touch voltages (e.g. to 50Vrms), can still allow higher voltages to be experienced during an ground/earth-fault, when it is not uncommon for touch voltages to rise momentarily to 160V. Electrical storms and other high-energy transient surges in the ground/earth network can cause this voltage to rise momentarily to considerably more than 160Vrms.

These high voltages can cause serious damage to electronic circuits not designed to cope with them, and could give a nasty shock to any personnel subjected to them.

Bonding screens to ground/earth at only at one end provides no screening protection from certain orientations of magnetic fields, which require a current to be driven along the length of the screen to provide the screening effect. This applies both to emissions from the cable, and its susceptibility. The final problem with bonding screens at one end only is that the unbonded end will provide a "window" in the overall high frequency shielding, through which external fields will couple with the inner conductors. Screening must be complete for the entire route of a signal. Even small discontinuities in the screening, e.g. holes, and lack of 360 dgr screen bonding (such as the use of pigtails) can degrade screening effectiveness unacceptably. Traditional practices were not so concerned about screen bonding at both ends, or the use of pigtails instead of 360-degree bonding, because the frequencies in common use were not as high as they are today.

With the high data rates commonly used these days, and the worsening of the electromagnetic environment, it is no longer acceptable to waste the EMC performance of expensive screened cables by only bonding their screens at one end, or using pigtails instead of 360-degree screen-bonding techniques. Transient interference poses a further threat; in a paper describing the advantage of screened ribbon cable, the author describes in wry fashion some of the ESD susceptibility tests that were performed: First, the shield was brought very close to the connector, then cut off, as might be done to avoid ground loops. The voltage on the center conductor [during an applied 10kV ESD ] was estimated to be far greater than 500 volts. After this test, our oscilloscope was returned to the manufacturer for repairs.

Present day best-EMC-practice for bonding cable screens is as follows:

++ use equipotential 3-dimensional mesh ground/earthing techniques (MESH-BN) to reduce the ground/earth potential differences between electronic units (that need to communicate over metallic conductors) to < 1V rms;

++ 360-degree bond cable screens to the local ground/earth at both ends, preferably directly to the wall or backplate of equipment cabinets;

++ run all cables (whether screened or not) very closely along elements of the MESH-BN ground/earth system, using them as Parallel Ground/earth Conductors ( PECs), along their entire run.

PECs can also be constructed to provide significant assistance with emissions and immunity (and crosstalk between cables) at frequencies much higher than 50/60Hz, potentially allowing the use of lower-cost, lower-specification cables than would otherwise have been necessary.

Special cases

Co-axial RF cables must always have their screens bonded at both ends, and will always generate a high-frequency common mode (CM) current path between their outer conductor (braid) and ground/earth. This is because the braid of a co-axial cable carries the return current of the signal carried by the centre conductor, and is why twisted-pair or twin-axial cables (which do not use their braid to carry return signal currents) are preferred for LF and wideband EMC and signal integrity reasons. To reduce the emissions and immunity problems caused by the CM currents of co-axial cables they should always be routed close to a PEC , and it may also be necessary to bond their screens to the PEC at irregular intervals averaging every 5 to 10 meters for long cable runs to prevent the CM currents leaking from the braid from becoming too troublesome.

If a cable screen is not bonded to local ground/earth at both ends, this should be for a very good technical reason, usually because the manufacturer of an EMC-compliant apparatus has specified this deviation from general good practice, and not because of some traditional proscription against ground/earth loops. Where cables interconnect units of electronic apparatus it is normal to bond their screens to the cabinets at both ends.

Screens bonded at one end only are usually only associated with certain models of transducers (often plastic-bodied ones) that contain no electronics.

LAN connections

When a network cable is run from one equipotential area to another (e.g. between IBNs), if the cable screens cannot be bonded at both ends because of the resulting high levels of circulating currents in the screens, then galvanic isolation must be used.

Measures are also required to make sure no-one can touch the un ground/earthed end of the screen (e.g. insulation to the maximum potential transient potential). Surge protection devices (SPDs) may also need to be fitted between the signal conductors themselves and their cable screen ("in-line" types), and from cable screen to the local ground/earthing terminal (parallel types). Such SPDs will certainly be required where such a cable goes between buildings (even underground). For coaxial (BNC) screened cables, the RF ground/earthing is maintained along with the isolation by capacitively coupling the screen (outer) terminal to the equipment case.

ECMA 97 recommends that LANs be isolated from ground/earth, because they cannot be guaranteed to stay in one equipotential area, and recommends the use of galvanic isolation barriers rated at >1500V and preferably 2kV, but states that 500V rated isolation may be used in specified circumstances. Since ECMA 97 was written the best- practices have shifted away from the use of IBNs inside buildings, and now we can see that when we have a proper implementation of a MESH-BN throughout a building it is preferable to bond LAN cable screens 360-degree at both ends, and run LAN cables closely along elements of the MESH-BN at all times, using them as PECs.

Alternatives to bonding cable screens at both ends

In some instances it is impossible or undesirable to bond cable screens at both ends.

There are a few alternatives.

Capacitive bonding at one end of a cable screen Capacitor bonding to ground/earth at one end of the cable screen, with direct RF bonding to ground/earth at the other, is sometimes suggested. This avoids the need for equipotential bonding, MESH-BNs, or PECs, by providing RF bonding at both ends without creating troublesome " ground/earth loops". Capacitor values of 1nF to 100nF are commonly used for this technique, with the lower values being better for higher frequencies but less effective at low frequencies.

Where no true equipotential bonding is available, capacitive screen bonding is an option, but you should be prepared to use adequately engineered application-- specific components for this, rather than just connecting wired capacitors to ground/earth. This is because it is difficult to assemble a wired capacitor between the screen of a cable and the ground/earth and still have it operate effectively over a wide range of frequencies.

This technique also suffers from the fact that where the cable is not run over a well constructed MESH-BN over its whole route, it does not help to prevent the unbonded end of the screen from flashing over, or protect the electronics from overvoltage damage, during ground/earth-faults or electrical storms. Also, the capacitors used will need to be rated for the full fault and surge voltages expected, which are likely to be high where the ground/earth bonding is poor. Measures are also required (e.g. adequate insulation to cope with lightning surges) to make sure no-one can touch the unbonded end of the screen.

Of course, creating a true equipotential MESH-BN or SRPP over the whole route of the cable, and running the cable close to its elements at all times, will remove the problems of flash-over and exposure of electronics and people to high voltages, but then the screen may as well be bonded to ground/earth at both ends anyway, since the additional ground/earth loop created is more likely to be an advantage overall than a problem.

So capacitor screen bonding cannot in general be recommended as a best practice for either EMC or safety. However, it may prove adequate in some situations when carefully implemented by an experienced engineer aware of all the issues. It may also be a helpful remedial technique for dealing with specific interference problems on a pre-existing "traditional" installation which does not have equipotential bonding or PECs, and where cable screens had been bonded at only one end.

Galvanic isolation

The only wholly commended alternative to the use of PECs and bonding cable screens both ends, is to use galvanic isolation barriers rated at full fault/surge voltage. Fiber- optics, wireless, microwave, laser, and infrared (e.g. IrDA) communication techniques are very suitable for communicating data without metallic connections at all. All of these techniques are becoming more readily available and their cost is falling.

In some instances, manufacturers of wireless data systems are claiming that the overall cost of installing their system on an industrial site is less than the cost of installing a single cable. Wireless systems should still be treated with caution for other EMC reasons, though. Any radio-based system is inherently more susceptible to blocking by many different sources of radio interference. Intelligent digital signal processing can mitigate but not remove this problem.

Fiber-optic communications are probably the most EMC-friendly of any of the available options, capable of supporting almost limitless common mode potential differences. A minor disadvantage is that the electronic receivers and transmitters at each end may add a weak point for interference susceptibility and unreliability.

When using fiber-optics, care must be taken with any cables that use metal elements, such as for strengthening or armoring. Their metal elements may accidentally compromise the cree page and clearance distances required for safety, and/or may arc over during surge events and cause intense (though localized) high- frequency disturbances and/or fire risks. Such metal elements should be cut, isolated, and protected where they cross the boundary of an equipotential zone.

Infrared communications tend to be line-of-sight, and microwave and laser communications are always line of sight, so if they are used thought must be given to the possibility that the communication path might be blocked, for example by a maintenance worker.

The problem of ground/earth faults

Ground/earth loop currents are mostly caused by voltage differences in the ground/earth structure due to heavy power consumption, so are generally at 50 or 60Hz and related harmonics. The low impedances of PECs at low frequencies diverts most of ground/earth current away from the cables' screens, allowing RF bonding of cable screens at both ends without fear of cable overheating.

Magnetic fields are another source of ground/earth loop currents. These currents are reduced to negligible amounts by running the cables concerned very close to their PEC. "Close" means no more distant than 25mm from the base of a tray, duct, or conduit or no more than a few millimeters away from a dedicated green/yellow ground/earth wire or copper strip being used as a PEC. A problem frequently encountered in practice is the effect of ground/earth loop currents caused by ground/earth faults, that is, when a conductor that is ground/earth-bonded at one point but is intended to be isolated from ground/earth along the rest of its length, becomes linked to ground/earth at some other point through an insulation failure. This can happen either to screened cables, passing near an ground/earthed structure where the jacket is abraded or pierced, or to the neutrals of supply cables which are not RCD-protected. (It can also happen to live conductors, of course, but in that case the fuse or circuit breaker should operate, which alerts maintenance personnel to the problem immediately!)

--- The ground/earth fault

The effect of an undiscovered ground/earth fault can be pernicious. The current that flows in the conductor which is faulted depends on its impedance and the ground/earth potential difference at the position of the fault. This may be low enough that, although a fault current flows, its effect is negligible and the fault is never manifest. On the other hand, it can be several tens of amps, in which case it is usually discovered as a result of the cable damage. In between, though, there are several potential hazards:

++ longer-term fire hazard: the cable heats up enough to present a fire hazard, but not enough to signal its presence quickly;

++ magnetic field interference due to the high current flow over a large loop area- the "wobbly monitor" effect is a typical symptom of this sort of fault;

++ induced interference in telecommunications systems if the affected cable is part of, or close to, such a system.

Some of these effects are quite difficult to diagnose on-site, and result in several man- days of wasted and expensive effort. In some cases, the potential for the problem is so acute that a system will be deliberately designed to be isolated from ground/earth and incorporate an integral ground/earth fault detector, so that maintenance engineers can be alerted to a fault occurring and trace and correct it quickly. The use of a properly installed MESH-BN will reduce the impact of such faults - by reducing or eliminating the ground/earth potentials that are the root of the problem- and often will result in an installation being able to tolerate much higher levels of fault occurrence.

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