Installing cable systems

Cable classification

Cables can be divided into four Classes with respect to their interference potential.

These classes must be physically segregated and preferably run in different trunkings as described . Their trunking must keep the cables close to a parallel ground/earthing conductor along their whole length, and if cable classes must cross they should only do so at right angles.

Cables CLASS 1 -- very sensitive signals

CLASS 1 generally applies to all low-level signals such as thermocouples, thermistors, RTDs, strain gauges, load cells, microphones, etc.; also high-rate digital and analogue communications such as Ethernet, video, RF receiver cables from antennae and all other signals with full-scale range less than 1V, 1 mA, or where their source has an impedance higher than 1k~2, or their signal frequency or rate is greater than 1MHz.

CLASS 1 cables must always use good quality highly-screened twisted-pair (or similar) cables and shielded connectors with no breaks in 360-degree screening and no screw terminals. Cable screens may be terminated to backplates using clamps, and/or to the walls of shielded cabinets using glands, as long as the cable screen remains unbroken over the entire signal path. Where CLASS 1 cables are closer than 1 meter to medium and high voltage power cables (1kV and above), they should be segregated within a covered metal conduit.

Always ask the manufacturers of the electronic units using such sensitive signals for their recommendations for cable types, connectors, and wiring routes, and make sure that their recommendations are followed exactly.

Cables CLASS 2- for slightly sensitive signals

This covers ordinary analogue (e.g. 4-20 mA, 0-10V, less than 1MHz) and low-rate digital bus communications (e.g. RS232, RS422, RS485, Centronics). These signals should use screened cables. Flat ribbon cables must be screened either with flat screening jackets or screened "round and flat" (a flat cable is rolled up in a round cable with an overall braid screen). CLASS 2 also covers digital (i.e. on/off) inputs and outputs such as limit switches, encoders, non-data-bus low rate signals, etc., and the outputs of internal DC power supplies (but not the DC links of power converters or motor drives, which are CLASS 4). For these, use screened cables, multi-conductor cables, flat cables, or even single wires.

Externally supplied AC and DC power distribution (up to 230/415V) may be treated as CLASS 2 after it has been internally filtered in the cabinet. For such conductors single wires may be used, preferably twisted together, but in any case always run all the phases and the neutral associated with a load in the same bundle. (To allow a mains cable to be treated as CLASS 2 the supply filters used must be adequate for the full range of supply interfering phenomena and mounted properly. Refer to use of filters.) Twisted pairs/triples/quads/etc, are always preferred for both screened and unscreened cables for CLASS 2, but be aware of the problems that can occur where an electronic unit only provides a single return terminal or connector pin when there are a large number of return wires. You may have to make special arrangements for connecting all the return conductors to the return terminal.

Cables CLASS 3- for slightly interfering signals

CLASS 3 covers externally supplied AC (< 230/415V) or DC power which does not also supply noisy external equipment, e.g. welders, drives, power converters, etc.

(External equipment must be considered to be noisy unless it is known to have emissions in line with harmonized EMC emission standards. Such units will generally be fitted with internal supply filters.)

CLASS 3 also covers control circuits with resistive and inductive loads where all inductive loads (e.g. the coils of solenoids and contactors) are fitted with suppressors recommended by their manufacturers.

Induction motor (e.g. squirrel cage) cables are CLASS 3 providing they are on/off controlled or run from adjustable speed drives with sinusoidal output filters, and providing their AC supply is itself CLASS 3 or better.

CLASS 3 signals and power can use screened cables, multi-conductor cables, or single wires. Screw terminals are fine as long as exposed conductors are less than 30mm long.

As for CLASS 2, twisted pairs (or triples, quads, etc.) are always preferred.

Cables CLASS 4 - for strongly interfering signals

CLASS 4 applies to the cables associated with the power inputs and outputs and DC links of adjustable speed motor drives (DC, AC, steppers, servos, etc.), welding equipment, and similar electrically noisy equipment.

Noisy equipment may be known to be noisy, or there may be some doubt which may be reinforced if it is clear that some or all of its power inputs and outputs are unfiltered.

Always check with the drive (or other) manufacturers whether they specify or recommend special cables, routing, or other details such as ferrite toroids/clips or filters -- and then always follow their advice exactly.

Cables to on/off controlled DC motors or slip-rings are also CLASS 4, although sparkless types such as pancake motors, or motors fitted with spark suppressed rotors or integral filters could be CLASS 3.

All the above CLASS 4 cables should use good quality braid screened types with properly made screen connections. Do not use pigtails for CLASS 4 screened cables.

The conductors in all CLASS 4 cables should preferably be twisted pairs, triples, quads, or as required, to twist together all the send and return current paths associated with one signal or load. Screw terminals are generally fine as long as no exposed conductors are more than 30mm long, but some manufacturers' instructions may provide more stringent requirements, so always check.

CLASS 4 also covers control cables to unsuppressed inductive loads (relays, contactors, solenoids, etc.), which may use unscreened twisted pairs but only as long as they operate infrequently (every few minutes or longer). If they operate more frequently than every few minutes they should either be suppressed, or use screened cables as above.

CLASS 4 can include all LV supplies which also power external noisy electrical equipment (e.g. drives, power convertors, welding, etc.) which may not meet their relevant harmonized EMC emissions standard, or which lack supply filters. These may use unscreened cables or bundles of single wires containing all associated phases and neutral. (The use of unscreened cables for CLASS 4 supply cables is forced on us by practicality and by historical installation practices, not because it is acceptable for EMC.) Unscreened Class 4 power cables should always run close to a PEC, which can be the cable armoring where this is available.

Cables to RF transmitting antennae are also CLASS 4, and must only be exactly as specified by the manufacturer of the electronic unit concerned, for cable type, termination, and routing. For such RF cables, no breaks in their 360-degree screening are allowed.

CLASS 3 also covers control circuits with resistive and inductive loads where all inductive loads (e.g. the coils of solenoids and contactors) are fitted with suppressors recommended by their manufacturers.

Induction motor (e.g. squirrel cage) cables are CLASS 3 providing they are on/off controlled or run from adjustable speed drives with sinusoidal output filters, and providing their AC supply is itself CLASS 3 or better.

CLASS 3 signals and power can use screened cables, multi-conductor cables, or single wires. Screw terminals are fine as long as exposed conductors are less than 30mm long.

As for CLASS 2, twisted pairs (or triples, quads, etc.) are always preferred.

Cables CLASS 4 - for strongly interfering signals

CLASS 4 applies to the cables associated with the power inputs and outputs and DC links of adjustable speed motor drives (DC, AC, steppers, servos, etc.), welding equipment, and similar electrically noisy equipment.

Noisy equipment may be known to be noisy, or there may be some doubt which may be reinforced if it is clear that some or all of its power inputs and outputs are unfiltered.

Always check with the drive (or other) manufacturers whether they specify or recommend special cables, routing, or other details such as ferrite toroids/clips or filters-- and then always follow their advice exactly.

Cables to on/off controlled DC motors or slip-rings are also CLASS 4, although sparkless types such as pancake motors, or motors fitted with spark suppressed rotors or integral filters could be CLASS 3.

All the above CLASS 4 cables should use good quality braid screened types with properly made screen connections. Do not use pigtails for CLASS 4 screened cables.

The conductors in all CLASS 4 cables should preferably be twisted pairs, triples, quads, or as required, to twist together all the send and return current paths associated with one signal or load. Screw terminals are generally fine as long as no exposed conductors are more than 30mm long, but some manufacturers' instructions may provide more stringent requirements, so always check.

CLASS 4 also covers control cables to unsuppressed inductive loads (relays, contactors, solenoids, etc.), which may use unscreened twisted pairs but only as long as they operate infrequently (every few minutes or longer). If they operate more frequently than every few minutes they should either be suppressed, or use screened cables as above.

CLASS 4 can include all LV supplies which also power external noisy electrical equipment (e.g. drives, power convertors, welding, etc.) which may not meet their relevant harmonized EMC emissions standard, or which lack supply filters. These may use unscreened cables or bundles of single wires containing all associated phases and neutral. (The use of unscreened cables for CLASS 4 supply cables is forced on us by practicality and by historical installation practices, not because it is acceptable for EMC.) Unscreened Class 4 power cables should always run close to a PEC, which can be the cable armoring where this is available.

Cables to RF transmitting antennae are also CLASS 4, and must only be exactly as specified by the manufacturer of the electronic unit concerned, for cable type, termination, and routing. For such RF cables, no breaks in their 360-degree screening are allowed.

Cables CLASS 5 and 6- for MV and HV

Cables carrying AC or DC supplies at voltages higher than 400/230V are commonly called High Voltage (HV), although more correctly these should be divided into Medium Voltage (MV) and HV according to the IEC's definitions. MV and HV cables are much more exposed to external disturbances such as lightning and powerful transients and surges caused by the operation of heavy-duty switchgear, than are LV cables.

MV power distribution cables are CLASS 5, and HV power distribution cables are CLASS 6. The types of cables used are defined by the power network engineers, and rarely by EMC engineers. However, running all the conductors associated with a given load in a single cable or bundle of cables, and using its amour as a PEC, is possible where load currents allow, and will help ensure that these cables don't cause unnecessary disturbances.

Some details common to all cable classes

All the conductors associated with a signal or a load must always be run together in a bundle over their whole length, even if this uses more copper (see section on cable routing below). Each multi-conductor cable, or bundle of single wires, if not armored or screened, should include an ground/earth conductor bonded to the local ground/earths at the equipments to which the cables are terminated, to create a PEC. (Armored cables can also use their armoring, although it is a good idea to include a separate ground/earth conductor as it is less likely to be disconnected during future modifications.) This additional ground/earthing conductor acts to reduce magnetic coupling at low frequencies, and also helps to reduce crosstalk.

When bundles are run over properly bonded MESH-BN PECs in a properly meshed site ground/earthing system, this ground/earth conductor probably need not be any greater cross- section than the other wires in the bundle. If the PEC or rest of the ground/earth structure is lacking in some way, this ground/earth conductor will provide a PEC (at least for the lower frequencies)

but will need to be sized appropriately for the continuous and surge currents it may be called upon to handle.

As has been mentioned before, screened wires must use 360-degree RF bonding (described above) at all connectors and glands, unless their signals really are non- aggressive and insensitive, when very short "pigtail" screen terminations may possibly be acceptable.

Signal cables from PLGs and computers

PLC and computer digital control and analogue I/O signals may be thought to be benign because the signals they carry are slow or very infrequent. However, if poorly designed they may carry electrical common mode noise generated by the operation of their internal digital processing, and this has a much higher frequency content than would be expected from the signals themselves. So, unless the manufacturer or supplier of the equipment or systems clearly states otherwise (and you have confidence in him), always treat them as Class 1 cables, using 360-degree screened cables along their entire length with 360-degree RF bonding at all connectors and glands.

External cables where powerful emitters are nearby

Where the electromagnetic environment suffers from a high-level electromagnetic field, for example from a nearby radio or TV broadcast transmitter, medical diathermy, plastic welding, induction heating or similar apparatus, additional measures may be required. These strongly polluted environments may only exist for a small area near to an item of equipment, and the best approach is segregation - avoiding placing apparatus or their cables in these areas altogether. Where this is not possible, the following methods may be used.

Using better quality screened cables

All signal or power cables that have to be present should use an additional screening layer which is 360-degree RF bonded at all joints, connectors, or glands. This is regardless of the cable Class based on the signals or power carried by the cable itself, so a Class 1 cable would require a double-screened cable. (Double screens do not need to be isolated from each other, except for the most demanding performance. A braid screen simply wrapped over a foil screen layer can make a large improvement in a cable's screening effectiveness.) Using covered cable ducts or conduits

If the cables are only run through this environment in rectangular ducts (used as PECs) fitted with conduit with RF-bonded covers, or in circular conduits, all RF bonded using techniques appropriate to the frequencies concerned, then it may be possible to use the normal cables for their Classes. The covered duct or circular conduit acts as another screened layer to the cable.

Circular conduit with 360-degree bonded joints and terminations can provide the most excellent screening performance at almost any price, so this is a remarkably cost- effective technique for extreme electromagnetic environments.

Using additional bonds between cable screens and PECs

Another technique, where the strong fields concerned are below 20MHz, is to expose an inch or so of cable screen or armoring every 10 meters or so, and bond these exposed places to the PECs the cables are following. The distances between these bonds should not be regular, and should be randomly varied between (say) 6 and 12 meters.

Capacitors can be used instead of direct bonds, to help keep 50/60Hz currents out of the cable screens, but this should not be necessary where proper ground/earth mesh equipotential bonding has been followed throughout PECs and the site.

Measures such as this have been used as a remedy to protect computer systems from electromagnetic disturbances created by nearby electric train services and/or marshalling yards. (Electrified railway systems usually produce their maximum radiated disturbances in the 1-20MHz region.) Although time-consuming and costly, as a remedial measure 10-metre screen bonding may be much easier to apply than the above two techniques. It suffers from the problem that the exposed cable screens or amour may allow water ingress or corrosion, so steps may need to be taken to seal these bonding points. Where the electromagnetic environment contains high levels of higher frequencies, screen bonding every ten meters may not be frequent enough. But bonding more often than this starts to be less cost-effective and the other two techniques above become more appealing.

What if you can't easily tell what Class a cable belongs to ?

Sometimes there may be some doubt about the "noisiness" of an item of apparatus or its cables. This often occurs when a new apparatus is being installed in a pre-existing site which already has apparatus and cables in place. A simple and quick test with a portable spectrum analyzer and hand-held close-field probe is all that is required to identify electrically the "noisier" cables and apparatus. The probe (or probes) and analyzer should cover the frequency range from 10kHz to 100MHz, at least.

The above four cable classes are relative, not absolute, and field-strength values cannot be given for the close-field probe results. However, a little time spent with the probe will soon show which cables are to be kept well away from the others. The problem with "noisy" cables is that their function may not clearly reflect their disturbance capability, especially power cables, and this is where this close-field probe method can save a lot of time and difficulty.

Unfortunately, it is not so easy to identify the sensitivity of cables with a portable probe (it can be done, but takes much more time and effort). Happily, most site electrical engineers will know what the cables are for, and cross-checking against the functional lists provided for Classes 1 and 2 should soon identify them.

It is quite likely that, in existing installations, different cable classes will be mixed up together and not segregated. This does not mean it is adequate to continue in this fashion. New cables should be segregated from each other, and from existing cables.

The additional time and cost will be well spent when, once every few installations, it prevents unreliable operation.

Segregation and routing

Physically segregating cables by their classes

The purpose of determining which Class a cable belongs to is to be able to choose the correct cable type and terminations, but it is also so that different classes of cables may be run segregated from each other to prevent them from interfering with each other. The minimum separation distances that should be maintained between the different cable classes.

--- Minimum spacings between cable classes when run over a single PEC

This assumes a continuous flat metal PEC under them all. Where separate PECs are used, a larger gap between them is required, although this may be offset by the use of PECs with side-walls (e.g. trays, open ducts), the taller the side-wall, the better. In the absence of a nearby PEC (not advised), the spacings between two numerically adjacent classes of cable should be at least 10 times the diameter of the larger bundle . Where the PECs used completely enclose the classes of cables (e.g. covered ducts, circular conduits) and the PECs are correctly RF bonded along their length and at both ends, there is no minimum spacing between PECs. Even so, it is still best practice to arrange the ducts or conduits in order of cable class so that Class 1 is always the furthest from Class 4.

When covered ducts are used for their EMC performance, it is important to ensure that all covers are replaced and properly RF bonded after any maintenance or modification work. Similar checks are needed for any part of the ground/earthing structure or PECs (have all the multiple fixings at a joint been re-fitted? is the cable amour still 360-degree bonded at all joints?). This is because many engineers may not realize that these metal structures are doing a lot more than merely providing mechanical protection or support.

Remember that all cables must run close to their PECs at all times (see PECs). "Close" means touching or within a few tens of millimeters - certainly not as much as 100mm away unless the PEC is a large circular or covered conduit containing only one class of cable.

If cable classes must cross, they should only do so at right angles with as much space between them as possible. It is best to try to avoid cable class crossings wherever possible, or else arrange for a PEC to run between them, this is especially important when classes 1 and 4 must unavoidably cross (in which case enclosing one in a circular conduit or covered duct is not unreasonable). Narrow metal ducts or conduits (or circular conduits) should only contain one Class of cable, if they are too narrow to maintain the minimum spacing between classes inside them.

The above figure only section the first four cable classes. Class 5 (for MV power distribution cables) should be segregated from Class 4 by 150mm at least (and from Class 1 by 750mm minimum), and Class 6 (HV power distribution) should be a further 150mm away from Class 5.

Remember, all these spacings are minimum values, for cables run for 30 meters over a common PEC. Where Class 5 or 6 cables are closer than 1 meter to Class 1, the Class 1 cables should be run in a covered metal duct.

Segregation within Classes

So far, this discussion of segregation has assumed that all the cables in a Class may be bundled together, but this may not always be advisable, especially for the more extreme Classes. Sensitive analogue Class 1 a cables should not be bundled with high-rate digital signals in twisted-pairs (Class 1 b) and neither of them should be bundled with high rate digital in co-axial cables (Class 1 c). It is recommended that these sub-classes be bundled separately and not run too close to each other (separation of at least 10mm between each pair in the order: 1 a, 1b, 1 c), keeping each bundle as close as possible to the metal surface of the PEC at all~ times.

Different Class 4 cables may also require individual routings. The cables from adjustable speed inverter drives to their motors are often specified by the drive manufacturers to have 600mm or more spacing from any other parallel run of cable.

Quite how this should be interpreted when running a dozen drive cables along the same general route is something that should be taken up with the drive manufacturers. It is difficult to make general rules for segregation within Class 4, because a cable may be very noisy in its own right, but still sensitive enough to pick up sufficient interference from neighboring Class 4 cables to upset the electronics it is connected to. This is another reason why the use of sinusoidal output filters for inverter drives might be preferred: such motor cables become Class 3 and may be bundled together with each other and with other Class 3 cables with few worries, as long as the drives are compatible with the filters.

Routing

All cables between equipment units should follow the same route All the cables between items of equipment should ideally follow a single route along a single PEC, whilst also maintaining their segregation by Class (see above for details of PECs and cable classes).

--- Installation cable routing

The two main principles of cable routing:

++ the cables between two items of equipment must always follow the same route; and

++ there should be a single interconnection panel for each item of equipment.

The single connection panel per unit helps prevent the inevitable circulating currents in the cables and metalwork from flowing through the apparatus and possibly causing interference with the internal electronics. Where there has to be a multiplicity of routes and/or connection panels, each should have its own PEC, and higher PEC ground/earth currents should be expected.

--- Segregation of cable classes in trays

Stacking cable trays along a route

Because of the minimum segregation distances required between cable classes, it is generally impossible to run cables of all four classes along one cable tray (they are usually not wide enough). ---- how this is overcome by running a "stack" of cable trays. In this example each tray is carrying just two cable classes.

The cable trays are stacked vertically and electrically bonded together at all of their support pillars. They all follow the same route between two items of equipment. Where cables are segregated vertically in uncovered trays, their vertical separation according to their respective classes should equal or exceed the horizontal minimum spacings.

Running all send and return conductors close together at all times

--- the technique of running all the send and return current conductors involved with any signal or electrical load together along all the entire route (even where this uses more copper). This technique is important to reduce magnetic field coupling.

---Routing send and return together

Often, this technique is not applied. For example, when the live lead to a load passes through a switch or relay, the neutral is usually routed directly or even "picked up locally" to the load. This is bad practice, as shown by the first example in the sketch.

Send and return current paths must always be as close together as is possible, which is why twisted pairs are preferred for signal cables.

Of course, in this example the unswitched conductor cannot follow exactly the same route, since it does not pass through the switch, relay, contactor, or whatever. Even where double-pole switching is used (or triple-pole, for three-phase supplies) there will always have to be some physical path difference between conductors in the vicinity of the electromechanical elements. The idea is to minimize all path differences everywhere else.

Dealing with heavy power conductors

The send and return conductors of heavy power cables are usually not twisted together (as is preferred for signal cables) due to their size. Such cables are best laid side-by-side (over their common PEC) as close as they can be together, to achieve the maximum EMC benefits.

Unfortunately, where cables are carrying very heavy powers the mechanical forces on their copper or aluminum conductors can be strong enough to make them move enough to wear out their insulation. These mechanical forces are created by the very magnetic fields that placing the cables close together is intended to partially cancel.

Because of this it may be necessary to place the cables further apart and forgo the EMC benefits. Extra segregation distances from sensitive equipment should then be provided.

A recent example of an 8000 amp DC drive, where the send and return conductors were 30 meters long and 1 meter apart along their length, showed that standard VDU screen displays could suffer unacceptable distortion and movement at distances of nearly 30 meters from the cables. This could have had a strong impact on the siting of the control room, but in this case LCD display screens were used instead of VDUs because they are not sensitive to magnetic fields. Running the DC motor cables closer together would have significantly reduced their magnetic field emissions, but could have given the cables a reduced lifetime (and re-wiring costs were estimated at s Connections to cabinets

There should only be a single connector panel for a cabinet. All external cables should enter a cabinet at only one side, rear, top, or bottom, and they should also enter the ground/earthed backplate along one of its edges.

This is so that, in conjunction with the other techniques described here, the high- level circulating currents flowing in the long cables in many industrial situations will flow from cable-to-cable via the connector panel or backplate edge via the screen- terminations or filters mounted in that area, and will not flow through the rest of the cabinet or backplate structure where they might upset the electronic units.

Parallel Ground/earth Conductor (PEC) techniques

Modern best practices for EMC in installations (according to IEC 61000-5-2:1998 and prEN 50174-2) also require the use of cable trays, conduits, and even heavy- gauge ground/earth conductors as Parallel Ground/earth Conductors (PECs) to divert power currents away from cables and their screens. The cabinet and backplate should provide the means for the connection of the necessary PECs.

Constructing PECs

The first function of a Parallel Ground/earth Conductor (or PEC) is to divert heavy ground/earth loop currents from both screened and unscreened cables. Since ground/earth currents are usually at 50/60Hz, and the surges from lightning events have most of their energy below 10kHz, it is enough for this purpose that the PEC has a very low resistance and a sufficient current-carrying capacity. Most cable support systems have enough metallic cross- sectional area to provide this low resistance and current capacity, especially on ground/earth- meshed (MESH-BN) sites where ground/earth currents in PECs are lower.

Any screen or ground/earth conductor external to a cable should be treated as a PEC and bonded to ground/earth at both ends. So, cable armoring should always be regarded as a PEC. It is vital to ensure there are no breaks in the electrical continuity of any amour used for this purpose. Cable installers traditionally regard amour merely as mechanical strengthening or protection, and may not be used to bonding it at joints and to the local ground/earth at both ends.

To reduce the loop currents flowing in cables due to magnetic fields (which can be extremely intense in the vicinity of some industrial processes, such as induction heating or magnetic stirring), cables must be run very close to the metal of their PEC throughout the length of their run.

PECs can also control higher frequencies. A variety of types of PECs, and ranks them by high-frequency performance. Some types of parallel ground/earth conductors (in order of increasing HF effectiveness)

Cable trays are usually perforated with slots to make cable fixing easier, but these can detract from its high frequency performance. The problem is exactly the same as has been described earlier on shielding effectiveness: slots and gaps interrupt current flow and therefore increase the transfer impedance of the structure. Because of their open construction, ladder- and basket-type cable support systems are poor as PECs, except for the control of 50/60Hz disturbances. Even then they may not have a substantial enough metal cross- section, so a wired PEC may be required as well.

PECs must be electrically bonded to the local equipment ground/earth at both of their ends, and to all their support structures and any other ground/earthed metalwork at every available opportunity. This is so that they help to create a meshed ground/earth structure, and it also helps the PEC to function effectively.

--- Slots and gaps in structures used as PECs

Joints and end-terminations in PECs must be bonded using methods appropriate for the frequencies to be controlled (RF bonding techniques). Cable trays and rectangular conduits will need to make electrical bonds directly to the cabinet wall (or floor, top, or rear) using U-brackets or similar with multiple fixings.

Round conduit can bond to the cabinet wall with circular glands, remembering to remove the paint first (to ensure a 360 deg. bond) and apply corrosion protection. PEC conductors will need appropriately-sized and positioned ground/earth terminals.

PECs may have to carry high continuous currents, and must be capable of handling the worst possible fault currents without overheating or other damage, so they must have an adequate metallic cross- section. Conductively coated plastic conduit or trunking will obviously not be adequate, for this reason, and if used will require a heavy-gauge copper wire PEC inside them to handle any heavy currents.

--- Using structural members as PECs (I-beam example)

--- Branching PECs from the main structure

Using structural steelwork as a PEC

In a building fitted with an equipotential ground/earth mesh any of the ground/earth-meshed metalwork of the building may be used as a PEC (1-beam girders, building steel, etc.). Because transfer impedance is affected by geometry, different positions relative to the structural member will have different high-frequency attributes. Running cables inside hollow metal structures is the best technique, but may be difficult to achieve and not easy to maintain. Running cables deep inside included angles on the outside of structural metalwork is a good practical alternative. Where there are too many cables for them all to be run in included angles, the noisiest should be run in one angle, and the most sensitive should be run in another. Less aggressive or less sensitive cable should use the less protected routes.

Providing electrically continuous PECs

Cables should run very close to their respective PECs at all points along their routes.

Where a cable splits off from one cable route to take a different route, an electrically continuous PEC should be provided for it, and it should remain close to its PEC at all times. A variety of techniques for continuing a PEC where a cable enters or leaves a given cable tray. Similar considerations apply to other cable support types such as ducts or conduits.

Dealing with large gaps in trays and ducts

Breaks in cable trays and ducts often appear to be required (and sometimes for structural reasons are unavoidable) where cables pass through walls or floors. In such cases the PEC trays or ducts should run right up to both sides of the difficult region and the gap in the PEC bonded with at least one short wide braid strap or heavy-gauge ground/earth wire (preferably more than one spread over the width of the tray). These bonding conductors should be suitably rated to withstand the worst possible ground/earth-fault or other current (which could be very high since equipotential mesh bonding is probably also restricted by the wall), and should follow exactly the route taken by the cables.

Such bonded-across gaps in PECs are weak points for high frequencies, so must be used with care. If high-frequency problems arise it may be possible to wrap metal "knit-mesh" around the exposed lengths of the affected cables, bonding this additional overall screen to the cable tray (PEC) at both sides with a large P-clip, saddle-clamp, or similar. Different classes of cables (e.g. "noisy" and sensitive cables) should not be included in the same overall knit-mesh. They should use different overall knit mesh shields, each bonded to the PEC at both sides of the gap.

Where it is difficult to install cable trays or conduits at all, a PEC may consist of heavy-gauge wire or a copper strip, strapped along the whole length of the cable. Such a PEC is only useful for controlling low frequencies, such as 50/60Hz and its first few harmonics, and the low frequency content of surges such as those from lightning.

When a complete PEC is not possible

It is sometimes difficult to use trays or conduit over the whole cable route. It is not good EMC practice for a cable to be run without a PEC, but it sometimes appears necessary where a new apparatus is being connected to a pre-existing installation. The preferred techniques for bonding cable screens as they leave or join a PEC. Unscreened cables should have any protective ground/earth conductors bonded to the PEC at the point of joining. Where cables must leave the protection of a PEC, allowances should be made in the project plan for subsequent remedial EMC work such as the running of a wired PEC.

--- Cables leaving or joining PECs

Next: Filtering--Interfaces: Attenuating noise

Prev: Unscreened cables

top of page  Article Index   Home