Electromagnetic compatibility EMC concerns all electromagnetic phenomena including power-frequencies, so these EMC techniques also help prevent "ground loop" problems and achieve good analogue and digital signal quality. As a result, the techniques described here generally reduce development and commissioning timescales and help ensure reliable operation, as well as help comply with the EMC Directive. The extra time and costs they require should be more than made up for by savings later on, and reduced commercial risks, with greater savings being made the earlier the techniques are applied. Some of these techniques contradict what some designers and installers may regard as "traditional" or "conventional" methods. This paper should be treated as merely a guide to some of the modern EMC techniques for use inside installations such as buildings.
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Electromagnetic compatibility EMC concerns all electromagnetic phenomena including power-frequencies, so these EMC techniques also help prevent "ground loop" problems and achieve good analogue and digital signal quality.
As a result, the techniques described here generally reduce development and commissioning timescales and help ensure reliable operation, as well as help comply with the EMC Directive. The extra time and costs they require should be more than made up for by savings later on, and reduced commercial risks, with greater savings being made the earlier the techniques are applied.
Some of these techniques contradict what some designers and installers may regard as "traditional" or "conventional" methods. This paper should be treated as merely a guide to some of the modern EMC techniques for use inside installations such as buildings.
It does not cover the assessment of the electromagnetic environments; specification, design, assembly, or proving of equipment; filtering; shielding; lightning protection or surge suppression; all of which play their part in ensuring that a lack of EMC does not cause problems in the operation and compliance of an installation.
External services and communications are not covered here. A large part of this Part 2 is taken from reference . This is easy to do very early on, and can save enormous amounts of time and cost later. Examples of noisy equipment include adjustable-speed motor drives; metal or plastics electric welding; power rectifier systems for electrochemical processes; radio, television, and radar transmitters; and power conversion.
Examples of sensitive equipment include radio, television, and radar receivers, instrumentation for temperature, flow, weight, pH, humidity, pressure, etc. Figure 2A shows an example of such segregation. The different classifications of apparatus and their power and signal cables should be powered via different distribution networks which run separately from each other, and the two or more classifications of low voltage equipment should ideally be powered from different distribution transformers.
The different classifications of apparatus should be spaced well apart from each other metres rather than centimetres , and any other cables associated with each type should be routed well away from each other. Where equipment cabinets contain both noisy and sensitive equipment, their manufacturers would be expected to achieve internal segregation so that one did not interfere with the other. At the initial design stage it is easy to allocate these facilities to well-separated locations, and costs nothing to do.
If interference problems are discovered during commissioning or afterwards the very significant costs of moving one of the facilities and its cabling well away from the other are likely to be dwarfed by the lost production due to the delays incurred. Similar problems can arise in the food industry, where sensitive checkweighers and metal detectors are followed closely by packaging machines employing noisy radio-frequency RF plastic welding techniques for sealing plastic bags or cartons.
If not designed-in from the beginning, the distance required between the packaging machine and the other equipment may not be achievable without wholesale re-structuring of the production line, at great cost. Twisted pairs, triples, quads, etc. Busbars are best spaced apart by thin layers of solid dielectric. Very heavy individual cables should at least be run as close together as they can be taking full account of other physical limitations. Figure 2B shows preferred and non-preferred routes where one of the conductors is switched, for example by manual switches in a room lighting circuit, or by relays or contactors in a control panel.
This technique reduces both differential and common-mode the couplings of magnetic and electric fields between the cable and its electromagnetic environment see Part 1. Although Figure 2B shows a power application this technique is vitally important, for both emissions and immunity reasons, whatever electrical signals or power a cable may be carrying, from weak transducer signals, through high-speed data, to heavy power. Screening cables for EMC is of little use unless both send and return conductors are enclosed together in a single screen.
The better the physical balance between the send and return current paths, and the better the electrical balance between the signals they carry, the better will be the electromagnetic performance of the cable whether it is screened or not.
Power cables also benefit from close proximity of send and return conductors. For a delta connected three-phases system this would just involve the three phases, but in a star-connected system it would involve all three phases plus their neutral. Very heavy power cables, such as those feeding the main DC motors in a steel rolling mill, could suffer high mechanical stresses on their conductors if placed too close together from the electro-motive forces created by the interactions of their magnetic fields and this could damage their insulation.
Air-insulated high-voltage cables also have to be separated by considerable distances to prevent discharges between the conductors. In both of the above cases, or whenever power send and return conductors do not follow virtually identical paths in close proximity, it must be accepted that high levels of electric and magnetic fields will occur in their vicinity, and may upset the operation of nearby electronics.
For example, problems with image stability on CRT type VDUs are often caused by the magnetic fields from nearby power cables where the send and return conductors are widely separated. A problem with this technique can occur for cables carrying very heavy currents. The electro-motive forces on the cables due to their powerful magnetic fields can subject their conductors to very high mechanical forces, and can cause the insulation to wear out too quickly.
Where power conductors have to be separated for this reason, it will be very important indeed to keep these cables segregated well away from any sensitive electronics.
Where such powerful magnetic fields exist and they are easily calculated they should also be checked against personnel exposure limits to protect staff and third parties from health hazards. These days the environment is highly polluted with frequencies up to thousands of MHz, and getting more so by the day. At the same time the signals used by electronic communications now regularly extend to 30 MHz and above. At the frequencies in typical use these days the stray couplings due to mutual inductances and capacitances creates circulating currents that flow through air or insulation for at least part of their paths.
These cannot be effectively controlled by star bonding. For more on stray coupling, see Part 1 of this series. The half-way house of partially-meshed bonding generally has the problems of both and the benefits of neither.
This achieves a low impedance from 50Hz up to higher frequencies depending on average mesh size , and is an important element of modern installations with their concentrations of high-technology equipment including computers and telecommunications.
This is the terminology I shall use throughout the rest of this Part. Heavy current equipment requires a closer mesh to prevent high voltage drops in the case of leakage or fault currents. Sensitive instrumentation often requires a smaller mesh size because it is vulnerable to voltage differences in the CBN over a wide range of frequencies.
Meshing the CBN helps protect equipment against the damaging effects of lightning surges, and surge protection devices SPDs function better when they are connected to a low-impedance CBN.
For lightning protection where most of the energy occurs at frequencies below 10kHz , it is generally recommended that no part of a site should have a CBN whose mesh size exceeds 3 or 4 metres, in any dimension. It is recommended to use so-called natural metalwork, such as re-bars, girders, structural metalwork, and any other metalwork to help achieve a MESH-CBN, as shown in Figure 2E.
Bonding should ideally be metal-to-metal, and seam welding is best. Short conductors may be used instead with a reduction in high-frequency performance, as shown by Figures 2F and 2G. Where high-integrity systems are required to continue functioning despite environmental extremes especially nuclear and military control rooms the mesh size of the CBN may need to be reduced so far that metal floors or walls, or even seam-welded metal rooms, become practical options. Incoming cables and metallic services such as air, gas and water pipes, ventilation ducts, etc.
Sometimes the main earthing terminal may need to be a large plate, to allow filters to be properly bonded see Part 4. Figure 2H shows an example of a BRC for one floor of a building. The general idea, which Figure 2H tries to show, is that the BRC travels around the perimeter of the area concerned, at least the floor of each building or each level of a structure, and is connected by single short bonding conductor to each item of apparatus and the power conductors this should always be routed physically very close to this bonding conductor.
Where an item of apparatus has to be some distance from the BRC, two or more bonding conductors should be used, widely separated from each other, to attempt to reduce the inductance of the connection to the BRC. A lot more detail on the construction of BRCs will be found in  to . Sometimes it is necessary to segregate an area within a floor or level to better control especially noisy or sensitive apparatus e.
Such a segregated area is sometimes called a zone, especially in IEC lightning protection standards. But it should be recognised that this is not an ideal situation and the cable or service may need to be re-routed if problems arise.
I understand that some internet server manufacturers are now specifying a maximum ground potential difference of no more than 15mV at any frequency of concern over the area occupied by their interconnected computers - a difficult specification to meet. Where the environment suffers from high levels of high frequency disturbances, control of the local reference potential plane may be important for the correct operation of low-frequency electronics, such as instrumentation or audio systems.
Figure 2J sketches a common type of construction. It is sometimes possible to use the metal support structure for the computer flooring, or metal-covered computer flooring tiles, as a bonding mat. Each bonding mat requires a BRC around its perimeter.
Although each application will differ, a mm meshed bonding mat as sketched in Figure 2K ought to be able to achieve the old IBM specification of ground potential differences of no more than 1V over a computer installation.
It is not unrealistic in some applications to install a seam-bonded metal floor, stand the equipment cabinets directly on the metal floor, and bond them to the floor with very short straps, maybe even one at each corner or one every mm or so of cabinet perimeter.
NAVAIR AD  has some good graphs of the effectiveness of braid bonding straps with frequency, and shows that a single 9" inch long braid strap is useless or even counter-productive at bonding equipment to a metal plane above 10MHz. A single 2 inch long strap is required to provide any reliable mat bonding at frequencies of 30MHz.
The use of multiple straps, widely separated from each other, allows longer straps to provide effective bonding to the mat at 30MHz.
However, bonding cable screens at both ends, and bonding cable trays and ducts to equipment cabinets at both ends both discussed later in this Part , also helps to improve the local reference potentials by providing lots of parallel bonding paths. Some authorities and standards suggest the use of an insulated meshed bonding network MESH-IBN for the IT and telecomm and other new electronic additions to such buildings.
Although MESH-IBNs can work very well indeed when first installed, they must be designed taking into account lightning and surge protection for personnel.
MESH-IBNs are also very vulnerable to technicians and engineers, who may decide to run a copper data or power cable between two areas or rooms, or even between floors, not realising that they are compromising the isolation of the new area.
For high-speed or sensitive signals, the local EMC-earth will generally be the shielded enclosures of the associated electronic units.
Earth potential differences arise in a number of ways, but all are reduced if the impedance of the earth structure is reduced, which is one of the reasons why meshed earth-bonding networks, as described above, are generally recommended for modern installations.
Surges due to nearby lightning activity and high-power transient events such as switching a large load, or earth-faults in cables and equipment give rise to momentary voltage differences between parts of the earth structure.
Because all these involve relatively low frequencies, the currents involved will divide up generally according to the various path resistances they are offered, so the provision of low-resistance PECs ensures that the earth-potential currents in cable screens which are bonded at both ends is reduced to acceptable amounts although it is never eliminated. Screened cables are capable of much greater EMC shielding performance than is traditionally achieved by screen-bonding at one end only.
Saddleclamps, P-clips, and the like, may be acceptable alternatives in situations where the frequencies concerned are low or the EMC performance required not very high. Part 3 has more on this topic. Tests have shown that even a 25mm pigtail can ruin cable screening even at the low frequency these days of 70MHz. Cables should remain close to their PECs at all times, to minimise circulating currents caused by power magnetic fields.
For very long cable runs or very polluted EM environments e. This technique is no longer effective, except in special circumstances, because of the poor EMC performance achieved from the screened cables, increasingly incompatible with the high frequencies and sensitivity of modern electronics.
What was often overlooked when the screens of long cables were connected at one end only is that during earth-faults and lightning surges, the whole surge potential can become concentrated at the end with the unterminated screen. This can cause flashover, with consequent fire hazards, but is much more likely to cause actual damage to electronics and expose operators and maintenance personnel to electric shock hazards. Ethernet, for instance, is designed to have only one end of its cable screen earthed, but where dangerous surge voltages can arise the installer should fit surge protection devices to limit them to safe levels.
Where a good quality equipotential MESH-CBN is not available a common situation when retrofitting in older buildings , it may be possible to run all cables over newly-installed PECs, with substantial CSAs capable of handling the possibly high fault currents. To function as PECs, these should of course be electrically bonded at every joint and to the local CBN at both ends usually an equipment cabinet wall or motor frame.
Possibly the best method to employ where earth structures are poor and it is not desired to install a lot of PECs is to achieve complete galvanic isolation to 20kV or more through the use of fibre-optics use types with metal-free cables , infra-red, or wireless communications for all signals and data, making sure that their transmitters and receivers have adequate EMC performance for the application.
Unfortunately, it is sometimes not appreciated how vulnerable ordinary analogue instrumentation circuits are to suffering severe errors when exposed to frequencies of hundreds of MHz, and so some instrumentation lacks the filtering it needs to be able to use screen-bonding at one end without suffering EMC problems. Clearly, the safety standards must be followed, and where this means that achieving adequate EMC is likely to be made more difficult as a result especially likely where modern electronic technologies are also employed then this is unavoidable.
Achieving galvanic isolation for all data and signals using infra-red or metal-free fibre-optics is often a good technique to use in such situations. Projects of this type may also need to use other EMC techniques not discussed here.
Of course, PECs must be bonded at each end to the equipment cabinets that the cables enter. They must also be effectively bonded at all joints all along their lengths.
EMC for Systems and Installations: Part 2
Preview Abstract Describes performance requirements, test methods and classification procedures for degrees of protection provided by empty enclosures against electromagnetic disturbances for frequencies between 10 kHz and 40 GHz. The shielding protection is measured for the purpose of demonstrating that the enclosure provides adequate shielding of electromagnetic energy to support acceptable performance of the complete assembled units when tested to applicable IEC standards. The purpose of this standard is to provide a repeatable means for evaluating the electromagnetic shielding performance of empty mechanical enclosures, including cabinets and subracks, and to specify a marking code to allow a manufacturer to select an enclosure with a known capability for attenuating electromagnetic fields. The requirements for immunity to various types of electromagnetic disturbances, including lightning and high-altitude electro-magnetic pulse HEMP will need to be considered by manufacturers when determining the need for application of this standard for specific equipment and applications, and for the specific enclosure shielding requirements which are necessary as a function of frequency. Look inside.
IEC System of Conformity Assessment Schemes
BS IEC 61000-5-2:1997