Building a meshed facility ground ( earth)

Having explored the techniques of bonding between structural components, we now look at implementation of meshed ground/earth systems in particular instances.

Constructing SRPPs and bonding mats for system blocks

Best-practice for typical commercial and industrial computer and telecommunication rooms (and the like) is to create a System Reference Potential Plane (SRPP) for each system block from a local bonding mat, as described. Bonding mat meshes are often made from wire of 6mm diameter or larger, or 25 x 3mm copper strip (of the sort used for external lightning protection). The advantage of using the "lightning down conductor" copper strip is that standard jointing clamps are available to ease assembly.

The bonding mat's mesh size must be related to the frequencies it’s desired to control, whether signal or interference frequencies. The rule of thumb for general applications is that the mesh size should be no more than one-twentieth of the shortest wavelength of concern (i.e. the highest frequency), so For example: to have some control over signals and interference at 30MHz requires the mesh size of the bonding mat to be no greater than 500mm square. Higher frequencies and /or better control (i.e. more attenuation of interfering frequencies, better signal integrity) will require smaller mesh sizes.

The frame of each equipment cabinet should then be bonded to its nearest ground/earth mesh conductor using the shortest heavy-gauge wire possible, better still a short wide braid strap. Round wires should not be used to bond the cabinet frames to the bonding mat where the frequencies of concern (either for signals or interference) exceed 10MHz. Where the wavelengths of concern are shorter than the longest cabinet dimension (e.g. >150MHz for a 2 meter cabinet) each cabinet may need multiple connections to the bonding mat from points on its frame separated by no more than one- tenth of the relevant wavelength, with a minimum spacing of 300mm.

--- Ground/earth bonding mat for IT room

--- Application of vertical bonding (MESH-BN technique)

As discussed earlier, it’s important that the mesh is not made too regular. If it’s made very regular, then at some frequencies every mesh element will resonate together, and the ground/earthing provided by the mesh could effectively "disappear" at those frequencies. A random allocation of +20% dimensional differences will randomize the resonances and help maintain ground/earthing integrity to higher frequencies. Even so, in some high-technology installations solid metal flooring may be required (described in the following section). The bonding mat for a system block is nowadays recommended to be an integral part of the overall building's common-bonding network (CBN), and be bonded to it at as many points as possible. A continuous bonding ring conductor (BRC) should surround the bonding mat (which itself surrounds the system block it’s the SRPP for) and be bonded to all the mat's conductors. This BRC then bonds to the building's CBN. In larger installations a system block may comprise a whole floor, in which case its BRC follows the perimeter of the building. Smaller system blocks may only take up part of a floor, so their BRCs will only enclose that part of that floor. Ideally the building's CBN vertical and horizontal meshes should be no more than 3 meters square.

--- two IT/telecomms floors with meshed SRPPs, but the principle of bonding the CBN to each floor's BRC has universal application.

In computer/telecomm rooms with raised metal-coated flooring panels (usually used with conductive carpet materials for control of static build-up) an alternative is to use the metal-covered floor panels as the ground/earth mesh, instead of creating one from copper wire or strip. The metal computer-flooring panels are already bonded together via the metal bridging contacts on the tops of their support pedestals. Equipment frame ground/earthing leads are then merely fitted with a ring tag and screwed directly to the nearest floor panel (underneath the carpet). It’s important that all the bridging contacts on the floor pedestals are in good condition and make a good electrical bond to each panel.

Where galvanic bonds cannot be guaranteed at every comer of the metal-covered floor panels, or at the crossing point of the framework, a grid of > 10mm 2 csa copper conductors may be strung underneath the floor, bonding to the floor panels or stringers at every upright, although every second or third upright may be adequate. Because the metal floor panels are only bonded by pressure contacts at the comer pedestals, this does not count as a solid metal floor system, but it should give equal or slightly better performance than a copper wire or strip mesh system (providing the pedestal bonds remain good). A possible disadvantage of using the raised metal flooring system is its intrinsic regularity, but the effects of bonding the floor to the various equipment cabinets, each with their own protective ground/earth lead and cable screen bonding should help to randomize the bonding mat's frequency response.

Even where the raised-floor panels are not metal covered, it’s often possible to employ floor stringers instead as a bonding mat. These are often used to add stability to higher floors, and may be used to provide a bonding mat instead or as well. When stringers are used as a bonding mesh they should not be used for protective ground/earthing as well. Bonding mats don’t have to be underneath the system blocks they serve - they can be installed above them instead. As we will see later on, all the cables to and from the equipment cabinets must be run in close proximity to conductors forming part of their MESH-BN or SRPP bonding mat, and this may make an overhead bonding mat preferable. Overhead bonding mats may also be the best way to upgrade existing installations with minimal upheaval.

Despite the ideal of creating an integrated ground/earth-bonding system, it’s generally a good idea not to connect air conditioning, heaters, motors, and other electrical apparatus to a bonding mat used for an IT or telecommunication or similar electronic system block. They should be bonded instead (depending on the size of the building) to the building's main ground/earth terminal, or the floor's main ground/earthing terminal, or to the bonding ring conductor (BRC) that surrounds the system block, taking account of touch voltages during fault conditions and other safety issues.

Ground/earth meshing computer/telecomm rooms for critical requirements

Where the electromagnetic environment is extreme t, and /or when absolutely the best reliability is required from the apparatus in the room, the mesh ground/earth described above may not be adequate. The bonding mat type of SRPP should instead be made from metal sheets of substantial thickness, all of them seam-welded to each other along all their mating edges. The equipment cabinets should be stood directly on this floor with any insulating feet removed, and RF bonded to the metal floor at each corner of their frame.

The solid ground/earthing system needs to be complemented by providing a Single Point of Connection (SPC) to the rest of the building's structure. Here, the SPC will generally be a region of the floor, or a vertical metal plate seam-welded to the floor, and all the cables and services entering will be electrically bonded all around their circumference at this point. Cables and services may not enter or leave the protected area at any other point. All unscreened cables will be filtered at this point, with the filter bodies bonded metal-to-metal to the SPC. For such rooms, metal-free fiber-optic cables will be preferred for all data and signals.

For installations required to survive the nuclear electromagnetic pulse, or which are so crucial to national security that they must emit no detectable electromagnetic disturbances, nothing but rooms made entirely of seam-welded metal, with specially screened doors, windows, and ventilation apertures will do. These rooms are essentially the same as the metal screened rooms used for some EMC tests, and what began as an ground/earthing technique has turned into a shielding technique. For data fiber-optic cables must be used exclusively, and any metallic conductors entering the room must be as few as possible and all fitted with very substantial filters.

Applying the mesh-floor technique to other types of apparatus

Although the above refers to rooms intended for computers and /or telecommunications, the technique is also applicable to rooms where sensitive instrumentation is to be used.

A good example might be a scientific laboratory where experiments require high powers and /or high frequencies as well as sensitive detectors, e.g. work with particle accelerators and cyclotrons. Segregation of the high power and /or high frequency equipment from the detectors is required (preferably by putting them in different all-metal shielded rooms), and each apparatus area should employ the techniques described above.

These extreme techniques of interference control are included in a program known as TEMPEST. Once restricted to military and government, TEMPEST is now available to the private sector to counter the increasing amount of commercial, financial, and industrial blackmail, terrorism, and espionage which relies upon the vulnerability of modern computers and computer networks to electromagnetic disturbances, and on their propensity to broadcast their data over a large geographical area where it may be picked up by sensitive receivers. In the USA these practices are becoming known as "information warfare".

Improving the ground/earth-bonding of older buildings

The improvement of the ground/earthing and bonding of an older building which does not have a common-bonding network (CBN) requires consultation on-site. It’s recommended to move towards an adequate CBN by enhancements to the ground/earthing and bonding network to reduce the mesh apertures. It will also often be necessary to improve the outdoor lightning protection system. The existing BN should be augmented with dedicated bonding ring conductors per room and per floor, and the bonding of cable ducts, troughs, racks, supporting metalwork, etc., at every possible point to create a CBN, and eventually a MESH-BN, although for practical reasons this may need to be accomplished over a period of time.

In contrast to the traditional practice of using a small number of heavy-duty conductors, it’s recommended to aim for a large conductive surface, e.g. by providing bonding at both side bars for joints in the run of a ladder type cable rack.

Where independent ground/earthing systems exist (they even used to be recommended by some older standards for specific purposes) these should be recognized as potentially hazardous and brought into the full CBN. Even if not fully meshed, they should at least have a single connection to the building's main ground/earthing terminal.

TN-C type supplies (which use PEN conductors) within a building are very bad indeed for EMC and signal integrity, and it’s recommended that such buildings are converted to TN-S supply systems (although TT and IT systems may be acceptable alternatives) with individual conductors for ground/earth and neutral throughout the building. Where for some reason PEN conductors are practically unavoidable within a building, improving the BN to a CBN and then to a MESH-BN may still not create favorable conditions for modem electronic technology. Such buildings may need to use the insulated bonding network approach described below.

Application and construction of insulated bonding networks (IBNs)

In existing buildings which don't (or can't) have a full 3-dimensional equipotential ground/earth mesh, ground/earth currents (especially during an ground/earth-fault or lightning event) can be very localized within the building. As mentioned above, TN-C supply systems (which use combined PEN conductors instead of dedicated ground/earth and neutral) can also cause problems for modem electronics due to their powerful ground/earth currents. In any of these three situations, bonding the ground/earth mesh for an electronic system block at numerous points to the building's existing ground/earth structure could encourage these undesirable continuous or transient ground/earth currents to flow via the new mesh and give rise to very powerful electromagnetic disturbances in the vicinity of the system block, upsetting operation and even causing actual damage. These considerations have, in the past, led to the belief that ground loops are bad for signal integrity and EMC. The recommended approach to this problem when installing modem electronic equipment is to modify the existing buildings as quickly as possible by:

++ improving the lightning protection system to provide adequate protection for modern electronic equipment;

++ using TN-S (or IT or TT) supply systems everywhere internally;

++ creating a highly interconnected MESH-BN type of CBN, with a multiplicity of small ground loops (ideally no larger than 3 or 4 meters on a side).

However, sometimes one or more of these objectives are not possible, or other factors prevent their achievement, and an insulated bonding network (IBN) needs to be used instead.

IBN's are never totally isolated from the CBN of the main building, but will have just a single point of connection (the SPC) between their "insulated" bonding network and the building's CBN (actual isolation may be required in truly exceptional circumstances, but only after the approval of a comprehensive safety case by expert safety engineers). The net current flowing through an IBN's SPC is ideally zero, and achieving this requires special measures to prevent the flow of common-mode currents (described later). When an IBN is completely disconnected at its SPC from the building's main bonding network it should be able to withstand at least 10kV of potential difference without arcing or leaking to the main building, so that it won’t allow surge currents from the rest of the building to flow into the IBN during foreseeable lightning events.

An IBN is an example of a single-point (or star) connected bonding network, and a building might contain a number of them, of a number of different configurations. A two or three-dimensionally meshed IBN is known as a MESH- IBN, and any IBN that contains system blocks may include bonding mats to provide SRPPs for these blocks. The construction of these mats is as described earlier, including their bonding ring conductor, except for the fact that they are not connected to the building's CBN other than at the SPC.

--- Examples of various star IBNs and star-MESH IBNs

The combination of one or more IBNs with the CBN of a building is called a hybrid bonding arrangement in IEC 61000-5-2.

Increasingly, modem EMC standards are taking a tougher line than IEC 61000-5-2 towards the use of IBNs. Partly this is because the proper maintenance of an IBN is increasingly difficult, and partly because their utility is steadily being eroded as the electromagnetic environment becomes more polluted with ever-higher frequencies, and as the operating frequencies of electronic equipment continues to increase. IBNs can never be truly insulated at all frequencies: the inevitable parasitic capacitances to the surrounding equipment and its building CBN will cause increasing currents to flow into and out of the IBN at higher frequencies, destroying its effectiveness.

Insulated mesh-bonded systems are described by a number of standards, including ANSI/TIA/EIA-607, but are probably most fully defined by ITU Recommendation K.27, which is referenced by many other more recent standards. ---considers that IBNs are only appropriate for telecomms buildings in exceptional circumstances or due to some unavoidable incompatibility with pre-existing equipment.

When implementing an IBN, says that co-ordination is needed regarding the routing of cables and the bonding of their screens, and also says that inspection, monitoring, and maintenance procedures are needed throughout its operational life to protect it from degradation.

The single-point connection (SPC) is the unique location in a MESH-IBN where a connection is made to the CBN of the rest of the building. The SPC is also used for bonding all the metallic services to the insulated area (water pipes, air ducts, etc.) as well as all cable amour, cable screens, the filters of unscreened cables, surge protection devices. Power cables should also have their protective ground/earth conductors bonded to the mesh at the SPC, as should any other cables containing a protective or functional ground/earth conductor. In reality, the SPC is not usually a point, but must have sufficient size for the connection of conductors. The single-point connection window (SPCW) is the interface or transition region between an IBN and the CBN (for the building). All metallic conductors and services (including any fiber-optics which are not metal-free) must enter the IBN via this window. Maximum dimension of an SPCW is typically 2 meters. The SPC can be a copper bus-bar, such as a normal ground/earthing bar. However, where a number of cable shields or coaxial outer conductors (e.g. armor), or filters and surge protection devices, are to be connected to the SPC, it would be better realized as a frame with a grid or sheet-metal structure. Reference suggests using a "transient suppression plate" at the SPCW. This plate should have an area > 1m^2 and be used as a potential reference onto which all EMC-related components are bonded.

The ideal connection to building ground/earth for the SPC (SERP) of an insulated mesh is the metalwork of the low-impedance cable tray or duct that carries the cables for the new room. Where such a duct does not exist, a bond with as low an impedance as possible should be made to the nearest part of the building's ground/earth structure. Consider also fitting the cables to the room with wired parallel ground/earth conductors ( PECs) at least for each different cable route ( PECs). There is a lot more to the successful realization of an IBN than its ground/earthing and bonding. Insulated systems attempt to interrupt the common-mode current by creating high impedances to its flow. Methods include un ground/earthed systems, isolation transformers, opto-couplers, fiber-optics, power conditioners, and Class II equipment (i.e. double insulated and un ground/earthed). But such attempts at isolation are fraught with problems and great care must be exercised if they have to be used in the absence of any alternative. The most trouble-free way to prevent common-mode currents from flowing into IBNs is to use metal-free fiber optics.

--- The insulated bonding network in an unmeshed building

Problems and special concerns with IBNs

Metalwork which is connected to the building's BN, but is near to an IBN, may need to be ground/earthed to the IBN's SPC to prevent electric shock or flashover in the event of a lightning strike. Other safety considerations, including insulation breakdown voltage and common-mode withstand voltage, and disconnectability, all need to be taken into account when designing the insulation means for an IBN and design guidelines and installation instructions are given in. There is a concern that craft-persons and other non-expert personnel may accidentally compromise the insulation of an IBN. Violation of an IBN's insulation due to maintenance (or any other) work may lead to failures in system operation or even physical damage during lightning or power faults. Hence the successful use of IBNs relies upon implementing appropriate control procedures, including regular maintenance and inspections. Looking back at successful star ground/earthing installations (which we now recognize as IBNs) that have helped to create the traditional view that the creation of ground loops is a bad thing, we notice that they were often broadcasting facilities (such as the BBC or ITA) or other enterprises with a high proportion of very skilled electrical engineers.

The skilled engineers were needed on a daily basis to ensure continued operation of the facility, and in many cases design and /or maintain the electronic equipment, and this used to be considered an acceptable overhead. Comparable modern facilities contain a very great deal more electronic technology operating at vastly higher frequencies, the detailed internal operation of which is mostly unknown even to the now very small proportion of skilled electrical engineers who may be employed. Some modern installations employ electrical engineers with almost no knowledge of modern electronics technology at all, this being left to the IT department whose computer science courses left them completely unaware about electrical safety or any of the issues covered by this guide. Most programming or IT support staff, and quite a few operational staff, would feel equal to the task of stringing a long data cable between two computers in different parts of their building (or even between buildings), or running an extension lead from an available mains socket in a nearby room, and would not even conceive that there were vital electrical safety and reliability issues involved. This is a serious worry, and one of the main reasons why IBNs and star ground/earthing techniques are increasingly not recommended for general use.

Maintaining ground/earth-bonding networks

Regular inspection and testing of ground/earth-bonding networks should include the continuity testing of:

++ cable supports

++ bonding straps

++ cable screen terminations

++ connections and also the checking and testing of:

++ cable entry points to segregated areas

++ corrosion effects (especially on ground/earthing systems)

++ integrity of equipment zoning (segregation)

++ surge protection devices and filters

++ the insulation of any IBNs.

In addition, stringent monitoring of all electrical installation extensions and alterations must be implemented, and records kept.

Non-IT installations

So far these guidelines on ground/earthing and bonding have tended to reference IT (i.e. information technology, including telecommunications) standards and conference papers. This is because these are very large industries in their own right with a strong commitment to standards making.

However, the best practice techniques described here are not limited to areas intended for computers and /or telecommunications, they are of general applicability, especially where electrically sensitive equipment (such as some type of instrumentation) or electrically noisy apparatus (e.g. metal welding, induction heating, plastic welding) is involved.

Many industrial installations consist of machines with associated dedicated control panels. As long as these are individually mesh-bonded to the metalwork of their machines (assuming the machines use metal construction) by taking advantage of all the cable trays, conduits and armor and adding additional bonding conductors where necessary, there should be no EMC problems as long as the panel only communicates signals to and from its own machine. Increasingly, machines are networked together, and /or to some central SCADA or other managerial system. As long as the interconnecting cables use a galvanically-isolated communications method specifically intended for the electromagnetically harsh industrial environment (Fieldbus, CAN, etc.) the vast majority of installations won’t need to go any further (e.g. by using an SRPP, such as a bonding mat). However, some industrial processes (such as the manufacture of DRAMs and similar semiconductors) are extremely sensitive to electromagnetic disturbances, and require the full metal flooring and mesh-bonding technique, even sometimes to the extent of screening entire rooms or even whole buildings.

A good example of another difficult installation might be a scientific laboratory where experiments require high powers and /or high frequencies as well as sensitive detectors, such as work with particle accelerators and cyclotrons. Segregation of the high power and /or high frequency equipment from the detectors is required (preferably by putting them in different all-metal shielded rooms), and each apparatus area should employ the ground/earthing techniques described above for "demanding requirements". Hospitals sometimes have similar requirements, e.g. the use of sensitive and life- critical anesthetic machines on a patient simultaneous with the use of high-powered RF diathermic "knives". Clearly in such a situation the option of placing each item of equipment in its own metal room is not feasible, and the only recourse is to specifying sufficiently robust equipment and checking that the final system works as intended, taking remedial actions as necessary.

Professional audio or video installations can also be difficult installations because of their use of low-level signals and the very high signal-to-noise ratios demanded.

Indeed, it’s commonplace for complex audio and video systems not to function to specification when first installed. Installers of such systems should take heart from the fact that the best-practices described in these guidelines are known to work very well in pro-audio applications. Unfortunately it appears that many manufacturers of pro- audio/video equipment don’t design them in such a way as to allow the benefits of best-practice ground/earthing and bonding techniques, especially when it comes to I/O design and cable screen bonding. Also unfortunately, many professional audio and video installers have a traditional horror of ground/earth loops, and as a consequence spend many days, or even weeks, "debugging" their traditionally designed installations before they even begin to function adequately, sometimes even compromising safety by removing protective ground/earth conductors to try to break a ground loop.

Embracing these guidelines in full, replacing "traditional" pro-audio/video techniques where necessary, and using equipment designed to take advantage of these modem techniques won’t only save time and money and reduce commissioning times, but will also provide improved signal quality, higher reliability, and a lower risk that future extensions and modifications will have a negative effect on signal quality.

Overall, the cost of ownership will be reduced by using these EMC best practices.

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