NOVEMBER 2006 Effective earthing of screened cables All metallic containment items, other extraneous metalwork and the screens of telecommunications cables in an information technology environment must be earthed; for two reasons; 1. For safety purposes; so that no dangerous voltage can exist on exposed metal items in the circumstances of a fault arising. It is a legal requirement in the UK 2 to earth so-called extraneous metalwork 2. To provide a low impedance path to ground to high frequency electrical noise. A safety earth or protective earth may be very effective at 50 Hz but may exhibit much higher impedance at higher frequencies All exposed conducting surfaces and other extraneous conductive parts must be connected to an earth point for safety purposes. This is to ensure that if by accident a live conductor touched these parts the resulting circuit made to earth would cause such a large fault current that a fuse or a circuit breaker would blow somewhere. If the extraneous conductive part was not connected to earth, or left floating, then it would maintain the live voltage on it until somebody came along and touched it; and then that person s body would form the conductive path to earth. In a structured cabling system the equipment racks, the active equipment, the metal patch panels and conduit, tray, trunking and all cable containment would all be considered as extraneous conductive parts and must be effectively earthed. In a screened cabling system all of the screening elements of the cables, patch panels and connectors must also be earthed for the screening process to be effective. Floating cable screens would be completely ineffective against interference and would also be considered a hazard as extraneous conductive parts.
Definitions There is some difference between American and British English in the terminology. Grounding is an American expression not often seen in European standards where the expression earthing is more common. There is also a difference with screened cable terminology. Americans would tend to use the term shielded (although the American standard TIA/EIA-568-B uses the abbreviation ScTP to describe them) whereas British English users would call them screened cables. Both expressions amount to the same thing. Earthing Connection of the exposed-conductive parts of an installation to the main earthing terminal of that installation (BS 7671) Equipotential Bonding Electrical connection putting various exposed conductive parts and extraneous conductive parts at a substantially equal potential (EN 50174-2) Grounding A conducting connection, whether intentional or accidental, between an electrical circuit (e.g. telecommunications) or equipment and the earth, or to some conducting body that serves in place of earth. (TIA/EIA-607) It is not intended to offer protection to either the user or the equipment. Examples of the uses of a Functional Earth are; To provide a zero-volt reference point To enable an electromagnetic screen to be effective To provide a signalling path for some types of communications equipment Electromagnetic performance Cabling screens should be properly bonded to earth for electrical safety and to optimise electro-magnetic performance. All cabling components that form part of a screened channel should be screened and meet screening requirements. Cable screens shall be terminated to connector screens by low impedance terminations sufficient to maintain screen continuity necessary to meet cabling screening requirements. Suppliers instructions as how to make low impedance terminations shall be asked for and observed. Work area, equipment cords and the equipment attachment should be screened and shall provide screen continuity. Functional Earth A Functional Earth, FE, is provided only to enable equipment to function properly.
Earthing Earthing screened cables Earthing and bonding shall be in accordance with applicable local electrical codes or IEC 60364-1. All screens of the cables shall be bonded at each distributor. Normally, the screens are bonded to the equipment racks, which are, in turn, bonded to building earth. The bond shall be designed to ensure that the path to earth shall be permanent, continuous and of low impedance. It is recommended that each equipment rack is individually bonded back to an earthing terminal, and not looped from one rack to another. This is to assure the continuity of the earth path should any rack be individually disconnected. What the standards say about earthing screened cables First of all, all of the standards agree that that screened cables must be earthed. EN 50174 clause 6.3.2 Screen not bonded to equipment, not recommended En 50310 clause 5.5 Cable screens shall be bonded directly to racks, cabinets or where required directly to the dedicated Signal Reference Potential Plane. Circumferential connections are most effective. The cable screens provide a continuous earth path to all parts of a cabling system that are interconnected by it. This bonding ensures that voltages that are induced into cabling (by any disturbances) are directed to building earth, and so do not cause interference to the transmitted signals. All earthing electrodes to different systems in the building shall be bonded together to reduce effects of differences in earth potential. The building earthing system should not exceed the earth potential difference limits of 1-volt r.m.s 1 between any two earths on the network. (Root Mean Square: this is like an average because it can mean any waveform whose total energy content is the same as would be delivered by one volt d.c.) BS7671 clause 413-02-2 To comply with the regulations it is also necessary to apply equipotential bonding to any metallic sheath of a telecommunications cable. (The consent of the owner must be obtained). IEC 60364-4-41 clause 413.1.2.1 The main equipotential bonding shall be made to any metallic sheath of telecommunication cables. However, the consent of the owners or operators of these cables shall be obtained. ANSI/TIA/EIA-568B-1.2 clause 4.6 The screen of ScTP cables shall be bonded to the telecommunication grounding busbar (TGB) in the telecommunication room.
Grounding at the work area is usually accomplished through the equipment power connection. Screen connections at the work area shall be accomplished through an ScTP work area patch cord extending from the TO to either the equipment, or to the personal bonding terminal. At the work area end of the horizontal cabling, the voltage measured between the screen and the ground wire of the electrical outlet used to supply power to the workstation shall not exceed 1.0 V r.m.s. and shall not exceed 1.0 volt d.c. The cause of any higher voltage should be removed before using the cable. Effective screen connections Screened cables must be effectively earthed for the screening elements to be at all effective. Ideally the cable screen would be effectively bonded to earth at both ends for maximum EMC protection, but in many cases this is not possible, with the outlet end terminating inside a plastic box inside a dry plaster wall. This is the only point where an installer can really make any guarantee that at least one high quality bond to earth has been made on the screened cabling. The screen itself should be 360 0 bonded to the patch panel earthing mechanism. Simply connecting the drain wire seen in some foil-screened cable to an earth terminal is not enough. The whole screening element must be covered by a 360 0 clamp and clamped down to an earth plate whose surface area is larger than the end cross-sectional area of the cable. Figure 1 shows the difference in impedance of simple green earth wire compared to a copper bonding strap across a range of frequencies 3. We can see that all sorts of resonance effects come into effect depending upon the length of the earth connection. It is presumed that the cable screening will find its way to earth via the screened jacks connecting with the screened patchcords which in turn connect through the terminal equipment and into the earth connection of that equipment s mains supply. It is essential therefore to ensure an effective equipotential earthing connection is made to the screened patch panels in the equipment / telecommunications rooms racks. Figure 1
Earthing the rack Before the patch panel can be earthed within an equipment rack we need to ensure that the rack itself is correctly earthed. The following list is adapted from the TIA Technical Information Notice 016 4. 1. An earth cable must be installed from the cabinet to an earth bar or earth terminal. The main connection point for the room is recommended to be a Telecommunications Ground Busbar, TGB, minimum dimensions 6 mm thick by 50 mm wide 5. 2. The earth connections at either end must be a screw terminal. Earthing via a 13 amp mains plug is not acceptable. 3. The cross sectional area of the earth cable shall not be less than 4 sq mm or 6 sq mm if the distance is over 10 metres. Note that American standards 5 require a minimum conductor size of 6 AWG or 4.1 mm diameter, 13.3 sq mm 7. Earth connections should be kept as short as possible and as straight as possible to avoid creating a high impedance path to high frequency EMI 8. Do not coil earth cables or loop them back on themselves 9. If the cabinet is to accommodate a shielded cabling system it is good practice to use two earth connections of different lengths to avoid resonance effects at different frequencies. It is even better to use earth bond copper braid straps. The ratio of width to length should be less than 5:1 6 10. It is bad practice to use non-copper conductors for earthing, e.g. steel armouring or conduit. 11. Metallic items within a cabinet, e.g. shelves, panels and cable organisers etc, shall be bonded. This should be with a 2.5 sq mm bond as a minimum. 4. The earth cable must be insulated and should not exceed 20 m in length 5. Each cabinet must be earthed to the terminal bar separately, not daisy-chained 6. A permanent label or tag should be used at the connection point at the cabinet marked with the words Safety earth Do not remove
Earthing within the rack 1. It is recommended that a Connecting Hardware Screen Bonding Conductor Busbar, SBCB, be formed within the cabinet. The ideal is a copper bar running the height of the cabinet with short connections from each piece of rack equipment that needs earthing. 2. Alternatively one short busbar may be formed at the top or bottom of the rack where the hardware bonding conductors are brought back. The minimum size of the screen bonding conductors should be 14 AWG 7,8 i.e. 1.6 mm dia, 2.08 sq mm. The nearest metric equivalent is 2.5 sq mm, which is recommended. Figure 2 3. The third option is to use the metal frame of the cabinet/rack itself as the SBCB and to connect each item hardware to that. The rack must them be connected back the TGB The screen bonding connector busbar/rack must be connected back to the TGB by copper cables described in point 3 of the earthing the rack table above. Figure 2 gives an example of an equiment rack with an internal busbar. Figure 3 shows an example with a rack earthing stud. Figure 3
Connectix and screened cabling systems Connectix has many years experience of supplying screened cabling products into the European market. The introduction of 10GBASE-T has re-ignited the screened versus unscreened debate as IT professionals now consider the practicalities of trying to keep alien crosstalk, and all external interference, out of UTP cables at frequencies of up to 500 MHz. as the C5e and C6 panels namely a 600-mm length of 2.5 sq mm earth wire with a ring terminal at the end. Connectix also supplies a complete range of equipment racks and frames which have 2 x M6 Earthing studs in the base and 2 in the top. Additionally each door and removable panel has an earth tag - which by use of the optional earth bonding kit, may be bonded back to the earth stud. Connectix has recently introduced Net 10G, a fully shielded and independently verified, cabling system for the new 10GBASE-T market. The range consists of screened jacks, which can be used at the wall outlet or in the patch panel, a screened cable and screened patch cords. The screened cable consists of foil-insulated pairs with an overall copper braid. The combination of aluminium foil and copperbraid gives much better EMC protection over a wider range of frequencies than designs using aluminium foil alone. The copper braid also allows better electrical connection at the termination points and gives a more effective ground when only one end of the cable can be bonded, e.g. in the equipment rack. Connectix server cabs have additional provision for an optional 10-way 80A earth bar (with screw terminals). Conclusions The ideal earthing system for an IT installation would consist of a Signal Reference Grid or SRG, which is a copper mesh or grid located beneath the raised floor of the computer room. Everything crossing the grid, from cables to equipment racks, would be connected to it by low impedance bonding straps. Effective bonding at the patch panel is essential for the successful operation of the cabling system and the Connectix patch panel frame bonds together all the metallic shells of the individual connectors. The Connectix Net 10G a panel has the same earthing provision
The metallic pedestals and stringers of the raised floor must also be earthed and the American standards require every 6th pedestal to be connected either to an SRG, if there is one, or the main earth terminal for the room. EN 50174-2 suggests that every second or third leg be connected via a 10 sq mm earth cable. Ultimately the cable installer can only make use of the earthing system presented to him and must take responsibility for correct and optimal termination of the screened cable, earthing of the screened patch panel and earthing of the cabinet within which they reside. References 1. ISO 11801 2nd Edition Information technology cabling for customer premises 2. BS 7671:2001 Requirements for Electrical Installations ~ IEE Wiring Regulations Chapter 54 Earthing Arrangements and Protective Conductors. SI 1989 No 635 The Electricity at Work Regulations requires at: Regulation 8 Precautions shall be taken, either by earthing or by other suitable means, to prevent danger arising when any conductor (other than a circuit conductor) which may reasonably foreseeable become charged as a result of either the use of a system, or a fault in a system, become so charged, Regulation 9 If a circuit conductor is connected to earth or to any other reference point, nothing which might reasonably be expected to give rise to danger by breaking the electrical continuity or introducing high impedance shall be placed in that conductor unless suitable precautions are taken to prevent that danger. 3. IEEE 1100:2005 Recommended Practice for Powering and Grounding Electronic Equipment 4. TIN 016:2003 Telecommunications Industry Association, Technical Information Notice sixteen 5. ANSI/TIA/EIA-J-607 Commercial Building Grounding and Bonding Requirements for Telecommunications 6. EN 50174-2, Information technology Cabling installation Part 2:Installation and planning practices inside buildings 7. ANSI/TIA/EIA-568-B.1-2-2003 Commercial Building Telecommunications Cabling Standard - Part 1: General Requirements - Addendum 2 - Grounding and Bonding Requirements for Screened Balanced Twisted-Pair Horizontal Cabling 8. NEC 800.100(A)(3) requires at least a 14 AWG [1.6 mm (0.063 in)] stranded or solid insulated conductor for connecting the ITS protectors and associated metallic cable sheaths to the selected ground. Other NEC ground requirements generally indicate a 6 AWG [4.1 mm (0.16 in)] minimum. A 6 AWG [4.1 mm (0.16 in)] stranded conductor should be used because this accommodates different code requirements and allows future changes.
9. JSP 480 9th Ed 2006 Defence coordinating installation design authority manual of regulations for installation of communications and information systems Bibliography ANSI/NECA/BICSI 568-2001 Standard for Installing Commercial Building Tele-communications Cabling BS 6701:2004 Code of practice for installation of apparatus intended for connection to certain telecommunications systems IEC 60364-4-41 Electrical installation of buildings - Part 4-41: Protection for safety - Protection against electric shock IEC 60364-5-548 Electrical installation of buildings - Part 5: Selection and erection of electrical equipment Section 548: Earthing arrangements and equipotential bonding for information technology installations IEC 61024 Protection of structures against lightning IEC 61140 Protection against electric shock BS 7671 Requirements for Electrical Installations, also known as the IEE Wiring Regulations 16th Edition and its corresponding Guidance Note No. 5 Protection Against Electric Shock EN 41003 Particular safety requirements for equipment to be connected to telecommunication networks EN50310 Application of equipotential bonding and earthing in buildings with information technology equipment ISO/IEC TR 14763-2 Information technology Implementation and operation of customer premises part 2: Planning and installation ITU-T K.27:1996 Bonding configurations and earthing inside a telecommunication building ITU-T K.31:1993 Bonding configurations and earthing of telecommunication installations inside a subscriber s building ITU-T Handbook Earthing of telecommunication installations (Geneva 1976) ETS 300 253 Equipment Engineering (EE) Earthing and bonding of telecommunication equipment in telecommunication centres IEC 60364-1 Electrical installation of buildings Part 1: Fundamental principles, assessment of general characteristics, definitions. UK Head Office: Connectix 33 Broomills Industrial Estate Braintree, Essex CM7 2RW