Brian, Referring first to the IEV (IEC 60050) we get these related definitions:
IEV number 195-01-13 <http://www.electropedia.org/icons/ecblank.gif> <http://www.electropedia.org/icons/ecblank.gif> <http://www.electropedia.org/icons/ecblank.gif> <http://www.electropedia.org/icons/ecblank.gif> functional earthing, functional grounding (US) <http://www.electropedia.org/icons/ecblank.gif> <http://www.electropedia.org/icons/ecblank.gif> earthing a point or points in a system or in an installation or in equipment, for purposes other than electrical safety IEV number 195-02-15 <http://www.electropedia.org/icons/ecblank.gif> <http://www.electropedia.org/icons/ecblank.gif> <http://www.electropedia.org/icons/ecblank.gif> <http://www.electropedia.org/icons/ecblank.gif> functional earthing conductor, functional grounding conductor (US) <h tp://www.electropedia.org/icons/ecblank.gif> <http://www.electropedia.org/icons/ecblank.gif> earthing conductor provided for functional earthing <http://www.electropedia.org/icons/ecblank.gif> IEV number 195-02-16 <http://www.electropedia.org/icons/ecblank.gif> <http://www.electropedia.org/icons/ecblank.gif> <http://www.electropedia.org/icons/ecblank.gif> <http://www.electropedia.org/icons/ecblank.gif> functional bonding conductor <http://www.electropedia.org/icons/ecblank.gif> <http://www.electropedia.org/icons/ecblank.gif> conductor provided for functional-equipotential-bonding <http //www.electropedia.org/icons/ecblank.gif> IEV number 195-02-17 <http://www.electropedia.org/icons/ecblank.gif> protective earthing and functional earthing conductor, protective grounding and functional grounding conductor (US) conductor combining the functions of both a protective earthing conductor and a functional earthing conductor >From IEC 60204-1-05 we get: 3.27 functional bonding equipotential bonding necessary for proper functioning of electrical equipment 3.43 protective bonding equipotential bonding for protection against electric shock NOTE Measures for protection against electric shock can also reduce the risk of burns or fire. 3.44 protective bonding circuit protective conductors and conductive parts connected together to provide protection against electric shock in the event of an insulation failure 3.45 protective conductor conductor required for protective bonding by some measures for protection against electric shock for electrically connecting any of the following parts: – exposed conductive parts; – extraneous conductive parts; – main earthing terminal (PE) [IEV 826-13-22, modified] 8 Equipotential bonding 8.1 General This Clause provides requirements for both protective bonding and functional bonding. Figure 2 illustrates those concepts. Protective bonding is a basic provision for fault protection to enable protection of persons against electric shock from indirect contact (see 6.3.3 and 8.2). The objective of functional bonding (see 8.3) is to minimize: – the consequence of an insulation failure which could affect the operation of the machine; – the consequences of electrical disturbances to sensitive electrical equipment which could affect the operation of the machine. Normally functional bonding is achieved by connection to the protective bonding circuit, but where the level of electrical disturbances on the protective bonding circuit is not sufficiently low for proper functioning of electrical equipment, it may be necessary to connect the functional bonding circuit to a separate functional earthing conductor (see Figure 2). I've made Figure 2 available at http:// ww.dougnix.net/Life/IEC_60204-1_Figure_2.html Based on all the forgoing, I would define the two as follows: Protective earth,and therefore symbol 5019, is the system used to prevent shock and maintain touch voltages on exposed conductive metal parts of the product to non-hazardous levels. The system also provides a means to ensure that protective devices will open the supply circuits to the product in the event that a fault occurs between a phase conductor and the chassis or a conductive metal part of the product. Functional earth and therefore symbol 5017, is the system used to provide an equipotential reference point for the electrical system EXCLUDING the safety functionality provided by the Protective Earth system. This could be used as a reference point for instrumentation (although this is more commonly done with a "noiseless earth" connection), or a common for any other non-safety related electrical function. I would argue that the Neutral Conductor represents a Functional Earth application, as it is the common reference in systems that use it, and it does not have a safety function. Referring to Keith Armstrong and Tim Williams book "EMC for Systems and Installations", p.102/103: 5. 1. 1. 1 Safety earth The purpose of the safety earth is to guarantee personnel safety under fault conditions. The lEE Wiring Regulations (BS 7671, [114]) define "earthing" as Connection of the exposed conductive parts of an installation to the main earthing terminal of that installation. Earthing ensures the provision of a low impedance path in which current may flow under fault conditions. Exposed conductive parts are those conductive parts of equipment which may be touched and which may become live in the case of a fault. The earthing connection prevents such live parts from reaching a hazardous voltage. Equipotential bonding, in this context, is a means of electrical connection intended to maintain various exposed and extraneous conductive parts at substantially the same potential under both operational and fault conditions. The protective conductor (typically colour coded green-and-yellow) provides this means and also connects the conductive parts to the installation's main earthing terminal. The prospective touch voltage within the installation is then the product of the impedance of the protective conductor and the earth fault current. Equipotential bonding therefore creates an equipotential zone within which exposed and extraneous conductive parts are maintained at "substantially" the same potential. It is worth noting that although the voltages within such a zone may be safe, they are not necessarily, and not even usually, zero. Continuous currents from various sources, including equipment earth leakage, are likely to be flowing, even in a "healthy" circuit. Allowable earth leakage levels from individual items of equipment are covered in section 4.4.1.1 on page 97. An "equipotential" zone may protect people but may not protect equipment or wiring. Moreover, within a large installation there may be more than one "equipotential" zone for safety purposes and the voltages existing between them may well be large and undefined; special precautions need to be taken for signal wiring (such as local area networks) that may cross such zones, to prevent them importing potentially hazardous voltages. Protection against electric shock is typically provided by earthing in conjunction with automatic disconnection of the supply. For this purpose, it is vital to co-ordinate selection of the protective device (for instance, fuse, circuit breaker or RCD) with the installation's earth fault impedance, to ensure that disconnection occurs sufficiently rapidly to prevent the touch voltage from rising high enough to cause a shock. The sizing and hence resistance of the earth protective conductor will therefore be determined largely by the prospective fault current available from the rest of the system. In the case of protective multiple earthing (PME), the protective conductors may take substantial continuous currents, even when the supply is off, as a result of circulating currents in the bonding network, and therefore may need over-rating. Since the concern is currents at low frequencies, it is resistance rather than inductance which determines the conductor impedance; this is not the case for high-frequency earths, as we see later. 5.1.1.2 Functional earth In order for an electrical circuit to interface correctly with other equipment, there must be a means both of relating voltages In one equipment to those in another, and of preventing adjacent but galvanically separate circuits from floating. This is the purpose of the functional earth and it must be distinguished from the safety protective earth. Earth conductors which are used for functional purposes only need have no requirements for sizing according to safety but should be coloured cream (according to BS767l) to identify them. Even so, because of the threat of circulating currents and potential differences between earthing zones, there may be other practical constraints on the widespread use of functional earthing on large systems. Signal circuits of equipment should normally be specified for a maximum common mode voltage, which will be the voltage that appears between different parts of a functionally-earthed system. If this voltage is likely to be exceeded, implementation of a Common Bonding Network (CBN, see 5.1.3.3) in the system is recommended. If this is impractical or inadequate, isolated circuit interfaces are the normal solution. I hope that helps! -- Doug Nix, A.Sc.T. IEEE PSES Toronto Chapter Toronto Section, Ontario, Canada [email protected] mobile (519) 729-5704 fax (519) 653-1318 Find me LinkedIn at http://www.linkedin.com/in/dougnix On 22-Sep-09, at 2:10 PM, Kunde, Brian wrote: Functional Earth Terminals - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]>
