Why Earthing So Important Electrical Safety for your Equipment's and Human Livrs

Electrical Safety for your System Know your Earthing and Protection

In Project Banks and Insurance Agencies prefer to know about Electrical Safety Like Earthing , Lightning and Design to ensure Safety and minimize of RISK Grounding can be the ultimate common cause due to its ubiquitous nature of connecting everything electrical together, and a great unknown because much of it and its operation are invisible to us.

This article discusses what makes up a good instrument grounding system and why. It doesn’t cover all the details about instrument grounding systems (which would take a book or two), but rather some of the basic principles that lead to good practices, as well as their technical basis.

First, we must divide the instrument systems into the incoming power side, which is nominally the AC side, and the instrument side, which is nominally the DC side (Figure 1). The DC side can be further divided into the DC power side (nominally 24 VDC), control and signal. The AC and DC power sides are normally isolated through transformers, while on the DC side, the power and signal are shared. Our discussion will be primarily about the DC side, but for reasons described later, the grounding system is typically shared by both sides.

Why we ground The main reasons we ground our systems are: Personnel safety, Electrical system protection, Lightning protection, Electrostatic discharge protection, Electrical noise control, Intrinsically safe circuits, Power quality, and To provide a reference plane for our electrical and electronic circuits and systems.

Personnel safety: This is primarily a concern on the AC (high-voltage) side of the instrument system.

Maintenance personnel test and repair instrument systems, and the operator may also interact with the instrument system’s front end and field instruments. Because of the low voltage (nominally 24 VDC), the instrument side is often worked without concern about electrical shock. An instrument tech may be in for a rude surprise, even a fatal one, when working on an instrument circuit where an ungrounded DC instrument circuit has come into contact with a higher voltage source (e.g. 120 or 277 VAC) that didn’t trip the high-voltage circuit overcurrent protection.

The National Electrical Code (NEC) Article 250 and other application-specific NEC articles provide requirements for grounding for personnel safety. This is a U.S. code, also known as National Fire Protection Association (NFPA) 70, which is used worldwide. However, each country or legal identity may have their own electrical code, or provide additional requirements to the NEC. You can’t take exception to requirements of the NEC or similar codes just because the system is an instrument system, not an “electrical” system. Equipment grounding and bonding are used to help ensure that there's a low impedance path back to the source during the fault conditions. This allows the system overcurrent protection to open up, protect the electrical system and remove dangerous voltage from the circuit in a timely manner.

Common codes and standards for grounding for personnel safety are: NFPA 70, National Electric Code (NEC), Article 250 and specific application articles.

IEC 60364, Electrical Installations for Buildings, Part 5, Section 54 IEEE 142 Std. – IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems (commonly called the Green Book) IEEE Std. 80 - IEEE, “Guide for Safety in AC Substation Grounding” Electrical system protection: The NEC also provides requirements for electrical protection to limit the damage to equipment and wiring. This is also safety-related as it minimizes the potential of a fire caused by an electrical source. A properly designed grounding system for the AC side of our instrument system can minimize the potential damage to equipment from an electrical fault, surge, lightning strike, etc., and contribute to the reliable operation of the equipment.

Common codes and grounding standards for this are essentially the same as those for electrical safety given above. NEC Article 250-50 also requires that all grounding electrodes that are present at each building or structure served shall be bonded together to form the grounding electrode system (commonly called a ground grid in petrochemical facilities).

This has given rise to the one of the most controversial aspects of grounding—whether it is wise or necessary to connect the DC side of the instrument system to that noisy, nasty electrical safety ground. In the early days of DCS systems, manufacturers commonly called for an isolated, clean ground.

This requirement has for the most part been superseded, but still raises its ugly head occasionally, both as a manufacturer’s requirement and in questions raised on various Internet forums. The answer to the question by NEC is a solid “yes.” Later, we'll talk about why this is actually a good idea, as well as the fact that there is no such thing as a “clean” ground.

Electrical noise control: Noise is any electrical signal present in a circuit other than the desired one. All electrical and electronic circuits have noise, which becomes interference when it has an undesirable or detrimental effect on the operation of a circuit or system. Always a concern in instrumentation and electronic systems, noise is not as significant in electrical systems (though modern electrical equipment often has digital monitoring and communication systems).

A common refrain is that ground is a place to drain your noise. The ground is not a sump for noise, and can actually be a source of noise. A basic principle of circuit electricity is that electricity always seeks to return to its source. This principle applies to noise: once coupled into a circuit, noise always works in complete circuits, and ground can serve as a return path for noise.

Most noise of interest is coupled into the instrumentation circuits by four methods: capacitive, inductive, radiated or conducted (Table 1). We will talk more about this when we talk about grounding of shielding.

Intrinsically safe circuits: For facilities that use intrinsically safe (IS) circuits to satisfy the requirements for instruments in classified hazardous areas, grounding can be an issue for certain types of intrinsic safe barriers. Zener barriers (Figure 2) require a high-integrity ground to shunt any dangerous electrical energy. The nominal grounding specification is a maximum resistance of 1 ohm. The intrinsic safety ground is connected to the plant safety ground grid. For the intrinsic safety ground, the concept of an equipotential ground plane is important to ensure that the ground potentials in the intrinsically safe circuit are as equal as we can make them to prevent a spark due to a voltage differential between circuit parts and ground. Transformer or galvanically isolated barriers typically do not require a ground connection.

IS Zener barrier Figure 2: Zener barriers in intrinsically safe (IS) systems require a high-integrity ground with a maximum resistance of one ohm. The concept of an equipotential ground plane is important to prevent a spark due to a voltage differential between circuit parts and ground.

The standards for intrinsic safety are: ISA RP 12.06.01, “Recommended Practice for Wiring Methods for Hazardous (Classified) Locations Instrumentation Part 1: Intrinsic Safety” ANSI/ISA 60079-11 (12.02.01) – “Explosive Atmosphere – Part 11: Equipment protected by intrinsic “i”” NEC Article 504 – “Intrinsic Safe Systems” Power quality: A stable ground reference is important for power quality, particularly with distributed control systems. Good grounding practices are also necessary for surge protection devices to work properly. Following the grounding practices of the NEC and the IEEE std.1100, “Recommended Practice for Powering and Grounding Electronic Equipment Grounding is also used to establish a common, stable voltage reference, so complex and sometimes widely distributed instrument systems can understand each other’s signals. Circuits work much better if they have a common reference between them.

Common ground Grounding can be a technically difficult subject primarily because of its complexity (breadth of scope, infinite number of potential connections, internal and external actors and bad actors, etc.,) and its uncertainty due to invisibility (can’t see what is going on below the surface, the ground is different anywhere you look, unknown influences, limited available models that can help the practicing engineer, etc.). It also can raise the specter of many electrical engineers’ least favorite subject—electromagnetic fields and Maxwell’s equations—when circuit theory isn’t enough. Fortunately, much of the basics can be understood by analogy and a bit of circuit theory. This can lead us to some good engineering practices in regards to grounding instrumentation systems.

We discuss three topics that commonly arise when engineering instrument grounding. The first is here and the others are in Part 2: What should the ground resistance be?

Do I have to connect my clean instrument ground to that dirty power ground?

Do I connect my shields to ground at one end or both?

What should the ground resistance be?

A common grounding question is what the ground resistance should be for a DCS/PLC system (this question applies to the DCS/PLC ground prior to any connection to other grounding systems). The National Electric Code Article 250.53 specifies that a second ground rod is required if the resistance of a single ground rod is greater than 25 ohms. The various DCS manufacturers have a recommend resistance range from one to five ohms. Communication sites specifications are typically on the order of five ohms or less.

The question also arises as to whether we should be concerned about impedance rather than resistance.

For instrument systems that have high-frequency components in the ground circuit, impedance is generally a concern for the above-ground part of the ground system as conductors tend to change from resistors to inductors as the frequency goes up. It is not as much a concern for the below-ground part of the instrument ground system.

In general, you should make every attempt to meet the manufacturer’s recommended ground resistance specification. If this is not possible, the DCS ground should be equal to or better than the associated power system ground resistance specification. Various grounding resistance specifications are given in Table 2.

Lightning has high-frequency components, and the response of the overall ground grid is impedancedriven, which should be taken into account in the design of the power ground grid. A common engineering specification for a DCS is one ohm or less to ground. There is, however, no technical reason why a DCS system will not operate at a higher ground resistance. For example, a DCS will operate correctly if it's been constructed on top of rock.

Grounding or Earthing Scheme in DCS or PLC Systems The basic difference between grounding and earthing is that when the system is grounded, it is still not connected to earth. The system has a ground bus bar inside or outside located at an appropriate place to which all internal grounding connections are returned. Once the final ground bus bar is connected to an actual earth pit or earth grid that the system gets finally earthed.

Whenever the DCS or PLC systems are grounded, they still not connected to the earth. The system has a ground bus bar inside located at an appropriate place to which all internal grounding connection is returned. Once the final ground bus bar is connected to an actual-earth pit or earth grid that the system finally earthed.

Improper earthing or grounding of Distributed Control System (DCS) or Programmable Logic Controller (PLC) may result in either mal-operation of the control system or a controller or failure of electronic cards or sometimes even embedded software gets erased.

In the case of DCS or PLC each cubicle is having a ground bus bar to which the controller chassis, shields can be connected. These bus bars are then returned to a final ground bus bar from where the connection is then taken to earth pit or earth grid.

The earth pit must have a small earth resistance (much less than 1 ohm). usually, the earth resistance can be measured by a three-probe method.

The voltage here is applied between probes E and P, and the current is measured in the loop E, C, and in between earth path. The resistance is then calculated by using Ohm’s law. The current should be accurately measured using milliampere meter. Table -1 gives distances between probes E and P and probes E and C against the depth of ground probe at point E.

Grounding or Earthing Scheme

There are four layers required for the correct and effective grounding and earthing.

Isolated local Ground (G1) Isolated Common Ground Reference (G2) Control System Ground (G3) Dedicated plant earth Grid (G4) Isolated local Ground (G1) Isolated local Ground (G1) is where power supplies, internal power component enclosures, etc., are grounded on a bus bar. This refers typically to one control system.

Isolated Common Ground Reference (G2) Isolated local Ground (G1) connection from each of the control systems, there within the realm is terminated along with the frame or cabinet or overall enclosures are individually terminated for grounding to create an Isolated Common Ground Reference(G2).

It should be noted that the enclosure earthing minimized the results of Electromagnetic interference.

Control System Ground (G3) It is where the incoming power supply isolation transformer secondary is grounded along with ground connection obtained from Isolation common ground reference(G2). This control system ground is considered as the final earth pit for that location. It can be terminated on the dedicated plant earth Grid (G4).

The control system ground (G3) or the final earth pit connected to the local control systems should be separate earth pit. Which then can be connected to a dedicated plant earth system. This control system G3 should not be shared with other plant systems.

Dedicated Plant Earth Ground (G4) The Dedicated plant earth ground (G4) may or may not exist. If exists it is supposed to have the lowest impedance. It consists of many earth pits in a grid fashion.

The cable used for grounding should be in green with yellow marks.

The ground bus bars to be used should be copper bus bars with approximately 10 mm as the thickness and 50 mm in width.

These recommendations are summarized as below.  Use a separate power distribution system for each location containing the control systems. This needs to be strictly adhered to.  Use good regulated power supply with distortion less than 2%. Tap the highest available power system voltage for feeding the isolation transformers used for electronics. If this is not possible, use UPS for supplying the power supply loads (control electronics in Power Electronic controllers and power supplies in Distributed Control Systems). However, keep the distance between UPS and the load to minimum or locate it locally.  The power supply should be free from non-repeating interruptions greater than 20 milliseconds. Otherwise, it can cause loss of system data, damage to embedded software, and finally mal-operation of the control systems.  Isolation transformers are recommended because they provide good line regulation and transient filtering.  Use proper surge suppressor devices after the isolation transformers to feed the power to the control systems. EMI filters also need to be used to avoid common mode noise injected into the control systems. UPS with isolation transformer(s), surge suppressors, and EMI filters helps in deriving the proper power supply for the control systems.  The signal and other control cables should run separately away from ac power lines, transformers, rotating electrical machines, solenoids, and other high power equipment. The recommended separation distance between signal and power cables

 The signal wires should be shielded and the shield should be connected to earth only on one side.  As far as power supply connectors are concerned, avoid ac and dc connections coming to the same connectors, especially in case of I/O boards. If not avoidable, use sufficient separation All grounding connection lengths should be kept as minimum as possible. The cables to be used for grounding should usually be green (with yellow marks), should be made of maximum number of strands (rather than a single conductor), and the sizes should be as given below

The size is also based on the incoming power supply conductors or cable size. Thus, between the two (power supply conductor or cable size If it is not possible to get the required resistance (particularly in cases where earth grid is not available), more than one pits need to be constructed and then paralleled to obtain the required earth resistance.

The Distributed Control Systems usually consist of following.  Incoming 220/110 V AC, 50 Hz/60HZ, single-phase supply normally obtained from UPS (please see the note at the end for 240 V single-phase AC supply)  Controller and related housing panels / cubicles  I/O’s communicating with field equipment. These are also called as Field / instrumentation input / outputs  Operator workstation UPS: The UPS supplies power to the system at one location or sometimes two systems at different locations. This needs to be avoided. It should supply power to only one location. The UPS should be as close to the system as possible. The system load determines the kVA of the UPS and usually at one location it does not exceed 50 kVA. The UPS voltage distortion on no load (without system connected) should be less than 1% and on load it should not exceed 2.5%. Usually, the UPS has an inbuilt step up isolation transformer whose secondary side is 110 V ac. This is the supply taken for the control system.

Cables from the UPS should be traceable, should run separately with proper separation distance in case required with respect to near by other power cables, and the neutral can be grounded provided isolation transformers are used after the Power Distribution Boards Power Distribution Boards: In each location, there are many panels / cubicles receiving the UPS power through these Power Distribution Boards. From these boards, further power distribution takes place to derive +24 V dc, and other dc regulated voltage required for the controller and I/O’s. Isolation of low kVA ratings need to be used to distribute the power supply to the DC power inputs. One terminal of the primary side of these transformers is connected to one screen, core is returned to earth and one of the terminals of the secondary is then connected to the second screen. for the connections. One isolation transformer can supply power to one or two regulated dc power supplies.  DC power supplies of SMPS type take isolated inputs as they have inbuilt EMI filters. The isolation transformers supply this power Power Electronic Systems (PES) involve an input converter or converter inverter set feeding power to dc or ac motors or acting as shunt converters. Typically, the system will take input power from a transformer or may be connected on the same ac power supply bus on which other power electronic systems are already present. Each system will have power electronic power modules (along with cooling system), controller, control logic (which can be an integral part of controller many a times), ac–dc switchgear and finally the load as the motor. Depending upon the system or the drive rating, there may be a dedicated transformer or there may not be a one. There is also a possibility that many systems may exist in one

location simultaneously. These could be from one manufacturer or from different manufacturers.

Accounting the scenario above, the earthing recommendations are given below. Grounding and earthing recommendations The isolation transformer (separate from one used for control power supply) provides necessary isolated power and dedicated grounding and earthing for the Power Electronic / Drive System. inside components of one given system (such as power modules, controller chassis, power supply chassis, cooling fans etc.) should be connected to an “Isolated Local Ground (G1a)”. This is applicable to each system. The cubicles / panels are connected with each other and returned to a ground called as G1b,. The two grounds G1a and G1b are now connected to one ground reference called as “Isolated Common Ground Reference (G2)”.

This ground G2 is now connected to an earth pit, which is called as the “Control System Ground (G3a)”.

The incoming neutral of the power supply can be connected to this ground G3a (in case it is not earthed near the transformer. Preferably, it should be earthed near the transformer in a separate earth pit). The neutral should be sized based on minimum of half the rated line current rating.

The cable connecting the inverter and ac motor (in case of VFD’s) should be an armored cable. It should be a three-core cable with symmetrically place current carrying conductors and three ground conductors, symmetrically embedded in it. The armor should be of corrugated aluminum. The armor and the ground conductors should be earthed at both ends (drive as well motor end). These end earth connections from the armor and ground conductors, and the motor frame / enclosure earthing connection should be returned separately The incoming isolation transformer body or frame should be connected to a separate “Control System Ground (G3c)” and this then should be connected to a separate earth pit This Earth pit connection can now be returned to “Dedicated Plat Earth Grid Some reference to the VFD switching frequency versus cable length

The lower switching frequency also reduces the EMI effects considerably. Higher cable lengths also produce reflecting voltage waves. LC filters on the output side of the inverter allow increased length of the cable.

Checklist for Distributed Control Systems (DCS)  Confirm the UPS capacity is adequate from factory.  Check that the UPS no load and on load voltage distortion is less than 1% and 2.5% respectively.  Check as to how far the UPS is located from the DCS. It should be within less than 20 meters.  If the same UPS is feeding two locations, check the capacity is adequate from factory and the no load and on load distortion is less than 1% and 2.5% respectively. Further check that both the DCS stations are in the vicinity of 20 meters.  Check that cables from UPS are routed separately and are not in vicinity of any other power cables. The safe distance in from other power cables should be as per Table –2.  Check that all the signal wires received from field

are shielded and are grounded as in fig. 4. Further check that these wires are not in vicinity of any power cables. If so, the separation distance should be as per Table –2.  Check that the grounding and earthing scheme is as per fig. 4 and all connections to grounds and to earth are radial as explained in Chapter 6.  Check that the earth connecting bus bars from grounds G3’s to earth pits and the earth grid G4 If the UPS has three-wire output (from secondary of its internal output transformer), the center-tapped connection should be taken to earth Grid  Check that the earthing resistance is much less than one ohm for all the earth pits Check that there is an earth inspection schedule agreed and drawn for future use.

Checklist for Power Electronic Systems (PES)  Check that the incoming power to the PES is coming from an isolation transformer and that its neutral is earthed.  Check that transformer capacity is adequate for the PES and that voltage distortion at the Point of Coupling is within acceptable limits (less than 2.5%).  Check that cables from transformer are routed separately and are not in vicinity of any other signal cables. The safe distance for the signal cables from the transformer cables.  Check that all the signal wires received from field are shielded and are grounded Further check that these wires are not in vicinity of any “other power cables”. If so, the separation distance Check that the grounding and earthing scheme and all connections to grounds and to earth are radial  Check that the earth connecting bus bars from grounds G3’s to earth pits and the earth grid G4 are sized.  Check that the earthing resistance is much less than one ohm for all the earth pits  Check that there is an earth inspection schedule agreed and drawn for future use.

Figure 1: Isolation transformer

Figure 3: Typical distribution system with grounding and earthing definitions

Figure 4 Grounding and Earthing Scheme for Distributed Control System

Important NOTE Discussion Although public distribution has made enormous headway in terms of availability of electrical power, this availability is still not always sufficient, and generator sets and uninterruptible power supplies are thus used. c the housing sector no longer accepts power cuts; c tertiary is a major computer consumer; c industry has set up in rural areas, is a major automation system consumer and is increasingly using static converters; for example, motors are controlled by a speed controller and functionally linked to a PLC. In all buildings, intelligent devices are increasingly being controlled by technical management systems (process - electrical distribution - building utilities). These digital systems, including distributed computing, nowadays require the problem-free joint existence of high and low currents; in other words, electromagnetic compatibility (EMC) is vital. A clash of technical cultures is inevitable: c electrical engineers have problems with the harmonics generated by static converters. These harmonics cause temperature rises in transformers, destruction of capacitors and abnormal currents in the neutral; c electronic engineers place filters upstream of their products, which do not always withstand overvoltage’s and lower network insulation; c lamp manufacturers are unaware of the problems caused by energizing inrush currents, harmonics and high frequencies generated by certain electronic ballasts; c computer engineers (same applies to designers of distributed intelligence systems) are concerned with equipotentiality of frames and conducted and radiated interference. These specialists sometimes have problems understanding one another and do not necessarily all have the same approach. Also, very few of them are familiar with earthing systems and their advantages and drawbacks faced with the evolution in

the techniques described above. earthing systems and disturbances in electronic systems Electromagnetic disturbances assume many different forms, namely: c continuous or occasional; c high or low frequency; c conducted or radiated; c common or differential mode; c internal or external to the LV network. Choice of earthing system is not a neutral one as regards: c sensitivity to disturbances; c generation of disturbances; c effects on low current systems Readers wishing to improve their knowledge in this area should study the following «Cahiers Techniques»: c n° 149 - EMC: Electromagnetic compatibility; c n° 141 - Les perturbations électriques en BT; c n° 177 - Perturbations des systèmes électroniques et schémas des liaisons à la terre. This section will only review the most important aspects, without describing earthing system behavior faced with MV (50 Hz) faults. Faced with harmonics The TN-C should be avoided since rank 3 harmonics and multiples of 3 flow in the PEN (added to neutral current) and prevent the latter from being used as a potential reference for communicating electronic systems (distributed intelligence systems). Moreover, if the PEN is connected to metal structures, both these and the electric cables become sources of electromagnetic disturbance. Note The TNC-S (TN-S downstream from a TN-C should also be avoided even though risks are smaller). Faced with fault currents c short-circuits: avoid separating the live conductors; otherwise the Icc creates an electromagnetic pulse in the resulting loop; c electrical earthing fault: the PE must follow the live conductors as closely as possible, or, better still, be in the same multi-conductor cable. Otherwise, as above, the transmitting loop effect appears. The higher the fault current, the greater this effect. The TT earthing system will thus be preferred, as the TN and IT (2nd fault) can develop currents a 1,000 times greater. In TN and IT, do not connect the PE to the metal frames of the building as the return currents may take a variety of paths and turn into transmitting antennae. The same also applies to the power cable, incorporating the PE, in which the sum of currents is no longer zero.

With respect to equipotentiality of frames, the TN and IT (on the 2nd fault) are equivalent since frame potential at the fault point suddenly rises to ≈ Uo/2 whereas it remains at 0 V at the origin of the installation. This leads to certain specialists specifying in TN and IT the creation of a low current frame circuit separated from the earth circuit (PE), both being connected to the earth connection at the origin of the LV installation. The TT with distributed PE throughout the installation is the best system in this respect (small Id and same potential reference for all the communicating devices), (see fig. 19). Faced with lightning and operating overvoltage’s These overvoltage’s, of common or differential mode and with a frequency of 1 kHz to 1 MHz can damage certain electronic devices if they are not fitted with an isolating transformer with a small primary/secondary capacitive coupling. As regards differential mode overvoltage’s, all the earthing systems are equivalent. The solution consists in: c implementing surge reducing at disturbance source level (e.g. RC on contactor coil); c protecting sensitive equipment by installing a surge limiter (varistor, ZnO lightning arrester) directly at their terminals. As regards common mode overvoltage’s (lightning), ZnO lightning arresters should be installed at the origin of the LV installation with the shortest possible earth connections. In this case, although the TN and TT earthing systems may seem more suitable than IT but overvoltage’s are transmitted on LV phases. In actual fact, at the frequencies considered, the phase/neutral impedance of the LV windings is very high (the phases are as though they were «unearthed» even if the neutral is earthed). Faced with HF disturbances: All the earthing systems are equivalent. Advice for minimizing the effects of HF disturbances: c use the Faraday cage effect for buildings (metal structures and meshed floors), or for certain rooms in the building reserved for sensitive equipment, c separate the frame network (structural and functional frames) from the earth network (PE), c avoid loops which may be formed by the high and low current circuits of communicating devices or place low current links (frame surfaces - ducts/metal screens - accompanying frames) under a «reduction effect», c avoid running them too close to power cables and make them cross at 90°; c use twisted cables, or, even better, shielded twisted cables. There are still not many standards in this area and they are often prepared (EMC standards) by electronic engineers. Installation standard IEC 364, sections 444 and 548, should provide increasingly more recommendations.

Evolution of the TN The original aim of this earthing system was simplicity, efficiency and minimum installation cost (see the American TN where the neutral is not even protected); Safety of personnel is guaranteed, but that of property (fire, damage to electrical equipment) is less so. Proliferation of low current power electronics is increasing and will continue to increase complexity of its implementation.

Derived from the TT of the nineteen twenties, the TN was a solution for controlling fault current value and ensuring that all insulation faults could be eliminated by a SCPD. It grew up in Ango-Saxon countries where rigour of installation designers and users is excellent. The logical evolution is TN-C → TN-C-S → TN-S → TN-S with fault current limitation to limit fire hazards, damage to loads and malfunctioning’s due to widespread use of distributed electronics (see fig. 20). A survey carried out in Germany in 1990 showed that 28 % of electrical (electronic) problems were due to EMC. In terms of protection, the TN system often uses fuses; already hindered by an overlong breaking time when limit safety voltage UL is 25 V, they will be further hindered in the long term if LV networks with voltages greater than 230/400 V are developed. The use of RCDs (impedance-earthed TN-S) solves this problem. Evolution of the IT The earliest electrical installations (1920) were produced in IT. However, double faults quickly gave this system a bad name (failure to master loop impedances). Standards gave it official status in the sixties in order to meet continuity of service requirements of process industries and safety requirements in mines.

Today, the IT system closely resembles the TN-S as regards installation (an additional surge limiter and insulation monitor). It is the champion of continuity of service and safety on the first fault, if this fault is promptly tracked and eliminated. Following widespread use of the distributed PE throughout the installation (as in TN), this system, in which the second fault current cannot be limited, will not really evolve, except for the rapid fault tracking techniques. As the likelihood of a double fault increases with the number of feeders and size of the installation, its use should be reserved for parts of the network and for control and monitoring circuits with, naturally, use of isolating transformers (see fig. 21). On these small circuits, use of the impedance-earthed IT allows signalling RCDs for fault tracking. Evolution of the TT To begin with, electrical distribution in France was in single-phase 110 V, followed by two-phase 220 V. Earthing of frames, combined with use of RCDs, aimed at de-energising consumers with insulation faults and cheaters. The development of electric household appliances led to protect people against indirect contacts. Protection against indirect contacts by RCD with standardized operating times was made official in the nineteen sixties. Today, the tendency is (as in TN and IT) to distribute the PE

throughout the installation and thus to use only one application earth connection (see fig. 22). This tendency should continue with the use of the LV neutral earth connection only (as in TN and IT), but maintaining the advantage (damage, fire, EMC) of a small insulation fault current. choosing the earthing system Choice may be determined by normal practice in the country. Choice of earthing system should be influenced by electrical power users and network operators (electrical service). Experience shows however that the choice is mainly made by the engineering firms designing the installation.

These both demand absolute DEPENDABILITY; electrical power should thus always be available and be completely risk-free, i.e. ÂŤout of sight, out of mindÂť. The elements making up installation dependability: c safety; c availability; c reliability; c maintenability, must therefore be optimised. In addition, a new requirement, electricity must not disturb the numerous low current devices. These are the criteria used to

make the best choice according to: c type of building; c the activity it houses; c whether or not an electrical service is available. In safety terms, the TT is the best, In availability terms, the IT is the most suitable, In maintenability terms, fault tracking is fast in TN (thanks to the SCPD) but repair time is often long. Conversely, in IT, tracking of the first fault may be more difficult, but repairs are quicker and less costly. The TT is a good compromise. In reliability terms, the protection devices used are reliable, but reliability of the installation and loads may be affected: c in TN-C by the fact that the PEN, not protected, may be damaged by harmonic currents; c in TN-C and TN-S; v by insufficient rigour for extensions, v by use of replacement sources with low short-circuit power, v by the effects of electrodynamic forces; c in IT, on a double fault, the risks inherent in TN described above also exist. However if tracking and elimination of the 1st fault are rapid, installation reliability is excellent. c in TT, by disruptive breakdown by return of the loads due to a fault in the HV/LV transformers. However the likelihood of this fault occurring is small and preventive solutions are available, e.g. use of surge arresters between one of the live conductors and the load earth connection. In disturbance terms, the TT is to be preferred to the TN-S whose high fault currents may be the source of disturbance.

reviews the strong and weak points of each earthing system: For installation designers Designing is simpler in TT, the same for extensions (no calculations). Designing complexity is equivalent in TN-S and IT. As regards costs: c the TN-S is the least costly to install, for example if the neutral is neither protected nor switched. But be warned: the cost of curative maintenance can be high; c the IT is slightly more costly to install (insulation monitoring and insulation fault tracking devices). Search for maximum availability of electrical power requires the presence of an electrical engineer, whose action will minimise curative maintenance; c the TT, if enough discriminating RCDs are installed, is slightly more costly to install than the IT, but fault tracking is simple and curative maintenance less costly than in TN. In terms of complete cost over 10 to 20 years, all three earthing systems are equivalent. The right choice In a certain number of countries, for some buildings or parts of a building, the choice is laid down by legislations or standards,

e.g. for hospitals, schools, navy, worksites, mines, etc. In other cases, certain earthing systems are strictly prohibited, for example the TN-C in premises with explosion risks. Apart from these compulsory choices, the DEPENDABILITY objectives (safety, availability, reliability, main tenability and proper operation of low current communicating systems) are those which should determine which earthing system is chosen for a specific building type. The degree of development of the country should also be taken into consideration, as should be national practices, climate.... If we plot an axis from North to South, as regards public distribution, we find the IT earthing system in Norway, TN-C in Germany, TT in France and in most African countries. In temperate, industrialized countries, all three earthing systems are used in private installations. Finally, it should be noted that it is possible and even advisable to mix the earthing systems

Conclusion The three earthing systems (TN - IT - TT) and their implementation are clearly defined in installation standards (IEC 364). Their respective use varies from country to country: c mainly TN in Anglo-Saxon countries; c TT often used in the other countries; c IT used when safety of persons and property, and continuity of service are essential. All three systems are considered to guarantee personnel protection.

Two major changes have had a considerable effect on choice of earthing systems: c search for optimum continuity of service; c proliferation of high current (disturbers) and low current (disturbed) electronic devices, which are increasingly set up in communicating systems. Thus the general tendency for earthing systems, in both MV and LV, is to limit insulation fault currents. At present, the fault currents of traditional LV earthing systems have the following standard values: c IT (1st fault): Id < 1 A; c TT: Id â&#x2030;&#x2C6; 20 A; c TN: Id â&#x2030;&#x2C6; 20 kA; c IT (2nd fault): Id â&#x2030;&#x2C6; 20 kA. Limiting fault currents: c simplifies maintainability of the electrical installation, thus increasing availability; c minimises the fire hazard; c can reduce contact voltage; c and, for sensitive systems, minimises disturbance due to electromagnetic radiation and common impedance. Moreover, in view of the proliferation of communicating digital systems (computers, video, automation, TBM etc., it is vital that earthing systems provide a potential reference which is not disturbed by high fault currents and harmonics. Consequently, future evolution should favour earthing systems generating fault currents which do not exceed a few dozen amps. TT earthing systems should therefore be increasingly used.

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