IEC 364-4-41

IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 34, NO. 5, SEPTEMBER/OCTOBER 1998

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A Summary of the IEC Protection Against Electric Shock  Giuseppe Parise,   Member, IEEE 

This paper paper provi provides des a summar summary y of the IEC propro Abstract— This tectio tection n agains againstt electr electric ic shock. shock. This This prote protecti ction on is provi provided ded by appropriate basic measures as follows: 1) for protection both in normal normal service service and in case of a fault fault (again (against st both both direc directt and indirect contact), use low and safe voltage of 50 V and below; 2) for protection in normal service (against direct contact), use insulation and/or enclose live parts or use isolation distance; and 3) for protecti protection on in case case of a fault fault (again (against st indir indirec ectt contac contact), t), prevent conducting parts not normally energized from becoming live. live. This is accomplis accomplished hed by groundin grounding g and automatic automatic disconnection of the supply, by use of Class II equipment (as double or equivalent insulation), or by separating the supply from ground. IEC publication 364-4-41 “Electrical installations of buildings,” (Part 4, Chapter 41) classifies types of system grounding as TNsystem, TT-system, and IT-system. Development of this summary is based based on actual actual hazard hazard risk risk analys analysis is of potent potential ial incide incidents nts to suggest criterion by which the appropriate measures can be applied to avoid or mitigate the injury or damage.  Index Terms— Electric risk, protection against electric shock, system grounding.

NOMENCLATURE CP

Circuit protection. Direct, indirect contact. Risk index of electric contact. Probability that someone touches conductive part. Probabili Probability ty that electric electrical al equipment equipment remains mains in servic servicee withou withoutt failur failuree on the conductive enclosure that bridges the isolation distance. Probability that the conductive part under consideration does not perform the condition . Automatic operating current of the disconnecting necting protecti protective ve device device within within the time stated in Table II. Current with s. Minim Minimum um curren currentt causin causing g the instan instantataneous tripping of the overcurrent protective device.

Paper ICPSD 97–48, presented at the 1997 Industry Applications Society Annual Meeting, New Orleans, LA, October 5–9, and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS   by the Power Systems Engineering Committee of the IEEE Industry Applications Society. Manuscript released for publication March 4, 1998. The author is with the Electrical Engineering Department, University of  Rome “La Sapienza,” 00184 Rome, Italy. Publisher Item Identifier S 0093-9994(98)05189-5. 0093–9994/98$10.00

SELV, SELV, PELV, PELV, FEL FELV

�   1998

IEEE

Rated residual operating current of the protective device. Disconnec Disconnecting ting time of the protecti protective ve device. Current passing through the human body. Fault current of the first fault (IT-system). Ground-fault current. Line conductor. Neutral conductor. Ground-fault conductor. Resist Resistanc ancee of ground ground electr electrode ode for the equipment system grounding. Resistance of body, hand to feet. Resistance of ground electrode for the neutral point of supply. Resistance of ground electrode for the system grounding. Nominal voltage, ac rms. Conventional voltage limit, that is, the admissible missible limit value of the touch voltage voltage persisting for a time that exceeds or is equal to 5 s (in normal conditions, 50 V ac rms or 120 V ripple-free dc). Conv Conven enti tion onal al vol volta tage ge lim limit it in in time time stated, stated, that is, the admissible admissible limit value of the touch voltage persisting for a time that that does does not not exce exceed ed seco second nds. s. Voltage to ground; ac rms. Prospective touch voltage; ac rms. Impe Impeda danc ncee of line line cond conduc ucto tor, r, sour source ce impedance included. Impedance of neutral conductor. Impedance of ground-fault conductor. Impe Impeda danc ncee of grou ground nd-f -fau ault lt cond conduc uc-tor—main circuit and feeder. Impe Impeda danc ncee of grou ground nd-f -fau ault lt cond conduc uc-tor—branch circuit. Impedance Impedance of the complete complete ground-fa ground-fault ult circuit (fault loop impedance). Impedance of prospective touch voltage. Residual impedance equal to . Safety, Safety, Protecti Protective, ve, Function Functional al Extra-Low Extra-Low Volta oltage ge,, not not exc exceed eeding ing the the upp upper limit mit of  50 V ac, supplied from one of the safety sources sources (a safety safety isolating isolating transformer transformer,, a source of current providing an equivalent

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that results in an electrical connection between the energized conductor or circuit part and a metal enclosure, or when the intervening airgap/isolation distance is accidentally bridged. Unless all metal enclosures have been grounded in an effective manner, there exists a tendency to raise the enclosure to the same electrical potential that exists on the power conductor. The probability of the failure can be evaluated by the insecurity , where is the security of the electrical equipment.  B. Does Someone Touch the Conductive Part? Fig. 1. Direct contact

and indirect contact

.

degree of safety, an electrochemical source). IPXXB Degree of protection such that the text finger cannot touch live parts (the degree of protection against contact with live parts is indicated by the designation IPXXY, where XX is two numbers in relation to the ingress of solid foreign bodies and liquid, respectively, Y is a letter in relation to degree of the possible contact, i.e., IP10B). MPDC, MPIC, Measures of protection against an electric MPBC shock, Preventing by the Direct, Indirect, and Both Contacts. I. INTRODUCTION: EVALUATION OF THE ELECTRICAL RISK

A

N ELECTRICAL contact, whether directly with live parts in normal service or indirectly with exposed conductive parts of electrical equipment as a result of a fault in the basic insulation, could cause physical injury or other harm to persons or domestic animals. The IEC standard 3644-41 [1] prescribes technical design methods that provide protection against electric shock (Table I). A criterion for selecting appropriate measures to achieve the objective of a sufficient level of protection against electric shock can be determined by evaluation of actual risk of   electric contact. The actual risk in any time depends on the following: 1) probability that a conductive part is live; 2) probability that someone touches it and related to the actual value of the touch voltage: (1) where the parameters in the following.

and

will be introduced

 A. Is the Conductive Part Live?

It is quite obvious that direct contact, which is by definition an actual contact with parts that are live in normal service, must be prevented to avoid injury or damage, if the value of  touch voltage is not very low. Indirect contact can accidentally occur when a failure of the basic insulation develops along a conductor or in a circuit part

The effective danger of an indirect contact, occurring with fixed or portable (mobile) electrical equipment is related to the probability of someone simultaneously touching the conductive enclosure or the conductive element that bridges the isolation distance. While not as predictable as the direct contact, indirect contact must be prevented, too. In the case of portable (mobile) electrical equipment, for which , it is suitable to prevent the appearance of  the electrical potential. In the other case of fixed electrical equipment, for which , it can be sufficient to limit the persistence of the electrical potential by grounding and automatic disconnection of the supply. C. If a Current Can Circulate, What is the Value of the Touch Voltage?

Fig. 1 shows an operator which can touch the artificial point “ ” of a live part in normal service (direct contact) or the point “ ” of an exposed conductive part of a piece of electrical equipment during a fault (indirect contact). Following a contact, a fault current flows. A fault-current path involving the ground or the grounding system is exclusively considered in this paper. For a given current path through the human body, the danger to persons depends mainly on the magnitude and duration of  the current flow ( in Figs. 2 and 3). Figs. 2 and 3 show, respectively, the two cases of a solidly grounded system or an ungrounded system. In the case of Fig. 3, the leakage current can be negligible and not dangerous. The touch voltage is defined as the product of current through the body and the total operator impedance (the sum of the total body impedance and of the footing impedance). The relationship between current and voltage is not linear, because the impedance of the human body varies with the touch voltage. The impedance from one hand to both feet is 75% of  the impedance from hand to hand , while the impedance from hands to both feet is 50% [5]. For different current paths, currents with the same magnitude give different dangers of ventricular fibrillation. The effect of current (e.g., 16 mA) which passes through one of the following paths:—left hand to left foot or right foot or feet;—both hands to feet, can be assumed as reference . The body current (e.g., 40 mA), passing through the path left hand to right hand has a reduced effect, as the 40% of the reference mA and (e.g., 20 mA), through the path right hand to left foot or right foot or feet, as

PARISE: A SUMMARY OF THE IEC PROTECTION AGAINST ELECTRIC SHOCK

TABLE I IEC PROTECTION MEASURES

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Fig. 2.

Solidly grounded system: fault current path during a direct or indirect contact (operator impedance

Fig. 3.

Ungrounded system:

).

includes the capacitive fault impedance of the system.

PARAMETERS

AND

TABLE II VALUES FOR SAFETY CONDITIONS

the 80% mA [5]. In practice, for designing protection against an electric shock, the necessary criterion is the admissible limit of touch voltage as a function of time. In Table II, column 1, the values of the

maximum admissible time are reported and, in column 3, the corresponding admissible limit values of touch voltage . Table 41-A of [2] gives the values reported in column 2. Reference [1] does not give values.

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(a)

(b) Fig. 4.

(a) General scheme for protection by automatic supply disconnection: fault-current path during a fault. (b) General scheme:

Reference [4] defines the conventional voltage limit , that is, the admissible limit value of the touch voltage persisting for a time that exceeds or is equal to 5 s. In normal conditions, is assumed equal to 50 V ac rms or 120 V ripple-free dc. Lower values may be required in special conditions. The probability of the part of the power system under consideration to perform the condition means a touch voltage does not exceed the values for the time in which the conductive part remains live (any ) or the failure persists (generally coincident with disconnecting time of  protective device). It is evaluated by the unit step

The function is zero for all negative values of the variable and equal to unity for all positive values. Obviously, if or is guaranteed less than for any time or even is always equal to zero ( intrinsically). II. THE  ABC’S OF THE  IEC APPROACH: PROTECTION MEASURES The aim of the protection measures against electric shock is to maintain at the lowest value the risk of contact, containing at least one of the components parameters or close to zero value, as far as possible. To introduce the IEC approach on the protection against electric shock, it is useful to make the following general considerations. It is quite obvious that there is not an actual shock hazard for the operator if the following conditions exist. 1) The electrical equipment is not energized or and so , or its exposed conductive parts cannot be energized . 2) The value of the nominal voltage , or of the voltage to ground , or of the touch voltage does not exceed the conventional voltage limit .

 

evaluation.

3) Touching the live parts is prevented (in normal service or by fault) . 4) A current cannot circulate or, on the contrary, a sufficient current is promoted to flow for a fast disconnection of supply (Figs. 2 and 3 show, respectively, the two cases of a solidly grounded system and an ungrounded system. In the case of Fig. 3, the leakage current can be negligible and not dangerous. Fig. 4(a) shows a general simple scheme in which the system and the exposed conductive parts are grounded. The equipment grounding allows elimination of the fault by disconnecting the protective device in time. So, the probability of an electrical contact for the first operator who will touch the equipment after the fault [Fig. 4(b)] is only reduced at the duration time . The IEC standard [1] proposes measures for the following: a) protection against electric shock in normal service caused by direct contact; b) protection in case of fault, caused by indirect contact; c) protection both in normal service and in case of fault. The Measures of protection against electric shock, Preventing by the Direct, Indirect and Both Contacts (MPDC, MPIC, MPBC) are drawn by the above-mentioned items (Table I). Item 1) motivates the active measure of protection based on preventing the persistence of the electrical potential by automatic disconnection of supply (MPIC based on ) and the passive measure of protection based on double insulation or by equivalent insulation (IEC Class II equipment) preventing the contact (MPIC based on ). Item 2) motivates the basic measure of protection based on using SELV, PELV, FELV, low and safe voltage with special circuits (MPBC based on intrinsically) and the passive measure of protection based on providing main

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Fig. 5.

TN-system: fault-current path during a ground fault.

Fig. 6.

TT-system: fault-current path during a ground fault.

and local equipotential bonding (MPIC based on intrinsically). Item 3) motivates protection by insulation of live parts, which can only be removed by destruction; protection by barriers or enclosures providing at least such a degree of  protection as IPXXB; for the cases to prevent unintentional contact with live parts, protection by obstacles and protection by placing out of reach [simultaneously accessible parts at different potentials must not be within arm’s reach (MPDC based on )]. Item 4) motivates the definition of types of system grounding (MPIC based on ). The system groundings are classified as TN-system, TT-system, and IT-system. The first letter, T or I, represents a system solidly grounded or ungrounded, respectively, and the second letter, N or T, represents the connection of the exposed conductive parts to the same grounded point of the supplying power system (Fig. 5) or to an independent ground electrode (Fig. 6), respectively. The TN-system and TT-system are realized to promote the circulation of ground-fault current and to favor automatic disconnection of supply, such that a touch voltage cannot be maintained at any point of the installation in excess of the voltages and in times stated in Table II. On the contrary, the IT-system is realized to limit and to control the circulation of the leakage current. Finally, the protection by electrical separation is realized to prevent the same circulation (protection by electrical separation of an individual circuit supplied through a separation source, i.e., an isolating transformer).

Fig. 7. Maximum prospective touch voltage to columns 2 and 3 of Table II.

duration curves according

Item 4) also motivates an additional measure of protection against direct contact (MPDC based on ), which is allowed by residual current devices. The use of residual current devices, with rated operating residual current not exceeding 30 mA, is recognized as additional protection against electric shock, in normal service, in the case of  failure of other protective measures or negligence by users ( for which ). The use of such devices is not recognized as a sole means of protection and does not obviate the need to apply one of the previous protective measures: the value of the actual current that flows through the body cannot be limited.

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Fig. 8.

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General scheme: fault current path during a ground fault. Local equipotential bonding.

III. TYPES OF SYSTEM GROUNDING  A. Generality

To realize the grounding of exposed conductive parts in each building, the performance of the grounding arrangements must satisfy the safety and functional requirements of the electrical installation. The selection and erection of the equipment of the grounding arrangements and of the protective ground-fault conductors ( ) must be such that the following exist. • The grounding resistance is in accordance with the protective and functional requirements of the installation and is expected to be continuously effective. • Ground-fault currents and ground-leakage currents can be carried without danger, particularly from thermal, thermomechanical, and electromechanical stresses. • They are adequately robust or have additional mechanical protection appropriate to the assessed conditions of  external influence. Simultaneously accessible exposed conductive parts must be connected to the same means of grounding. A main equipotential bonding system must interconnect the following conductive parts: 1) main protective conductor; and 2) main ground-continuity conductor (main water or gas pipes, risers of central heating and air-conditioning systems). The additional interconnection of metallic parts of the building structure and other metal pipework is recommended. In some cases, it can be useful or necessary to provide local bonding (reduction of , Fig. 8). This supplementary equipotential bonding must include all simultaneously accessible exposed conductive parts of fixed equipment and extraneous conductive parts, including, where possible, the main metallic reinforcement of constructional reinforced concrete. The equipotential system must be connected to the protective conductors of all equipment, including those of socket outlets. The exposed conductive parts of electrical equipment must be connected to the protective conductor under specified conditions for each type of system grounding. The use of the TN-system or TT-system is conditioned upon the property of the high/low voltage transformer, i.e.,

in a power system supplied by a utility transformer, only the TT-system can be realized. Figs. 5 and 6 show the TN- and TT-systems. The impedance of the complete ground-fault circuit should be low enough to ensure sufficient flow of groundfault current, for fast operation of the proper circuit protective devices, and to minimize the potential for stray ground currents on solidly grounded systems, as described in [3]. To contribute to a ground-fault current path of low impedance, the grounding conductor must be run adjacent to the power cables with which it is associated, i.e., inside the same conduit or the same raceway. In the TT-system, ground-fault current is normally determined by the electrodes resistances , considering that, by comparison with these resistances, the other impedances of ground-fault circuit (as are, in general, negligible . The impedance of   prospective touch voltage is equal to (in the IEC publication, includes also the value of ). In the TN-system, the impedance of prospective touch voltage is equal to . The ground-fault currents have, in general, higher values than in the TT-system, considering that the electrode resistance is out of the ground-fault circuit . So, in the TN-system, the use of the overcurrent protective devices can be sufficient, whereas, in the TT-system, the use of residual-current protective devices can be necessary.  B. TN-Systems (Figs. 5 and 10)

The TN-system is, in general, the case of a building plant supplied by its own transformer substation. All exposed conductive parts must be connected to the grounded point of the power system by protective conductors ( ). Generally the grounded point is the neutral point. The protective conductors must be grounded near each power transformer or generator of the installation. In addition to providing the proper equipment grounding in such a substation, step and touch potentials also must be maintained at a safe level. “TN-S system” is the name given to the most general case of a TN-system. This presents the neutral and the protective ground-fault conductor as being separated. Besides, the

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TN-C system provides in the same conductor ( ) both the function of the neutral and the protective ground-fault conductor. For this system, protection must be provided by overcurrent-operated protective devices; it is not possible to use residual protective devices. Finally, the TN-C-S system is a combination of these two solutions. C. TT-Systems (Figs. 6 and 11)

The TT-system is, in general, the case of a building plant supplied by a utility transformer substation. All exposed conductive parts, collectively protected by the same protective device, must be interconnected by protective ground-fault conductors with a ground electrode of resistance , common to all those parts. Where several protective devices are utilized in series, this requirement applies separately to all the exposed conductive parts protected by each device. The neutral point (or, if one does not exist, a phase conductor) of each generator or transformer must be grounded with a proper ground electrode.  D. IT-Systems (Fig. 4 with

and Fig. 6 with

2) the conventional touch voltage limits (Table II, column 3) that define the correspondent maximum times (Table II, column 1) in which can persist (Fig. 7); 3) the time–current characteristic curve – of the protective device. Following a fault in a part of the installation supplied by , the touch voltage cannot be maintained at any point of the installation in excess of the maximum time , admissible for the voltage ([1], clause 413.1.1.1). It is necessary to adopt a protective device with an operating current ensuring the automatic disconnection within the time stated in Table II. These requirements are met if the following conditions are fulfilled: (2) (3) (4) (5)

)

IT-systems are isolated from ground or connected to ground through a sufficiently high impedance, either at the neutral (star) point of the system, or at an artificial (star) point. The fault current is low in the case of a single fault to an exposed conductive part or to ground; disconnection is not imperative. Measures must be taken, however, to avoid danger in the event of two faults existing simultaneously. No live conductor of the installation must be directly connected to ground. To reduce overvoltage or to damp voltage oscillation, it may be necessary to provide grounding through appropriate impedances or artificial neutral points. Exposed conductive parts must be grounded individually, in groups or collectively. IV. PROTECTION BY  A UTOMATIC SUPPLY DISCONNECTION  A. General Conditions of Safety

The protection by automatic disconnection of supply provides that the exposed conductive parts of electrical equipment must be connected to ground. A general scheme is shown in Fig. 4(a). The component elements to consider are the following: 1) the faulting electrical equipment and the supplying part of the power system; 2) the operator; and 3) the protective device. The two measures “grounding with disconnection of supply,” after the occurrence of a fault, are respectively intended to immediately “sound the alarm” and to prevent touch voltage from persisting for such time that a danger could arise [Fig. 4(b)]. Consequently, the design parameters that characterize this protection measure at any point of the installation are as follows: 1) the fault current and the touch voltage (since is generally fixed, depends on the fault loop impedance and depends on the ratio , where is the impedance of prospective touch voltage);

[Note that the third term of expression (3) is obtained substituting the value of the current (2).] That is, if it is known that , and V, on the basis of (2) and (3), the values of  A and V are determined. In Table II, it is possible to find the admissible time limit s in column 1 in correspondence to of column 3 or, in this case of intermediate values of voltage, to the next higher value in the column ( V V, or, using the curve of Fig. 7, the actual value s in correspondence to V). Therefore, at the faulted point of installation, the condition (5) must be fulfilled, adopting a protection device with an operating current A, and ensuring the automatic disconnection within the time s. It is clear that this protective measure necessitates coordination of the following: 1) characteristics of protective devices; 2) type of system grounding (TN-system, TT-system, or IT-system). In the TN-systems and TT-systems, use of the following protective devices is recognized: 1) overcurrent protective devices; 2) residual-current protective devices, useful to protect the cases with the lowest values of current, such as, in particular, the arcing fault or the fault current since the beginning evolution. The following must be considered. • Overcurrent protective devices with an inverse time characteristic are characterized by a long disconnecting time, which, in any case, must fulfill . • Overcurrent protective devices with an instantaneous tripping characteristic or residual-current protective devices are characterized by a short disconnecting time , which can guarantee high values of until . In general, at each point of installation where the conditions (4) and (5) cannot be fulfilled or are very difficult to be realized, the following should be noted.

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Fig. 11. Grounding arrangement for ground-fault protection in TT-system, three-phase, four-wire circuits. Fault-current path through ground-fault conductors and earth.

Fig. 12.

variable: case of correct protection coordination.

• Realize supplementary equipotential bonding. Important considerations are the following. • In the TT-system (Fig. 5), , where is the effective ground-fault current, is the ground-potential rise of the ground electrode; the impedance is generally very low in comparison to . • In the TN-system, is the touch potential. It is also possible to see the following. • For TN-systems, it is very important to have supplementary equipotential bonding for reducing the touch potential from the value at the value, as shown in Fig. 10 for the operator. • For the TT-system, it can be sufficient to have a good main equipotential bonding system for reducing the effective touch potential. This reduction of to is not considered in Fig. 11 for the operator, exposed to . 3) IT-Systems ([1, Clause 413.1.5])   According to  Rule 2, the following condition must be fulfilled: (11)

Fig. 13.

variable: case of bad protection coordination.

Solutions to the handicap of not following the conventional approach can be the following. • Adopt a residual-current protective device, which can make the above-mentioned protection coordination easier.

If an insulation monitoring device is provided to indicate the occurrence of a first fault from a live part to exposed conductive parts or to ground, this device must release an audible and/or visual signal, or automatically disconnect the supply. It is recommended that a first fault be eliminated with the shortest practical delay. Conditions for disconnection of supply in the event of a second fault as specified for TT-systems and TN-systems must apply, depending on whether all exposed conductive parts are interconnected by a protective conductor (individually or in groups for TT-systems, collectively for TN-systems). In the case of TN-systems (see [1]): — if the neutral conductor is not distributed, the relation shown by (6 ) is applied considering: — in place of , — the loop impedance of the line conductor and of the equal to and — for the values correspondent in the same row

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in Table II, column 1:

REFERENCES (6 )

If the neutral conductor is distributed, the relation shown by (6 ) is applied considering: — in place of — as the loop impedance of the neutral conductor and of the and — for the values correspondent in the upper row to in Table II, column 1: (6 ) . If such safety conditions cannot be fulfilled, it is necessary to provide a local bonding (supplementary equipotential bonding). In IT-systems, use of the following protective devices is recognized: 1) insulation monitoring devices; 2) overcurrent protective devices; and 3) residual-current protective devices. V. CONCLUSION If the conventional approach is adopted according to [1], a criterion to avoid or mitigate the injury or damage of  an indirect contact occurring with electrical equipment is as follows. In the case of portable (mobile) electrical equipment, it is necessary to prevent the appearance of the electrical potential, whereas, in the case of fixed electrical equipment, it can be sufficient to limit the persistence of the electrical potential by grounding and automatic disconnection of the supply.

[1]   Electrical Installations of Buildings, pt. 4, “Protection for safety,” ch. 41, “Protection against electric shock,” IEC Pub. 364-4-41, amendment 1, 1996. [2]   Electrical Installations of Buildings, pt. 4, “Protection for safety,” ch. 41, “Protection against electric shock,” IEC Pub. 364-4-41, 2nd ed., 1982. [3]  Recommended Practice for Electric Power Distribution for Industrial Plants, ANSI/IEEE Std. 141-1993, ch. 7. [4] Electrical Installations of Buildings, pt. 2, “Definitions,” ch. 21, “Guide to general terms,” IEC Pub. 364-2-21, 1993. [5]   Effects of Currents on Human Beings and Livestock , pt. 1, “General aspects,” IEC Pub. 479-1, 1994. [6] “Measures against indirect contact by automatic disconnection of supply,” International Electrotechnical Commission, Geneva, Switzerland, Tech. Rep., type 3, 1st ed., 1996-03, no. 1200-413, 1996.

Giuseppe Parise  (M’82) was born in Luzzi, Italy,

in 1947. He received the Degree in electrotechnical engineering from the University of Rome, Rome, Italy, in 1972. From 1973 to 1979, he was Researcher and Assistant Professor, University of Rome, and, in 1980, he was appointed Associated Professor of Electrical Power Systems. His research, design, and consultant activities cover the areas of design, planning, safety, security, and energy management of power systems. Since 1983, he has been a Member of the Superior Council of the Ministry of Public Works, as an expert in power systems. He is member of the Italian Electrotechnical Commission (CEI) CT/SC 11A, “Generation, Transmission and Distribution Systems of Electric Power,” and President of the Electrical Commission of the Engineers Association of  Rome’s Province. Mr. Parise is a member of the IEEE Industry Applications Society Power Systems Grounding Subcommittee.