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On Human Life Risk-Assessment and SensitiveGround Fault Protection in MV Distribution Networks

José L. Pinto de Sá, Senior Member, IEEE, and Miguel Louro

Abstract—The minimum phase-to-ground fault current re-quired to be sensed by protection systems in medium-voltage(MV) networks can be as low as 0.7 A in a few countries, leadingto a lot of undesired relay trips and poor service quality to cos-tumers. However, these settings raise the protection thresholdabove the minimum fault current that concerns network operatorsregarding human safety, although they cannot be practiced whengrounding is distributed. The purpose of this paper is to present arisk assessment foundation to determine the required protectionsensitivity to ensure human safety in MV distribution networks.The proposed approach is based on a biophysical model includedin IEC standards, the consideration of current paths models fortypical faults and Monte Carlo methods to deal with nonlinearity,and the many involved random variables. Downed conductorsand line-to-concrete pole faults are investigated and sensitivityanalysis performed to highlight some important determinants ofthe results.

Index Terms—Monte Carlo methods, overcurrent protection,safety.

I. INTRODUCTION

H IGH resistance phase-to-ground faults are dangerousevents, because they can be very hard to detect. Of these

faults, downed conductors have motivated a lot of investigation[1]. Don Russell and his team [2] developed a very detailed re-search to find that a number of detection methods were efficientbut also that it could be most difficult to discriminate a faultsituation from “normal” electrical events on the distributionfeeders. As those authors noted, if an algorithm regularly iden-tifies normal system activity as a “fault,” it is virtually uselessin practical relay applications.

On the other hand, in overhead line networks, faults withvery high resistance can appear due to trees leaning against aconductor, for instance. VTT researchers in Finland have devel-oped a very sensitive algorithm [3] capable of coping with thesefaults, but field monitoring has shown that a number of nonfaultevents could also produce the voltage disturbances upon whichthe authors’ methods were based.

Some of the aforementioned protection methods have beenrefined and are now suitable for the processing power of modernmicroprocessor-based protective relays [4], but they all present

Manuscript received December 18, 2009; revised May 23, 2010; acceptedJune 14, 2010. Paper no. TPWRD-00944-2009.

J. L. Pinto de Sá is with the Instituto Superior Técnico, CIEEE, Lisbon 1049-001, Portugal (e-mail: [email protected]).

M. Louro is with EDP Distribuição, Lisbon 1050-044, Portugal (e-mail:[email protected]).

Digital Object Identifier 10.1109/TPWRD.2010.2053564

some limitations [5]. Meanwhile, many ordinary zero overcur-rent relays, either directional or nondirectional, are in the field.

Utilities make different assessments about the safety risk ofhigh-resistance ground faults, but they all are pressed to pro-vide the best possible service, which brings necessary com-promises between protection sensitivity and security [6]. Whilesome utilities are comfortable with settings of 10% of rated cur-rent and above for ground fault protection in distribution net-works, others have a tradition of more sensitive protection. InEurope, where there is no neutral conductor in MV feeders andsensitive ground relaying is common, a number of Distributionutilities require the detection and elimination of ground faultswith as low as 1 A [7] and even less [8]. To cope with non-transposed lines asymmetry and charging current unbalances,which also produce nonfault zero currents, this high sensitivitywas traditionally the role of a relay dealing with the currentof the substation grounding connection, which is nonselectiveby nature. Zero overcurrent relays in feeders were set above5–7.5 A to avoid tripping with nonfault zero currents [8], andthe relay at the substation grounding connection was set to 1A or less and timed to 2 to 3 min. Usually, this last relay trig-gers an automatism, which works by sequentially disconnectingthe feeders until the relay resets and the faulted feeder is found.For insulated grounding systems, a zero overvoltage relay at thesubstation bus is used instead. However, excessive use of thispractice led to many consumer complaints and with time, newprotection approaches have been developed which are more se-lective [9].

Anyway, many utilities around the world still face a dilemmabetween guaranteeing sensitive ground faults relaying andavoiding power-quality (PQ) complaints, especially for feederswhich are disconnected too often. Although the quest for sen-sitive and yet secure ground fault protection keeps deservinga lot of research, to these authors’ knowledge, a quantitativeevaluation of how dangerous a high resistance ground fault canreally for human life in MV distribution networks was neverpublished. Exceptions exist only in some particular industrieswhere risk conditions are well characterized, as in coal mines[10].

At this point, we should stress that a direct contact between aperson and an ungrounded live part of an MV power system willprobably lead to the appearance of an arc flash or to tissue elec-troporation [11]. In either case, it is unlikely that a person willsurvive without severe injuries. Electroporation occurs in just5–40 ms and, therefore, it is impossible for a relaying systemto prevent its consequences to the victim [12]. However, it ispossible to minimize the amount of energy transmitted to an arcflash by means of fast tripping [13]. Anyway, for arc flash and

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2 IEEE TRANSACTIONS ON POWER DELIVERY

Fig. 1. Occurrence of the vulnerable period of ventricles during the cardiaccycle (from [15]).

electroporation, currents are so large that the issue is protectionspeed, not sensitivity.

The origin of most high-resistance faults, according to theexperience of a few utilities, including the one for which oneof the authors works for, is the conductor touching the ground,MV/low-voltage (LV) transformer faults, and cable failures be-tween tower anchor clamps [9]. Therefore, the purpose of thispaper is to offer a human safety risk assessment for the mosttypical high-resistance ground faults endangering human life inMV distribution networks in order to provide a quantitative basisfor the definition of an informed tradeoff between sensitivity andsecurity requirements in ground-fault protection.

II. REVIEW OF THE PROBABILISTIC LETHALITY

OF THE ELECTRIC CURRENT

In high-resistance ground faults, small currents are present,and that is why protection sensitivity is an issue. For these smallcurrents, the most important lethal effect is ventricular fibrilla-tion and, therefore, some established facts about this phenom-enon must be reminded [14].

As Fig. 1 illustrates, ventricular fibrillation can only occurif an electrical current overlaps the nervous electric heart con-trol in the recovery period of the cardiac cycle, which lasts foronly 10% of the entire cycle. This current has also to reach aminimum threshold, but if it persists in time, only 5% of it isrequired to disturb the cardiac cycle, reducing its duration andlowering the fibrillation threshold. This is the reason for the “S”shape of the threshold-time functions illustrated in Fig. 2 (from[15]).

Since the number of heart cycles is about 70–80 for a healthyperson and can reach a hundred under physical effort, the ven-tricular fibrillation threshold starts lowering only after 0.1 s, toreach a steady 5% of its initial value in about 2 s.

Another important fact is that the fibrillation threshold variesfrom one person to another. In Fig. 2, there are three curves:c1, c2, and c3. Under c1, the fibrillation probability is zero, c2is reached for 5% of the people, and c3 for 50%. This means,for example, that even though the absolute minimum fibrillationthreshold may be 0.04 A, for a current of 1 A lasting less than0.1 s, approximately 95% of the people will survive.

Fig. 2. Conventional time/current zones of effects of ac currents (15 Hz to 100Hz) on people for a current path corresponding to left hand to feet ([15]).

For a given contact time, ventricular fibrillation can be mod-eled by a lognormal probability function that fits the three curvesof Fig. 2, as shown by animal experiences.

It must also be reminded that the curves in Fig. 2 are for acurrent passing through the heart from the left hand to the feet.For other body paths, the values for the current differ and areusually larger, which is accounted for by the IEC 60479 standardthrough “heart factors” relating the fibrillation currentin a body path and this reference path

(1)

Besides ventricular fibrillation, muscle tetanic contractionmay be a severe hazard when stepping on a conductor on theground, because it can cause knees to flex and the person tofall. C. Dalziel has conducted a lot of research which is not incontradiction with IEC 60479 assumptions and provides furtherdata [16] about the current thresholds for tetanic contraction.Although Dalziel’s research was focused on hand muscles’tetanization, and hence, on “let-go” current thresholds, recentresults point to the validity of extending those results to otherbody extremities [17].

III. MODELING COMMON GROUND FAULTS INVOLVING

HUMAN CONTACTS

In what follows, line-to-tree faults will not be addressed be-cause they do not define an immediate danger for human safety,although it is known that they can ignite fires.

A. Downed Conductors

Undetected downed conductors are a human safety issue be-cause the distances between live parts of the power system andthe public are diminished beyond acceptable levels. A downedconductor may have a very low fault current even for low resis-tivity soils; for example, if the open circuit is from the sourceside and the line is back fed from MV distribution transformers,or in resonant or insulated networks.

Step Voltage Near a Downed Conductor: When somebodytouches a downed conductor with a foot, the current will flowfrom one foot to the other (Fig. 3).

The situation presented in Fig. 3 is represented in Fig. 4 tohighlight the associated electrical phenomenon.


PINTO DE SÁ AND LOURO: ON HUMAN LIFE RISK-ASSESSMENT AND SENSITIVE GROUND FAULT PROTECTION 3

Fig. 3. Step voltage in case there is foot contact with a downed conductor.

Fig. 4. Step voltage in case there is foot contact with a downed conductor (pro-file view).

The ground potential rise (GPR) from the downed conductoris obtained by

(2)

where

length of the downed conductor in contactwith the soil;

grounding resistance of the downedconductor;

short-circuit current to ground.

The voltage applied to a person stepping on the downed con-ductor can be determined by the following equation (refer toFig. 4):

(3)

where

step voltage (in relative value of GPR);

step length.

The step voltage can also be obtained from Fig. 5 and ex-pressed by

(4)

The resistance of the foot contact to ground can be rep-resented as a function of the top soil resistivity (assuming a

Fig. 5. Electrical equivalent of step voltage near a downed conductor.

two layers model) and a constant that depends on the toplayer depth and the difference between top soil and bottom soilresistivity [18]

(5)

These expressions follow what is usually derived for substa-tions grounding design and are not new [19], [20].

The calculation of the grounding resistance fora conductor laid on soil can be done through the formula ([19,p. 167])

(6)

where

radius of the conductor;

2L buried horizontal wire length (a portion of );

s/2 depth of burial.

In this formula, all of the variables are to be defined. It hasbeen found that for a particular point of a downed conductor,the fault current to soil tends to be random in magnitude be-cause of varying contact effects of grass, roots, bushing, androcks that cause intermittent ground contact paths of variableimpedance. Arcing disturbs the ground path by rapid heating,drying, and expansion of soil, charring of glass and the meltingof sand into glass, in a complex and dynamic way. However, atleast for conductors lying on soil for a few yards (or meters), itappears that experience confirms the findings in [21] in whichthe arcing from the conductor to the ground is seen as equivalentto a flat plate. This arcing contact is modeled in (6) by a depthof burial.

Substituting (5) in (4), the following equation is obtained:

(7)

For the case represented in Fig. 5, (8) can be derived by re-placing (7) in (3)

(8)

Of course, in (8), all of the involved variables are random,namely, the length of the laid conductor, the step length, the soilresistivity, the type of shoes and their electrical resistance, the



Fig. 6. Touch voltage when touching a faulty MV pole.

body electrical resistance, and the person fibrillation threshold,although they exist between some limits.

B. Pole Faults

Touch Voltage Near a Pole: Another common indirectcontact can occur when there is a fault to a metallic or steel-re-inforced concrete pole and a person accidentally touches it.This hazard is more probable when a pole-mounted distributiontransformer has a fault to its grounded tank, particularly when itis located near houses, but it also occurs with broken insulators.

Touch voltage is the most limiting aspect because the proba-bility of the electric current passing through the heart is higherthan for step voltage.

The case of an indirect contact is illustrated in Fig. 6. It isassumed that the pole in Fig. 6 does not have a ground returnwire, as is most usual in MV lines. Should the pole have a groundreturn wire, the results would probably be less constraining.

Considering Fig. 6, the touch voltage to the human body canbe calculated through

(9)

The pole resistance depends on the placing of theground electrodes and the soil resistivity. The placing of theground electrodes is uniform, so we have

(10)

In Fig. 7, the touch voltage is presented as depending on theresistance of the human body, the shoe resistance, the groundingresistance of the feet, and the current passing through the humanbody. The touch voltage can be obtained by using

(11)

Equation (12) establishes the relationship between the short-circuit current to an MV pole and the current passing through aperson’s heart

(12)

Fig. 7. Touch voltage across the human body.

Besides heart fibrillation, a current from hand to feet canalso cause tetanic muscle contraction in those body extremities,for values much lower than those causing fibrillation. Anyway,by observing (12), it is apparent that it also depends on manyrandom factors, such as soil resistivity, shoes electrical resis-tance, body path for the current and its heart factor, appliedvoltage, and the person fibrillation threshold.

IV. ASSUMPTIONS FOR A RISK ANALYSIS

A. Probabilistic Approach

A probabilistic approach is how these authors propose dealingwith the randomness of the variables involved in the studiedevents. Risk analysis as related to electric injuries resulting fromground faults is not a new idea [22], but to the authors’ knowl-edge, it was never applied to protection practices.

Traditional grounding theory uses pessimistic values for allof the random variables present in danger assessment. This im-plies the assumption that all of the unfavorable situations willoccur simultaneously, which rarely occurs. This prudent prac-tice makes sense when designing grounding systems for instal-lations with an expected life of several decades and with opera-tional conditions not entirely foreseeable.

However, the goal of this paper is to determine the fault cur-rent that is harmful for people stepping on downed conductorsand touching poles under fault in real conditions. A determin-istic analysis based on the worst conditions would lead to a pes-simistic result of almost null probability and requiring too muchprotection sensitivity. On the other hand, it appears wise to relaxthe protection sensitivity to determine the associated risk. Thus,in what follows, we start by proposing probability distributionsfor the involved random variables.

B. Probabilistic Assumptions for Common Random Variables

Soil Resistivity : According to data from a local utility(EDP), its most common soil resistivity in dry weather is 150

m and the largest identified value was 37 k m. It was consid-ered that the soil resistivity follows a lognormal-type distribu-tion and, fitting that data, a median value of 150 m with 95%of the soils having a resistivity below 2000 m was found. Soilresistivity depends a lot on seasonal moisture and regional vari-ations, and in other regions, other statistic parameters can fit thedata better.

Nonuniform Soil : The larger the soil resistivity is, theworse it is. The top soil layer usually has a larger resistivity thanthe average soil because it is more exposed to water evaporation.



TABLE ISHOE PROPERTIES AND PROBABILITY OF OCCURRENCE

In reference [23], a 30% resistivity increase in the top soil layerdue to evaporation is presented.

According to the weather profile of Portugal, it was found thatfor 150 days of the year, the top layer resistivity is identical tothe average resistivity. In the remaining days of the year (dryweather), the top layer resistivity should be 30% above the av-erage resistivity.

Human Body Resistance : Statistics for the human bodyresistance are presented in [16], which follow a lognormal-typedistribution [24]. However, its average value depends on the pathconsidered for the current: it is somewhat larger for foot to footthan for hand to feet. IEC 60479 provides means to evaluate therelative impedance for different current paths.

Heart Current : If the heart current surpasses a cer-tain threshold, ventricular fibrillation is probable. The data pre-sented in [15] are used for the fibrillation current. A lognormalprobability function is the most adequate for accessing the fib-rillation current [24].

Shoe Resistance : Grounding design guides usuallyassume barefooting in their models. However, for risk assess-ment in real life, barefooting is uncommon, at least in industri-alized regions. Data on shoe resistances can be found in [25]and are being used for grounding design in Australia and in theU.K. Table I shows those data, with minor adjustments to localconditions.

The voltage withstand thresholds of the shoes have to becompared to the actual value calculated in the analysis. If thethreshold is violated, any considered algorithm must be ad-justed for this circumstance. This, of course, makes the analysisof the statistics very nonlinear.

C. Specific Probabilistic Assumption for Downed Conductors

Conductor Section and Burial Depth : To simulate adowned conductor, a very low burial depth can be assumed (e.g.,0.002 m). For illustrations purposes, a 25-mm section was alsoassumed as typical of MV conductors. These parameters have areduced effect on the conductor-to-ground resistance; thus, theywere not taken as random. For example, a 10-times increase inthe depth burial (justifiable to model arcing into the soil nearthe conductor contact) only reduces the grounding resistance in13%.

Fig. 8. Electric potential near a downed conductor.

Conductor Lengths : This is a most determiningvariable for a conductor-to-ground resistance, which is in-versely proportional to the contact length defined in (6). Onone hand, the average span between MV poles dependson the voltage level. For the lowest voltage in MV overheadlines, it can be around a hundred yards (or meters), varying upto 1/3 that value. On the other hand, when an overhead linefalls on ground, only a portion of that maximum valuewill be in touch with the soil—or close enough for arcing toperform the grounding.

Reports from maintenance crews point to two types of reasonsfor a line to break in regions where it is unusual to have linesrunning on roadsides and, hence, driving accidents are not acause for line falling. The most usual reason is simply breakingon the points where tensile efforts are at maximum, which areat the poles suspensions. Trees and human artifacts cause otherfractures, which have a higher occurrence at midspan, where theconductors are at their minimum height. Considering all of theseaspects would lead to a complex probability distribution, butwe consider that a uniform distribution is a reasonable model,taking into account that it probably underestimates .

Step Voltage : The step voltage near adowned conductor depends on the step length and the potentialnear the conductor. Fig. 8 represents the potential near a downedconductor. This data were obtained through a simulation ([26]).

Step Length : The standards for grounding design al-ways take the step length at its maximum, 1 m. However, as forbarefooting, that model is too pessimistic for modeling reality;hence, a step length with a uniform distribution probability be-tween 0 and 1 m was assumed. Probably a lognormal distribu-tion would fit reality the best, but in the end, the convergence ofall random variables will always lead to results approximating alognormal distribution, regardless of the assumed distributionsfor each variable, thanks to the central limit theorem.

D. Specific Probabilistic Assumptions for Faulted Poles

Pole Grounding Factor : This factor represents the re-lationship between the pole grounding resistance and the soil av-



Fig. 9. Touch voltage dependence on arm length.

erages resistivity. In this study, some particular grounding prac-tices for MV poles yield [27]

(13)

Arm Length : The IEEE [19] states that for project pur-poses, an arm length of 0.8 m must be used, but this is too pes-simistic for average real conditions. Therefore and in order toobtain a realistic assessment of the arm length, a uniform distri-bution probability between zero and 0.8 m is proposed.

Touch Voltage : The touch voltage (obtained from[27]), which depends on the arm length, is represented in Fig. 9.

Heart Factor : The heart factor depends on the pointof entry of the electric current in the human body. If this pointis the left hand, then the factor is 1, but if it is the right hand, theheart factor is 0.8 [15]. We have assumed a probability for thepoint of entry being the right hand and equal to that of the lefthand (50%).

V. RISK ANALYSIS THROUGH MONTE CARLO METHODS

Monte Carlo methods work by repeated random sampling tocompute results. With readily available inexpensive personalcomputers and powerful software, such as MATLAB, MonteCarlo methods are an easy approach to estimate risk for the situ-ations under study, particularly considering the large amount ofrandom variables and the nonlinear behavior of the shoes. Forexample, for computing equations (8) and (12), the method wasapplied as follows.

1) In the Monte Carlo simulation, random numbers thatfollow the presented random variables probability distri-bution were generated.

2) Five million random data were calculated for each randomvariable (using each random variable probabilistic dis-tribution functions) by using MATLAB, followed by thefinal equations calculation for that amount of data. Finally,the five million results for the equations were sortedand the thresholds verifying particular probabilities weredetermined.

Fig. 10. Fault current for 0.5% probability of heart fibrillation when steppingon a downed conductor: —with random shoes,�—barefooted.

Fig. 11. Fault currents versus probability of a fall when stepping a downedconductor. —with random shoes,�— barefooted, X- with only one type ofshoes (dry new black rubber).

A. Case for Downed Conductors

A first investigation was done for the fibrillation risk whenstepping on the downed conductor. Fig. 10 shows the achievedresults.

Perhaps surprisingly, the fault currents causing immediatefibrillation are quite large, even for barefooted people. It mustbe reminded that this is not the current through the person, butthe fault current from the conductor to ground, for a 0.5% prob-ability of fibrillation with random variability of all the parame-ters. For a typical step time of 2/3 of a second, more than 100 Aare needed even for barefooted people. One of the reasons forthis is that the current that passes through the heart in case ofthe current entering one foot and leaving the other is only 4% ofthe total body current [15].

Unfortunately, the current threshold that makes the leg mus-cles stop to respond to voluntary actions and causes knee flexingand loss of balance is much smaller [18]. Once a person falls,there is a large chance that the fall will be on the conductor, andthat is why this hazard is what must be considered for steppingdowned conductors.

Fig. 11 shows how the falling risk grows with the fault cur-rent. Since muscle contraction has nothing to do with the cardiaccycle, now the contact time is irrelevant.

Even a fault current of 1 A can make 1% of the peopl fall,and 5% of them if barefooted. The risk of falling for people withdry new black rubber shoes is equal to the average shoes up to10 A, but above this current, there are other shoes, with largerresistances, which increase the amps required to cause a fall.

Fig. 12 illustrates the importance of the soil resistivity and thelength of the conductor making contact with the ground, for the



Fig. 12. Fault currents versus fall probability when stepping a downed con-ductor. �—for a short portion of the conductor on the ground (5 m); X- forvery resistive soil (5 � ); - all random (reference); �—for low resistivity soil�� ; �—for a long portion of the conductor on the ground (95 m).

Fig. 13. Fault currents versus heart fibrillation probability for touching adowned conductor. —falling on it, with chest making contact, �—with thehand, standing 1.5 m from the point the conductor touches the ground.

risk of a fall. These results confirm how high-resistance soil anda small length of contacting conductor greatly increase the riskfor the same fault current, which must be taken into account ifthe probability for these occurrences is considerable.

Once a person falls, there is a chance to fall on the downedconductor. The risk of fibrillation can be estimated consideringas the worst condition the chest touches the conductor and bothbare hands touch the ground, at a random distance between 0and 1 m from the conductor. Fig. 13 shows the probability forthis hazard, and the risk of fibrillation for touching the hangingportion of the conductor with a hand when standing, instead ofstepping on it.

The heart fibrillation probability when falling on the con-ductor is very similar to the probability of muscle tetanizationfor stepping on it, but the probability of both occurring is ap-proximated by their product; hence, it can be said that for theconsidered statistics, the risk of dying from stepping on the con-ductor will be only 0.5% if the fault current is less than around4 A. However, for this current, the risk of dying from touchingthe hanging conductor with a hand will be 4 times greater.

B. Case for Faulted Poles

The application of the same analytic methodology to the caseof a faulted pole provides the results of Fig. 14 for a growingrisk of heart fibrillation. The amount of time until heart fibril-lation occurs depends on the touch duration, but unfortunately,

Fig. 14. Time to heart fibrillation versus fault current for: different probabili-ties. From left to right—0.5%, 2.5%, 5%, and 25%.

Fig. 15. Time-to-heart fibrillation versus fault current for different assump-tions, all for a 0.5% risk of death.

the related “let-go” current is much smaller; therefore, once ahand touch is made to a faulted pole, it will likely remain untilheart fibrillation occurs, unless the current is small enough orthe fault is eliminated by some protection. For standing faults,the asymptotic current is what matters, for example, 4 A for adeath risk of 0.5%.

Of course, these results are for random variables in whichshoes have a determining importance. Fig. 15 illustrates howbarefooting as well as considering only a maximum distance(0.8 m) from the feet to the pole will affect the results (all of theremaining variables are still random). For touches lasting longerthan 2 s, barefooting reduces the current required for heart fib-rillation from 4 A to 2.9 A, with the same risk of 0.5%, underthe considered statistics.

These illustrative results can be compared as shown inTable II.

From the obtained results, it can be said that to reach a veryhigh safety level, fault currents to ground above 3 to 5 A shallnot be allowed, but below these values, the risk of death is verylow.



TABLE IIMINIMUM FAULT-TO-GROUND CURRENTS FOR RISK OF DEATH

VI. EXPORTING THE RESULTS OF THE RISK ANALYSIS TO THE

DESIGN OF SENSITIVE ROUND PROTECTION SCHEMES

The results reached in the previous section define fault cur-rents that can endanger human life for a calculated level of prob-ability. The analysis leads to values for currents because electriccurrent is the lethal agent in the human body, once electropora-tion (which has more to do with voltage) and arc flash are ex-cluded. Of course, if different assumptions are made about therandom behavior of the independent variables, particularly thesoil resistivity and the features of the shoes, the same risk levelswill be yielded by different currents. However, it is unlikely thatrealistic parameters however different will change these valuesin orders of magnitude.

On the other hand, the evaluation of the total risk depends onthe probability of the events themselves (people moving underoverhead lines, how the lines are maintained, and how they areclamped to their poles, the materials used to manufacture MVpoles, how these poles are grounded, etc.). In addition, of course,it is up to each community to decide on what the risk is that canbe afforded.

Notwithstanding, if a very low risk of casualties is desired,(e.g., 0.5%), phase-to-ground faults yielding more than a fewamps cannot be tolerated. Under this constraint, the suitable pro-tection schemes depend very much on the type of network andparticularly on its grounding philosophy. In what follows, sometypical scenarios will be addressed.

Networks With Resonant Grounding: Networks with reso-nant grounding (through arc-suppression coils, also named Pe-tersen coils), can limit the current of any phase-to-ground faultto very small values. A perfect resonance would eliminate thatcurrent, but real systems cannot do it, first because the coils arenot perfect inductances and always have some resistance, andsecond because a perfect tuning would create a large voltage un-balance even for very small phase asymmetries. Real systemscan reduce the current of a phase-to-ground fault to an extentcapable of suppressing most arcs, and that is precisely its bestvalue, but cannot eliminate the fault current from a downed con-ductor or to a pole from a pole-mounted faulted transformer.

However, from the risk analysis presented before, it can besaid that if a persistent fault current to ground is less than 3 to 5A, the risk of human casualties may be really small. Sustainedoperation with this fault is precisely the usual practice in Aus-trian and German utilities [28], where grounding through Pe-tersen coils is an old and established tradition and fault currentsare most often reduced to a few amps with 3 A being a typicalreference.

For a phase-to-ground fault to be detected in a feeder in thosenetworks, the current transformers (CTs) have to cope with very

low zero-sequence currents and still provide a measurement ac-curate enough for the use of sensitive wattmetric relays. Thisaccuracy usually demands a CT with a primary current vectoraddition or flux summation, in which the conductor cables passthrough the center hole of the core balance CT. For overheadlines, the use of this CT is only possible if the line has a cableportion between the substation bus and its first pole, which is ausual design in many European regions.

Nonresonant Networks With Neutral Grounded Only in theSubstations: In networks that are ungrounded or grounded onlyin the substations, without a neutral conductor in the feeders,phase-to-ground faults usually have considerable currents,which are easy to detect by protective relaying, particularly ifflux-summation CTs are available to provide good measure-ments of zero-sequence currents. However, there are two verydifferent situations to consider:

Backfed Downed Conductors and Faulted Poles: A downedconductor broken near the suspension point in the source-sidepole can have a considerable length on soil with good resis-tivity and still be characterized by small fault currents. A pole-mounted transformer with a phase fault to its inner groundedtank creates a similar situation if the fuse of the faulted phase isblown.

It can be shown that the phase-to-ground backfed fault cur-rent is limited to 1/3 of the prefault load current in the line,unless the fault resistance is really large, and that the residualcurrent ( the zero-sequence current) on the source side ofthe line is equal to that fault current [26]. Therefore, sensitivezero-sequence overcurrent relays can guarantee human safetyagainst the hazards previously investigated if they can be set tocurrents as small as 3–5 A, about the same amps to which reso-nant grounding usually limits any phase-to-ground fault current.

However, if the region served by the network is populatedby barefooted people, has poorly maintained equipment, andmoderate power-quality (PQ) requirements, a sensitivity as lowas 1 or 2 A may be a better option at the substation neutralconnection.

Nonbackfed Phase-to-Ground Faults: If zero-sequence over-current relays are set to 3–5 A, they will be very sensitive tofaults fed by the source, unless a very short length of conductormakes contact with very high-resistance soil. However, settingan overcurrent relay to 3–5 A can make it sensitive to chargingzero currents from the feeder to a fault somewhere else. Meansto guarantee selectivity include varimetric directionality in un-grounded networks and an inverse-time behavior for the othergrounding options.

Networks With Distributed Grounding: In networks withdistributed grounding, it may be difficult to distinguish by theamount of amps between currents flowing to ground throughplanned ground connections, or those from a downed con-ductor. For low-resistance faults, this is not an issue becausecurrents are large and easy to sense by usual relays. However,faults involving short lengths of conductors contacting high-re-sistance soils can yield low fault currents and be precisely themost dangerous (see Fig. 12).

Hence, there is no doubt that high-resistance faults in net-works with distributed grounding will be better identifiedthrough the signature of their waveforms, not through their



magnitude. In other words, those are the scenarios where thesolutions developed by Don Russell and others and now com-mercialized by a number of manufacturers are most valuable.

VII. CONCLUSION

Beginning with a biophysical model for human physiology, aset of random variables was identified as determining the risk ofheart fibrillation for hazards related to downed conductors andfaults to poles. By applying Monte Carlo methods, it was foundthat to guarantee a very low risk of death, fault currents above afew amps shall not be sustained. Downed conductors and faultedpoles yield similar fault current limits.

Fulfilling the requirement to limit ground fault currents toa few amps depends on how a network is grounded. Specialrelays are most valuable where other methods cannot be applied,as in multigrounded networks or in lines where sensitive zero-sequence overcurrent relays cannot be used.

REFERENCES

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[15] Effects of Current on Human Beings and Livestock—Part 1: GeneralAspects, IEC 60479-1, IEC, 2007.

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José L. Pinto de Sá (SM’90) was born in 1951. He isa Professor of Electrical Engineering at the InstitutoSuperior Técnico, Lisbon, Portugal. He has worked insubstations advanced automation since 1981. In the1990s, he led the development of multifunction pro-tective relays for EFACEC, and from 2001 to 2006,he was with LABELEC as a Coacher and a Consul-tant. His main research interests are in power sys-tems protection, protective relaying, and substationsautomation.

Miguel Louro was born in 1978. He received theM.Sc. degree in electrical and computer engineeringfrom the Instituto Superior Técnico, Lisbon, Por-tugal, in 2008

He was with LABELEC from 2002 to 2007. Hismain tasks were to provide expertise toward powersystems protection for power distribution and gener-ation. He is now in the Network Analysis departmentof EDP Distribuição, where he focuses on large-scaleincident analysis and protection systems.

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