Comparison of touch and step voltages between IEEE Std 80 and IEC 479-1.pdf

Comparison of touch and step voltages between IEEE Std 80 and IEC 479-1 C.-H.Lee and A.l?Sakis Meliopoulos

Abstract: A comparison of the safety criteria of two widely accepted standards, i.e. the IEEE Std 80 and the IEC 479-1 is proposed. The two standards differ in their definitions of permissible body current and body resistance. Another difference is that the IEC 479-1 does not provide guidance on human feeusoil contact impedances. It tacitly assumes that these impedances are computable by the designer. A comprehensive study of permissible touch and step voltages by these two standards is included for a wide range of conditions, which enables a direct comparison of the two standards. It is shown that differences exist and these differences are quantified.

1

Introduction

Since the early days of the electric power industry, safety of personnel in and around electric power installations has been a prime concern. A mechanism by which the safety of personnel is affected is the ground potential rise of grounded structures during electric power faults, and the possibility of humans touchng grounded structures and therefore subjecting themselves to voltages. The 50 or 60Hz electric current conducted through a human body as a result of an accidental conduct with a grounded structure, under adverse conditions, should be of magnitude and duration below those that cause ventricular fibrillation. Over the years, and after many investigations of electric current effects on humans, safe limits have been established

safety. On the other hand, globalisation of national economies provides an interest towards harmonisation of standards. The first step in this endeavour is the technical comparison of various standards addressing the same issue. This paper provides a technical comparison of two standards that address the safety of electrical installations.

and standards have been developed which provide permis-

sible values of body currents to avoid electrocution. Two such standards are the IEEE Std 80 and the IEC 479- 1. The IEEE Std 80 has seen three editions (1961, 1976 and 1986); it is being currently revised, and has been in use in the USA and several other countries. The IEC 479-1 was released in 1984. The premise of both standards is to establish safe (permissible) body currents. The underlying assumption is that the designer of grounding systems will make sure that these values are not exceeded under adverse conditions of accidental contact of humans with grounded structures. The phdosophcal difference between the two documents is that while the IEC 479-1 does not address all relevant computational issues that may be 'necessary in the design process (such as feethoil resistance, etc.), the IEEE Std 80 does address most computational issues and provides procedures and guidance for assessing safety of a grounding system [l]. With ever increasing fault current levels in today's interconnected power systems, there is renewed emphasis on OIEE, 1999 ZEE Proceedings online no. 19990586 DOL lO.l049hpgtd:19990586 Paper received4th May 1999 C.-H.Lee is with the Departmentof Electrid Engineering, I-Shou Univmity, Kaohsiung, Taiwan, Republic of China A.P. Sakis Meliopoulos is with the School of Elatrid and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0250, USA IEE Proc.-Gener. Transm. Distrib., Vol. 146, No. 5, September 1999

Fig. 1 Defmitwn of electric shock mdeljnirameters - touch voltage

A2

Fig. 2

2

h& equivalent circuit

Electric shock model

Electric shock may occur when an individual touches a grounded structure during a fault (touch voltage), walks in the vicinity of a grounding system during a fault (step voltage), or touches two separately grounded structures during a fault (metal to metal touch voltage), etc. While each condition can be examined separately and in detail, we shall only focus on touch voltages. The electric shock model is shown in Figs. 1 and 2, which dlustrate a human standing near the middle point of a ground mesh and subjected to touch voltage. The electric shock model is the circuit which determines the flow of electric current in the human body. 593

The human body may come into contact with a ground or soil at three points (hand and two feet), as illustrated in Fig. 1. The grounding system and soil are represented with a Thevenin equivalent at the points of contact. Fig. 1 illustrates the equivalent resistances between any pair of the contact points, B, Al, and A2. When a fault occurs, voltages will appear between any pair of the points of contact B, Al, and A2. The Thevenin equivalent in this case is a three terminal circuit (terminals B, Al, and A2), and can be computed with proper analysis methods [2]. A typical simplification is to assume that the voltage at points AI and A2 are practically the same, in whch case the Thevenin equivalent is simplified to that of Fig. 2. The Thevenin voltage source Veq equals the open circuit voltage, in this case the voltage at the points of contact when the human is not touching. The equivalent internal resistance, as shown in Fig. 2 between the points of contact, can be accurately computed with numerical techniques [ 2 4 . For the electric shock model of Fig. 2, the following definitions apply: Touch voltage (or Thevenin equivalent voltage): The open circuit potential difference between a grounded structure (point B) and the surface potential at the point where a person is standing (points A1 and A2). Body voltage: The voltage across the human body when the electric shock circuit is closed. Body resistance:The resistance of the human body between the points of contact, i.e., in the case of Fig. 1 between point B and points A1 and A2 (hand to two feet). It depends on many factors such as size, skin condition, pressure at the contact, etc. Touch Resistance (Thevenin equivalent resistance): The resistance of the soil between the point of contact of the human body with the soil (points A1 and A2) and the grounding system, i.e. req. Body current:The electric current through the human body. The described electric shock model is inherent in both documents. However, the two documents differ in the application of the electric shock model. Table 1 provides an overview of the differences between the two documents with reference to the electric shock model. In subsequent paragraphs, a more detailed discussion of the safety criteria adapted in the two documents will be presented, followed by a comparison.

3

Safety criteria - IEEE Std 80

The IEEE Std 80 is based on a simplified electric shock model. The parameters of the electric shock model are shown in the second column of Table 1. This model is usually translated into permissible touch (or step) voltages. As an example, the permissible touch, VTPerm,and step, Vs,perm, voltages for a 50kg person are

0.116

VT,peTm

0.116

VS,perm

h

Additional comments and observations regarding the IEEE Std 80 are given below. The permissible body current has been selected from statistical data and represents a 0.50/0probability of ventricular fibrillation. It is believed that the approximate formulae for the Thevenin equivalent resistance in IEEE Std 80 were derived as follows. The human foot can be modelled as a circular plate touchng the surface of the earth. The resistance of the plate to remote earth is approximately

where p is the resistivity of the earth and b is the radius of the plate. The human foot is definitely not a circular plate. However, it has been observed with scale models and numerical studies that the area of the foot in contact with the earth is the most important variable. For this reason, b can be approximated with

where A is the area of the foot in contact with the earth. For an adult with large feet, the area A of the person’s feet is approximately 200cm2.Using this value, the radius is b = 0.08m and the resistance of one foot touchng the earth is

where p is expressed in Qm. The IEEE Std 80 further assumes that the mutual resistance between the two feet (see Figs. 1 and 2) has negligible effect, and thus the equivalent resistance is simply the parallel combination of the two feet to soil resistances

The equivalent resistance, req, in Fig. 2 should also take into account the resistance of the grounding system. However, for practical grounding systems, this resistance is typically small compared to the resistance 1.5p, and is thus omitted. Cases in which the effect of the grounding system resistance can account for more than 2% are of academic importance only. The above equations for req apply to the case of uniform soil and neglect the effect of grounding system proximity or mutual resistance between feet. For nonuniform soil or for soil with a cover layer of high resistivity, the IEEE Std 80 provides a correction factor c,(h,, k). Specifically, the equivalent resistance req is given by

+

~ ~ ~ = -( 1 . 5 ~1000)

h

+

= -( 6 . 0 ~ 1000) ~ ~ ~

r e q= 6.0cs(hs,k ) p s Teq

= 1.5cS(h,, k ) p s

for step voltage

(3)

for touch voltage

(4)

Table 1: Electrical shock model differences between IEEE Std 80 and IEC 479-1

594

IEEE Std 80

IEC 479-1

Body resistance

100052

Voltage-dependent and Path-dependent (Figs. 5 and 6 )

Thevenin equivalent resistance

1.5cgSfor touch voltage 6.0c8,for step voltage

no guidance

Thevenin equivalent voltage

Simplified equations k; k, L l o r use of computer models is suggested

no guidance

Permissible body current

O.116Adtfor 50kg person 0.157Adtfor 70kg person

S-curves (Fig. 7) independent of human size IEE P r o c G e n e r . Tmnsm. D u t r i b . , Vol. 146, No. 5, September I999

where = (P

-

Ps)/(P

+

(5)

P S I

Note that the region of greatest discrepancy is for layers of 1 4 in (or 0.0254-0.1016 m) thck, which is the usual case.

py = resistivity of the upper layer, p = resistivity of the soil below the upper layer, h, = thickness of the upper layer, c, = reduction factor for derating the nominal value of the

surface layer resistivity determined as follows: (1) c, = 1.04 for uniform soil, and (2) for nonuniform soil, the IEEE Std 80 provides a graph (Fig. 3 of the standard) for the graphical determination of c, from k and h,y. Investigation with computer models revealed that the IEEE Std 80 approximate formulae are accurate for all practical purposes for uniform soil only. For soil with an upper layer of high resistivity stone the correction factor cs(hs,k) is in error, especially for the practical case of upper layer thickness of 1-4 in (0.02544.1016m).

0.12 0.18 layer thickness, m Reduction factor comparison of IEEE Std SO andprogrm SGSYS 0

Fig. 4

0.06

IEEEStd 80

~

4 4 - SGSYS computed

4

i8

4.0

Safety criteria - IEC 479-1

.

3.5

0

Fig.3

1.o

2.0

D Feet to soil resistances as a fwzction of feet separation and gravel

thickness

= 3925Rm. pyclt/ = 2108m, D = feet separation. ft

The computer-based method for evaluation of the correction factor c,(h,, k ) consists of computing an equivalent voltage source connected to the points of contact of the human body with the ground field as indicated in Figs. 1 and 2. The points of contact of the human feet with the earth’s surface are modelled with two metallic plates placed at the location of the feet. The shape and dimensions of the plates are shown in Fig. 3. The grounding system together with the contact model is then viewed as a system with multiple grounds. This system has three terminals Al, f i 2 , and B. The elements of the equivalent circuit are computed with the method of moments [2,4]. Standard network techniques are then employed to compute the Thevenin equivalent parameters veq, req, as illustrated in Fig. 2. It is important to note that modelling the feet as two plates (surface electrodes) provides a realistic analysis model. The correction factor c,(h,, k) is then computed from equation

7000 -

6000 t C: 5000 ai

5

4000

-

req

c s ( h s , k )= 1.5Ps

The value of reg, and therefore cs(h,, k), depends on foot size and spacing between feet. Using a foot model as shown in Fig. 3, the IEEE Std 80 model is matched exactly for uniform soil and assuming feet separation of D = 2 ft. Fig. 3 also illustrates the effect of the mutual resistance between the two plates representing the two feet. Note that for the usual standing position D = 1-2 ft, the effect of the mutual resistance is negligible. However, as the feet come closer than 1 ft, the effect of the mutual resistance is such as to increase the value of req. The computed values of c,(h,, k) are given in Fig. 4 superimposed on the present values of the IEEE Std 80. IEE Proc -Gener Trunsm Distrib , Vol 146, No 5, September 1999

The data of IEC 479-1 can be utilised in two ways: (a) actual body currents can be computed for an individual subjected to a touch or step voltage in a specific system and specific conditions, and (b) permissible touch and step voltages can be computed for a specific system. 595

a

Fi .6 Internal i m p e k e

Pa'

of the human

bo& as a fiction of the current

Numbers in brackets indicate current paths between both hands and corresponding part of the body. Numbers without brackets indicate impedance of several paths in the body as percentage of hand to hand impedance. Data from IEC 479-1 (1984).

3L zone 4

a

10-

E

zone2

10

'--.

Figs. 5 and 6. req,T is the feet to soil resistance for touch voltage, i.e. the two feet to soil resistances are in parallel. reqSis the feet to soil resistance for step voltage, i.e. the two feet to soil resistances are in series. The IEC 479-1 does not provide any data for r e , , or req,* For this reason, we shall use the data of IEEE Std 80, i.e. Eqns. 3 and 4. Note that application of above equations to obtain the permissible touch and step voltages is straightforward and involves the following steps: Step 1: For a given electric shock duration t and assumed probability of ventricular fibrillation, determine the value of permissible body current, ib,penn(t), from Fig. 7. Step 2 For the current ib,penn(t), determine the body resistances RbT and Rbs from the data of Figs. 5 and 6. For touch voltage it is expedient to assume that the path will be one hand to two feet (75% of the body resistance given in Fig. 5), and for step voltage the path is foot to foot (100% of the body resistance given in Fig. 5). Step 3: Compute req,Tand reqSfor IEEE Std 80. Step 4: Compute permissible touch and step voltages using eqns. 6 and 7 For comparison purposes with the IEEE Std 80, the permissible body current ib,pem(t) is computed for probability 0.5'1/0 of ventricular fibnllation by proper interpolation between the curves C1 and C2 of Fig. 7.

4.2 Computation of actual body current For a given touch or step voltage, the computation of the body current using the IEC data requires the solution of a set of nonhear equations. This solution can be obtained iteratively or with a graphical method described below. Step 1: Compute the Thevenin equivalent resistance, req, of the electrocution circuit. Step 2 For a given (or computed) touch (or step) voltage and equivalent resistance re from step 1, the actual body current is computed with t i e graphical method shown in Fig. 8. Specifically, the actual body current is determined by the simultaneous solution of the following two equations: &ouch

1

2

10

3 10

10

duration of current-flow, ms

Fig.7

Ib

Permissible bo4 currentfor IEC 479-1

4. I Permissible touch and step voltage IEC 479-1 The permissible (or allowable) touch, VTa,and step, Vf, voltages are computed from the following equations

v$ = i b , p e r m ( t ) v:

[RF ( i b , p e r m ( t ) ) $- r e q , ~ ] (6)

= i b , p e r m ( t ) [Rf

( i b , p e r m ( t ) ) $- Teq,S]

= V b / r b = T/b/f(Vb)

(9)

where the function rb = V b ) represents the nonlinear characteristics of the body resistance as a function of body voltage determined with the data of Fig. 5. Note that eqn. 9 represents a nonlinear function whch is illustrated in Fig. 8 as curve 1.

r1-

350

(7)

where ibgem(t) is the permissible body current for IEC 4791 for an electric shock duration t. This current is obtained from the data of Fig. 7. &T(ib,pem(t)) is the body resistance for the path specified by the touch voltage (typically hand to two feet) and for a body current equal to ibpem(t). This value can be obtained from the data of Figs. 5 and 6. R,,s(ib,pem(t)) is the body resistance for the path specified by the step voltage (foot to foot) and for a body current equal to ibgem(t). This value can be obtained from the data of 596

(8)

v b $- r e q i b

4

10

CUNe 1

50 B(Vtouch/req, 0)

0

0

50

I

1

100 150 200 250 300 350 400

body current, mA

Fig.8 Graphical methodof computing a c t d b o r f v current IEE Proc-Cener. Transm. Distrib., Vol. 146, No. 5, September 1999

Table 2: Permissible touch voltages per IEEE Std 80,1986 edition 50kg person, probability of ventricular fibrillation 0.5% Shock

Soil resistivity

duration

50Qm

100Qrn

200Qrn

500Qm

1000Qrn

3000Qm

0.05s

526.9V

559.2V

599.7V

680.6V

923.4V

1328.0V

2946.6V

0.10s

372.5V

395.4V

424.0V

481.3V

652.9V

939.1V

2083.6V

0.15s

304.2V

322.9V

346.2V

393.0V

533.1V

766.7V

1701.2V

0.20s

263.4V

279.6V

299.8V

340.3V

461.7V

664.0V

1473.3V

0.25s

235.6V

250.1V

268.2V

304.4V

413.0V

593.9V

1317.8V

0.30s

215.1V

228.3V

244.8V

277.9V

377.0V

542.2V

1202.9V

0.35s

199.1V

211.4V

226.7V

257.3V

349.0V

502.0V

1113.7V 1041.8V

0.40s

186.3V

197.7V

212.0V

240.6V

326.5V

469.5V

0.45s

175.5V

186.4V

199.9V

226.9V

307.8V

442.7V

982.2V

0.50s

166.6V

176.8V

189.6V

215.2V

292.0V

420.0V

931.8V

Table 3: Permissible step voltages per IEEE Std 80,1986 edition, 50 kg person, probability of ventricular fibrillation 0.5% Shock

Soil resistivity

duration

50Qm

100Qm

200Qm

500Qm

1000Qm

3000Qm

0.05s

551.1V

680.6V

842.5V

1166.2V

2137.3V

3755.9V

10230.1V

0.1os

389.7V

481.3V

595.7V

824.6V

1511.3V

2655.8V

7233.8V

0.15s

318.2V

393.0V

486.4V

673.3V

1234.0V

2168.5V

5906.4V

0.20s

275.6V

340.3V

421.2V

583.1V

1068.7V

1877.9V

5115.0V

0.25s

246.5V

304.4V

376.8V

521.5V

955.8V

1679.7V

4575.0V

0.30s

255.0V

277.9V

343.9V

476.1V

872.6V

1533.3V

4176.4V

0.35s

280.3V

257.3V

318.4V

440.8V

807.8V

1419.6V

3866.6V

0.40s

194.9V

240.6V

297.9V

412.3V

755.7V

1327.9V

3616.9V

0.45s

183.7V

226.9V

280.8V

388.7V

712.4V

1252.0V

3410.0V

0.50s

174.3V

215.2V

266.4V

368.8V

675.9V

1187.7V

3235.0V

Table 4 Permissible touch voltages per IEC 479-1,5% body resistance values, probability of ventricular fibrillation 0.5%, hand to two feet Shock

Soil resistivity

duration

10Qm

50Qm

lOOQm

200Qrn

500Qm

1000Qm

3000Qm

0.05s

342.4V

375.8V

417.6V

501.2V

751.8V

1169.6V

2840.9V

0.1os

319.0V

349.3v

387.1V

462.7V

689.6V

1067.7V

2580.2V

0.15s

287.5V

313.8V

346.7V

412.5V

609.9V

938.9V

2254.9V

0.20s

256.7V

279.5V

308.1 V

365.1V

536.1 V

821.2V

1961.6V

0.25s

222.2v

241.3V

265.2V

313.1V

456.6V

695.9V

1652.9V

0.30s

187.9V

203.7V

223.4V

262.7V

380.9V

577.8V

1365.3V

0.35s

148.8V

160.9V

176.0V

206.2V

296.9V

448.2V

1053.0V

0.40s

121.7V

131.0V

142.7V

166.1V

236.1V

352.9V

820.1V

0.45s

101.1V

108.5V

117.9V

136.6V

192.6V

286.1V

660.0V

0.50s

88.9V

95.3V

103.4V

119.5V

167.8V

248.2V

570.1V

IEE Proc-Gener. Transm. Distrib., Vol. 146. No. 5. September 1999

591

Table 5: Permissibletouch voltages per IEC 479-1,50% body resistancevalues, probability of ventricular fibrillation 0.5%, hand to two feet Shock

Soil resistivity

duration

108m

508rn

1009rn

2008rn

5008m

10008rn

0.05s

449.4V

482.8V

524.6V

608.2V

858.8V

1276.7V

2947.9V

0.10s

415.5V

445.7V

483.6V

559.2V

786.1V

1164.2V

2676.6V

30008rn

0.15s

374.5V

400.9V

433.8V

499.6V

697.0V

1026.0V

2342.0V

0.20s

334.6V

357.4V

385.9V

442.9V

614.0V

899.1V

2039.5V 1720.4V

0.25s

289.6V

308.8V

332.7V

380.6V

524.1V

763.4V

0.30s

245.1V

260.8V

280.5V

319.9V

438.OV

634.9V

1422.4V

0.35s

193.8V

205.9V

221.0V

251.2V

341.9V

493.2V

1098.0V

0.40s

152.9V

162.2V

173.9V

197.3V

267.3V

384.1V

851.3V

0.45s

128.8V

136.3V

145.6V

164.3V

220.4V

313.9V

687.7V

0.50s

116.0V

122.5V

130.5V

146.6V

194.9V

275.4V

597.3V

Table 6: Permissible step voltages per IEC 479-1,5% body resistance values, probability of ventricular fibrillation 0.5%, hand to two feet Shock

Soil resistivity

duration

108rn

50Bm

1008m

200Bm

5008m

10008rn

3000Qrn

0.05s

367.5V

501.2V

668.3V

1002.5V

2005.3V

3676.5V

10361.5V

0.10s

341.7V

462.7V

614.0V

916.4V

1823.9V

3336.4V

9386.2V

0.15s

307.2V

412.5V

544.1V

807.3V

1596.9V

2913.0V

8177.1V

0.20s

273.8V

365.1V

479.1V

707.2V

1391.4V

2531.8V

7093.2V

0.25s

236.5V

313.1V

408.8V

600.2V

1174.4V

2131.5V

5959.6V

0.30s

199.7V

262.7V

341.5V

499.0V

971.5V

1759.1V

4909.3V

0.35s

157.8V

206.2V

266.7V

387.7V

750.6V

1355.4V

3774.8V

0.40s

128.7V

166.1V

212.8V

306.2V

568.5V

1053.6V

2922.3V

0.45s

106.7V

136.6V

174.0V

248.7V

473.1V

846.9V

2342.4V

0.50s

93.7v

119.5V

151.7V

216.1V

409.2V

731.1V

2018.7V

Table 7: Permissible step voltages per IEC 479-1,50% body resistance values, probability of ventricular fibrillation 0.5%. hand to two feet Shock

Soil resistivity

duration

109m

0.05s

474.5

608.2

0.1os

438.2

559.2

0.15s

394.3

499.6

0.20s

351.7

442.9

0.25s

304.0

0.30s

50Qm

1008rn

2008rn

5008m

775.3

1109.5

2112.3

3783.5

10468.5

710.4

1012.9

1920.4

3432.9

9482.7

631.2

894.4

1684.0

3000.0

8264.2

557.0

785.1

1469.3

2609.6

7171.1

380.6

476.3

667.7

1241.9

2198.9

6027.1

256.9

319.9

398.6

556.1

1028.7

1816.2

4966.4

0.35s

202.8

251.2

311.7

432.7

795.6

1400.4

3819.8

0.40s

159.9

197.3

244.0

337.4

617.7

1084.9

2953.5

0.45s

134.4

164.3

201.7

276.5

500.8

874.7

2370.1

0.50s

120.9

146.6

178.8

243.2

436.3

758.2

2045.8

Figs. 9-12 provide the data of Tables 2-7 in graphical form. The coordinates are the permissible touch or step voltages of the two standards, respectively. Each point represents permissible voltages, as allowed by the two standards computed for the same parameters of soil resistivity and shock duration. By construction, then, each point on the diagonal of the graph represents a case where the two standards yield the same permissible voltages. Points above the diagonal represent cases where the IEC 479-1 is more conservative than the IEEE Std 80, whde points below the 598

1000Qm

30008m

diagonal represent cases where the IEEE Std 80 is more conservative than the IEC 479-1. Note that the points are about evenly distributed around the diagonal. Finally, Figs. 13 and 14 compare the body resistance value used for the computation of the permissible touch and step voltages with the two standards. Note that for the usual shock durations 0.25-0.5s, the 5% body resistance of the IEC 479-1 standard is near 100052 or higher. Ths is useful information for persons questioning the use of 100OQ in the IEEE Std 80. IEE Proc.-Gener. Transm. Distrib., Vol. 144, No. 5, September 1999

1600 1400 -

IEC 479-1 more

1200 -

d 5

1000 -

c

.I

p! A

600 -

13

400-

U

IEEE Std 80 more conservativethan

-

800

Fi 9 Permissible touch voltages er IEEE Std 80 against IEC 479-1; 5% bog resistunce values, probability ofvenntricular fibrillation 0.5% hand to two

feet

11 10

01 0

A

1 2 3 4 5 6 7 8 9 1011 permissible step voltage, kV (IEC 479-1)

Fig. 10 Permissible step voltages per IEEE Std 80 vs IEC 479-1; 5% body resistme values, probubility of ventriculrufbrilhtion 0.596, hund to two feet

r

3.0 2.5 2.0

IEC 479-1 more conservativethan IEEE Std EO

-

SQ

1.5-

2g .g 2

1.0

-

x x

’/

,LEE Std 80 more Conservativethan IEC 479-1

0.5

0.5 1.0 1.5 2.0 2.5 3.0 permissible touch voltage, kV (IEC 479-1)

Fi .I1 Permissible touch voltagesper IEEE Std 80 ugaht IEC 479-1; 50% b a resistance values, probability of ventricular fbrilhtion 0.5%, hand to two feet

600 400

1

0

I

’

‘

I

0.50

IEEE Std 80

0.10 0.20 0.30 0.40 electric shock duration, s

0.50

Fi . I 4 Body resistance aguinst electric shock h a t i o n at mm permissib!! t a c h voltage; 50Y( I body resistance values, hand to twofeet

6

c’o

’

IEC 479-1

IEEE Std 80 more conservativethan IEC 479-1

2

6

’

touch voltage; 5% body resistance values, hund to two@et

x5

B ’m

I

Fi . I 3 Body resistance against electric shock duration at mwcimm permis-

sib!

g” 1000 2 800

86

I

0.10 0.20 0.30 0.40 electric shock duration, s

IEC 479-1 more

3

IEEEStd 80

-

200 0.5 1.0 1.5 2.0 2.5 3.0 permissible touch voltage, kV (IEC 479-1)

-

-

Summary and conclusions

The safety criteria of the IEC 479-1 and IEEE Std 80 have been compared and their differences have been quantified. There are cases in whch the IEEE Std 80 is more conservative than the IEC 479-1 and vice versa. The IEC 479-1 safety criteria are rather complex, while the safety criteria of IEEE Std 80 are simplified. The opinion of the authors is that simplicity is important. Given the fact that the safety criteria include comfortable safety margins, one can conclude that the simplicity of the IEEE Std 80 does not compromise safety in grounding system design. Another major difference is that the IEC 479-1 does not address all relevant computational issues, while the IEEE Std 80 provides approximate equations and formulas which are useful to a designer. The IEEE Std 80 also provides useful procedures for grounding systems safety assessment. References

11 IEC 479-1 more conservativethan IEEE Std 80 7 6 5 t

/ x

IEEE Std 80 more conservativethan IEC 479-1 1 2 3 4 5 6 7 8 9 10 11 permissible step voltage, kV (IEC 479-1)

Fi .I2 Peimirsible step voltages er IEEE Std 80 against IEC 479-1; 50% bo!y resistance values, probability of ventricularfibrillation 0.5%, hund to two feet IEE Proc.-Gener. Transm. Distrib., Vol. 146, No. 5, September 1999

\

ANSVIEEE Std 80-1986: ‘IEEE guide for safety in AC substation grounding’. 1986 SAKIS MELIOPOULOS, A.P.: ‘Power system grounding and transients: an introduction’ (Marcel Defier, Inc., New York, 1988), pp. 119-133 International Electrotechnical Commission IEC Report: ‘Effects of current passing through the human body, part 1: general aspects’. 4791, IEC 1984 SAKIS MELIOPOULOS, A.P., XIA, F., JOY, E.B., and COKKINIDES, G.J.: ‘An advanced computer model for grounding system analysis’, IEEE Trans. Power Deliv., 1993, 8, (l), pp. 1%23

Appendix: Analytical expressions of body resistance vs. body voltage

This Appendix presents analytic expressions for the voltage-dependent body resistance which closely match the IEC 479-1 data. These formulae can be used in lieu of the 599

Table 8 Total body impedance, per IEC 479-1 (data from IEC 479-1 (1984)) Values for the total body impedance (9)that are not

Touch

exceeded for a percentage (percentile rank) of

voltage (v)

5% of the population

50% of the population

95% of the population

25

1750

3250

6100

tabular data of the IEC 479-1. Table 8 presents a reproduction of body resistance data in the IEC 479-1. The Table contain three sets of data corresponding to the stafistical values of 5%, 50% and 95% of population (see IEC 479-1). Each one of the sets can be approximated with one analytic function of the form Rb,model(Z)

50

1450

2625

4375

75

1250

2200

3500

100

1200

1875

3200

125

1125

1625

2875

220

1000

1350

2125

700

750

1100

1550

1000

700

1050

1500

2000

677

1084

1464

&,model

50% of the population

668.381

1080.02 1

1427.296

bl

15344.426

7751.063

73049.270

C1

515.588

2552.197

21 19.462

Vn

311.673

80.182

131.727

( ~ 2b ,2 ~ 2 ~ 2 0 )

bzib

-k C2

*

The coefficients a l , bl, c1 and vo or the vector X = [al, bl, c1, vo] and the coefficients q,b2, c2 and i,, or the vector X = [a2,b2, c2, io] will be different for each one of the three sets of data. For each data set, the unknown vectors x = [al, bl, c1, volT and x = [a2, b2, c2, &IT are computed by the weighted least squares method, i.e. by solving the following optimisation problem. m

Minimise J =

95% of the population

a1

(z)= &,model = U 2 -k

Table 9: Computed coefficients of the analytic expressions for body resistance 5% of the population

= R b , m o d e l (a1> b l >c 1 , 210)

wir: = rTWr i= 1

where r = - Rb,mode,(~), w iis weight for the residual ri, and W is a diagonal matrix, the diagonal elements being the weights w i The solution to the above nonlinear estimation problem is obtained with the following algorithm

Table IO: Computed coefficients of the analytic expressions for body voltage

5""

= X u - (HTWH)-lWTWIRb,modei(Zu) - b]

where b = Rb,mpanued, Rb(xV+')- R&")

+ H.(x"+~ - x"), and

H = dRb(X)/aXIX=XO 5% of the population

50% of the population

a2

140.6

63.1

9

0.6

1.o

95% of the population

The computed analytic expression for the three sets of data, using the above algorithm with wi = 1.0 for all i, are listed in Tables 9 and 10. The accuracy of the derived analytic expressions as compared to the IEC 479-1 data is illustrated in Tables 11 and 12. For all practical purposes, the analytic expressions can be used in lieu of the data of Table 8.

77.8 1.4

Q

-130.1

-50.5

-68.0

io

241.1

48.1

26.9

Table 11: Comparison of the proposed formula for resistance to the IEC 479-1 data Values for the total body impedance (a)that are not exceeded for a percentage (percentilerank) of Touch voltage

.-.

5% of the population

50% of the population

95% of the population

IVI

IEC 479-1

600

Proposed formula

IEC 479-1

Proposed formula

IEC 479-1

Proposed formula 6102.3

25

1750

1758.0

3250

3258.6

6100

50

1450

1414.4

2625

2603.1

4375

4338.3

75

1250

1278.3

2200

2185.0

3500

3600.7 3149.8

100

1200

1195.9

1875

1890.8

3200

125

1125

1136.4

1625

1678.9

2875

2832.2

220

1000

992.7

1350

1279.4

2125

2158.3

700

750

744.9

1100

1091.5

1550

1542.1

1000

700

704.6

1050

1087.8

1500

1501.4

2000

677

668

1084

1080.0

1464

1427.0

IEE Proc-Gener. Transm. Distrib., Vol. 146, No. 5, September 1999

Table 12: Comparison of the proposed formula for body current vs. body voltage to the IEC 479-1 data Values for the body voltage (V) that are not exceeded for a percentage (percentile rank) of

5% of the population Body current (mA)

14.3 34.5 60.0 83.3 111.1

IEC 479-1

25 50 75 100

95% of the population

50% of the population Proposed formula

26.5 48.6 75.2 98.7 125.4 220.8 677.7

Body current (mA)

7.7

IEC 479-1

19.0 34.1

25 50 75

53.3 76.9 163.0 636.4

100 125 220 700

220.0 933.3 1428.3

125 220 700 1000

1000.1

952.4

1000

2954.2

2000

1913.1

1845.0

2000

IEE Proc.-Gener. Transm. Distrib.. Vol. 146, No. 5, September 1999

Proposed formula

27.7 47.9 71.9 99.2 128.9 222.5

Body current (mA)

4.1 11.4 21.4 31.3 43.5

IEC 479-1

25 50 75 100 125 220

Proposed formula

25.1 49.1 76.7 99.7 124.3

692.2

103.5 451.6

700

219.4 701.8

1004.6 1908.1

666.7 1366.1

1000 2000

998.9 1990.3

60 1