Page 757 - Industrial Power Engineering and Applications Handbook

Table 22.6 Safe (tolerab1e)potential difference

1 (i) ~ Soil resistivity, p L2m ~ 70
(ii) ~ Surface resistivity for cnncrctc of thicl\ne\\ /I, ~
ab 250 mm, p\, <I111 550
(iii) Surface resistivity for gravel of thicknc\\ h, a\
1 50 rnm, ps2 Qn1 - 2500
Duration of fault 5 i I .o I .0
1 (maximum clearing time of the interrupting device) 1
I
Averdge weight of a huinaii body (kg) 70 70
:. Safe touch voltage.
0 157
E, = (I s c, p, + inno) x
\t
70 - 550 70 - 2500
i Reflection tactor k=- k= ~
70 + 550 70 + 2500
= -- 4*0 = -0.77 = -~
2430 = - 0.95
620 2570
:. C, from Figure 22.14 C, = 0.85 C, = 0.68
0.157
:. E, = (1.5 x 0.85 x 550 + 1000) x ~
\/ 1
= (701.25 + 1000) x 0.157 = (2550 + 1000) x n. I 57
= 267 V = 557 v
0.157 0.157 = (6 x O.6X x 2500 + 1000) x o.157
Step voltage, E, = (6 c, p> + 1000) x T = 16. x 0.85 x 550 + 1000) x ~ \'7
\' f ;I
= (2805 + 1000) x 0.157 =(l0200+ 1000) ~0.157
= 591 V = 1758V
-
, Ground re\istance (62)
R,> = /L (equation (22.12)) L being Iargc
= 4\A
1 therefore
i\ ignored cor eax of calculation
1 knuinption\ hy experience: Area 90 000 120 000
~
Ground rcsi\rancc

' Total ground resistance of power plant and 0.103 x 0.089
' switchyard areas interconnecled in K,, = 0.103 + 0.089
parallel (recommended practice) where 12 = 0.0177