ANALYSIS OF SOME SAFETY ASSESSMENT STANDARD ON GROUNDING SYSTEMS

Transcription

1 ANAYSIS OF SOME SAFETY ASSESSMENT STANDARD ON GROUNDING SYSTEMS Shang iqun, Zhang Yan, Cheng Gang School of Elecrical and Conrol Engineering, Xi an Univeriy of Science & Technology, , Xi an, China, Abrac. There are differen afey aemen andard of grounding yem in differen counrie. Some of andard are widely adoped in he world, uch a IEEE , IEC-471-1, EA-TS41-24 and BS7354. In order o chooe reaonable afey aemen andard in he deign of grounding yem of ubaion, hee four andard are compared uing a pracical cae. I howed ha BS7354 i more conervaive han IEEE and EA-TS41-24, and he definiion of allowable curren are differen beween IEEE and IEC eyword: grounding yem, afey aemen andard, ouch volage, ep volage. Inroducion Grounding grid i an imporan guaranee for ubaion afe operaion. I can no only provide public reference for variou equipmen inide ubaion, bu alo excree faul curren rapidly, reduce he rie of ubaion' ground poenial and enure he peron' and equipmen' afey when he yem i failure. The main elecric parameer of ubaion grounding grid i grounding reiance, ouch poenial and ep poenial. The grounding yem afey aemen i a e which compare he maximum ouch poenial and ep poenial of he human body wihand wih given afey andard. For he afey aemen of grounding yem, differen naion have differen andard. Some of andard are widely adoped in he world, uch a IEEE , IEC-471-1, EA-TS41-24 and BS7354. In order o provide a reference for afey aemen of acual grounding yem, hoe four andard are analyzed and compared via a pracical example in hi paper. Circui Model Before dicuing he volage ha human body can wihand, we hould briefly explain he curren which human body can wihand. The effec when curren hrough he vial organ of human body depend on he ime of curren paing hrough he body, he ineniy and frequency of he curren. The greae danger i ha he curren can caue venricle remor and breahing op when he curren hrough he body. In he range of 0.03 econd o 3 econd, he curren which will no caue venricle remor and breahing op, I B, depend on he energy aborbed by he body, S B. S = I 2 (1) B B 111

2 Where, i he ime of curren hrough human body. Touch volage and ep volage can be expreed in he following equivalen circui, Fig.1 H V h I B R B Z h Fig. 1. equivalen circui of ouch volage and ep volage In he Fig.1, R B i he reiance of he body, V h i he volage beween H and F when he elecric hoc doen' occur; Z h i he equivalen impedance of power ource when he elecric hoc occur. Comparion of he Definiion of Safey Sandard IEEE Sd IEEE Sd propoed he curren which he human body can wihand, I B. For hoe whoe weigh were 70g, F I B = (2) The calculaion formula of he andard ouch volage E and he acual ouch volage E m.are alo given in hi andard a following equaion (3) and equaion (4). E = ( C ρ ) (3) Where, C S i he reducion coefficien of reiiviy, ρ i he reiiviy of he urface gravel layer. E m G m i = (4) m Where, m i geomeric facor of grounding grip. i i error correcion facor of m, m i effecive lengh of buried conducor, ρ i reiiviy of he oil, I G i influx faul curren. 112

3 Thi andard comparing he value of he acual ouch volage and he andard ouch volage o deermine wheher he ouch volage i afey. I hould be poined ou ha he reiance of he hoe i no conidered in hi andard. BS Sd The ecion heading are in boldface capial and lowercae leer. Second level heading are yped a par of he ucceeding paragraph (lie he ubecion heading of hi paragraph). BS Sd 7354 provide he calculaion formula of he acual ouch volage, V T. 0.5 V {ln( h / d) + C} ρ V T = π R i (5) Where, C S i reducion coefficien of reiiviy, R i he meh reiance, i he oal lengh of grounding conducor, h i he burial deph of grounding, d i he diameer of he conducor, V i meh poenial, i =(0.15n +0.7), n i he oal number of parallel conducor. In hi andard, V T i defined a he um of he volage a one meer ouward along he diagonal of grounding grid and he volage of grounding grid o he urface. The calculaion formula of andard ouch volage i no given in BS Sd 7354, bu i provide he equivalen circui of ouch volage, a howed in Fig.2. Where, R F, R RF are he reiance of lef and righ foo hoe, R C i ouch reiance, R B i body reiance. R B=1Ω V T R F=4Ω R C=3ρ R RF=4Ω RC=3ρ Fig. 2. he equivalen circui of ouch volage Uing he equaion (6), he curren hrough he human body, I T, can be calculaed, compare I T wih he curren ha he human body can wihand, which i given in IEC RF + Rc VT = IT ( RB + ) (6) 2 EA-TS Sd The calculaion formula of he acual ouch volage, E, wa given by EA- TS41-24 a following equaion (7), e d E ( grid) = (7) I i imilar o he formula in IEEE , bu he parameer calculaion ha ome difference. Where, e i he facor concerned wih he diribued curren, d i modified coefficien o uneven diribued elecrode curren, i he lengh of all conducor, including verical grounding rod, p i he perimeer of grounding grid. EA-TS41-24 require ha here mu have 113

4 fence around he grounding grip and he diance i wo meer, he ouch volage formula of he fence i a following equaion (8), f d E ( fence) = (8) Where, f = 0.26 e. EA-TS41-24 didn' given he formula of he andard ouch volage, bu uing he curren v. ime curve which wa given in IEC o deermine wheher i i afe. Fig. 3. curren curve effec on human body in IEC andard In Fig.3, line a i he line ha he elecric hoc curren i 0.5mA, which people ha feeling and reacion. On he lef of line a, people ha no feeling and no reacion o he elecric hoc curren, called no reacion zone, howed a he "region 1" in Fig.3. On he righ of line a, he area i called "region 2", in which people ha feeling and reacion o he curren. ine b i called afey curve. "Region 3" which beween line b and line c i non-faal pahological and phyiological effec area, in hi area he elecric hoc curren may caue convulion, difficuly breahing and hear funcion diorder. On he righ of line c i "region 4", in which i may caue faal venricular fibrillaion and evere burn danger. IEC didn' give he calculaion formula of andard ouch volage and acual ouch volage, i only provide he effec curve of curren hrough human body, and body reiance curve in differen condiion. In hee andard we uually conider ha ep volage i le han ouch volage, and generally believed ha if he ouch volage mee he afey value, he ep volage alo mee i. Example Analyi Now we can give ome analyi hrough a concree engineering example. A 115V ubaion: lengh a=60m, widh b=50m, grounding grid area A=2867m 2, hicne of gravel layer h =0.1m, reiiviy ρ =5000Ω m, reiiviy of oil ρ=52.33ω m. The maximum faul curren i 10A, and he faul ime i 1, grounding grid pacing D=5m; he number of grounding conducor n A =n B =12, oal lengh of grounding grid C =1200m, deph 114

5 h=0.8m, diameer d=0.0125m, he grounding rod are a he edge of he grounding grid, he oal number n=34, and he oal lengh r =81.6m, diameer d 2 =0.02m, each lengh 2 =2.4m. IEEE Calculae he acual ouch volage G Em = Where, m i he geomeric facor of grounding gird, m =0.6. i i he error correcion facor of m, i =2.72, he value i M i he effecive lengh of buried conducor r M = C + [ ( )] =1330m, i can be drawn ha acual ouch volage E R m i 2 2 x + y Gmi Em = =642V M Sandard ouch volage E i, E = ( C ρ ) =781V, Where, C i he reducion coefficien of reiiviy. We can find ha acual ouch volage E m i le han andard ouch volage E, o he grounding grid i conidered o be afe. BS Calculae he acual volage ρ V 0.5 VT = π R {ln( h / d) + C} i = 848V R i he grounding reiance, i can be calculaed according o he formula and i value i Ω; V i ground poenial riing, i value i 4784V. RF + Rc VT = IT ( RB + ), 2 R B =1000, R F =4000, R C =15000, he curren paing hrough he human body I T can be obained, I T =81mA. According o he curren v. ime curve a howed in Fig.3 in IEC-471-1, hi grounding grid doen' mee he afey andard. EA-TS The ouch volage of he fence i, f d E ( fence) = = 0.26E ( grid) = V E = IT ( RB + R f ), o he curren paing hrough he human body i I T =4mA. According o he curren v. ime curve a howed in Fig.3 in IEC-471-1, hi grounding grid mee he afey andard. M m i 115

6 Concluion According o calculaion reul of a pracical engineering example, i can be found ha BS 7354 i more conervaive han he oher wo andard. The andard of he curren which he human body can wihand in IEEE i larger han ha of human body afey curren which i propoed by IEC So he afey aemen andard hould be applied according o he pracical iuaion. Reference [1] Chen Xianlu, iu Yugen, HuangYong. Grounding. Chongqing Univeriy Pre, [2] Chen Jiabin. Grounding Technology and Grounding Device. China Elecric Power Pre, 2002 [3] iuyan, Ding Daiyong. Analyi of he Difference and Relaionhip beween he Two ind Grounding Syem Safey Sandard. in Elecrical Engineering Technology, vol. 36(1), 2007, pp [4] Wang Hewen. Grounding Grid Deign of Foreign Counry Subaion Projec in Nonferrou Meal Deign, vol. 3, 1995, pp42-49, 21 [5] ANSI/IEEE , IEEE Guide for Safey in AC Subaion Grounding [6] Inernaional Elecroechnical Commiion Repor, Effec of Curren Paing hrough he Human Body Par 1. General Apec IEC,