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TM HIU IN P BC V CCH PHNG TRNH TAI NN INTrong qu trnh vn hnh, do nhiu nguyn nhn khc nhau, vic dy dn in b t ri xung t gy nguy him cho con ngi l iu khng th trnh khi. Bi vit a ra mt s phn tch v hin tng trn a ra nhng kin thc phng trnh tai nn in do in p bc.I. HIN TNG DNG IN I TRONG T V S PHN B IN TCH TRN MT T Trng hp dy dn b t ri xung t s c dng in chm t. V phng din an ton, dng in chm t lm thay i c bn trng thi ca mng in (in p gia t v dy dn thay i, xut hin cc in th khc nhau gia cc im trn mt t v gn ch chm t). Dng in i vo t s to nn im chm t mt vng dng in r trong t v in p trong vng ny phn b theo mt nh lut nht nh (hnh 1.1).

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Hnh bn cu

Hnh 1.1

Bng tnh ton v thc nghim, ngi ta thy rng in p trong vng ny phn b theo dng hypebn v c ng cong phn b in p nh sau:

Hnh 1.2

in p ti mt im no y trn mt t (im A) gn ch chm t c tnh theo cng thc:

Ux = U. X/ X = K. 1/X(CT 1.1)

Trong :

X: Khong cch t im A n im chm t, Ux: in p ti im A

U: in p trn vt ni t. X : Bn knh vt ni t.

K = U. X(CT 1.2)

Ti im chm t U = Umax.

Trong vng cch vt ni t gn 1m c khong 68% in p ri.

Nhng im trn mt t nm ngoi 20m cch ch chm t thc t c th xem nh ngoi vng dng in nguy him (hay cn gi l nhng im c in p = 0 t).

Trong khi i vo trong t, dng in tn b in tr ca t cn tr. in tr ny gi l in tr tn hay in tr ca vt ni t.

R = U/I(CT 1.3)

Trong :

U: in p ging trn vt ni t.

I: Dng in chy qua vt ni t vo trong t.

II. IN P BC Khi 1 pha chm t hoc 1 thit b no b chc thng cch in, ta c s phn b th nh sau: in p i vi t ch trc tip chm t: U = I. R

Hnh 1.3in p gia 2 chn ngi do dng in chm t to nn gi l in p bc.

Ub = Ux - Ux+a = K{1/X - 1/(X+a)} = K.a /X(X+a) (CT 2.1)

Trong :

K = U. X

a: di bc (khong 0,8 m)

x: Khong cch n ch chm t. T cng thc trn ta thy:

+ Cng xa vt ni t, Ub cng nh.

+ Nhng im cch xa vt ni t 20m Ub xp x bng 0

+ Nhng vng trn ng th (hay nhng mt cu ng th) l nhng vng trn (hay mt cu ng tm) m tm im l ch chm t. Ub = 0 khi 2 chn ngi ng trong vng trn ng th.

in p bc c tr s kh ln nn d khng tiu chun ho Ub nhng m bo an ton, qui nh l khi c chm t phi cm ngi n gn ch b chm t vi khong cch sau:

+ T 4 m 5 m i vi thit b trong nh.

+ T 8 m 10 m vi thit b ngoi tri. Dng in i qua chn ngi, khng qua c quan h hp tun hon nn t nguy him hn, nhng vi tr s in p bc khong 100V n 250V, cc bp c c th b co rt lm ngi ng xung lm thay i dng in i qua ngi gy nguy him ti tnh mng

III. PHNG TRNH TAI NN DO IN P BC trnh ti nn in do in p bc gy ra, ti khon 2, iu 61 Qui trnh an ton in qui nh khi thy dy dn t, ri xung t phi c bin php khng cho mi ngi ti gn di 10 mt, k c bn thn. Ngoi ra cn lu nu ang ng trong phm vi nh hn 10 mt th hai chn phi ng trn vng trn ng th, mun di chuyn ra ngoi phi tin hnh nhy l c m bo an ton. Trn Vn Cng Phng KTAT PC Qung Tr