Solve the problem 9.21 Can we remove the voltage on the chassis in the U.S syste

Question

Solve the problem 9.21

Can we remove the voltage on the chassis in the U.S system? Can we depend on the ground path to trigger the circuit breakers? Why? For the system in Figure 9.27. the circuit breaker is rated at 20 A and the source voltage is 120 V. The system parameters are R_n = R_h = 0.5 Ohm. R_g_man = 500 Ohm, R_man = 1000 Ohm, R_g = 10 Ohm, and R_g-xfm = 20 Ohm. Assume that the circuit breaker failed and did not clear the fault; compute the current through the person. For the system in the figure, what is the voltage on the chassis during heaving loading condition? Why? Modern wiring system.

Explanation / Answer

If Cb breaker is not open applying KvL in the loop : Vs=If(Rh+Rn). Here internal circuit is shorted hence Rload =0

Maximum Fault current observed = Vs/(Rh +Rn) = 120/0.5+0.5 = 120A

Current will be more in less resistance path.

hence current will be Irg> Irgxfm,>Irgman

Applying current division of parallel circuit :

as Rg<Rgxfm<Rgman

Current divisionin parallel circuit is given as : Irg= (Itotal*Rtotal)/Rg

Rtotal= 10II20II1000= 6.622 ohms

Irg = (120* (6.22))/ 10= 74.64A

Irgxfm= 120*6.22/ 20= 37.32A

Irg man = 120* 6.22 / 1000 = 0.7464A

If fault current issomething other than 120 then also same procedure is to be achieved .

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