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# Complex Imaginary, Question Five, CI 5.

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Hello All,

A little help would be appreciated on this one.  In the Complex Imaginary Test, Morning Session, Question 5 (the one through iBooks) the question asks about the fault current after a switch is thrown.  The circuit shows an inductor on the source side of the switch.  I went to solve this problem believing that this should be an instantaneous switch (there is no indication components are not ideal) and the fault would also occur immediately following the switch being thrown.  I recalled form my basic college courses that current cannot change instantaneously in an inductor.  However the answer is given by way of circuit analysis including the downstream resistor.  Was this an improper assumption?

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• 10 months later...

In case anyone else needs the solution...

IIRC, the current I1 is listed in the problem, before the switch is moved.

So:

X1 = V / I1 = 1500 / I1

Then when the switch is moved:

Zeq = R1 + jX1

I1 = V / Zeq

The only trick here is remembering that the calculated impedance of the inductor is a reactance. So, Zeq = R + jX, not Ze = R + X.

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