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Again:the confused Qs about 2382-10


tcfreeman08

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Hi all,

again

Q: The fault current due to an earth fault of negligible impedance in a 400V, three phase, four wire circuit having an earth loop impedance of 0.3 ohms, is

a.1383A

b.767

c.124.5

d.72

the ans is b, why not is a, by cal. Zs x IAn < Uo, is 1333.3A

what is wrong?

thanks

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I think its because the fault is between Live and neutral (old terms) and not live to live. Hence 230v not 400v. I wait to be corrected though

Applaud Smiley:D

I would tend to think you would be agreed with, not corrected there Tooold!;)

i.e.

Hi all,

again

Q: The fault current due to an earth fault of negligible impedance in a 400V, three phase, four wire circuit having an earth loop impedance of 0.3 , is

a.1383A

b.767

c.124.5

d.72

the ans is b, why not is a, by cal. Zs x IAn < Uo, is 1333.3A

what is wrong?

thanks

As Tooold says..

"an earth fault of negligible impedance in a 400V, three phase"

is a 'Short' between 'Earth' (neutral or the star point) potential and ONE of the three phase conductors.

NOT a short between two of the phase conductors!.

The earth loop impedance reading is the resistance between a phase conductor and earth...

In 3phase the earth loop reading across each of the phases to earth should be substantially the same value!

so each phase only has 230v to earth,

resistive short of 0.3ohm,

V/R = I 230/0.3 = 766.666666 etc etc etc.....:^O

thus PFC is 767Amps!

;):D

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Applaud Smiley:D

I would tend to think you would be agreed with, not corrected there Tooold!;)

i.e.

As Tooold says..

"an earth fault of negligible impedance in a 400V, three phase"

is a 'Short' between 'Earth' (neutral or the star point) potential and ONE of the three phase conductors.

NOT a short between two of the phase conductors!.

The earth loop impedance reading is the resistance between a phase conductor and earth...

In 3phase the earth loop reading across each of the phases to earth should be substantially the same value!

so each phase only has 230v to earth,

resistive short of 0.3ohm,

V/R = I 230/0.3 = 766.666666 etc etc etc.....:^O

thus PFC is 767Amps!

;):D

special spot on.

i came across this question and struggled for ages, wish had known about this forum when i was doing it. as soon as your told why, it all makes sense.

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