17th edition test questions

[noel2507]

hi i'm revising for my 17th edition and having problem with a couple of question. know the answer but don't understand how to get there. any help i would be greatful

i have supplied the question and the answer but i don't understand how to get this answer could you please explain.

20 The fault current due to an earth fault of negligible impedance in a 400 V, three phase, four wire circuit

having an earth loop impedance of 0.3 , is

a 1383A

b 767A

c 124.5 A

d 72A.

Answer b 411.4.5

37 A circuit is protected by a 10 A BS 3036 semi-enclosed (rewireable) fuse. The minimum permissible

current rating (Iz) of a conductor protected by this fuse would be

a 7.25A

b 10A

c 13.8A

d 20A.

Answer c appx5 5.1.1

 

[Dean182]

First one.

I=V/R

I=230/0.3= 766.6A

Second One.

Rating factor for a 3036 is 0.725

Rated current of protective device is 10A

10A/0.725=13.79

Not 100% on the first one, as i dont know if there asking for the measured value before it's doubled for 3phase. However 415/0.3=1383A

 

[revjames]

Dont forget its an earth fault therefore 230V - earth.......

 

[Dean182]

Dont forget its an earth fault therefore 230V - earth.......

Read it wrong, I presumed it was asking for the PFC on a 3phase system with a Ze of 0.3

 

[revjames]

Another example of the need to read the full question!

 

[Dean182]

Guilty