17th edition 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

 

[revjames]

Look them up in the big red book, as far as the first one goes its a bit of a trick question because a 400V 3 wire system is still only 230V to earth! therefore the fault current is worked out by ohms law: I=V/R which is 230/0.3 which is 766.67A Prospective Short Circuit current is usually double this figure on a 3 phase system.

Don't know the second without looking it up.

 

[noel2507]

i've been in the red book now for a few days really sick of the sight of this book now. it was a straight forward answer just me looking to far into things. if you get time the second question would be a big help. thanks