If your ever caught short without your `BIG BROWN BIBLE BS7671`and need to know a Z`s figure use this simple formula.....
For a type B MCB....
1.Multiply the nominal setting of MCB by 5
Then using Ohms law...
2.Divide 240 (volts) by the sum you got in the above calculation.
I.E for a 25 amp type B
25A x 5 = 125A
240V / 125A = 1.92 ohms
For a type C MCB substitute the 5 for a 10
25A x 10 = 250A
240V / 250A = 0.96ohms
For a type D MCB substitute the 10 for a 20
25A x 20 =500A
240V / 500A = 0.48ohms.
For a type B MCB....
1.Multiply the nominal setting of MCB by 5
Then using Ohms law...
2.Divide 240 (volts) by the sum you got in the above calculation.
I.E for a 25 amp type B
25A x 5 = 125A
240V / 125A = 1.92 ohms
For a type C MCB substitute the 5 for a 10
25A x 10 = 250A
240V / 250A = 0.96ohms
For a type D MCB substitute the 10 for a 20
25A x 20 =500A
240V / 500A = 0.48ohms.