Calculation help needed!

[chrisgc]

Hi all with regard to BS 7671 page 48 (411.4.5) can someone tell me in plain English how to do these calculations.

As I am trying to calculate the following question:

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

a) 1383 A
b) 767 A
c) 124.5 A
d) 71 A

If someone can explain in simple terms how to calculate this I would be grateful as dont understund the signs in BS 7671 (the _ & left arrow sign).

Also does anyone know of a link which has the formulas on but explained in laymen terms.

Chris

 

[Mark.W]

It is simply your Uo (Nominal Voltage) / Zs. So 230V / 0.3 ohms!

 

[ian.settle1]

400v divide by SQRT of 3 then this answer is divided by 0.3 giving resultant answer.

 

[chrisgc]

Hi

Answer is:

c) 767 A

How did you guys come to this and how would I put it into a caclulator ie what buttons would I press? given the info in the first thread?

Chris

 

[Mark.W]

400 / √3 or 400 / 1.732 = 230.94 then 230.94 / 0.3 = 769.8

I tend to just use 230V / 0.3 = 766.66

 

[chrisgc]

400 / square root of 3 (looks like a tick) or 400 / 1.732 = 230.94 then 230.94 / 0.3 = 769.8

I tend to just use 230V / 0.3 = 766.66

What does the square root of 3 come in?

Is that cos its a 3 phase?

Chris

 

[Mark.W]

Yip you got it, it's cos it's 3 phase!

 

[chrisgc]

Ah great thanks for that.

Is there not a way of knowing what the mathmatical signs are in the BS 7671 as they are not really self explanetory?

Chris

 

[Mark.W]

Not sure I know what you mean, sign as per BS7671? Signs for what?

 

[chrisgc]

In laymens terms really.

BS 7671 can be quite difficult to understand at times.

I am going through loads of sample questions but I am struggling a bit when it comes to searching for the correct answer (even though i have answer sheets - I am trying not to cheat myself by looking at them).

I passed my part p domestic installers but have my 17th edition exam on Friday and am thinking I am not even going to manage to answer all 60 questions at the rate i am going.

Its mainly the formulas etc I am struggling with as on the domestic course they tend not to spend much time and focus on it as crammed in a bit.

Any help or info at all.

Chris

 

[Mac]

Try this mate. Useful Formulas

 

[chrisgc]

Am I worrying over nothing as most people seem to pass the 17th edition but i am really thinking I am struggling especially time with time restraints etc?

Chris

 

[Tag]

Try this mate. Useful Formulas

Printed!

...regarding volt drop do they use feet/foot in the 17th exam?

 

[Mark.W]

or meters!?!?!?