Studies Help Please

[diGreez]

Hello everyone,

I am working on my C&G 2357 unit 304 task A and one of the questions is:

Determine the minimum cross-sectional-area(csa) for each of the following:
a) earthing conductor, where the incoming live conductor are 25mm2.

I am up and down the regs and web to find something on this, and all I found is this formula:

S =​	Ö(Ia²t)​
​	k​

can someone please direct me right way on this please...

Thanks

 

[Richard Burns]

If you have been given no information on protective devices and their characteristics then you cannot calculate using the adiabatic equation.
Based on the information given, look at table 54.7 in the regs.
You also need to comply with 543.1.1 (not a problem in this case)
If it is a buried earthing conductor you need to look further.

 

[diGreez]

THANKS A LOT Richard!

 

[diGreez]

hello again,

can I use amp law A = W/V on three phase?
is it right:

three-phase supply of 400V and machine load 5kW;

5kW = 5000W; 5000/400 = 12.5A;

so design current (lb) = 12.5A?

Thanks,

 

[telectrix]

on 3 phase, your 5kW is shared between the 3 phases so P = √3 VI. so I = 5000/ (1.732 x 400) = 7.215A

that's without allowing for any power factor correction.

 

[diGreez]

so does it mean that each phase will have load of 7.215A current or that machine will be using total of 7.215A?

 

[telectrix]

each phase will be 7.215A. so you select your cable and MCB accordingly. bear in mind that a motor draws a high start up current and will generally require a type C MCB.

 

[diGreez]

Thanks a lot telectrix!

 

[telectrix]

if the load is a motor, the pf will be between 0.8 and 0.9 ish. so then divide the current by 0.8 to allow for this. in the above case, your current would then be 9A.

 

[Baker1988]

If you have no info on protective devices then you will just have to look into the regs book for min CSA which I think is 16mm for 25mm incoming and is this the type of questions that are in the tasks as I have just started my 304 unit and the questions I have seen so far don't seem difficult at all

 

[diGreez]

Baker, a agree, it is not difficult, it just getting my head round it. and I am working on task B q.:1a.

 

[Baker1988]

Is they any way you could pm me a few of the questions so I know what to prep air my self for you don't have too if its too much trouble

 

[diGreez]

how do you get or where from pf? is there a table in regs/osg?

 

[telectrix]

the pf is determined by the actual motor design. nothing in the regs. sometimes it's printed on the motor rating plate. if not, then you have to make an educated guess. 0.8 is about the worst you'll get so if you use that value, you''ll be erring on the side of caution.

 

[diGreez]

and this figure pf 0.8 is in A?
and what about start up, how do you know what A will be required how high it is(or is it another experience thing)?