R2 Calculation

[Sck]

Help with a calculation please?

R2 = (Zs - Ze) x 1000
(L X1.2) - R1

R2 = (3.83-0.4) x 1000
(30 x 1.2 ) - 7.41

= 87.87 mOhms/m

How come i keep coming up with 3401.41??

Next problem

Ze = 0.4 Ohms

Zs = 0.4 ( (7.41 + 18.10) x 30 x 1.2 )
1000
= 1.32 Ohms

How come I come up with 0.93276

Going Stir Crazy here, please help me out on this one guys

Thanks

 

[Sck]

Received with thanks

 

[Sck]

Thanks my friend I got that
LOL.

 

[Geoffsd]

What's going on?

All you need to do to find R2 is multiply the length (30m) by the resistance per metre of whatever csa you are using.

No need to involve Ze or Zs.

in fact, Zs cannot be determined until after you have calculated R2 (and R1)

 

[Sck]

Thanks for everyones help. Sleep well

 

[Guest55]

What's going on?

All you need to do to find R2 is multiply the length (30m) by the resistance per metre of whatever csa you are using.
you may not yet know the size of your cpc yet for design purposes , but if you know the load size and circuit length , you can select your phase conductor size , which will give you R1

No need to involve Ze or Zs.

in fact, Zs cannot be determined until after you have calculated R2 (and R1)

again for the purpose of design calcs , you use the max Zs allowed for the chosen circuit device to help determine your minimum cpc size.

 

[Geoffsd]

Yes, I did get lost but this thread does seem to be rrrr about face.

Shouldn't the equation be -


R2 (mΩ/m) = ( Zs - Ze ) x 1000 - R1(mΩ/m)
. . . . . . . . . . .( length x 1.2 )

or -

Total R2 = ( Zs - Ze ) - R1

 

[Guest55]

Yes, I did get lost but this thread does seem to be rrrr about face.

Shouldn't the equation be -


R2 (mΩ/m) = ( Zs - Ze ) x 1000 - R1(mΩ/m)
. . . . . . . . . . .( length x 1.2 )

try it with the OP's original figures for Zs , Ze and circuit length and see if you get a credible value for the max resistance of the cpc.

 

[Geoffsd]

But the max. res. of cpc is just Zs - Ze - R1


If you mean max. mΩ/m of R2 then

R2(mΩ/m) = (1.32 - 0.4) x 1000 / (30 x 1.2) - R1(mΩ/m) = 25.55 - R1(mΩ/m)


25.51 is the mΩ/m value of 2.5/1.0

 

[Guest55]

you've used 1.32 for Zs , the op gave a figure of 3.83.
have you deducted the R1 of 7.41 he gave in the 1st post ?
the cable described was pvc singles , not t&e.

 

[Geoffsd]

R2 = (Zs - Ze) x 1000
(L X1.2) - R1

R2 = (3.83-0.4) x 1000
(30 x 1.2 ) - 7.41

...

Zs = 0.4 ( (7.41 + 18.10) x 30 x 1.2 )
1000
= 1.32 Ohms

you've used 1.32 for Zs , the op gave a figure of 3.83.

Ok. I read the wrong figure.

Sorry, but the method is correct and not as you wrote in post #15 - the equation 'upside-down'.

have you deducted the R1 of 7.41 he gave in the 1st post ?

No, i didn't actually do it but that can be done for whatever value of R1 is chosen

the cable described was pvc singles , not t&e.

The resistance values are the same.


My confusion started by not realising this was an exercise but thinking that the OP was using an actual installation and then the thread deteriorated by inaccurate quotes and equations being inverted and spurious numbers introduced.


So, to answer the original question -

R2 = (3.83-0.4) x 1000
(30 x 1.2 ) - 7.41

should read

R2(i.e. mΩ/m value) = ((3.83 - 0.4) x 1000 ) - 7.41
. . . . . . . . . . . . . . . . (. . . (30 x 1.2) . . . .)

R2(i.e. mΩ/m value) = (3430) - 7.41
. . . . . . . . . . . . . . . . (. 36 .)

R2(i.e. mΩ/m value) = 95.28 - 7.41

R2(i.e. mΩ/m value) = 87.87

 

[Guest55]

youre correct , well spotted
just checked and was reading from the wrong section of my notes lol , the 1000 was upside down
made things interesting for while tho

 

[Geoffsd]

:wink_smile: