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

 

[Englishsparks]

Is this a ring circuit or a radial circuit?

 

[Sck]

Thanks for asking this would be a radial circuit.

Thanks

D

 

[Geoffsd]

On 2.5/1.5 T&E R2 = R1 (or Rn) x 1.66.

You cannot work it out from Ze and Zs measurements.


What are you trying to do?

 

[Englishsparks]

Is this a ring circuit or a radial circuit?

Ze = External earth loop
Ze + (R1 + R2) = Earth Loop (Zs)

 

[Sck]

Sorry Scenario is:

4.5 Kw fixed load single phase heater, using 70 degrees thermoplastic cables, running in PVC conduit, over 30metres. Measured Ze is .4 ohms
Iz is 20A using a BS 3036
No correction factor for any thermal insulation needed
Table 4D1a and 4D1B gives me a voltdrop of 10.56v
Using singles I need to determine which size CPC to use

Zs = Ze + (R1 + R2) X 30m x 1.20
1000
= 3.83 which brings me into the question in the first post

Many thanks

D

 

[YoungBart]

You need to get your R1+R2 values from table B1 in GN3. If its the same type of question.
The first part of the equation you need is something like
R1+R2 X the factors from table B2 X legnth / 1000)

 

[SYKRAPS]

Maths on a Sunday ????????? You must be mad

 

[Geoffsd]

Ze = 0.4 Ohms

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

Those are the figures for 2.5/1.0 cable

How come I come up with 0.93276

If you mean that is the actual measurement -

because of parallel paths - which is why you cannot work it out backwards

and because the cable is not hot - which is why you multiplied by 1.2

 

[Sck]

can we have the full question if possible? I've seen a question like this and you need to get your R1+R2 values from table B1 in GN3. If its the same type of question.

Thanks for the reply

Basically I wish to know the size of the CPC to be used for the final circuit, also how I come up with the answer pasted below

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



Many thanks

D


​

 

[Sck]

Yes I know, I must be mad, but this has got me!

 

[Sck]

Dear Sir, yes I multiplied by 1.2 using table E.2 54.3 FROM GN 1

 

[Sck]

I Have actually got 18.10 from GN 1 TABLE E.1 Page 155,
Am i going mad?

 

[YoungBart]

once youve worked out your R1+R2 (from the formula in my edited post) you add that to the Ze, This gives you your Zs, (Providing that complies with the max permitted) You then need to work out your fault current,
So 230v/Zs Should give you an answer in Amps.
Then go to your time current graphs for the relevant breaker and find out how long it takes to operate. With this you can work out your csa size. Basically (square root) fault current sqaured X time / Conducor material factor found in tables 54.2-54.6
Thats my understanding, but I could be wrong, and I'm sure someone will be along soon to tell you a definate answer

 

[Guest55]

you've got the - R1 bit in the wrong place , it should be subtracted after the division by 1000

so it should read ....

R2 = ( Zs - Ze ) x length x 1.2 - R1

1000


edit ; the 1000 wont centre under the line lol.