RFC cross connect formula

[gazdkw82]

I may have asked this question before but I forgot what was said and the problem has reered it's ugly head again.

I was doing some RFC testing the other day. Did my cross connecting but when I was doing the calcs I kept getting the wrong figure. It seemed I was not pressing = before dividing by 4. Having corrected the way I did the formula (R1+R2 =, then ÷4) I got the correct figure.

However, I went on to do another RFC on the same installation and I got the opposite problem. If I did R1+R2= then divide by 4 it was the wrong predicted reading. Doing R1+R2÷4 (without the =) predicted the correct result.

What is the correct way to do it?

 

[Ian1981]

You add your line-line end to end resistance with your cpc-cpc end to end resistance then divide by 4 to get your calculated R1+R2 for the ring final circuit

 

[gazdkw82]

so if you had a line to line of 0.11 and a cpc to cpc of 0.15 you would have the following formula.

0.26÷4

 

[Ian1981]

so if you had a line to line of 0.11 and a cpc to cpc of 0.15 you would have the following formula.

0.26÷4

Yes giving around 0.07 ohms

 

[Deleted member 26818]

I prefer using the measured r1 + r2.
I only really use the calculated value, to confirm correct connection of the outgoing and returning legs of the conductors, when I can’t easily identify them.

 

[Ian1981]

Agree as we are required to measure and confirm protective conductor continuity etc for initial verification

 

[Deleted member 26818]

I measure r1, rn and r2 and record the results.
I then connect the Line and CPC and take another measurement.
If that measurement matches the calculated r1 + r2, then I finish cross connecting and then go out into the field to measure the R1 + R2 at the sockets.
If it doesn’t match, I swap the end of one of the conductors and re-measure.

 

[gazdkw82]

It's all well and good but I have to keep to the methods stipulated in the testing schedule. After my AM2 I can do it however I think works best

 

[Deleted member 26818]

It's all well and good but I have to keep to the methods stipulated in the testing schedule. After my AM2 I can do it however I think works best

How would you verify correct cross connection if all the conductors were singles?

 

[AJshep]

Just remember if you have to document it in your AM2 to use the correct terminology.
So use the "r" rather than the capital "R"
r1+r2 ÷ 4 = R1+R2