# RFC cross connect formula

Discuss RFC cross connect formula in the Electrical Courses and Electrical NVQ's area at ElectriciansForums.net

#### gazdkw82

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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

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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

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Trainee
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

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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

D

#### 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

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

D

#### 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

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Trainee
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

D

#### 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

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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

#### gazdkw82

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Trainee
How would you verify correct cross connection if all the conductors were singles?
It's taught at college to verified by cross connecting and measuring at the outlets. If the readings are increasing then you have the wrong combination

#### Pete999

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It's taught at college to verified by cross connecting and measuring at the outlets. If the readings are increasing then you have the wrong combination
bit confusing though Gaz, if you are taking readings at spurred sockets.

#### gazdkw82

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Trainee
bit confusing though Gaz, if you are taking readings at spurred sockets.
I agree Pete. In reality that is not how we would approach it. It's just the college way

#### Pete999

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I agree Pete. In reality that is not how we would approach it. It's just the college way
Don't you think that is wrong Gaz? the college is wrong imo, hence the confusion

#### gazdkw82

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Trainee
Don't you think that is wrong Gaz? the college is wrong imo, hence the confusion
Tbh iv used this method before in the workplace and it's proved to work. However that was on a RFC with no spurs so it was easy to figure out. I had them cross connected the wrong way and the readings just increased as I went around the ring, then started to decreas after the centre point of the ring

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