2391

[ezekiel]

started 2391 at college last night and was bit concernd at looking stoopid in front of all the sparks, as it turns out i think im the only one who has a qualification not a spark in the class - bit worrying really as the 2391 is quite a big qualification......

anyway why is r1+rn/4 is it purely because the conductors are interconnected

 

[davelx]

started 2391 at college last night and was bit concernd at looking stoopid in front of all the sparks, as it turns out i think im the only one who has a qualification not a spark in the class - bit worrying really as the 2391 is quite a big qualification......

anyway why is r1+rn/4 is it purely because the conductors are interconnected

Try and draw an equivalent circuit with resistors and then use the normal formulae for resistors in series and parallel you should be able to work it out. In fact if you draw it out it should become clear by inspection without needing to use the formulae.

 

[Grinning Grandad]

When Phase & Neutral are cross connected at the CU you are creating one loop which is double the resistance value of the individual Phase & Neutral conductors loop value.

When you do a check at a socket at this point you are reading two identical resistances in parallel.

The value of two identical resistors in parallel is half the value of any one resistor.

If for example r1 = 0.3ohms and rn = 0.3ohms the reading at the socket will measure 0.15ohms. r1+rn / 4.

I was told an algebreric (?) formula to prove this. (assuming the denominator X = 4)

0.3 + 0.3 = 0.15
X

Therefore

0.6 = 0.15
X

Rearranging for X =
0.6 = 4
0.15

Therefore the denominator has to be the value of 4.


Confused? You will be....

 

[ezekiel]

because the resistance is reduced in parallel ?

 

[Spark123]

Yeah, if you have two identical cables in parallel the conductance is doubled, conductance is the opposite to resistance hence resistance is halved.

 

[ezekiel]

thnx very much