2391 revision

[sarni31]

Firstly I would like to say Hi

Been a member for a little while but this is my first post..

I'm doing some 2391 revision and come across this question..

Explain briefly, why the measurement obtained at each socket outlet, when carrying out Stage 2 of the continuity of ring final circuit conductor test, is expected to be

line loop + Neutral loop / 4

ok so cross connected line and neutral = r1+rn but cant remember why its divided by 4 , can anyone help

 

[Andy™]

Because its a ring, it has 2 routes back to DB, reducing resistance

 

[ianmacd]

( r1 + r2 ) / 4 approximates to ( r1 x r2 ) / ( r1 + r2) which is for resisters in parallel.

E.g. r1=0.45 & r2=0.75

( 0.45 + 0.75 ) / 4 = 0.30

( 0.45 * 0.75) / ( 0.45 + 0.75 ) = 0.405

E.g. r1=0.12 & r2=0.21

( 0.12 + 0.21 ) / 4 = 0.083

( 0.12 * 0.21 ) / ( 0.12 + 0.21 ) = 0.077

It's easier to remember r1+r2/4 than r1r2/r1+r2 and they are fairly close (especially when r1 & r2 are lower than an ohm). This is why the ISG says R1+R2 should be approximately the same as r1+r2/4.

Ian.

 

[ianmacd]

Please replace r2's and R2's with rn's and Rn's. Just reread the question.

 

[sarni31]

Thanks Ian

its looking vaguely familiar now ..........