CPC Calculation

[RobinMitchell]

Hey all,

To calculate the resistance of the CPC you end up with an equation with (R2 + RN) / 4 = R1 where R1 is the line conductors resistance, R2 is the CPC resistance and RN is the resistance of the neutral wire.

My question, Is this method below also an acceptable way of doing it (if the line and neutral conductors have the same CSA)?

Measure Line + Earth resistance
Measure Line + Neutral resistance
Line resistance = (Line + Neutral resistance) / 2
Earth resistance = Line + Earth Resistance - Line Resistance

It might even be the same and it's just me being really tired! Also I wanted to post this in a non qualified section but not too sure where to do just that.

All the best,
Robin

 

[telectrix]

no. you measure a ring end to end ( r1,rN,r2) note the use of lower case letteres. then R1+R2 should be (r1+r2)/4. this is because you ve 2 parallel conductors ( divide by 2) and your R1+R2 is half way round ( divide by 2 again). it's a bit more complicated in real life, as you cross connect the legs for your R1+R2 to be the same at every point. bear in mind that this divide by 4 only aspplies to ring final circuits.

 

[telectrix]

pm paul.m for access to the training forum. there trainee questions can be asked without any sarcastic comeback. electrics is a different ball game to electronics, but the basic theory is still electrons shuffling around. lol.

 

[RobinMitchell]

Ok will do just that. But one other question...

You mentioned Ring circuits which I agree with but what if this mesaurement was being taken on a radial circuit where it does not return to the consumer unit? Does the situation change then?

 

[telectrix]

yes. there's no r1,r2,rN on a radial the use of lower case letters is specific to ring circuits. you would calculat R1, Rn, R2 by multiplying the mΩ/m (from tables) by the length. this is at the design stage so that you know that you will have an acceptable loop impedance. R1+R2 is then measured using a low ohms meter ( or MFT), with the line/cpc shorted at one end, before the circuit is energised.

 

[RobinMitchell]

So just to clarify....

If I wanted to test the earth wire resistance of a light on a radial circuit the method I described would work because it is a radial circuit.

Many thanks!

 

[telectrix]

yes. if you short one end and measure R1+R2 then R2 will be half, assuming that the 2 conductors are the same size. with T/E, e.g. 2.5mm, the ratio is R2 = R1(2.5/1.5) as the cpc is 1.5mm, i.e 1.66666666666666and on forever.

 

[RobinMitchell]

Fantastic! Thanks for the help and clarification :laugh3:

 

[telectrix]

you'e welcome. please make cheques out to Campaign for Afghan Spiritual Healers. ( if you cant fit it in, just use the initials.... CASH. :mickey: