2394 practice question

[Jonny Bothwell]

How would one figure out the following question?

The resistance of a 2.5mm copper cable is given as 7.41M Ohm per metre at 20 celcius and that of a 1.5mm copper cable is given as 12.1M Ohm per metre. A circuit wired with 25m of 2.5/1.5 twin with cpc sheathed cable would have an expected R1+R2 value measured at 20 celcius of ?

 

[Darkwood]

Break it down - this one isn't too hard to work out...

What does R1 stand for?
What does R2 stand for?

When taking an R1+R2 measurement you are getting the combined resistance of 2 cores of the T&E in this case, going up one core to a joint you made between them then back down the other core.

You know the length of each core and you also know the milliohm per meter of both legs.(not M Ohm as you have put which means Mega Ohms)

Have a go and post your sums and we will guide you

 

[Jonny Bothwell]

so i get, 7.41x25 = 185.25
12.10x25 = 302.50

so 487.75 x 0.001 = 0.49

would this be your method?

 

[Leesparkykent]

so i get, 7.41x25 = 185.25
12.10x25 = 302.50

so 487.75 x 0.001 = 0.49

would this be your method?

which ever way you find best. I add both miliohm resistances together, divide by 1000 and then x by the length.

 

[DuaneMHunt1976]

10^12 Terra = T
10^9 Giga = G
10^6 Mega = M
10^3 Kilo = K
Unit
10^-3 milli = m
10^-6 micro = u (It should be the greek letter MU but its a lot easier to do a u)
10^-9 nano = n
10^-12 peco = p

(R1+R2)xLength
(7.41+12.1)x25
19.51x2 5=487.75mOhms or just 0.48775 Ohms Might round it to 2dp and be 0.49Ohmx

 

[Mike Blair]

remember because it says 20 Celsius you need to add correction factors x by 1.2 "20%" you are working out a calculation on a dead circuit but once it is pulling current the cable will get warmer and create more resistance

same with bs7671 you get the zs readings example 32a type b bs60898 = 1.37 ohm


80% of 1.37 ohm
1.37 ohms x 0.8 = 1.09ohms

i think gn3 and on site guide have this factor already applied

Edited misread the question as it said AT 20 Celsius