2391 calculations

[nick1986]

Hi guys I'm studying for 2391/new 2395 I would really be great full for help I've been studying the gn3 I have an exam simulator by electa courses and this question really gets me I prolly bang my head when u tell me how it's done but I carnt think here goes

Given that the resistance per meter of 2.5mm2 cable is 7.41mohm/m at 20oc what would the approximate r1+r2 reading be at the extreme end of 25m long 20a radial socket out let circuit a)o.37 ohm, b)0.74 ohm c)0.27ohm d)0.17ohm

Thanks for your help guys I no that 20oc is a rule of thum is 1.04 the closest I've got is 29ohm

 

[Zeespark]

The answer lies in the back of the OSG. You will need to factor in ambient temperatures first though I THINK.

AndyGuinness

 

[brav]

show us how u got 29 ohm first mate?

Remember what u wrote ... Given that the resistance per meter of 2.5mm2 cable is 7.41 MILLI ohms per METRE

 

[Technician]

Page 166 of the OLD OSG has the figures in

milliohms per metre run for (R1 + R2) calcs.

It is the same table in the new OSG as table

I1.

As Zee says, check for temperature but the

table is for 20 deg Cel.

Yes, how did you get 29 ohm? Have you

dropped a factor somewhere?

Use the table to calculate (R1 + R2) where

you select a line conductor size AND one

for a protective conductor.

The 1.04 factor is in another table which you

might not need because the question is for

20 Cel.

Have a go and come back in.

 

[park1]

Ok

R@ 20c = R@ 20c x (1 + coefficient of copper x rise in temp)

Coefficient of copper = .004

R@ 20c = 7.41 x 1 as the value is already given at 20 c

So

R = R@20c x length/ 1000

To find R@70c

7.41 x (1 + 50 x .004)/1000 the 50 comes from (70-20)

 

[nick1986]

Thanks guys for your help

 

[steptoe]

have you got it sussed nick ?

 

[chris_k]

Its been a long time since ive done some of these, but here goes.

The question doesnt state whether the circuit is T&E or singles so ill do them both.

Temperature is not an issue as 20deg c

Suppose the cable is T&E

Length of cable 25m

Resistance of cable/m r1 = 7.41

r2 = 12.1 (resistance of 1.5mm cpc)

r1 - 25m x 7.41 / 1000 = 0.185

r2 - 25m x 12.1 / 1000 = 0.303

0.185 + 0.303 = 0.488 ohm Not one of the answers given. Must be using singles.

Lets try using r1 + r2 same csa (singles)

Length of cable 25m

Resistance of cable/m r1 = 7.41

r2 = 7.41

r1 - 25m x 7.41 / 1000 = 0.185

r2 - 25m x 7.41 / 1000 = 0.185

0.185 + 0.185 = 0.37 One of the answers i believe.

 

[nick1986]

Yeh from the help on here I got the answer thanks