How to calculate temperature adjusted maximum Zs values

[mick525]

Hello,
I have my practical 2391 test on Friday and have been told by the examiner that we will be required to complete the calcuation for the maximum operating temperature for Zs. I am unsuse what to do with the calculation.

From guidance notes three (second example down, pg 113), it shows the following equation. I cant seem to come to the final result that they do:


Ztest (max) = 1/1.20 {7.67+0.35 (1.20-1)}
Ztest (max) = 6.45 ohms


I come out with 6.65 so I am not sure where I am going wrong, can anyone help please? If you can, can you also list how to do it step by step on a calculator. Please note, I know I could use the rule of thumb method, but the examiner has specified to calculate using the method above.


Cheers

Mick

 

[malcolmsanford]

Zs = Ze + (R1 + R2)

So your calc for R1 or R2 will be mΩ/m ( cable resistivity) X length x 1.2 ( temp) /1000

 

[mick525]

Hi Malcolm,
Thanks for this but 7.67 is the Zs, 0.35 is Ze and 1.20 is the correctional factor to go to 70'C. There is no mention of R1 or R2 in the calculation so I am still confused.

Cheers

Mick

 

[Silly Sausage]

Zs = Ze + (R1 + R2)

Your Z[SUB]e[/SUB] stays the same @ 0.35

R1+R2 = 7.67 - 0.35 = 7.32

Correct this value for increased temp by dividing by 1.2 => 7.32/1.2 = 6.1

Add Z[SUB]e[/SUB] => 0.35 + 6.1 = 6.45

 

[mick525]

Hi Archy,
thanks for this, however the 7.67 in guidance notes three on page 113 is the Zs value, not R1+R2. It also says in the book that you have to add the Zs to the Ze which works out as 7.76+0.35 = 8.02. Any ideas?

Cheers

Mick

 

[Silly Sausage]

Hi Mick,

You need to do some algebra...

Ztest <= (Zs + Ze(F-1))/F

expand it out =>

(Zs + ZeF - Ze)/F ...

Zs/F + Ze - Ze/F ...

Ze + Zs/F - Ze/F ...

Ze + (Zs - Ze)/F ... & from Zs = Ze + (R1 + R2)

Rtest = Ze + (R1 +R2)/F

Which is the result shown in the example.

Hope that's clearer!?!?

 

[mick525]

Thanks Archy

But still not clear on that. I will stick to the calc with out adding the Ze

1/1.2x7.67=6.39

 

[Richard Burns]

Ztest (max) = 1/1.20 {7.67+0.35 (1.20-1)}
Ztest (max) = 6.45 ohms

I come out with 6.65 so I am not sure where I am going wrong, can anyone help please? If you can, can you also list how to do it step by step on a calculator. Please note, I know I could use the rule of thumb method, but the examiner has specified to calculate using the method above.

I had to do it by rule if thumb but,
Ztest (max) = 1/1.20 {7.67+0.35 (1.20-1)}
assume you can tell 1.2-1 = 0.2
on a calculator
1/1.20 = 0.83333 put in memory
0.35 * (1.2-1) = 0.35 * 0.2 = 0.07
0.07 + 7.67 = 7.74
7.74 * memory recall =7.74 *0.83333
= 6.45

 

[mick525]

Thanks Richard you are a star this was starting to make my head burst thanks a lot makes sense now.

 

[spark 68]

If this is just for your 2391 practical,

Then all you need to do is your test, note the readings, then compare with the table in BS7671 (tabulated readings) times the BS7671 value by 0.8 (the 80% rule of thumb), use this 80% value to compare your results with, and say if your circuit complies.

Others above have given you the more in depth reasons and calcs.
Good luck!

 

[mick525]

thanks

 

[tony mc]

Unless they have changed the practicle I have to aggree with Spark 68 as this is the what I did on my practicle to make sure that the cct complies.

Best of luck!

 

[ian65]

Hi every one, i have been looking through this thread and taking note.

But calculation i am stuck on for some reason as i thought i had it sorted is the written exam Tabulated with a measured value Pass/Fail?

I am re taking my written on the 9 Feb i would be very greatful if some one could explain the way in which the formula works? As i can see if you get 3/4 value of Max this is the way it is done?

Help.. Ian.

 

[tony mc]

Have a look at this post.


[h=3]Max Zs in OSG and the Regs
[/h]
[h=3]

Remember when you take your ZS reading on the cct you are testing you record that reading.

EG a 32A 60898 type B mcb on a final ring cct and your ZS test is 1.18 ohms.

On checking BS 7671 the max ZS for this device is 1.44 ohms.

However BS 7671 has not incorporated ambient and operating tempertures of the conductors so you use the rule of thumb method by muliplying the 7671 value by 80% to compensate for this.

1.44 x 0.8 = 1.15 ohms

So your 1.18 measured value is above 1.15 so the circuit does not comply.[/h]Remember GN 3 and the OSG tables have already been corrected which is explained in the attached post.

Best of the luck with the exam!

 

[ian65]

Have a look at this post.


Max Zs in OSG and the Regs





Remember when you take your ZS reading on the cct you are testing you record that reading.

EG a 32A 60898 type B mcb on a final ring cct and your ZS test is 1.18 ohms.

On checking BS 7671 the max ZS for this device is 1.44 ohms.

However BS 7671 has not incorporated ambient and operating tempertures of the conductors so you use the rule of thumb method by muliplying the 7671 value by 80% to compensate for this.

1.44 x 0.8 = 1.15 ohms

So your 1.18 measured value is above 1.15 so the circuit does not comply.

Remember GN 3 and the OSG tables have already been corrected which is explained in the attached post.

Best of the luck with the exam!

Many thanks for info Tony i needed to check i was doing the calculation correct and it looks like i am, as you say GN 3 and On site guide are already set for the 0.8 80% and 1.2 diferants in temprature.

I am looking forward to tonight for my second attemp like a whole in the head lol. Many hanks once again for info, Cheers Ian.