formulae for Zs

[brucelee]

Hi everyone

what are two formulae for determining ZS is it
Zs =Ze+ (R1+R2) and
Zs = (mohm/m x mf x length)
divided by 1000
or Zs = Ze + (mohm/m x length) divided by 1000 ?

and if asked to determine the expected value for (ZS) for a circuit,assume ambient temp is 20degees C
ZE is 0.55 circuit length 15m using 1.5mm/1.0mm

 

[Spa Key]

Hi everyone

what are two formulae for determining ZS is it
Zs =Ze+ (R1+R2) and
Zs = (mohm/m x mf x length)
divided by 1000
or Zs = Ze + (mohm/m x length) divided by 1000?

and if asked to determine the expected value for (ZS) for a circuit,assume ambient temp is 20degees C
ZE is 0.55 circuit length 15m using 1.5mm/1.0mm

Bold is correct, and I don't quite understand your second question.

 

[wallyanker]

Question 2 : Look in the on site guide, not sure where abouts but definately in there, a table that gives resistance figures per meter for different size cables, times the figures by your length of 15m and then add to the ze, this will give you a zs

 

[Guest123]

Appendix 9, table 9A.

Also if you use your second formula, you need to apply the factor for maximum conductor operating temperature, table 9C.

 

[Tester Steve]

Right.
OSG app9 page 166.
1.5/1.0 per metre is 30.20 ohms. Correction factor for 20degs is 1 (page 167)

Resistance of the cable = mOhm/metre x length x correction / 1000

= 30.20 x 15 x 1 / 1000 = 0.45 ohms

Zs = Ze + (R1 + R2)

Zs = 0.55 + 0.45 = 1.

Here endeth the lesson.

Steve.

 

[Tester Steve]

Great minds, eh Lenny??

Steve.

 

[Risteard]

But to compare it with tabulated values you need to apply a correction factor. So multiply (R1+R2) by 1.2 and add to Ze.

 

[brucelee]

Bold is correct, and I don't quite understand your second question.

your answer doesnt take into account conductor operating temp at time of test so have to x by 1.2 which is the multiplying factor for conductor operating tenp off 70 degrees c for thermoplastic protective conductors bunched or a core in a cable i think

 

[brucelee]

thanks guys
so
Zs = Ze+(R1+R2) or
Zs = Ze +(mohm/m x MF x Length Divided by 1000)

because the tables in On site guide table 9A, and GN3 are for ambient temp at 20C so dont need correcting for ambient temp only for conductor operating temp