2391 Question

[m4tty]

Hi,

An earth fault loop impedance test on a 50A compressor circuit gives a result of 0.93 ohms. The result of the previous cpc continuity test for this circuit gives (R1+R2) as 0.47 ohms. The ambient temperature at the time of test was 25 degrees celcius (factor 0.98) and the cable is 70 degree pvc (factor 1.2). If the maximum tabulated value of Zs for this circuit is 1.09 ohms, show by calculation (not rule of thumb method) if the measured value of Zs is accepable.

Thanks for any replies

 

[Apache]

*bump*

 

[SPECIAL LOCATION]

Hi,An earth fault loop impedance test on a 50A compressor circuit gives a result of 0.93 ohms. The result of the previous cpc continuity test for this circuit gives (R1+R2) as 0.47 ohms. The ambient temperature at the time of test was 25 degrees celcius (factor 0.98) and the cable is 70 degree pvc (factor 1.2). If the maximum tabulated value of Zs for this circuit is 1.09 ohms, show by calculation (not rule of thumb method) if the measured value of Zs is accepable.

Thanks for any replies

1. Its toooo late for me to start calculating....

but..

2. Appendix B in Guidance note 3 goes over some bits about Max Zs

& methods to calculate & adjust for temp etc...

pages 107 thro 114

GuinnessGuinnessGuinnessGuinness

GuinnessGuinnessGuinnessGuinness

GuinnessGuinnessGuinness

 

[the doctor]

sounds like an old 2391 question where they ask about the 1.2- they tend not to do it nowadays

so.... how do you want it, with the 1.2 and no rule of thumb or the opposite?

alan

 

[the doctor]

woops, didnt read it properly.

i will get a bacardi and roll up made and will be back presently

 

[the doctor]

Hi,An earth fault loop impedance test on a 50A compressor circuit gives a result of 0.93 ohms. The result of the previous cpc continuity test for this circuit gives (R1+R2) as 0.47 ohms. The ambient temperature at the time of test was 25 degrees celcius (factor 0.98) and the cable is 70 degree pvc (factor 1.2). If the maximum tabulated value of Zs for this circuit is 1.09 ohms, show by calculation (not rule of thumb method) if the measured value of Zs is accepable.

Thanks for any replies

Ok

1 The Zs is 0.93 ohms and the R1+R2 at measurement time is 0.47 ohms....This gives you calculated Ze of 0.93-0.47 =0.46 ohms

2 this R1+R2 was took at 25 degrees C so we need to multiply the figure by the correction factor to get it back to 20 degrees C so....0.47 X 0.98 = 0.46 ohms

3 now we are at 20 degrees we need to jack the R1+R2 up to 70 degrees so we multiply our 20 degree figure by 1.2 so.... 0.46 X 1.2= 0.55 ohms

4 now we get our theoretical Zs by adding up the Ze to the R1+R2 (70 degC) so......Zs = 0.46 + 0.55 = 1.01 ohms

this is less than the 1.09 ohms max so it is good to go

NB While I am at it, I may as well do it the other way to avoid confusion. This is using the rule of thumb.....

1 measured Zs at 25deg C is 0.93 ohms

2 maximum Zs is 1.09 ohms

3 corrected maximum Zs to allow for temperature is 1.09 X 0.8 (rot)= 0.87 ohms

4 circuit fails now:coat