R1 + r2

[Peter Woodford]

Guys,

not done formulas in a very long time and need to get back into it. If I have a Zs 0f 0.48 ohms on a 230v supply how do i work out the R1 + R2 value. if poss can you send me the working outs on how to do this.

thanks in advance

 

[iceman]

hi there you need to find out wot Ze is thn take this value away frm ya Zs reading

 

[Des 56]

You need to know the Ze which will be deducted from the Zs to leave the R1 + R2 value

ZE + (R!+R2) = Zs

 

[Peter Woodford]

ze was 0.46 ohms

 

[Des 56]

ze was 0.46 ohms

Then that leaves 0.02 ohms
This could be the R!+R2 (f there are no parallel earth paths)

 

[xraytek]

If Zs was obtained using a loop impedance tester then it would include parallel paths, and give a lower value than Ze+R1+R2.

I suppose in theory you could loop test at the board with bonding still connected, then deduct Zs from that figure.
This is not what I would do, but just offered it for discussion.

The correct way would be taking the live out the MCB, put it on the earth bar and measuring at the other end of the circuit.

 

[zupos40]

You need to know the Ze which will be deducted from the Zs to leave the R1 + R2 value

ZE + (R!+R2) = Zs

You formulas missing the F factor (1.2) for 70 celcuis PVC
if designing the circuit the Zs design = to Ze + F(R1+R2) where Ze is 0.35 TNC-S or 0.8 TN-S and F = 1.2 (pvc conductors table 9C on site guide)

As well if you performing a periodic inspection and are unable to measure Zs the you can measure R1+R2 and then Ze, then finally Calculate the ZS for direct comparison with BS 7671:2008 usind the following formula

Zs calculated = Ze measured + 1.2(R1 +R2 measured) ( this is known as the summation method)

Personelly be told Zs-Ze does not = R1+R2 nor should it ever be used as a short cut to actual testing.

Guys,

not done formulas in a very long time and need to get back into it. If I have a Zs 0f 0.48 ohms on a 230v supply how do i work out the R1 + R2 value. if poss can you send me the working outs on how to do this.

thanks in advance

Peter have a read of GN.3 Appendix B, there are a number of worked examples that may be useful near the back of the book.

 

[chocolate]

or . . . measure R1+R2 directly using a low-resistance continuity test meter.