Discuss Schedule of Test Results (No 18: Zs) Calculation in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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Ashley2

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The SOTRs gives a column 18, for the Zs of each circuit. Does anyone have the workout/formula for this?
 
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kellogs64

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lol thanks for that,makes me wonder how i passed my coarses lol
 

HandySparks

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It's a column for test results. Ideally, you measure the Zs for the circuit using your MFT or earth fault loop impedance tester.

Then you compare your measured figure with the corrected maximum Zs allowed for the OCPD of the circuit for the disconnection time required in order to determine whether your measured Zs is adequate.
 
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Ashley2

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It may help if I rephrase this.

Lighting Circuit1: R1+R2, is obtained by carrying out a continuity test, by placing a temporary link between the Line & cpc at the DB, & then recording the resistance between the line & cpc, at the furthest point.

Lighting Circuit2: Is obtained in the same way, as per above.

How do I now get the Ze for each of the above circuits, in order to calculate the Zs, for each circuit? (or is the Ze figure the same for all circuits, with only the R1+R2 figures changing for each circuit, & therefore giving different Zs figures for each circuit).

 

HandySparks

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Yes, Ze, which is normally obtained by measurement, is the same for all the circuits of a board.

If the board is not at the origin of the installation, then Ze for the board is called Zdb and is the Zs of the distribution circuit supplying the board.
 

Jimmy Boy

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Ashley is this a homework/study theory question ? if it is that's fine mate I understand the questions, if this is an install you are doing or considering doing, you shouldn't be until you have understood the inspection and testing procedures and recording of results.

J
 
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sedgy34

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He's on the right track with his answers leave him to it
best way of learning
 
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Ashley2

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Since I posted this, I have come across the following question (Q5b), which gives the answer not as I would have stated. Bearing in mind that the question specifically asks: "b) Where on a circuit must a reading for Zs be carried out", as per follows:

Question 5b. http://www.djtelectraining.co.uk/downloads/ADL 2391 REVIS.pdf

Answer 5b: http://www.djtelectraining.co.uk/downloads/ADL 2391 REVIS.pdf

I would have thought that the reading for Zs is carried out at the origin (i.e. incoming supply side of the DB). And that Zs for the circuit is dependent on the R1+R2 for that specific circuit i.e. Zs = Ze + (R1+R2) & that it is the R1+R2 figure that is taken at the farthest point of any specific circuit. (R1 + R2) is then added to the Ze.

I know it may be being picky, but the SOTR specifically asks for the Zs of the circuit, which is derivable by reading the R1+R2 of that circuit, from the farthest point, & not the Zs.

Also, thanks for all your replies, it is appreciated.
 
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telectrix

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I would have thought that the reading for Zs is carried out at the origin (i.e. incoming supply side of the DB). And that Zs for the circuit is dependent on the R1+R2 for that specific circuit i.e. ZS = Ze + (R1+R2) & that it is the R1+R2 figure that is taken at the farthest point of any specific circuit. (R1 + R2) is then added to the Ze.

your Ze ( external loop impedance ) is measured at the origin. the Zs is measured at the furthest point of the circuit from the origin.
 
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Ashley2

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Yes, I am getting my Zs & Ze, intertwined, You can only have the Zs, if Ze & (R1+R2) have been measured. However, I go back to my initial question:

"(Q5b), Where on a circuit must a reading for Zs be carried out", as per follows:

Question 5b. http://www.djtelectraining.co.uk/dow...91%20REVIS.pdf

Answer 5b: http://www.djtelectraining.co.uk/dow...91%20REVIS.pdf

The reading for Ze is carried out at the origin (i.e. incoming supply side of the DB). (GN3: (p53): "Measurement of external earth fault loop impedence, Ze: The external earth fault loop impedence, Ze, is measured using an earth fault loop impedence tester at the origin of the installation."

Is the the Zs for the
circuit therefore not dependent on the R1+R2 for that specific circuit i.e. Zs = Ze + (R1+R2) & that it is the R1+R2 figure that is taken at the farthest point of any specific circuit, & not the Zs figure. (R1 + R2) is added to the Ze, to derive at the Zs figure.

I know it may be being picky, but the SOTR specifically asks for the Zs of the circuit, which is derivable by reading the R1+R2 of that circuit, from the farthest point. The only measurement is the R1+R2 at the farthest point of the circuit, surely it is not the Zs that is being measured at the farthest point.
 

telectrix

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yes it is. you measure R1+R2 before energising ( dead test ) as you described earlier. say for argument's sake that reading is 0.35 ohms. yo then measure Ze at the origin with the main earthing conductor dissed and the installation isolated. say this reading is 0.2 ohms. you would then expect the Zs at the furthest point to be 0.35 + 0.20 = 0.55 ohms,,, yes? Ok, reconnect the main earth. energise the installation , and measure Zs at the furthest point. you will usually find that it's less than expected due to parallel paths through bonding etc.
 

Jimmy Boy

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You have to measure the Zs at the furthest point as you are trying to establish the total impedance of the circuit to compare against the values in the OSG/BGB, the highest value you obtain is the one to compare against the values to make sure you comply with your disconnection times,now on a single point of use radial like a cooker or shower this would be straight forward, on a ring not so, because you may have spurs off the ring which will increase your resistance and therefore your Zs, so you would check all of the points of use, to obtain the max Zs.

J
 
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