# Schedule of Test Results (No 18: Zs) Calculation

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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Zs=Ze+(R1=R2)

Arms
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#### kellogs64

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.

#### widdler

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

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#### Ashley2

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

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

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#### Ashley2

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:

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

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

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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Arms
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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#### Ashley2

So its a less accurate Zs of the circuit, because it is being measured whilst the main earth is connected, & hence, as you say, parallel earth paths are introduced into the equation.

The true Zs of the circuit would therefore be if two tests were made. The first, for the Ze, (at the origin of the installation, & thereby avoiding parallel earth paths) & the second, the R1+R2 at the specific circuit, & then combining the two.

Although, as you say, the two could be combined into one test, but this then is less accurate then the above, due to parallel earth paths.

#### Strima

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It's a truer figure of the actual installation.

I did start rattling on but my head isn't working properly and it didn't make sense, time to get the bike out and clear some cobwebs.

#### Jimmy Boy

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Calculating Zs on a circuit is an acceptable way of getting the figure, some *cough* so called bodies say you must do this on any circuit where you can't plug a tester in, I don't agree with calculation, just as I don't agree with asking the distributor for a Ze value for an installation, let me ask you a question Ashley, what advantage does measured have over calculated ? think about what you are testing,and what you are trying to determine

J

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#### Ashley2

Thanks for the reply. I see where you are coming from. I am actually looking at the question/answer in line with what officialdom would require, i.e. those who set the question & decide what the right answer is. That is why I put this into this forum, as even GN3 cannot & does not cover everything/every eventuality.

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#### Guitarist

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It is arguable that carrying out a Zs test will give you a "real life" reading and so is actually more accurate. This is fine, just so long as any parallel paths giving you an adequate reading don't change (such as a water mains being changed to a plastic incomer).
The advantages of Ze+(R1+R2) are that you are isolating that circuit and so are not dependant on the rest of the installation remaining the same. By linking your line and cpc and testing at each point, you will also be confirming polarity and line switching.
Personally, I use both measured and calculated methods depending on the situation at the time, and I agree with all the reasons why people prefer one method over another.

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#### pencilpusher

It is arguable that carrying out a Zs test will give you a "real life" reading and so is actually more accurate. This is fine, just so long as any parallel paths giving you an adequate reading don't change (such as a water mains being changed to a plastic incomer).
The advantages of Ze+(R1+R2) are that you are isolating that circuit and so are not dependant on the rest of the installation remaining the same. By linking your line and cpc and testing at each point, you will also be confirming polarity and line switching.
Personally, I use both measured and calculated methods depending on the situation at the time, and I agree with all the reasons why people prefer one method over another.
I always start with calculating ZS as I know this will give me a worst case scenario. My ZS is then measured afterwards to give me the feel good factor (hopefully!)

#### Jimmy Boy

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I have no argument with that mate...on the day you energise the installation, you rely on the actual Zs to meet the disconnect times, but I agree if the calculated fall within the value good to go...BUT what if the circuit under test once reconnected after the R1/R2 test has a fault on it ? lets say the MCB termination has been nipped up on the insulation ? this is why I like to see a good measured Zs..
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#### Ben Butcher

Coming back to this trade after some years messing about with other things, I find myself quietly in awe and humbled that there are so many of you out there discussing things to such depth. I doubt that the average Joe that sees a NIC or NAPIT etc sticker on someones van knows just what that represents! I have spent a long time working in a job where there was a lot of "Oh it'll do wonnit?" going on, and I look forward to coming back to where, well, frankly there will be a bit more "How do we know it will do?" !!!...

#### Guitarist

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I have no argument with that mate...on the day you energise the installation, you rely on the actual Zs to meet the disconnect times, but I agree if the calculated fall within the value good to go...BUT what if the circuit under test once reconnected after the R1/R2 test has a fault on it ? lets say the MCB termination has been nipped up on the insulation ? this is why I like to see a good measured Zs..
J
Totally agree mate. What I should have added is that even when I have calculated my Zs on a new installation, I will always go around before I hand it over and carry out at least one measured Zs on every circuit for the very reasons you have just pointed out. This is why my testing takes so long and I usually end up out of pocket time-wise (not that I care as long as I'm fully satisfied with my results).

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Coming back to this trade after some years messing about with other things, I find myself quietly in awe and humbled that there are so many of you out there discussing things to such depth. I doubt that the average Joe that sees a NIC or NAPIT etc sticker on someones van knows just what that represents! I have spent a long time working in a job where there was a lot of "Oh it'll do wonnit?" going on, and I look forward to coming back to where, well, frankly there will be a bit more "How do we know it will do?" !!!...
There are still plenty of us who care mate

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#### Ashley2

(a) Deriving Zs by calculation: (with Ze, having been tested for previously, & (R1+R2) having been tested for previously, to give you the theoretical Zs, &

(b) Deriving Zs by practical: Testing the Zs, i.e. without having to carry out the Ze test.

The issue of parallel earth paths leads to one question mark, whilst the other issue is having to carry out a Ze, which entails disconnecting the supply earth, & then insuring that you do reconnect this.

Is there not a case for doing both also, i.e. theoretically & practically, because both have their advantages & disadvantages, therefore would some installations not benefit from both being carried out.

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#### oldtimer

I did an EICR on Friday in an empty house GREAT I did measure the Ze Zs and did the R1+R2 but we are missing a point here the Schemies are the first to berate you if you work out R1+R2 by calculation ie take the Ze reading away from the Zs and fill the box in but because the Schemies are happy to badge companies who do what i call drive by testing surprise surprise it is them who are cutting corners

#### Jimmy Boy

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Is there not a case for doing both also, i.e. theoretically & practically, because both have their advantages & disadvantages, therefore would some installations not benefit from both being carried out.

Ashley, you have to do the R1+R2 (As well as giving you a value it gives you a continuity test on the CPC) and you have to do a Ze ( your obliged to test for the provision of an Earth and check it's value), so you will always have the calculated value ! as stated anyone worth their salt wouldn't rely on this and would compare the tested against the calculated to make sure something hadn't gone belly up when you reinstall the board or accessory connections.

J

#### Guitarist

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Unfortunately, there seem to be many electricians out there who don't actually understand what they are testing, or why.
That's why we get the "It works dunnit?" brigade.

#### Jimmy Boy

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I think you will find its " it works innet" lol

J

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#### oldtimer

Yep a bit like the PAT testing you get guys who have done it for years yet when you ask a technical question and you get I just push the button and the tester says pass or fail a bit like little Britain the computer says no

#### Guitarist

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I think you will find its " it works innet" lol

J
The "innet" brigade haven't made it to Norfolk yet mate. I'm sure it's only a matter of time tho..... :sailor:

#### Jimmy Boy

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LOL mate..well sorry they have made it the Suffolk borders..web feet and all..ah Norfolk many a happy day there at Sea Palling beach..Awesome waves

J

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#### SirKit Breaker

Thanks for the reply. I see where you are coming from. I am actually looking at the question/answer in line with what officialdom would require, i.e. those who set the question & decide what the right answer is. That is why I put this into this forum, as even GN3 cannot & does not cover everything/every eventuality.
GN3 covers more than enough for Testing, and certainly covers in depth Ze and other basic tests.

Cheers..............Howard

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#### Ashley2

But GN3 cannot cover everything, it is just a progeny of BS 7671. However, I do grant that it is THE MOST IMPORTANT DOCUMENT, relating to Inspection & testing, I myself revere it in that regard.

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#### PEG

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...was up for revering it till i read Figure 2.4b Page 41........want me 30 squid back........:smilewinkgrin:

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#### Ashley2

Yes, would agree with you, circuit-breakers should be in the off position. But I doubt if there is a single book out there that did not make at least one mistake. That does not detract from the high value content of GN3.

I also like the fact that it is around the 100 page mark, which is another refreshing aspect, because they could so easily have doubled this to 200+, by more detail & explanation.

#### ruston

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...was up for revering it till i read Figure 2.4b Page 41........want me 30 squid back........:smilewinkgrin:

Is there much difference between the red and green version as I have'nt bought one . I was'nt going to either but a few time my finger has been hovering over the one click buying on Amazon lol. Is it worth buying for the changes? Ta.

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#### oldtimer

Is there much difference between the red and green version as I have'nt bought one . I was'nt going to either but a few time my finger has been hovering over the one click buying on Amazon lol. Is it worth buying for the changes? Ta.
Naa just leave it for another year then you can buy the updated one thats after you have bought the new orange OSG whilst you save up for the replacement ISI&TEE (formally) PAT

#### ruston

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Naa just leave it for another year then you can buy the updated one thats after you have bought the new orange OSG whilst you save up for the replacement ISI&TEE (formally) PAT
I vowed to myself I would buy no more "updates" but my curiosity gets the better of me. Amazon makes a fortune out of me with electrical books lol, sucker for them, same as gear .To old now to changeresent:

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