Schedule of Test Results (No 18: Zs) Calculation

[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]

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]

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

 

[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.​

 

[Guitarist]

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.

 

[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]

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

 

[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]

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).

- - - Updated - - -

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

 

[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.

 

[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]

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]

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]

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

J

 

[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