A Zs observation for all

[ian932]

I was told by my NAPIT guy that I should be putting 80% of the value BS7671 in the Max Zs column. He said the value in BS7671 is for an electrical designer who will apply the correction factors to the value when designing the installation. 80% is just a rule of thumb, the actual calculated value of ZsMax will beZsMax = Ze + (((1/Fa)*Fc)*(R1+R2))

where Fa is the ambient correction factor and Fc is the cable correction factor and Ze is the the Earth Fault Loop Impedance external to the origin.

Can you tell I am just in the process of doing my C&G2391 exam revision?

The question though is the column on the form for the ultimate max zs allowed or adjusted and I find no ruling anywhere to say either is right or wrong. In fact that column does not even exist on the model form page 341 BRB so it may be one that scam providers put in some time ago.

It appears that it

 

[apprentice87]

Hi all, Now i am only a beginner at all this, so please do not all shoot me down and might all criticism be constructive please!!!!

Sooooo, here we go!!!

For a 32 amp type B mcb on 230V the MAX Zs is 230 / 32 x 5 = 1.437 ohms

Now, i have a copy here of an NICEIC eic form, and, sure enough, the question is "Maximum Zs as permitted by BS 7671"

So, the correct answer to the question as written, is definitely, no matter what, 1.437 ohms

Now then, this refers to the maximum Zs in actual service conditions, in other words, at the operating temperature of the conductors, [which we all know increases with temperature]

When you are designing a circuit, the figures you will find in the various charts for the resistivity of the conductors are all given at the standard temperature of 20C.

So the calculation you would do would be something like;

The length of run of the cable, x [R1 + R2] x around 1.2 [to correct things to allow for the fact that the conductors will be assumed to be at 70C in use instead of 20C] all divided by 1000 to convert from milliohms to ohms.

When you are verifying an installation on site, it is assumed that everything will be all cooled down and back at ambient temperature. So we need to "reverse" the original x 1.2 bit that we did in our design calcs. The reciprocal of 1.2 just happens to be about .8 !! Sooooooo, if we make sure that the Zs as measured with our meters is not more than .8 of the "calculated" "design" figure, we can be sure all will be well when the cable IS at operating temperature.

I think it should make clear on the forms whether they are asking for the Zs as calculated and corrected to the cable max operating temperature, OR whether they want the MEASURED value at normal ambient temperature..

So, everyone is right, for all i know they might have changed the wording of the question on different age forms..

john..

 

[ian932]

I think it should make clear on the forms whether they are asking for the Zs as calculated and corrected to the cable max operating temperature, OR whether they want the MEASURED value at normal ambient temperature..

Yes

 

[apprentice87]

I had a good idea! all go out and get you rpad of forms, and then post the actual question on here, Perhaps all the different forms from the different providers [nearly said scam providers then] are actually different.

Have a look at old ones and new ones as they might have sneakily changed the forms too!!

john..

 

[ian932]

Just got my NICEIC Inspection, Testing, and Certification book out and page 141 actually says not to use corrected.

I suppose what that means is if your with the NIC then that

 

[steptoe]

well NIC put the 80% value on the inside cover of their cert books.!

so thats a bit contra of them, ,,,,

 

[apprentice87]

Hi again, a few minutes ago i just downloaded specimen forms from the NIC and the question on them is definitely; "Maximum Zs as permitted by BS 7671"

So for the NIC at least, it would appear that you should put what ever you come up with as a result of 230V divided by the device operating current, times 5, 10 or 20 for type B, C or D devices respectively

Interestingly though, the "model forms" in the BRB and the OSG ask for Ze and you must state whether you obtained it "by measurement or enquiry" and then they ask for R1 + R2 and there is also a column entitled "Earth loop impedance Zs" and that is all.

But, they do not say whether they want calculated or measured R1 + R2 and whether they want it corrected to conductor operating temperature, [say 70c] or 20c, or merely just the measurement at the ambient temperature on the day!!

Soooo, as it is obvious that, as in the BRB they are not sure what they want at all, it is open to the schemes to come up with their own, hopefully more precise, design of forms.

So, just go by the wording of the particular forms your own scheme issues!!

Great if you go to do a PIR some years later though, and you have no idea how the figures on the original EIC [if it is still there] were arrived at in the first place though!!!!!

john..

 

[Nicky Tesla]

I always write "80%" at the top of the column.

 

[ian932]

well NIC put the 80% value on the inside cover of their cert books.!so thats a bit contra of them, ,,,,

The way the NIC assessor explained it to me was you fill in the form with the max zs figure from the BRB with no correction, but you cross check your measured value with the design figure i.e. the 80% table. That

 

[Chr!s]

The way the NIC assessor explained it to me was you fill in the form with the max zs figure from the BRB with no correction, but you cross check your measured value with the design figure i.e. the 80% table. That

 

[ian932]

Is that not for the schedule of circuit details.The schedule of test results states "Max Measured".

You've lost me there mate, we are not be debating what goes in a measured value field/column, measured is what your meter tells you or Ze + (R1+R2), there is no correction made to that figure before recording it as its tested at ambient. You could multiply it by 1.24 to get to the max operating value, assuming we are operating at an ambient of 10deg and the cable has a max operating temp of 70deg. But you would not record this figure.

The debate is about the 'Max Zs Permitted' field/column on a MW, EIC or PIR and what figure goes in there.

 

[Chr!s]

You've lost me there mate, we are not be debating what goes in a measured value field/column, measured is what your meter tells you or Ze + (R1+R2), there is no correction made to that figure before recording it as its tested at ambient. You could multiply it by 1.24 to get to the max operating value, assuming we are operating at an ambient of 10deg and the cable has a max operating temp of 70deg. But you would not record this figure.The debate is about the 'Max Zs Permitted' field/column on a MW, EIC or PIR and what figure goes in there.

Well as i stated in a earlier post, refer to the note at the bottom of the tables in BS 7671, as far as the IEE are concerned, column 12 of the "Schedule of Tests Results", the measured value with the appropriate adjustment is required.

 

[ian932]

Well as i stated in a earlier post, refer to the note at the bottom of the tables in BS 7671, as far as the IEE are concerned, column 12 of the "Schedule of Tests Results", the measured value with the appropriate adjustment is required.

I think that note on the tables is ambiguous and does not say record that value in column 12, The model for is simply that a model and every form I can recall using say's 'Maximum measured' Not 'Maximum measured and corrected' The maximum part means the highest Zs taken from all points throughout the circuit under test not what it would be if corrected.

This is certainly not what the NIC want recorded and I don't think it is clear enough in the BRB to do it any other way. I will follow the process of the NIC as the IEE is a guide and an ambiguous one at best.

Here is a Napit extract http://www.napitonline.com/downloads/CP%204%2007%20P%2014-15%2016th%20Zs%20Values%20Temperature%20adjusted.pdf it say

 

[apprentice87]

Hi,

"max measured Zs" is precisely that, the max measured Zs for the circuit concerned.

You do not, and are not supposed to, "correct" anything. All you must do, is to make sure that the value as measured, does not exceed .8 of the figure obtained at the design stage [based on the conductor being at an assumed temperature of 70c]

On the NICEIC form i am looking at right now, there are columns [in the section entitled "circuit details"] for the type and rating of the overcurrent device.

You know the presumed supply voltage, so it is simple enough to work out the "max Zs permitted by BS7671" that they ask for in the next column.

"Correcting" things to allow for cable temperature, is part of the DESIGN process to arrive at the right cable size long before you ever start to fill in EIC's

The question is not asking; "what is the value of r1 + r2 at such and such temperature to arrive at the max permitted Zs taking into account Ze" They simply ask; "what is the max Zs permitted by BS7671"

Then, further along on the form, in the section entitled "test results" it asks for "maximum measured earth loop impedance Zs"

All you/we have to do, is to enter our value "as measured" [because this is what they asked for] and then just make sure that it is not more than .8 of the figure in the "other" column that was arrived at, at the design stage. [like it explains in appendix 14 i think it is, of the BRB]

john.

 

[sellers]

I've just had my office visit this evening and he didn't mention anything about me using 1.44 as my max Zs for a 60898 B32,,, we did discuss the 0.8 correction though!!

I had mine earlier this week, I use the corrected values and was told this wasn't right. Allthough, I beleive on an EIC they should be as BRB but on a PIR as per OSG.