Do I use zdb or ze when calculating zs values on am2?

Uplight99 · May 21, 2023

Got my am2 coming up on Tuesday and I know I have to calculate my Zs readings

Maybe I’m overthinking this but am not sure if I need to use the Ze (which is apparently given to me) or my measured Zdb from my three phase panel in the equation. I have heard different answers, would be great if someone could clarify this for me. I assumed it would be Zdb+(R1+R2)

Also do I need to use the 80% OSG value or the value in bs7671

Aaron b · May 21, 2023

You are pretending that you are on a real installation so it will be a Ze. The main difference between that and Zdb is your disconnecting the main earth and is worst case. Your gas and water will be floating and less than a foot long so they will be the same anyway. I didn't have it given to me but centers probably differ.

The values requested are maximum Zs values but you will likely have a question on the 80% rule. You will need to compare your readings against the maximum Zs and show how you've got them.

Uplight99 · May 21, 2023

Aaron b said:
You are pretending that you are on a real installation so it will be a Ze. The main difference between that and Zdb is your disconnecting the main earth and is worst case. Your gas and water will be floating and less than a foot long so they will be the same anyway. I didn't have it given to me but centers probably differ.

The values requested are maximum Zs values but you will likely have a question on the 80% rule. You will need to compare your readings against the maximum Zs and show how you've got them.

So I use the Ze value given to me that goes into the supply characteristics for the formula

The Zdb is irrelevant to the formula and I just put that on the results page

Don’t want to fail the testing for one little mistake

Also you say show you how you got the results as in write down the formula for each circuit on a separate piece of paper?

I am aware of having to write down r1+rn/4 and r1+r2/4 to prove my R1+R2 reading for the ring circuit

pc1966 · May 21, 2023

Uplight99 said:
I am aware of having to write down r1+rn/4 and r1+r2/4 to prove my R1+R2 reading for the ring circuit

Don't forget brackets!

r1+r2/4 is very different to (r1+r2)/4

Aaron b · May 21, 2023

Uplight99 said:
So I use the Ze value given to me that goes into the supply characteristics for the formula

The Zdb is irrelevant to the formula and I just put that on the results page

Don’t want to fail the testing for one little mistake

Also you say show you how you got the results as in write down the formula for each circuit on a separate piece of paper?

I am aware of having to write down r1+rn/4 and r1+r2/4 to prove my R1+R2 reading for the ring circuit

There a question paper as part of that section. There will be some about the ring circuit so you will end up writing it down on the question paper. There's questions like what page in the osg is the 80% values. You will be expected to use gn3 the bbb and the osg.

I don't think you would fail for a small mistake, it has a pass mark.

I believe net have made their version of the EIC forms available on the website it might be worth familiarising yourself with it.

https://www.netservices.org.uk/wp-content/uploads/2023/04/NET-18th-Ed-BS-7671-Certs-Blank-V1022-2.pdf

Uplight99 · May 22, 2023

pc1966 said:
Don't forget brackets!

r1+r2/4 is very different to (r1+r2)/4

I was under the impression this equation doesn’t need brackets because it’s little r1 and little r2 from the end to end readings. It would only be in brackets if it’s your (R1+R2) reading

Aaron b · May 22, 2023

Uplight99 said:
I was under the impression this equation doesn’t need brackets because it’s little r1 and little r2 from the end to end readings. It would only be in brackets if it’s your (R1+R2) reading

Your little r's are the end to end readings. The figure 8 test is to measure the (R1+R2) value and you can predict/validate the (R1+R2) value using the (r1+r2)/4 formula.

You can either use the r1 and the r2 values from the first step and add them together, using brackets or finding the sum before dividing, or measure between line and cpc after putting in the links for step 3 and measuring them both together and dividing that value.

My measured Zs values were worse than calculated and didn't meet the maximum Zs. It was probably worn sockets or rcd uplift and fortunately I was able to explain in one of the questions.

Uplight99 · May 22, 2023

I understand the equation but when I done the 2 day revision course last week they said the r1+r2 doesn’t go in brackets but you guys are saying it does so I’m a bit confused. On YouTube GSH electrical also done a video on this and didn’t put the r1+r2 in brackets.

Also the only Zs reading that I need to carry out as far as I’m aware is from the ring circuit and the rest are calculated

Aaron b · May 22, 2023

Uplight99 said:
I understand the equation but when I done the 2 day revision course last week they said the r1+r2 doesn’t go in brackets but you guys are saying it does so I’m a bit confused. On YouTube GSH electrical also done a video on this and didn’t put the r1+r2 in brackets.

Also the only Zs reading that I need to carry out as far as I’m aware is from the ring circuit and the rest are calculated

Well 1+1/4 is 1.25 whereas (1+1)/4 is 0.5.

Uplight99 · May 22, 2023

Aaron b said:
Well 1+1/4 is 1.25 whereas (1+1)/4 is 0.5.

This is how I was told to validate:

Let’s say my r1 is 0.20 rn is 0.20 r2 is 0.30

R1+Rn = r1+rn/4
R1+Rn = 0.20+0.20/4
= 0.40/4
= 0.10 ohms
So R1+Rn = 0.10 ohms

Then repeat for r1+r2 to validate the R1+R2 reading

I was not told to put anything in brackets

DPG · May 22, 2023

Uplight99 said:
This is how I was told to validate:

Let’s say my r1 is 0.20 rn is 0.20 r2 is 0.30

R1+Rn = r1+rn/4
R1+Rn = 0.20+0.20/4
= 0.40/4
= 0.10 ohms
So R1+Rn = 0.10 ohms

Then repeat for r1+r2 to validate the R1+R2 reading

I was not told to put anything in brackets

You've done the sum by doing the addition first. But in mathematical hierarchy (BODMAS) division is done before addition.

So thought you have done it in the correct order, it is actually written incorrectly.

Uplight99 · May 22, 2023

DPG said:
You've done the sum by doing the addition first. But in mathematical hierarchy (BODMAS) division is done before addition.

So thought you have done it in the correct order, it is actually written incorrectly.

This is exactly how they told me to write it out on my revision day. Not sure what’s right/wrong now

GSH electrical have also done it the same as me and they’re college tutors

Aaron b · May 22, 2023

GSH have written it as a fraction which is just another way. If you make sure you do the addition first, it will work or if your calculator lets you lay it out as GSH has. The equation is technically wrong without the brackets and all written on one line.

What you have written "R1+Rn = 0.20+0.20/4" = 0.25ohm

DPG · May 22, 2023

Uplight99 said:
This is exactly how they told me to write it out on my revision day. Not sure what’s right/wrong now

GSH electrical have also done it the same as me and they’re college tutors

You don't need brackets when it is written with items above and below the line as they have done. But the way you wrote it is incorrect, ie. R1 + Rn / 4

loz2754 · May 22, 2023

Uplight99 said:
This is exactly how they told me to write it out on my revision day. Not sure what’s right/wrong now

GSH electrical have also done it the same as me and they’re college tutors

That drawing is not the same as writing the equation in a straight line. In the drawing, the sum of r1+ rn is written above the divisor, which makes it obvious in which order to evaluate the equation.
When written in a straight line, we need to follow the rules of what order to do the evaluation, which is summarised using the acronym BODMAS, or BODMAS.
Just to back this up, page 66 of guidance note 3 shows the equation as (r1 + r2)/4, which clearly shows the use of brackets.

Do I use zdb or ze when calculating zs values on am2?

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