New Zs values

[Engineer54]

What a bloody convoluted exercise the IET have made of Max Zs values. Think i'll stick with values that has been in use from time immemorial and has worked perfectly well during that time....

 

[Leesparkykent]

Hi there
ok so you don't like the way I got to the figure fair enough, but as you have more knowledge than me answer these questions then

what are we proving with max Zs ?

What realtionship does this have with say a 32A breaker 60898 type B

there must be a relationship between breaker & the value otherwise there would be no value so what is it

so so if you have the above how do you get to 100% in the book

show me the math

forget my second Gn3 part although math might not be how you would do it I still get the correct answer

so show me your math just looking at how you get the figure in the book there has to be a reason ie like ohms law you can't change it it is a fact

The reason the loop impedance Zs value is required is to determine that the earth fault path will have a low enough impedance to enable a large enough fault current to flow to operate the protective device within the specified time.

The relationship between maximum zs and 60898 mcbs
Type B - Above 3 In up to and including 5 In
Type C - Above 5 In up to and including 10 In
Type D - Above 10 In up to and including 20 In*

I don't need to show you the maths you are obviously more than capable

For the Cmin factor x the final figure by 0.95

And lastly why are you getting the hump? Lol

 

[marc8]

Not getting the hump I am asking you to prove what the book says is correct where did the value come from as my formula may not or maybe correct I still get to the correct answer I already know the relationship between the value & the breaker.
I am asking you to prove it after all this is a forum which is to promote discussion & answers, if I am wrong I am happy to admit it, but the formula is givin the correct answer at the end how I get there I have shown without the use of the book, I am just asking you to prove how they get the answer or the relationship between the answer & the breaker sorry of I come across rude or if I have the hump I have not just looking to check the math

 

[Leesparkykent]

Not getting the hump I am asking you to prove what the book says is correct where did the value come from as my formula may not or maybe correct I still get to the correct answer I already know the relationship between the value & the breaker.
I am asking you to prove it after all this is a forum which is to promote discussion & answers, if I am wrong I am happy to admit it, but the formula is givin the correct answer at the end how I get there I have shown without the use of the book, I am just asking you to prove how they get the answer or the relationship between the answer & the breaker sorry of I come across rude or if I have the hump I have not just looking to check the math

It's all good if you are getting the same answer.

230/(OCPD rating X 5) for B type
230/(OCPD rating X 10) for C type
230/(OCPD rating X 20) for D type

X0.8 for 80% rule of thumb
x0.95 for Cmin

or even easier change x0.8 for x0.76 as this takes the Cmin in to account.

 

[marc8]

Thanks leesparkykent
did not have book with me that day but guy showed me the formula & without the book & knowing the value it made sense to me & was very quick on the old calculator.
Whats was more interesting to me was that when I did the 2360 & 2391 ect years ago we where told location of answer in the book, but not the relationship between the answer in the book & how you get there.

 

[andyb]

What a bloody convoluted exercise the IET have made of Max Zs values. Think i'll stick with values that has been in use from time immemorial and has worked perfectly well during that time....

That doesn't make sense Eng
Unless you can remember every max Zs for every device you are going to have to look them up.
So are you going to keep a copy of the old regs just to use the Zs tables?
Or are you just not going to get the new book and carry on working to the bgb?

 

[marc8]

#36 you don't need the book if you follow the formula

6amp type B

6x5 = 30
230/30 = 7.66
7.66 x 0.8
= your corrected ZS
value try it
change breaker to anything 20A 40A
the answers are as the book states

OCPD is correct from post #34 that is the relationship & that's gives you the formula & the formula gives you max ZS stated in BS7671 can't be any FFS whatever that is

 

[Engineer54]

That doesn't make sense Eng
Unless you can remember every max Zs for every device you are going to have to look them up.
So are you going to keep a copy of the old regs just to use the Zs tables?
Or are you just not going to get the new book and carry on working to the bgb?

It makes perfect sense to me, this project was designed in good part to the last ed' of BRB, but i do also have a copy of BGB. This will in all likelihood be my last project. And NO i'll not be bothering to get a copy of the latest load of old bull. ..lol!!

 

[GB.Kayak]

Max ZS for any type B breaker 60898 or 61009

Type B = 5 x
at present using 230volts before the change to 220volts

lets use 32A type B breaker for this example

32A x 5 = 160A
160 A is fault current that will trigger breaker

we have our 160A now what
230v our supply voltage Uo

230/160 = 1.43 this is 100% value for ZS
1.43 x0.8 is our rule of thumb
1.43 x 0.8 = 1.15 corrected ZS actually book states 1.16 table B6 OSG P119 green book

so now for our new GN3

32A x 5 =160A
supply Uo 220v

220/160A =1.37 max ZS
corrected value = 1.37 x 0.8 = 1.1 ohms



Using green book values for C & D
type C use x 10 32 x10 = 320 230/320 = 0.78175 0.78175 x 0.8 = 0.575 rounded up = 0.58

type D use x 20 32 x 20 = 640 230/640 = 0.359375 0.359375 x 0.8 = 0.2875 rounded up = 0.29

dont need the book as the sum won't change just one value which would be 230 going to 220v. the figure gives 100% but just x 0.8 to give the rule of thumb

Does GN3 indicate that U0 is 220 V? I have not yet invested in Amd 3 books yet; however, I understand that U0 is reduced by the Cmin factor of 0.95. If this is correct then the 'modified value' for U0 is not 220 V unless you are working to 2 significant figures, but is 230 x 0.95 = 218.5 V.

 

[shanky887614]

nominal voltage hasn't changed you are correct but the Cmin factor is applied to the nominal voltage when calculating the maximum zs of an OCPD to counter in the permitted tolerance of +10%/-6% in voltage variations. That is where GB kayak has got 218.5 volts from. you would of thought they would of given us a correction factor of 0.94 instead of 0.95 considering the minimum permitted voltage range is 216.2 volts (-6%) but the powers that be deemed 0.94 to be un realistic (another fine contradiction might I add).

i have never measured 230v though and thats what i was getting at, i have measured 232 or 226v etc so you do need to work it out each time based on your reading.

 

[Leesparkykent]

i have never measured 230v though and thats what i was getting at, i have measured 232 or 226v etc so you do need to work it out each time based on your reading.

The calculation is provided on the right hand side of the picture you have posted.

 

[andyb]

i have never measured 230v though and thats what i was getting at, i have measured 232 or 226v etc so you do need to work it out each time based on your reading.

I've measured 239v & 234v, but my favorite is 241v. I've only had that once.

 

[shanky887614]

The calculation is provided on the right hand side of the picture you have posted.

yeah i know, i took the picture with my phone using my own regs book

 

[Leesparkykent]

yeah i know, i took the picture with my phone using my own regs book

Cool, so how do you calculate the maximum Zs permitted by bs7671 of an OCPD device then? Because I can't see how measuring the voltage like you have said has anything to do with it. It's based on the nominal voltage which is 230volts.

 

[marc8]

Before you slate it try the formula not gn3 part try the first formula for any type B,C,D 60898 put in the parts of the formula & check it gives you the answer