New Zs values

[Silly Sausage]

:smilielol5:I know what that is without looking!

 

[SPARTYKUS]

Tel, its because them thar laws of physics changed again... Like when it became a great idea to run just two cores to everyones house instead of three. i think it is a similar event to that, you know, based on safety {*cough*} not {*cough*} money.

 

[nickblake]

here's a few 0.4 disconnection time 60898 B Type
40 amp = 1.09
32 amp = 1.37
20 amp = 2.19
16 amp = 2.73
6 amp = 7.28

 

[telectrix]

what about a banana. we've not done a banana yet.

https://www.youtube.com/watch?v=piWCBOsJr-w

 

[Knobhead]

I've a BYB the OP can buy, very cheep.













OK it's 16th edition

 

[Engineer54]

Just think of the millions upon millions of circuits that were designed, installed, and have been working for donkeys years to the previous max Zs values!! Now many or i would even say most of the actual Zs values would have been much lower, but those say close to, or right in the limit of those previous max Zs values are now deemed non compliant!! ...Bloody ridiculous. They could have better spent the time, sorting out the areas within BS7671 that actually ''DO'' need sorting out!!

 

[marc8]

#18 are you sure about 32amp

if the table is correct in the above post, nappit at 80% = 1.10 ZS for 0.4 60898 or 61009
if that is correct then 1.10 x 1.2 = 1.32 not 1.37 as in post 18.
Unsure now

 

[Engineer54]

#18 are you sure about 32amp

if the table is correct in the above post, nappit at 80% = 1.10 ZS for 0.4 60898 or 61009
if that is correct then 1.10 x 1.2 = 1.32 not 1.37 as in post 18.
Unsure now

And so the unnecessary confusion begins.... lol!!

 

[andyb]

#18 are you sure about 32amp

if the table is correct in the above post, nappit at 80% = 1.10 ZS for 0.4 60898 or 61009
if that is correct then 1.10 x 1.2 = 1.32 not 1.37 as in post 18.
Unsure now

Your maths is wrong

80 is 80% of 100 but 20 is 25% of 80.
Therefore you need to multiply 80% of something by 1.25 to get to 100%

There, that's cleared that up.

 

[stevethesparks]

The way I see it use the old tables and multiply by 0.95
or Instead of using the 80% factor use a 76% factor it works out the same

 

[nickblake]

#18 are you sure about 32amp

if the table is correct in the above post, nappit at 80% = 1.10 ZS for 0.4 60898 or 61009
if that is correct then 1.10 x 1.2 = 1.32 not 1.37 as in post 18.
Unsure now

Max for a .4 disconnection time is 60898 / 61009 is 1.37 table 41.3,page 58 BYB , these are the maximum not the recorded as what the 80% is

 

[marc8]

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

 

[Engineer54]

Where are you getting 220 Volts from FFS?? If anything you should be using 240 volts if you want to be accurate, that's the real nominal voltage in the UK. It'll make far more sense than using 230 volts... As for your 220 Volts, ...Well!!!

 

[Leesparkykent]

Maximum earth fault loop impedances given in tables 41.2, 41.3, 41.4 and 41.6 have been revised to take account of the Cmin factor. Cmin is the minimum voltage factor to take account of voltage variations depending on a number of considerations.

What you have done has over complicated it for no reason. They are 5% lower so all you have to do is the calculation exactly how you would of before and x0.95 it's simples :icon12:

 

[marc8]

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