Calculation of ZS

[rapier]

Evening guys


When calculting ZS u us the formula ze + r1 + r2


So i have 0.18+0.60+1.00 = 1.78

But i also have my r1+r2 result which is 0.40

0.18+0.40 = 0.58

so which 1 do i use then?

Cheers guys.

 

[Spudnik]

It should be Ze + (R1+R2)

 

[sparkymaz]

Evening guys


When calculting ZS u us the formula ze + r1 + r2


So i have 0.18+0.60+1.00 = 1.78

But i also have my r1+r2 result which is 0.40

0.18+0.40 = 0.58

so which 1 do i use then?

Cheers guys.

So where does the 0.60 and the 1.00 come into it? I take it 0.18 is your Ze? If your (R1+R2) is 0.40 then these are added together to give 0.58 as you said. How have you come up with the other result of 1.78?

You could measure it aswell

 

[Spudnik]

The 1.78 has come from Ze + r1 + r2

 

[ezzzekiel]

then that is zs
ze= 0.18
r1= 0.60
r2 = 1.00
so zs = ze + r1+r2 zs = 0.18+0.60+1.0 = 1.78
you are using r1+r2/4 value which is the outlet expected readings which is 0.60 + 1.0 / 4 = 0.40

 

[rapier]

so zs = ze + r1+r2 zs = 0.18+0.60+1.0 = 1.78
you are using r1+r2/4 value which is the outlet expected readings which is 0.60 + 1.0 / 4 = 0.40

So 1.78 is the answer.

what are u tryin to say about the last bit sorry mate.

So what happens if u have a lighting circuit, u dont have your r1 + r2 measurements as you wouldnt find these readings as its not on a ring.

But u would just get ur (R1+R2) by joining the live and earth together. So how would u find the measurement of zs by calculating.

 

[4x4 mark]

you join live to earth at the board and go to the last light and take the reading across live earth and hey presto r1+r2
for the ring were you testing opposite live and earth together or testing the live and the earth and adding them together

 

[ezzzekiel]

link line and cpc at db reading at last outlet is r1+r2

or to calculate add yor ze to tabluated figures for resistence of cable from onsite guide table 9a page 166
example a 2.5/1.5 cable has a resis of 19.51 ohms per metre so for say 15 metres:
19.51 x15 /1000 = a r1+r2 of 0.29 ohms

 

[jeremy]

There are other factors, ambient temp, max op temp to take into account. Please buy GN3. It looks really boring and can be but it's full of interesting stuff (unlike some of us!!)

 

[Spudnik]

So 1.78 is the answer.

what are u tryin to say about the last bit sorry mate.

So what happens if u have a lighting circuit, u dont have your r1 + r2 measurements as you wouldnt find these readings as its not on a ring.

But u would just get ur (R1+R2) by joining the live and earth together. So how would u find the measurement of zs by calculating.

I think you (and maybe a few others) are getting a little confused here.

Zs=Ze + (R1+R2) NOT Zs=Ze + (r1 + r2).

As you have said, the latter only applies to RFC's.

 

[maddfridge]

Hi all

common everbody

ZS=Ze+(R1+R2) then mulitplied by the correction factor for max operating temperature of the cable

onsite guide table 9c

page 168

cheers

 

[PC Electrics]

Rapier is confusing his r1+r2 with R1+R2.
He should also know that R1+R2=(r1+r2)/4 for a ring.

Have a look at the OSG under testing, esp page 82.

 

[Big Man]

you join live to earth at the board and go to the last light and take the reading across live earth and hey presto r1+r2
for the ring were you testing opposite live and earth together or testing the live and the earth and adding them together

And make damn sure you take out the neutral out at the CU (and earth it) too else you might get a little bit of a surprise , especially if you have flourescents on the lighting circuit!

 

[maddfridge]

I think you (and maybe a few others) are getting a little confused here.

Zs=Ze + (R1+R2) NOT Zs=Ze + (r1 + r2).

As you have said, the latter only applies to RFC's.

hi there

Jason i agree with you but you ALL MUST CONSIDER maximum ZS no one here has mentioned this !!!

It is not what zS is

it is what value is used for ie to make sure that the protective device operates within the correct disconnection time

Does the zs value EXCEED that of the values given in tables 41.1 to 41.5 etc of the regulations

then it is not a safe circuit etc

hope this helps a bit

cheers all

 

[snowplough]

hI Guys,

Interesting listening to our conversation as i am just learning myself and trying to get my head around testing .

When ou do the testing on a lighting circuit and you have to take your R1 and R2 reading at the last ceiling rose , is this because the resistance there should be at it highest?.

Secondandly, once you have taken that reading, do you have to test at every switch and ceiling rose after, that if so , where would you put your leads in the two way , and the intermidiate switch as there are three and four points .

As i remember from test board at college we did R1and R2 reading at rose , flicked switches for polarity, then we had to go in each switch as well , this is what im confused about , was this for continuity between earth and live.

Ive loads to ask as iam confused ,

Thanks for your patience,

Snowplough