pie in the sky by your to be no1 spark

[i=p/u]

ive asked this loads and always get wrapped up in a bundle....

i have always asked about when designing a circuit how do you know your zs complies, isit as simple as this? say you have 20A water heating circuit

20x5=100 x100= 10000x0.4=4000 ,square root of 4000= 63.2/115= 0.5 cpc

tell me it aint this easy after all this time

 

[i=p/u]

is distance of circuit missing

 

[Paul.M]

Hi mate, have a look at section 4 of the OSG regards to minimum cpc sizes (dont have mine on site atm, doh).

 

[i=p/u]

doesnt say... but id assume 1mm is minimum..

 

[KFH]

I would approach from the other angle. Use Table I1 from the OSG to get the resistance of the CPC and the line conductor and multiple each by the length of cable to give R1 and R2, add these to the Ze to give an idea of Zs.

 

[i=p/u]

so want a 25A circuit for a heating system choose 30A type B protective device = 4mm cable and that gives me 11v as volt drop over 40Meters. so say tn-c-s and have value of 0.18 ohms as Ze.

so max Zs of 1.24 -0.18= 1.06 ohms for R1+R2 = 88 meters..

am i good or am i good!! c

 

[Richard Burns]

I think you may be mixing terms slightly here, however:

Table 52.3 in the BGB gives the minimum CSA for conductors in various situations.

Zs is determined as compliant if the nominal voltage over the current causing disconnection in the required time is greater than the measured/ calculated value of Zs. i.e. Zs * Ia <= Uo

Your CPC csa can be calculated for compliance (subject to the minimums in table 52.3) using the adiabatic equation as you have used above.
Taking the disconnection time at the fault current (in your case you have used the 0.1s disconnection time current for a BS60898 type B breaker (five times current rating))
The fault current used above and
the factor k (where you have used the value for a cpc in a multicore cable)

Using the adiabatic equation this does give a minimum csa of 0.55 mm2, however for a power circuit minimum is 1mm2.

 

[Richard Burns]

so want a 25A circuit for a heating system choose 30A type B protective device = 4mm cable and that gives me 11v as volt drop over 40Meters. so say tn-c-s and have value of 0.18 ohms as Ze.

so max Zs of 1.24 -0.18= 1.06 ohms for R1+R2 = 88 meters..

am i good or am i good!! c

Numbers look right there.

 

[i=p/u]

yea , i would jump to the appropriate available size... as all i know is 1mm is lowest you can go, .....

this one is litttle tricky for brain or its nackerfed trying to digest a book....

Zs is determined as compliant if the nominal voltage over the current causing disconnection in the required time is greater than the measured/ calculated value of Zs. i.e. Zs * Ia <= Uo

so 1.24 x 100A= 124v so my 230v to earth is more so will comply..???

 

[KFH]

4mm T&E with 2.5 CPC has a resistance of 12.02 mOhm/metre so 88 m = 1.06 Ohms. Add Ze of 0.18 = 1.24.

I have just realised you had worked it backwards to give a max length. So by my reckoning you are right. Min CPC size using adibatic is of course a different question and answer

 

[Richard Burns]

yea , i would jump to the appropriate available size... as all i know is 1mm is lowest you can go, .....

this one is litttle tricky for brain or its nackerfed trying to digest a book....

Zs is determined as compliant if the nominal voltage over the current causing disconnection in the required time is greater than the measured/ calculated value of Zs. i.e. Zs * Ia <= Uo

so 1.24 x 100A= 124v so my 230v to earth is more so will comply..???

Yep looks good

 

[i=p/u]

its crazy if that works , anywondeer elecctricians are struggling , heads are mashed with numberws instead of sales

 

[telectrix]

now you've got my brain ticking over , looking at your figures.and it's coming up to beer o'clock. DOH

 

[i=p/u]

have punjanna an we can talkk electrics alllllllllllll night, you get waterey mouthed at beer oclock