New Zealand calculating main earth size

[jamesBoater]

i am just a little unsure if i need to use 5 seconds as my D/T or 0.4? PFC obtained through Ze test at origin.

i have been told 2 different answers and any guidance would be helpful.

thanks james

 

[D Skelton]

You could if you wanted an easy life, but then you'd be oversizing the earthing conductor on 90% of the circuits you install. Each to their own.

 

[GLENNSPARK]

You could just refer to table 54.7 of the regs book, where the sizing of the earth has been calculated for you I believe?

the adiabatic is good for calculating CPC/earth size for singles.....and also to make sure that any CPC within a cable is good for the time/current curves for the particular choice & setting of OPD being utilised...
say you had a ring final where the live conductors were intact but the CPC was broken....however all points served had R2....this is when the adiabatic would come in handy....it would confirm the difference between a pass and a fail.....

 

[stigwart]

Okay I see, because basically the c.s.a has halved but still able to carry the fault current without any damage occurring?

 

[tony mc]

Have a look at this sticky it may help Example of use of Adiabatic Equation.