Earth cable sizing calculations 1

[Mefi01]

Hello friends,


Can anyone explain how earthing cables sizes can be calculated to comply with BS7671 please? any supporting documents or materials would be appreciated.

Thank you

 

[ScottyUK]

For applications where the earthing conductor size isn't explicitly called out you can use the adiabatic equation with appropriate factors. Have a look at

http://www.napitonline.com/downloads/CP 3 07 P 10-11 16th Cable Sizing 2 of 2.pdf

for a starter. I think this article is from an earlier edition of the wiring regulations - probably the 16th Edition - but the method is still valid. Google [blue]BS 7671 adiabatic equation[/blue] and have a read of some of the hits.

 

[7anoter4]

I think you speak about Regulation 543 Protective Conductor.
If it is known the presumable maximum short circuit phase-to-ground and the fault clearance time [of the protection device] you can use the formula 543.1.3 S=sqrt(I^2*t)/k
If the live conductor is known then see Table 54.7.
If you want to know how this above formula was deducted you have to start with the assumption that all the heat produced in the conductor remains in conductor and is not exhausted outdoors.
If R.I^2*dt it is the energy loss and c.Vol*d*dT it is the heat deposited in the conductor where c it is heat capacity of the conductor material, d it is material specific gravity, Vol it is the conductor volume and dT it is the increasing temperature, R=ro*[1+a*(T(t)-T(o)]*length/S where ro it is material resistivity and a it is the resistance temperature factor then:
ro*(1+a(T-To)*length/S*I^2*dt=c.d.length*S*dT.
By integrating you’ll get the IEEE 80/2000 formula 37:
I=Amm^2√{[TCAPx10^-4/(tc*ror*ar)]xln[(Ko+Tm)(Ko+To)]}
where:
I is the rms current in kA
Amm^2 is the conductor cross section in mm2
Ko= 1/αo or (1/αr) – Tr in °C
Tm is the maximum allowable temperature in °C
Ta is the ambient temperature in °C
Tr is the reference temperature for material constants in °C
αo is the thermal coefficient of resistivity at 0 °C in 1/°C
αr is the thermal coefficient of resistivity at reference temperature Tr in 1/°C
ρr is the resistivity of the ground conductor at reference temperature Tr in μΩ-cm
tc is the duration of current in s
TCAP is the thermal capacity per unit volume from Table 1, in J/(cm3 • °C)