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What is the formula to calculate on size of Earthing cable used for Racks & power panels
In BS 7671 : 2008, under 543.1.3, following adiabatic equation has been provided for calculation of earth wire cross section area,
S = sqrt (I*I*t)/k
S = nominal cross sectional area of earth conductor in mm2
I = value in amperes (rms for a.c.) of fault current for a fault of negligible impedance, which can flow through the associated protective device.
t = is the operating time of the disconnecting device in seconds corresponding to the fault current I ampere
k = this value depend on the cable construction and method of installation. Please use following links to select proper value.
For further details, please refer Chapter 54 of BS 7678 and following link also may be helpful.
In BS7678 : 2008, it has been given as adiabatic equation
for calculation of cross sectional area of earth conductor as follows,
S = sqrt(I*I*t)/k
S = nominal cross section area of the earth conductor in mm2
I = value in amperes (rms of a.c.) of fault current for a
fault of negligible impedance, which can flow through the associated protective
t = the opening time of disconnecting device in second
corresponding to the fault current I amperes.
K = this value is based on the construction of the conductor
and usage of it. You can use following links to select suitable value.
For further details please refer BS 7678 and following link also may be helpful.