Earth Strip Calculation

Dear Venkatesh,

The minimum cross-sectional area of protective conductors (earth strip) shall be determined by the following formula:

S = (√I²t)/k

Where,

S = cross-sectional area, in square millimetres

I = value (a.c., r.m.s.) of fault current for a fault of negligible impedance, which can flow through the protective device, in amperes

t = operating time of the disconnecting device, in seconds

k = factor dependent on the material of the protective conductor, the insulation and other parts and the initial and the final temperatures.

k = 159 (for 200º C Max. Temperature) for Copper,

K = 105 (for 200º C Max. Temperature) for Aluminium,

k = 58 (for 200º C Max. Temperature) for Steel.

Note:

1) The initial temperature of the conductor is assumed to be 30 "C.

2) If application of the formula produces non-standard sizes, conductors of the nearest higher standard cross-sectional area shall be used.

3) Maximum permissible temperatures for joints should be taken info account.

Alternatively, value of k may also be calculated by:

k = √[Q_c (B+20)/Q₂₀) ln {1+(θ_f + θ_i)/(B+ θ_i)}]

Where,

Q_c = Volumetric heat capacity of conductor material [J/ºC mm³]

B = reciprocal of temperature coefficient of resistivity at 0 ºC for the conductor ºC]

Q₂₀ = electrical resistivity of conductor material at 20 ºC [Ωmm]

θ_i = initial temperature of conductor [ºC]

θ_f = final temperature of conductor [ºC]

Vinu_Answers Sure _Answers

Thank U , Very much sir . I am clear now

Very good reply. It will be immensely helpful to Q4 Electrical Engineers.

Thanks,

Manindra

The minimum cross-sectional area of protective conductors (earth strip) shall be determined by the following :

S = (It)/k

or

S = (I√t)/k

IN calculation the sqt and square will cancelled . so the above formula can also be mentioned . I hope i am right

Dear Venkatesh,

The minimum cross-sectional area of protective conductors (earth strip) shall be determined by the following formula:

S = {√(I²t)}/k

Hope, Formula is clear now.

Vinu_Answers Sure_Answers

British Standard 7671 is the only formula needed.

Requested you explain this formula with some example.

Like: earth strip size for 500KVA Transformer/ for DG set & other small panels as per their load in KW.

Please clarify....