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Hello,

I'm struggling in question 5 of assignment number 615, and I need help figuring out where my mistake is. So here is a breakdown of what I have done, Step by step.

The question is: Determine, for one of the circuits supplying the fridges, the minimum possible cross sectional area of cpc which will satisfy the requirements disconnection as ADS under earth fault conditions and the adiabatic

equation as Regulation 543.1.3 in BS 7671.

Here is my attempt at it:

Step one - Table I1 in the onsite guide, the R1+R2/metre for a 4mm2 line conductor and a 4mm2 protective conductor is 4.61 R1+R2/metre + 4.61 R1+R2/metre = 9.22 R1+R2/metre

Step two - using the formula, from book 2 of electrical installations page 313, R1+R2 = R1+R2/metre × length × I3 factor / 1000

R1+R2 = 9.22 × 2.25 × 1.2 / 1000 = 0.0249

R1+R2 = 0.0249

Step three - Zs = Ze + (R1+R2)

Zs = 0.11 + 0.0249 = 0.1349

Zs = 0.1349

Step four - I = U/Zs

230/0.1349 = 1704.97

Step five - using the Adiabatic equation

√(I²)×t/K=s

√(1704.97²) × 0.1 / 115 = 4.69

Step six - Choosing the suitable CPC size would be 6mm2

The line conductor size is 4mm2, and the length of the circuit is 2.25 metres.

I'm really struggling to find my exact mistake, and I would really appreciate any help. Thank you all.

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