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Cable calculation, step 14: the adiabatic check on the CPCWill the CPC survive until the device trips?
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S = minimum CPC size (mm²) · I = fault current (A) · t = disconnection time (s) · k = a factor for the conductor and insulation material and temperatures
k comes from Chapter 54, Tables 54.2 to 54.6. Ours is Table 54.3: a protective conductor incorporated in a cable or bunched with cables, initial temperature 70 °C or more. For 70 °C thermoplastic, copper: k = 115. The starred 103 is for conductors over 300 mm². Ours is 6 mm², so 115.
Working
S = √(550.4² × 0.1) ÷ 115 = √30 294 ÷ 115 = 174.1 ÷ 115 = 1.51 mm² The CPC of the designed circuit is 6 mm². Chapter 54 minimum allowable is 1.51 mm². 6 mm² ≥ 1.51 mm² ✓
BS 7671 · Chapter 54 · Tables 54.2–54.6 · values of k BS 7671 · Table 54.3 · k for a CPC in a cable or bunched (70 °C thermoplastic copper = 115)
That's the design finished. It works in normal use, and in a fault the protective device will operate before the CPC is damaged.
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