(b A final circuit load is fed from marks) sub-distribution board. The a 230 v T
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Question
(b A final circuit load is fed from marks) sub-distribution board. The a 230 v TN-s supply via a main distribution board and a final circuit from the sub-distribution board comprises 25 m of PVC multi-core cable with 6 mm twin conductors and a 4 mm c.p.c. It is protected by a general purpose Bs88 25 A fuse in the sub-distribution board. The sub-main circuit feeding the sub-distribution board is 35 m long and comprises mm conductors and a 6 mm 10 c.p.c. bunched with other cables. Assess this circuit to check if it will comply with the shock risk constraints in BST671, stating clearly the tables you have used. (8 marks) What other factors may need to be considered when assessing whether the earthing arrangements for this circuit are acceptable? (2 marks) (Total Q1: 25 marks)Explanation / Answer
1. For line conductors up to and including 16 mm 2 , the cpc should be at least the same size. 2. For sizes between 16 mm 2 and 35 mm 2 , the cpc should be at least 16 mm 2 . 3. For sizes of line conductor over 35 mm 2 , the cpc should be at least half this size.
In order to apply the equation we need first to calculate the earth fault current from: I =U0/Zs.
minimum size of cpc is given by: s = sqrt(i*i*t/ k )
In distribution cable feeding a distribution board in a factory. Here, then, are the eight basic steps in a simplified form:
1. Determine the design current Ib .
2. Select the rating of the protection In .
3. Select the relevant rating factors (CFs).
4. Divide In by the relevant CFs to give tabulated cable current-carrying capacity It .
5. Choose a cable size, to suit It .
6. Check the voltage drop. 7.
Check for shock risk constraints. 8.
Check for thermal constraints.
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