Cable size of tails supplying a CU

Question

I have a temporary CU board setup for rewires etc. (CU, isolator, 25mm tails, earth block, socket outlets) that I want to make smaller (and also want to know theses answers anyway). 
 If I made a new temporary CU board setup with a mains isolator to 4mm T&E tails, to single module 32A RCBO, to 4mm T&E final circuit cable, to a socket outlet or two, do you think the 4mm tails would be compliant? 
 Overload protection - The 4mm tails (and reduction in CCC) are protected from overload by the 32A RCBO and design current of 32A of the socket circuit. 
 Fault protection - Lets say most common main fuse in domestic is 100A (worst case) BS 1361 / BS 88-3 which has a max Zs of 0.27 ohms to achieve a 5 second disconnection time for a distribution circuit on a TN system (0.14 ohms for 0.4 seconds). So if the Zs is <0.27 it's OK. 
 Main (tails) earth size would need to be the same as the line conductor (4mm) to comply with table 54.7. 
 Or use the adiabatic equation (amusing a Zs of less than 0.27 ohms): 
 S = &radic; I2 x t / k 
 Where: 
 Zs = 0.27 ohms I (fault current) = 851A (230 / 0.27) t = 1s (850A on BS 88-3 time current graph) k = 115 (70* thermoplastic) or 143 if separate cable 
 &radic; 851 x 851 x 1 = 851 851 / 115 = 7.4mm2 or 851 / 143 = 5.9mm2 (if separate cable) 
 So 4mm supply tails with 4mm earth using table 54.7 would be adequate? Am I missing anything? Thanks.

gkenyon · Answer

You can't take the loop impedance at one point on the circuit only ... you need to consider fault currents along the whole length. 
 Best place to look as to how to do this properly is Section 8.6 of the IET Electrical Installation Design Guide. For fuses, you plot t=k 2 S 2 /I 2 for the conductor size you want to check, on the tripping characteristic curve in Appendix 3 of BS &671, which always comes out as a line on that graph-paper: 
 
 The results for 70 degree PVC cables with copper conductors when using a BS 88-3 fuse are shown in Table 8.10 of the EIDG. For a 100 A BS 88-3 fuse, fault current, needs to exceed: 
 
 1100 A to protect 4 sq mm cpc/line conductor (copper, 70 deg PVC) 
 1000 A to protect 6 sq mm cpc/line conductor(copper, 70 deg PVC) 
 600 A to protect 10 sq mm cpc/line conductor (copper, 70 deg PVC) 
 440 A to protect 16 sq mm cpc/line conductor (copper, 70 deg PVC) 
 
 So, for your example of prospective fault current (at the origin) of 850 A, you'd need 10 sq mm cable minimum ... but you can also see in the general case why the recommended approach is use 16 sq mm for the earthing conductor in TN supplies. 4 sq mm will definitely not do the trick in all installations (you'd need at least 1100 A at all points up to the 32 A breaker) ... between the 100 A fuse and the CU - if there is a fault it might well cook.

Cable size of tails supplying a CU

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