BS 7671 - Breakers & Cables

If you look in the front of the book there is a section headed definitions.

If in doubt, consult a qualified local Electrician.

References to BS7671 are to 17th Ed 2015 amendments - watch out because 18th edition applies in a few months, I do not have copy - a good contract will define effective date to avoid standard changing during contract. Lots of current codes I_d etc. are in 411.xx.yy of BS7671 - page 38 has "symbols used in standard" but writes I_f deleted by 2008 amendment.

1) Id is in part 411.6.2 of standard. Being 0.03 amp = 30 mA, I think there is RCD protection at 30 mA which sets maximum for differential imbalance. Id is S/C current to an exposed conductive part - which involves imponderable real earth rather than a protective conductor.

2) I_gt is not in standard. Roughly, the standard requires that a "short circuit" fault be disconnected in less than 5 seconds (0.2 seconds for socket outlets) to limit duration of "touch potentials" between conducting parts of faulted equipment and earth or other equipment. S/C current at end of line (most impedance) is the lowest value, which must be compared with maximum trip current curve of fuse/breaker at 5 seconds - if current is greater than (gt??) that current value on curve, then trip time will be less than 5 sec.. The standard 411.3.2 gives variations with system voltage & earthing type.

3) 4) I²t is fault current squared times maximum time for protection to disconnect. The real relevant number is I²tR joules energy,where R is cable resistance, versus energy to heat conductor to maximum permitted insulation temperature, assuming no heat loss - adiabatic - that's where K²S² applies . The calculation has to include increase of R due to heating during fault. Refer 434.5.2 and Table 43.1 of standard. Phase conductors in 3 phase circuit get maximum current for a symmetrical 3 phase fault, the current for a phase-neutral or phase-earth fault is lower. Example - for 1 ohm each wire Line & Neutral, S/C current is 230/(1+ 1) amp L-N; 400/(1+1) L-L : 230/1 for 3 phase symmetric. If line & neutral conductors are same cross-section area, it is not necessary to do neutral calculation for cable withstand but for disconnect time- which is max for end of line[lowest S/C current]. Theoretically, the maximum current seen by protection fuse is if outlet side of fuse is shorted - "start of line" - and it must be rated to break that current.

5) I do not know - because I cannot see value in table for normal load current - that is relevant for volt drop. I_n is rated current/setting of protection, not necessarily load current - see 433.1.1 of standard There are maximum lengths for volt drop at running and starting currents for given cable mm² and for S/C fault clearance in required usually 5 or 0.2 seconds. The table is filled with the same example values on every line, but LC1-D25 "rings bells" as a motor contactor Telemecanique/Schneider make, so looks like motor circuit.

The chart header says max volt drop 5%, but standard Table 4Ab in Appendix 4 has 3% for lighting and 5% at running current would give 30% at 6 x full load motor start - which could cause start trouble. The table has no column for motor start current & time - which are relevant to overload protection & not tripping out during start attempt.

I tried to guess the meaning of Lmax and % volt drop column 5 in "Switchboard Schedule".

Is it length of run for 5% volt drop :-

1/ 4 mm² copper wire is 4.35 mΩ /m @ 20 Celsius; per table 4D 4B/BS7671, 5.5 mΩ/m @ 70 Celsius. For first line of Schedule, Ib is 3.85 Amps, Loop drop 20 Celsius copper (Live + Neutral), single phasel is 2 x 4.35 = 9 mΩ/Amp/m x 3.85 x 514 metres = 17.8 volts - that's 7.7% of 230V. Three phase drop @ 70 'C copper is 9.5 mΩ/Amp/m x 3.85 x 514 m = 18.8V - that is 4.7% of 400V. N.B. 3 phase volt drop is 5.5 x 2 x sin 30 degrees = 9.5 mΩ/Amp/m - as Table 4D 4B, 4 mm² 3 phase. That fits 5% if there is an assumed 1.06 multiplier to current for 6% low voltage, or an assumed copper temperature of 35 Celsius, not stated in schedule.

2/ Considering 1/, 4.7%, where does 2.42 % volt drop on 5th line of Schedule come from?? It is not for 120m actual run, which would be about 1%. Lmax varies in proportion to I_b on each line of table, but % volt drop in 5th column does not.

Is it maximum length of run for disconnection in 0.4 seconds, per 411.3.2.2 of standard for Uo = 230V line-earth?

3/ Table 41.3(a) for 16 amp type B circuit breakers (MCB) or RCBOs gives max 2.73 Ω - that makes 2.73/2 = 1.365 Ω for earth or phase conductor - N.B. Temperature rise of a 29 amp rated cable is negligible at 4 amps. That would be 1365/4.35 = 314 metres run, well short of 514m Lmax in Schedule.

Considering the above, I am not surprised the OP is confused - he should ask maker of Schedule to explain!!!