Dear dipil,
First of all sorry for late reply.As per my knowledge and experiance i wrote my answer::::
1. The power supply cable length is determined by engineering values. Cables may be interlinked in series until the impedence or resistance gets too high. First sign will be degradation of the arc and weld, and heating of the cable, particularly at the couplings. However, generally, the power supply cable should be one length (what ever distance) with lugged connections.
2. Same applies for weld cable to piece to be welded as in no. 1 above. Rule of thumb states that you should not interconnect more than five cable lengths (typically one length is 16m.)
3. The weld machine must have internal ground to earth, both for quality and for safety. This is addressed in US by OSHA under assured equipment ground (not be confused with grounding requirements for portable tools and extension cords.)
4.A rectifier converts alternating current to direct current (or vice versa) and adjust phase, a transformer only converts one voltage to another. Rectfiers generally provide better control of the arc and heat, thus resulting in higher quality welds. From a safety stand point, anything that provides better control reduces risk.
One other point to remember - Best Quality practice and Best Safety practice is to connect the welding ground clamp to the piece within 0.5m of the area to be welded. This results in lowest weld resistance, and also controls the current path to ground. This is especially important when welding on mechanical or electrical equipment, as remote connection to ground may allow the current path to arc across bearings, camshafts, drive shafts, gears, circuit boards, etc. Some building fires have also been known to occure due to arcing from structural components (or pipes) that inadvertantly became the path to ground for the welding current.
Hope this helps
Please let me know.....