Document b5p7EdoLqxGY3wEMyOqDkwbgZ

(up to 10,000 amp) in resistance welding does not permit the use of plugs except for control circuits, usually 120 volts or less Power lead terminals should be securely and permanently attached with screws or bolts for the duration of service The magnitude of the secondary welding current is such that the cables for a portable welding machine must be heavy Even with sizes from 200,000 to 1,000,000 circular mils, power losses are high and m many instances water cooling is required, especially for high production portable, as well as multiple-spot station, welding machines The length of these cables vanes from 1 to 14 ft, the latter being the maximum considered reasonably economical The covering for the cable used with a portable welding machine is thick, high tensile rubber, or now, more commonly, neoprene These are now commonly concentnc--two cables m one with water cooling Abuse of the cables for resistance welding is severe The production requirements de mand the utmost of the cable materials used, and even the best cables need frequent re placement dunng a production year In use the cables are subjected to electrical pulsation, bendmg, and twisting Electrical pulses at the rate of 3600 per minute cause a continuous whipping of the two cables which leads to fatigue and eventual breakdown This condi tion is minimized by the use of concentric cables The secondary voltage presents little shock hazard, since the maximum voltage is about 30 volts, but the operator can be hurt by a cable blowout caused by steam pressure due to overheating from faulty water cooling cir culation A periodic check for weak spots m the cable covenng is good practice The use of concentric welding cables is now common be cause they do not have the undesirable fea tures of the pulsating cables Portable weld ing machines, including the cables, should have proper weight balance to permit opera tion without undue strain to the operator Electrodes and holders For resistance welding, copper or copper alloy electrodes are m general use Made in various shapes for particular applications, they are, with few exceptions, water cooled Re sistance-welding electrodes, unlike those used in arc welding, are not deposited on the work However, the high current density and con tinuous application of mechanical force finally cause wear and necessitate redressing and re placement Electrodes are placed in holders connected to the secondary circuit The weldmg pres sure is also applied through the holders Their designs cover a multitude of applica tions and vary considerably with the weld ing methods used Electrode holders and electrodes should be water cooled to mini mize softening of the metal from heat and to reduce the chance of heat bums to the operator Machine installation Installation of resistance welding equipment should conform to the NFPA National Elec trical Code, Standard No 70 Some items worthy of special attention are listed below 1 Control circuits should operate on low voltage, not exceeding 24/36 volts max imum for portable spot welders 2 Stored energy equipment (capacitor dis charge or resistance welding) having conbol panels involving high voltage (more than 550 volts) should be completely en closed Doors should have locks and con tacts wned m the control circuit to short circuit the capacitors when door or panel is opened A manually operated switch will serve as an additional safety measure, assuring complete discharge of the capaci tors 3 Back doors of machines and panels should be kept locked or interlocked to prevent tamjienng 4 A fused safety switch or circuit breaker should be located conveniently near the welding machine so that power supply circuits may be opened before a man services the machine and its controls 5 The pomt-of-operation hazard should be eliminated by suitable guards Enclosure guards, gate guards, two-hand controls, and similar standard guards as designed for punch press operations are applicable 6 A flash welding machine should have a 959