Why we do not feel current /voltage while doing spot welding?

Question

Why we do not feel current /voltage while doing spot welding?

Answer 1

Spot welding uses a very high current -- hundreds or thousands of amperes -- but only a very low voltage.  This is too small to be felt, since the human skin does not even begin to conduct at anything less than about 50 volts.  

Answer 2

Simply because you are not within the voltage circuit, spot welding is a closed circuit.

Answer 3

maximum potential energy converts into heat energy at the junction tht's remaining energy does not felt u danger

Answer 4

The voltage at the electrodes is very low typically less than 12 volts consequently a very very low current would flow through our body say a few micro amps. Since our first perception of current is at about 1 milliamp we cannot feel the electricity flowing through us.

The resistances of the metals being joined are very very low typically a few milliohms and so a very high current flows through the metal causing it to heat up, melt and so join together. If we touch the electrodes as it welds we are effectively putting our high resistance body in parallel with the low resistance metals so we don't feel anything, except perhaps heat from the welding process or hot electrodes.  

Answer 5

Dear Jabi....

                 The basic spot welder consists of a power supply, an energy storage unit (for example, a capacitor bank), a switch, a welding transformer, and the welding electrodes. The energy storage element allows the welder to deliver high instantaneous power levels. If the power demands are not high, then the energy storage element isn't needed. The switch causes the stored energy to be dumped into the welding transformer. The transformer decreases the voltage and increases the current. An important feature of the transformer is it reduces the current level that the switch must handle. The welding electrodes are part of the transformer's secondary circuit. There is also a control box that manages the switch, and may monitor the welding electrode voltage or current.

The resistance presented to the welder is complicated. [Geoff Shannon, Advances in Resistance Welding Technology Offer Improved Weld Quality and Reliability for Battery Manufacturers, Battery Power Products & Technology, July/August 2007, Vol 11, Issue 4, www.batterypoweronline.com.] There is the resistance of secondary winding, the cables, and the welding electrodes. There is also the contact resistance between the welding electrodes and the workpiece. There is the resistance of the workpieces, and the contact resistance between the workpieces.

At the beginning of the weld, the contact resistance is usually high, and most of the initial energy will be dissipated there. That heat and the clamping force will soften and smooth out the material at the electrode-material interface and make better contact (that is, lower the contact resistance). Consequently, more electrical energy will go into the workpiece and the junction resistance of the two workpieces. As electrical energy is delivered to the weld and causes the temperature to rise, the electrodes and the workpiece are conducting that heat away. The goal is to apply enough energy so that a portion of material within the spot melts without having the entire spot melt. The perimeter of the spot will conduct away a lot of heat and keep the perimeter at a lower temperature. The interior of the spot has less heat conducted away, so it melts first. If the welding current is applied too long, the entire spot melts, the material runs out or otherwise fails, and the "weld" becomes a hole.

The voltage needed for welding depends on the resistance of the material to be welded, the sheet thickness and desired size of the weld 'nugget'. When welding a common combination like 1.0 + 1.0 mm sheet steel, the voltage between the electrodes is only about 1.5 V at the beginning, but can fall as low as 1 V at the end of the weld. This decrease in voltage results from the reduction in resistance caused by the workpiece melting. The open circuit voltage from the transformer is higher than this, typically in the 5-10 V range, but there is a large voltage drop in the electrodes and secondary side of the transformer when the circuit is closed.

Due to changes in the resistance of the workpiece as it starts to liquefy, the welding process can be monitored in real-time to ensure coinsistent weld quality, using modern monitoring/feedback equipment. The resistance is determined indirectly, by measuring the voltage and current at the electrodes.