Dangerous Voltages  
  The following section is not intended as an absolute guide to electronics safety. Many factors, most of which are beyond the control of this author, contribute to the degree of hazard that you are subject to while working on electronic circuits. Use care and critical judgement at all times. Safety is your personal responsibility!
There are two different ways in which electriciy can injure or kill you: by burning, and by affecting nervous system functions. The human body is damaged by the flow of current through it. Tissue damage occurs via heating, just like a burn. When the current path passes through important organs (such as the heart, brain, spinal column) significant body functions (such as heart beat and breathing) can be disturbed or halted.
Dry skin has a fairly high resistance, so very little current flows and you don't feel anything when touching low voltages (say below 35V). Therefore, with some caveats, if your circuit is operating on +15V and -15V maximum, you can safely poke around in it while it is under power. Circuits that operate with higher supply voltages should be treated with more care.
Wet skin has a much lower resistance than dry skin. If you perspire, as we all do, your skin resistance can be lowered such that you may feel a minor shock from low voltages (I have felt 24 volts). Blood, having a saline component, is quite a good conductor. Accidental shock via an open wound is quite possible, and could be quite dangerous, even at low voltages.
If you accidentally come in contact with higher voltages, such as 240VAC, you'll know it right away because of the pain caused by the shock. But, if the voltage is high enough to cause significant current flow, and if the current path passes through important organs, you may find yourself unable to let go, or perhaps even electrocuted.
This is where the "one hand in the pocket" rule comes from. Think about getting a high voltage shock in one hand while the other hand is touching a ground point. Where will the current flow? Down one arm, across your torso (right through the heart) and then down the other arm. If the second arm was in your pocket instead of resting on the table, you might survive what otherwise would be a deadly mistake.
Metal jewelry is a very good conductor. If you have a power supply that can deliver high current, your class ring would be happy to provide the conduction path. Here the danger is heat rather than shock. The ring will conduct as much current as the supply will deliver. If you short out a car battery with your ring, the ring will literally vaporize! Imagine what that would feel like on your ring finger! Vaporizing metal wouldn't be fun in the eyes either!
In summary, always observe the following precautions:
  • Remove all rings, bracelets, necklaces, etc. before working on live electronic circuits.
  • Keep your hands dry at all times.
  • Wear full coverage bandages on all open wounds.
  • Respect all circuits, especially those that have high voltage or high current capability.
  • Keep one hand in your pocket when working on high voltage circuits.


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