Hi Ali,

1. This is a difficult question to answer. From an electric-shock point of view, many authorities consider somewhere around 50V to be the limit, and in fact this level figures prominently in many of the wiring rules in my part of the world (United Kingdom).

It's not quite that simple however, as the severity of a shock depends upon many other factors, such as which parts of the body the current flow through, for how long, and the intensity of the current. In extreme circumstances, it has been known for 32V to prove fatal, yet many people have survived contact with 120 or 240V domestic power.

2. At 12V the automotive battery doesn't represent a shock hazard, but the lead-acid battery is capable of delivering a very high current. If you short-circuit a car battery you can quite easily get in excess of 1000A flowing -- Quite enough to melt wiring and start a fire.

The difference between this and a 12V battery for a transistor radio is that the latter has a much higher internal resistance which limits the available current.

3. Direct contact is where somebody touches a part of the equipment which is energized in normal operation. If you were to open a panel cover and grab hold of a live busbar, for example, that would be direct contact (Don't try it! [Linked Image])

Indirect contact is where somebody gets a shock by touching a part which should not normally be live. For example, if you had an electric heater with broken connections inside so that the metal case became hot, then anyone touching the case would get a shock by indirect contact.

Hope this helps.