Any voltage greater than 30 volts is typically regarded as capable of delivering harmful shock currents. Even if the voltage is too low to induce shock damage, low voltages can still be deadly. They may be enough to shock the victim, leading them to flinch back and come into touch with something more deadly nearby. The thing they touch may be a live wire, or it may be an object that has been charged by contact with a live wire (such as a metal fence post).
The amount of current flowing through a person is proportional to the voltage across them. So for example, if the voltage is reduced to 10 volts, then the current will be reduced by ten times, or 100 milliamperes. A person can be killed by current ranging from about 1 ampere to well over 100 amperes.
The danger associated with low voltages comes from two sources: first, the opportunity for someone to come in contact with these low voltages, and second, the ability of such low voltages to cause injury if they do make contact with you. A power line falls into the first category; if it causes pain when touched, it is because it is conducting electricity and should not be handled carelessly. Objects connected to the wall socket fall into the second category; if they feel like they could hurt you when touched, they should not be touched without adequate protection being used.
30 volts is often regarded as a conservative threshold number for harmful voltage in the industry. Any voltage exceeding 30 volts should be regarded as dangerous by a prudent individual, who should not depend on normal body resistance to guard against shock. The human body can be injured by electricity just as it can by any other force. High-voltage power lines can cause serious injury or death if they fall on someone or damage someone's equipment. The U.S. government requires that electrical utilities provide ground fault interrupters (GFIs) on all new circuit panels used in residential buildings. GFIs are devices that identify abnormal conditions between a line and neutral conductor of an electric circuit. They operate to interrupt the circuit if there is no connection between line and neutral, indicating a problem with how you are wiring your home. A GFI will prevent people from working on or touching exposed parts of circuits that may contain high voltage, such as underground cables or transmission lines.
The human body is resistant to moderate levels of heat (98°F or 37°C), water, pressure, and the various other forces that comprise nature. It is only through carelessness or misunderstanding that most injuries occur. Even then, many accidents could have been prevented if those involved had only known what was happening around them.
What is a dangerously high voltage? Shock hazard usually starts at 30-40 volts and rises as the voltage rises. However, even lower voltages can be harmful in low-resistance conditions (e.g. when hands are sweaty, or more skin surface area is in contact with the voltage source). First and second degree burns start to appear at about 120-250 volts, so 110-120 volts is safe for hands without causing any harm.
High voltage means electricity that has been stepped up from lower levels in power lines or generators. It may seem like a large voltage, but it's actually only a small percentage of the total energy contained in the electrons flowing through the line. The actual danger comes from the current level reaching extremely high values: hundreds or thousands of amperes. This can happen if a circuit becomes open - no longer connected to earth, which acts as a drain on electric currents. Power circuits should never be left open because this allows current to flow indefinitely into earth, which could lead to serious damage or injury.
The human body is well protected against high voltage, since we normally feel only very strong signals as pain. A person can be killed by a voltage less than what they would receive from an ordinary light bulb. The heart receives its own electrical signal just before it is going to beat, so even if there is a delay after turning off a light bulb, the person would have time to escape before being hit by lightning.
Voltages greater than 30 Vrms, 42.2 V peak, or 60 VDC are considered dangerous enough to inflict a deadly electric shock in humans. The human body is resistant to high voltages because the cells will not conduct electricity until they are damaged by it. When this happens, the flow of electrons becomes excessive, which causes further damage to the cell membrane.
The risk of death from an electric shock increases with increasing current strength. This is because the heart and brain are the most likely to receive a fatal charge from an electrical hazard. Other organs may also be injured by current, but less seriously. For example, low levels of current can cause heat damage to skin and tissue near electrodes after long contact times. High currents can also cause smoke, carbon monoxide, and toxic chemicals to be released from circuit components during an accident.
Electrical hazards can occur anywhere in the world where electricity is available. However many deaths occur because people think an outlet is safe place to leave small children. They may not understand that even if the child does not touch the metal case of the plug, they can still get hurt by coming into contact with the power line when it rains or burns their hands on another object nearby.