Final answer:
A shock greater than 10 mA can cause involuntary muscular contractions and a 'can't let go' effect, making it dangerous. Larger shocks can lead to ventricular fibrillation or even burns.
Step-by-step explanation:
An electric shock from a current of 10 mA (milliamperes) or higher can stimulate sustained muscular contractions, similar to regular nerve impulses. This effect can be significant enough to cause involuntary movement, sometimes described by affected individuals as being 'knocked across the room.' Additionally, at currents of 10 to 20 mA, there arises a potentially dangerous 'can't let go' effect, where the muscles that close the fingers are stimulated stronger than those that open them, causing an involuntary grip on the source of the shock. This can extend the duration of the shock, increasing the risk of harm.
In more severe cases, larger shocks can cause ventricular fibrillation of the heart at 100-300 mA, which can be lethal if circulation is not quickly restored. Currents above 300 mA also have the potential to cause burns, particularly where the current is more concentrated. Understanding the severity of an electric shock is crucial for safety, which is why modern electric fences are designed to pulse on and off to allow a person to break free from the circuit.