Final answer:
Electricity becomes hazardous when it generates excessive heat (thermal hazard) or when it completes a circuit through the body (shock hazard). Water increases the risk as it conducts electricity, and visible sparks mean electrical discharge but are not direct hazards unless they lead to either of the aforementioned conditions.
Step-by-step explanation:
Electricity becomes hazardous to humans primarily in two scenarios: when it generates excessive heat, known as a thermal hazard, or through shock, which is when it completes a circuit through the body. The interaction with water also poses a risk because it can facilitate the electrical current to pass through a person more easily, but is not hazardous in isolation without completing a circuit through a body. Visible sparks are a sign of electrical discharge, and while they indicate the presence of electricity, they do not directly indicate hazard to humans unless they lead to completing a circuit through the body or to excessive heat which could start a fire.
Most importantly, a thermal hazard occurs when electric power is converted to thermal energy at a rate that is too fast to be safely dissipated, potentially causing a fire. On the other hand, a shock hazard is present when electric current flows through a person, ranging from slightly painful to lethal depending on several factors, including the amount of current, the path it takes through the body, the duration of exposure, and the frequency when alternating current is involved.