Electrical Safety: Understanding Contact Electricity and Protection Measures

Posted on Aug 3, 2024 in Physics

Definition of Contact Electricity

Contact electricity refers to the potential for electric shock when the human body experiences a potential difference, resulting in current flow. This occurs when a circuit is completed through the body, with the neutral point connected to ground and the phase factor contacting the person’s hand.

Factors Involved in Electrical Accidents

  1. Inadvertent contact
  2. Degree of danger based on the current’s waveform
  3. Value of current intensity
  4. Value of voltage
  5. Time of current passage
  6. Impedance of the human body
  7. Current trajectory
  8. Nature of the current
  9. Frequency of the current
  10. Reaction capacity of the organism

Value of Current Intensity

The maximum current a person can safely carry depends on the type of current (AC or DC) and its waveform. AC values must consider the average, effective, or maximum values.

Voltage Values

The voltage affecting the body is the value of current passing through it when subjected to a potential difference. The RBT (Electrical Regulations) sets safety voltage limits of 24V to 50V in wet rooms and higher values in other cases.

Time of Passage of Electric Current

The absolute threshold represents the maximum time a person can withstand current safely.

Electrical Impedance of the Human Body

The human body’s impedance depends on the current path, contact surface, and temperature.

Path of Electricity

Electric current follows the path of least resistance, affecting organs based on the path it takes.

Frequency

Higher AC frequencies have lower effects, while power frequencies (50/60 Hz) can be fatal. DC values of intensity are less dangerous than AC values for equal currents.

Protection Measures

Protection Against Direct Contact

  • Isolation of active parts: Covering with insulation that can only be removed destructively.
  • Barriers or enclosures: Placing active parts inside protective enclosures.
  • Obstacles: Preventing accidental contact with live parts.
  • Out of reach for removal: Limiting access to live parts.
  • Residual-current-differential devices: Complementing other protection measures.

Protection Against Indirect Contact

Using low voltage SELV safety, meeting specific conditions.

Physiological Phenomena Caused by Electric Current

  • Perception threshold: Value of current a person can tolerate.
  • Muscular control current limit: Maximum intensity a person can withstand and release the electrode (16mA for AC, 76mA for DC).
  • Protection threshold muscle: Violent muscular contraction, leaving the person attached to the conductor.
  • Ventricular fibrillation (VF): Uncoordinated contraction of heart muscle fibers, leading to immediate loss of heart function.
  • Ventricular fibrillation threshold: 500mA to 400mA for short periods, 50mA for longer periods.

Other Effects

  • Brain injury
  • Blockade of the epiglottis
  • Laryngospasm
  • Overall shock
  • Burns (mediated or immediate)