Electrical Hazards and Safety Measures
Unit 16: Electrical Risk Physical Agents
Electrical Hazards
An electrical accident is defined as receiving a jolt or shock, with or without causing personal damage or injury.
- Direct Contacts (34%): Contacts with parts of the installation normally under strain.
- Indirect Contacts (18%): Contacts with metal parts or elements accidentally energized.
- Arc Burns (48%): Produced by the union of two points at different potentials through a low electrical resistance element.
Factors Influencing the Electrical Effect
- Current Strength (Density):
- Electric Shock => Muscular Tetanization => Atrial Muscle => Cardiac Arrest.
- Applied Voltage:
- Safety Voltage: A voltage that can be applied indefinitely without danger to the human body. Applicable for both continuous or alternating current. Depends on location:
- Dry: 50V / Humidity: 24V / Submerged: 12V
- Duration of Electrical Contact:
- Perception Threshold: The minimum value of current that causes a sensation in a person is 0.5 mA, regardless of the time.
- No-Drop Threshold: The maximum value of the current for a person to hold the electrodes is 10 mA for times longer than 5 seconds. It increases significantly for times less than that value.
- Fibrillation Threshold: The minimum value of current that causes ventricular fibrillation is 40 mA for longer than 3 seconds. It varies between 500/400 mA for times between 10/100 ms.
- Electrical Resistance of the Human Body:
- Skin resistance decreases rapidly with increasing current. For contact voltages of 50V, it varies widely. For values above 100V, it decreases rapidly, becoming negligible when the skin is broken.
- The internal resistance of the human body depends crucially on the path of the current through the body.
- The Route of the Current Through the Human Body:
- Perpendicular paths are much more dangerous than transverse paths.
- Dangerous Paths:
- Left Hand => Right Hand
- Left Hand => Right Foot
- Right Hand => Left Foot
- Left Hand => Head
- Frequency of Current:
- For higher frequency electric current at 50 Hz, the dangerous effects of ventricular fibrillation decrease progressively, although thermal effects prevail in the current.
- Direct current, in general, is not as dangerous as alternating current, primarily because it is easier to let go, and the ventricular fibrillation threshold is much higher.
Effects of Electric Current
- With the Passage of Current Through the Body:
- Death by Ventricular Fibrillation (VF)
- Death by Asphyxiation
- Asphyxia and Respiratory Arrest
- Muscular Tetanization
- Internal and External Burns (Fatal or Not)
- Renal Block due to Toxic Effects of Burns
- Electrolytic Effect in the Blood
- Minor Physical Injuries from Falls, Bumps, etc.
- Without Passage of Current Through the Body:
- Direct Electric Arc Burns, Particle Projections, etc.
- Eye Injuries from Electric Arc Radiation (Conjunctivitis, Blindness)
- Injuries Due to Exposure to Gases or Vapors Released by Arcing
Protection Systems
- Class A Systems: Reduce the risk by preventing contact between conductive elements and masses and making contacts safe.
- Separation of Circuits
- Use of Small Safety Voltages
- Separation Between Live Parts and Accessible Masses via Insulation Protection
- Inaccessibility to Simultaneously Conducting Elements and Masses
- Coating Masses with Protective Insulation
- Bonding
- Class B Systems: Act as assets and disconnect or cut the power when dangerous conditions are detected, ensuring disconnection of the installation as quickly as possible.
- Low Sensitivity: Leakage Current of 300 mA
- High Sensitivity: Leakage Current of 30 mA
- Very High Sensitivity: Leakage Current of 10 mA