Electrical Safety in Construction
Preventing electrocution, burns, and arc flashes by managing temporary power systems, grounding, and applying Ohm's Law to site safety.
Overview
Electricity follows the path of least resistance to the ground. If a worker becomes part of that path, electrocution occurs. The severity of an electrical shock is determined by the amount of current (Amperes) flowing through the body, the path it takes, and the duration of exposure.
The Physics of Electrocution
Lethal Current Thresholds
Checklist
- 1 mA: Barely perceptible tingling sensation.
- 10-20 mA: "Let-go" threshold. Muscles contract involuntarily, and the victim cannot release the energized object.
- 50-100 mA: Ventricular fibrillation (heart rhythm disruption) occurs; highly lethal without immediate defibrillation.
- >1000 mA (1 Amp): Severe internal and external burns; cardiac arrest.
Key Takeaways
- Electrocution lethality is driven by current (Amperage), not voltage. Even extremely low currents () can cause fatal ventricular fibrillation.
- Once a worker reaches the "let-go" threshold (), they cannot break the circuit themselves, requiring external intervention.
Ohm's Law and Grounding
The current () that flows through a worker is determined by Ohm's Law:
Where:
- = Current in Amperes
- = Voltage (e.g., 120V or 240V)
- = Resistance of the human body in Ohms ()
Note
Dry skin may have a resistance of 100,000 , resulting in a harmless shock from a 120V source. However, wet or sweaty skin drops resistance to as low as 1,000 , resulting in a potentially lethal current.
Ohm's Law: Electrical Shock Hazard Simulator
Adjust the voltage and skin resistance to calculate the current flowing through a human body. Observe how physiological effects escalate dramatically with current (measured in milliamperes, mA).
Low VoltageStandard (120V)High Voltage
Wet/Broken Skin (1,000 Ω)Dry Intact Skin (100,000 Ω)
Calculated Current (I = V / R)120.00 mA(or 0.1200 Amps)
Physiological Effect
Severe burns. Cardiac arrest. Extremely lethal.
To prevent this, equipment grounding provides a low-resistance path (close to ) back to the source. If a fault occurs, the current rushes through the ground wire, tripping the breaker instead of passing through the worker.
Procedure
Lockout/Tagout (LOTO):
Before servicing any equipment or circuit, physically lock the power source in the 'OFF' position and tag it to prevent accidental re-energization by another worker. Verify de-energization with a multimeter before proceeding.
GFCI Implementation:
Use Ground Fault Circuit Interrupters (GFCIs) for all temporary 120-volt, single-phase, 15- and 20-ampere receptacles. GFCIs detect minor current imbalances (as low as 5 mA) between the hot and neutral wires and shut off power in 1/40th of a second.
Maintain Clearances:
Maintain a minimum clearance of 10 feet from overhead power lines for voltages up to 50 kV. Add 0.4 inches of clearance for every 1 kV over 50 kV. Use non-conductive ladders (e.g., fiberglass) and warn equipment operators of line locations.
Assured Equipment Grounding Conductor Program (AEGCP):
If GFCIs are not used or are unavailable, implement a strict, documented AEGCP to continuously test the continuity of all grounding conductors, cords, and tools on site, maintaining color-coded inspection logs.
Ground Fault Circuit Interrupter (GFCI)
A fast-acting circuit breaker designed to shut off electric power in the event of a ground-fault within as little as 1/40 of a second, preventing lethal electrocution before the current can reach dangerous levels.
Key Takeaways
- Current (Amperes), not Voltage, is the primary factor determining the physiological lethality of an electrical shock.
- Moisture (sweat, rain, or standing water) drastically lowers the resistance of human skin (), turning a minor shock into a lethal event according to Ohm's Law ().
- Equipment grounding provides a low-resistance path for fault current, ensuring it bypasses the worker and trips the overcurrent protection device.
- GFCIs and strict Lockout/Tagout (LOTO) procedures are the most effective engineering and administrative controls against electrical hazards on dynamic construction sites.