Electrical risks

Resources Persons

A wide variety of electrical equipment is present in research laboratories and buildings; they may present electrical risks if improperly or badly used. The most important rule to be respected is that any intervention which may expose the human body to electricity, from modifying an existing installation to resetting a circuit breaker or simply changing a light bulb, must only be done by someone who has the proper electrical authorization.

In case of electrical anomaly (e.g., electrical breakdown of equipment, sudden jumps of a circuit breaker, damaged equipment), you should contact the technical department of your establishment and the ACMO, they will give you instructions.

The only things someone with no authorization can do are the following:   

  • manipulating the available equipment for command, power shutdown, setting
  •    
  • plugging in equipment to electrical outlets
         

Risk

The risk is that electric current passes through the body of the person to a terminal of the generator or to the ground. The consequence may be electrification with possible burns, or electrocution.(electrification leading to death).

This will take place:
       
  • always, when someone touches both wires of a circuit, either directly or via conductors
  •    
  • in certain conditions, when someone touches one of the wires of the circuit and the ground
  •    
  • in certain conditions, when someone touches an incorrectly insulated equipment and the ground

An electrical ground return only occurs if the body or the clothes present a lower electrical resistance than the circuit, for instance when bare-foot, wearing wet non-insulating shoes or standing on wet ground.

The consequences depend on the intensity of the current going through the body, and thus on the voltage and the body resistance since:

 

I (intensity in amperes) =  U (voltage in volts) / R (resistance in ohms)


Thresholds of electric current danger

From 10 mA, there is a danger for human beings (shock induced tetanus).

The table below (source: INRS) indicates the risks depending on the intensity of the alternating current. These are only indicatory values; for DC, they should be multiplied by 3 or 4 for voltages < 1 kV).


Source : INRS


Other parameters also intervene in the gravity of the incident:   

  • the duration of the contact
  •  
  • the pathway through the body (e.g., via the heart…)
  •  
  • the surface area of the contact zone  

Risk prevention

Prevention includes:

        
  • The design of the installations:
    •    
    • a differential breaker on the incoming cable of each room
    •    
    • a ground connection
    •    
    • so as to avoid all direct contact with the current, live naked wires should be set apart, separated by obstacles (e.g., locked electrical cabinet), electrical circuits should be insulated (protective sheaths for cables)
    •    
    • emergency stop buttons which are well indicated and accessible
         
  • design of material to avoid any indirect contact:
    •     
    • the use of extra-low voltages (ELV) of 25 or 12 V under damp or wet conditions
    • insulation of frames                  

Source : INRS

        
  • individual prevention measures:
  •      
    • Do not turn on any equipment with a damaged power cable (cut,  cracked or degraded by chemical products)
    •    
    • Never pull on the cable to unplug equipment, always pull onthe plug itself
    •    
    • Unplug equipment before cleaning it
    •    
    • Do not touch an outlet with wet hands, even wearing gloves
    •    
    • Do not touch an outlet if your feet are wet
    •    
    • Never place a multiple outlet near a tank containing water or an electrolyte (buffer solution)
    •    
    • Never connect several multiple outlets in series to a same outlet
    •    
    • Never leave an extension cord connected to an outlet (and to no equipment) lying on the ground or an a lab table
    •    
    • Never make or use an extension cord with two male plugs
    •    
    • Any equipment with a bad contact of the power button should be repaired
    •    
    • If a power cable has a moulded plug, do not replace  it with an unmoulded one
    •    
    • Do not modify any electrical material (e.g., an electrophoresis tank), they are designed so that noone is exposed to electric current
    •    
    • Warn the technical department of any anomalies

In case of accident

 First, cut the current without putting yourself in danger .

Then, according to the first-aid conditions on the site, immediately contact:  

  • the laboratory SST (first-aid at work)
  •    
  • and/or the preventtive medecine service
  •    
  • and/or an ambulance (the SAMU in France)

In all cases, and it is even more important than in the case of other incidents, rapid action must be taken; the survival of the injured person depends on this.


  

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