Content:
General principles on electricity to know;
Electricity risks,
Electrical accidents,
Prevention of electrical risk,
Prevention of the risk of static electricity;
In our industrial society, electricity is the most widely used form of energy. Workers are required to use electrical equipment. This implies that any business can be faced with an accident of electrical origin. While the number of electricity-related accidents is steadily decreasing, they are often very serious.
The electrical risk includes the risk of contact, direct or not, with a bare live part, the risk of short-circuit, and the risk of an electric arc. Its consequences are electrification, electrocution, fire, explosion ...
The prevention of electrical risk is based, on the one hand, on ensuring the safety of electrical installations and equipment and, on the other hand, on compliance with safety rules when using them or when operating on or near them. electrical installations.
Electrical risk prevention measures are subject to codified regulatory requirements as well as associated standards.
Finally, static electricity, even if it does not present a significant physiological risk for people, can be the cause of serious accidents (fires, explosions, falls, etc.).
General principles on electricity to know
Electricity is a phenomenon directly linked to the structure of matter. It should be remembered that atoms are formed of a nucleus (positive) around which one or more electrons (negative) revolve. Atoms are electrically neutral, that is, they contain as many positive charges as negative charges.
Formation of static electricity and dynamic electricity
By rubbing two insulating materials against each other, part of the electrons of one are forced to leave their atoms and accumulate on the surface of the other. Atoms that have given up electrons are positively charged, and those that have accepted are negatively charged. These charges will remain momentarily on the surface of the body for a few seconds to several months depending on the materials and environmental conditions. These electric charges constitute what is called static electricity. The quantity of electricity formed is generally very small. The more a body is conductive, the less it is conducive to such an accumulation of charges.
Dynamic electricity is a flow of free electrons flowing in one direction. To create such a flux, it is necessary to use a conductive material connected at its ends to a generator (battery, dynamo, accumulator, alternator).
Some reminders
Electricity is made of a free electron flow. To create a flow of electricity, it is necessary to use a conductive material (metals, graphite, human body…) connected at its ends to a generator (battery, dynamo…).
Static electricity is formed when there is friction between two insulating materials. Under certain conditions, it can cause fires and explosions.
Components of an electrical circuit
In general, an electrical circuit consists of:
a direct current generator (one direction) or an alternating current generator (two directions),
conductor wires connected to the generator terminals,
one or more receivers.
Electricity risks
The risks associated with electricity for humans are of different kinds. These are mainly the risks of electric shock, electrocution and burns. These risks originate from direct or indirect contacts and electric arcs.
Direct and indirect contacts
Direct contact is a contact with a bare live part. It is for example the contact with a conductive part of a connection terminal, with the core of a bare conductor ...
An indirect contact is a contact with a conductive part setting accidentally energized. This is for example contact with a metal cabinet which is not connected to the earth and whose electrical equipment it contains has an insulation fault.
Direct or indirect contacts cause electrification or electrocution. Electric currents on the muscles of the human body can cause tetanization (motor muscles and the rib cage) or ventricular fibrillation which can cause the heart to stop.
Electric arc and its consequences
An electric arc is likely to appear when opening or closing a circuit. In fact, under the influence of the electric voltage created between the ends of the conductors that are separated or approached, the free electrons come out of the metal and violently collide with the air molecules of the interstitial space. This has the effect of snatching electrons from atoms in the air and suddenly making it conductive. This phenomenon is accompanied by the projection of molten metal particles (over 3000 ° C). It is the electric arc.
In general, electric arcs can shoot out between two conductors or two neighboring receivers brought to different potentials when the layer which separates them is not thick enough or when its insulation quality has been reduced. The resulting connection is first invisible (leakage current) then visible (electric arc). The lightning we observe during thunderstorms are electric arcs between two clouds or between a cloud and the Earth. In electrical installations, a short circuit causes an arc which can have significant consequences.
The electric arc can be, for humans, the cause of more or less serious burns and for fire or explosion installations .
What is a short circuit?
A short circuit results from an accidental connection between two conductive parts exhibiting a potential difference between them. The resulting short-circuit current is dangerous: depending on the location where it occurs, it can reach a very high intensity (50 kA and more).
The causes of short circuits include:
deterioration of insulation by aging or mechanical wear,
the rupture of a conductor,
the fall or introduction of a conductive tool in a circuit with bare live parts.
Electrical accidents
Various exposure situations
Voltage measurement at the terminals of an electric vehicle battery
Rare but often serious accidents
For 30 years, the number of work accidents, as well as serious accidents due to electricity, has been steadily decreasing. However, these remain particularly serious. Each year, around ten workers die from electrocution. The first few minutes after an accident are very important for the chances of survival: it is necessary to act very quickly, on the one hand by cutting the current without touching the body of the victim, on the other hand by immediately informing the emergency services. In addition, accidents related to electricity can cause fires or explosions.
Accidents of electrical origin occur above all during operations on fixed low voltage installations (cabinets, boxes, sockets, etc.) during the use of portable machine tools, or during interventions on or in the vicinity overhead lines, transformer stations and underground pipelines.
Electrification and electrocution
A person is electrified when an electric current passes through his body and causes injuries of varying severity. We speak of electrocution when this electric current causes the death of the person.
The electrification can occur by direct contact (with an active portion) or indirect (with a setting accidentally energized mass). Current only flows if the circuit is closed, i.e. if there is:
either two points of contact with bare live parts of different potential,
either a point of contact with a bare live part and another with the earth.
The first minutes following the accident are very important for the victim's chances of survival: you must therefore act very quickly. In addition, the intervention time of first aid is decisive in the evolution of the state of health of accident victims. It is therefore important that people working near live electrical installations have basic first aid skills .
How to rescue an electrified person
You must first turn off the power (using the switch, the circuit breaker, unplugging the plug, etc.) without touching the victim's body in order to avoid a further accident. If this is not possible, the injured person must be freed from contact with the live parts, taking care that no one else can electrify himself.
You must then call or have someone call for help : a first aid worker at work then the SAMU (15) or the fire brigade.
Then rescue the victim . You have to stay with the victim until help arrives. The first steps must be taken until help arrives by personnel trained in first aid.
Health effects
Electrisation can be more or less serious, it all depends on:
the frequency of the current,
current intensity (danger from 5 mA),
the duration of the current flow,
the surface of the contact zone,
the trajectory of the current,
skin condition (dry, damp, wet),
the nature of the soil.
Effects of alternating electric current according to its intensity on the human organism
0.5 mA: skin perception
5 mA: electric shock
10 mA: contracture resulting in inability to let go
25 mA: tetanization of the respiratory muscles (asphyxia beyond 3 min)
40 mA for 5 seconds: ventricular fibrillation
50 mA for 1 second: ventricular fibrillation
2000 mA: inhibition of nerve centers
In general, the current follows the shortest path, and therefore the least impedant, between the entry point and the exit point of the body: it can therefore damage all the organs that are in its path.
Fires
30% of fires are of electrical origin. The main causes are:
temperature rise in cables due to overload ,
the short-circuit causing an electric arc ,
an insulation fault leading to abnormal current flow between receiver and earth or between receiver and earth,
faulty contacts (such as loose connection or oxidized) resulting in abnormal resistance and heating,
lightning
electrostatic discharge.
Some factors can worsen overheating:
insufficient ventilation,
the accumulation of dust or grease deposits,
the storage of flammable materials near electrical installations,
the stacking of cables preventing the evacuation of heat.
What to do in the event of an electrical fire lower than 1000 V?
Give the alert
Switch off the installation and possibly neighboring installations
Close doors and windows
Attack the fire at the base using a suitable extinguisher (carbon dioxide, water spray *, powder)
After the fire has been extinguished, evacuate the toxic gases by ventilating and then check the atmosphere: carbon monoxide (CO), carbon dioxide (CO2), oxygen (O2)
* Beware of runoff water which can be conductive
Explosion in Electrical Risk Assessment
In areas at risk of explosion , electrical installations, both power and control, constitute a potential source of ignition for the explosive atmosphere. In order to reduce this risk, these installations are reduced to the strict minimum. In addition, the electrical equipment used in these areas respects the construction, assembly and operation conditions defined in standards.
Main factors of accidents of electrical origin
Failure to comply with safety rules when designing or modifying an electrical installation
Poor condition of equipment and insulation in particular (deterioration, cuts, etc.)
Inappropriate use of equipment (portable device, extension cable, etc.)
Electrical authorization not suitable for the operation to be performed.
Failure to respect safety distances from bare live parts
Prevention of electrical risk
The prevention of electrical risk is based on regulatory provisions contained in the Labor Code. It concerns the safety of electrical installations and equipment, right from their design. The objective is to avoid any contact, whether direct or indirect, with bare live parts or accidentally energized. In addition, the equipment must comply with the regulations in force in order to protect users.
Characteristics of electrical equipment and installations
Protection against direct contact
Direct contact is contact between a part of the human body and an active part (normally live part) of an electrical installation. There are several ways to prevent direct contact:
Remoteness : the remoteness depends on the environment (site, premises reserved for production, etc.) and on the voltage value.
Barrier or enclosure : they constitute an obstacle (screen, boxes, cabinets, etc.) with a minimum degree of protection (IP2X or IPXXB in low voltage, IP3X or IPXXC in high voltage) and can only be opened using a wrench or tool.
Insulation of active parts : this covers the active parts with insulation and can only be removed by destruction.
Very low voltage : this can be of the very low safety voltage (SELV) type or of the very low protective voltage (PELV) type. The conditions for obtaining these voltages are specified in standard NF C 15-100.
Protection against indirect contact
Indirect contact is contact between a part of the human body and a conductive mass that is accidentally energized. There are several ways to prevent indirect contact:
Earthing of the masses with automatic cut-off of the power supply: the earth connection diagrams are also called “neutral regimes”. They are defined in standard NF C 15-100,
Double insulation or reinforced insulation,
Very low voltage as for protection against direct contact.
Over-current protection
An overcurrent is a dangerous increase in the electric current flowing through a conductor or absorbed by a receiver (motor, radiator, etc.). There are two types of overcurrents:
overloads resulting from increased load,
short circuits which result from contacts of almost zero impedance between conductive elements brought to different potentials.
The existence within the circuit of a circuit breaker, a thermal relay or a fuse makes it possible to reduce the danger by opening the circuit when the current exceeds a given value for a determined time.
Voltage and classes of electrical equipment
The nominal voltage of the electrical equipment must match the supply voltage.
Standard NF EN 61140 divides electrotechnical equipment into 4 classes according to their design from a safety point of view:
insulation between active parts (normally energized) and accessible parts (metal masses),
the possibility or not of connecting the accessible metal parts to the earth.
Analyze the electrical risk
Ignorance of the electrical risk, mainly during non-electrical work, is still a source of accidents today. A systematic risk analysis makes it possible to foresee possible dangerous situations and to plan prevention as effectively as possible.
The main elements to be taken into account in the analysis of the electrical risk are:
the characteristics of the electrical installation (voltage ranges, primary and secondary sources, positioning of cables and insulated pipes, etc.),
the environment of the operation (proximity or vicinity of live parts, possibility of falling tools, moving machinery, etc.),
the tasks to be performed by the operators (operator's position, normal gestures to be performed and possible reflex gestures, wearing of protective equipment, etc.).
The electrical risk analysis must be carried out before each operation and updated if necessary throughout it.
It is the employer who is responsible for implementing this risk analysis. However, this analysis also concerns each actor, to the extent of their attributions, skills and responsibilities. In the preparatory phase of the work, the employer can designate a person to draw up the safety requirements and procedures. In the phase of carrying out operations, the works or site managers are responsible for the execution of the work and the implementation of the planned safety measures. The executors and those in charge of intervention are responsible for their own safety.
Work on or near electrical installations
General preventive measures to ensure the safety of personnel
A person carrying out an operation on an equipment or an electrical installation must be trained and authorized by his employer. Various protective measures must be implemented so that she can work in safety: signaling the room or the operation, isolating the electrical installation, implementing protective measures for work on or in the vicinity of the installations, checking facilities, provide personal protective equipment if necessary ...
For any operation exposing an electrical risk, preventive measures must be implemented to eliminate or reduce the electrical risks.
Rooms or locations presenting a risk of electric shock must be demarcated by means of obstacles and indicated by means of a regulatory warning sign of the electrical danger . Access to these premises or locations is reserved for authorized persons.
When opening an electrical cabinet with exposed live parts that are accessible, safety markings must be installed . This markup must not be able to be crossed inadvertently.
Record an electrical installation
The work carried out with the power off is the only one offering total safety with regard to the electrical risk, provided that it is sure that all voltage is effectively removed and that it remains so. To do this, the logging procedure defined in the NFC 18-510 standard must be applied .
Lockout procedure for an electrical installation
Separate the installation from any source of electrical energy concerned and previously identified
Lock the separation devices in the open position in order to prevent any re-energization.
Identify the part of the installation concerned to be sure that the work will be carried out on the planned installation
Check that there is no voltage
Ground and short circuit
Prevention of the risk of static electricity
An underestimated source of ignition
Often considered as a secondary parasitic phenomenon, static electricity is part of our daily life through its applications: photocopying, painting, spraying, dusting… It can however cause serious accidents: fires or explosions for example. Directly linked to the atomic structure of materials used or processed in industry, it is created spontaneously, under certain conditions, during manufacturing or handling operations. However, many insulating plastics which have a marked aptitude for the accumulation of electric charges are used in the most varied fields.
What is static electricity and how is it formed?
When two materials are rubbed together, some of the surface electrons of one are torn from their atoms and will accumulate on the surface of the other. These charges remain momentarily on the surface of the materials (from a few seconds to several months depending on the materials and the environmental conditions). They form what is called static electricity, which is usually present in small amounts. The more insulating a body, the more charges it accumulates.
Electrification can take place in different circumstances: passage of a liquid in a pipe, emptying a bag containing a bulk product, passage of a conveyor belt over a return pulley, impacts of particles on the wall of a separator, cleaning a container, moving a person on the ground ...
Static electricity hazards
If charges formed on the surface of a material cannot flow to earth or do not flow fast enough, these charges continue to build up and can reach a level that causes an electric shock. (by sparks for example). If this happens in an explosive atmosphere , then they can cause ignition.
Static electricity can cause accidents with disastrous consequences, in particular fires and explosions . These accidents are the cause of injuries, often serious (burns), of death as well as of often significant material damage (extension of fires to neighboring installations, etc.).
It can also have physiological effects on humans. Given the low energies involved, an electrostatic discharge is not dangerous in itself for a person but can be painful by its repetition and have serious consequences if it is the cause of falls for example. Walking on the ground and rubbing on the seats are two of the main sources of static electricity.
Main factors of accidents of electrostatic origin
It's about :
petroleum liquid transfer operations ,
tank cleaning operations and inappropriate use of extinguishing devices,
powders discharges in an explosive atmosphere (gas or flammable liquid vapors)
disruptive phenomena due to the electrically charged human person (because isolated from the earth),
the use of a solvent , in particular toluene, in installations having insulated metal parts or insulating surfaces,
the use of pipes not suitable for pneumatic transport,
lack of equipotentiality between equipment.
Prevention of fire-explosion risks
In order to implement the appropriate preventive measures, it is first necessary to identify the explosive atmospheres liable to appear and the types of electrostatic discharges that may occur, and to assess their ignition power.
These measures depend on:
the nature of the company's activity,
the raw materials used,
the design and installation of manufacturing and handling equipment,
the surrounding atmosphere ...
Main measures to prevent the risk of fire or explosion due to static electricity
Work environment and equipment
In an explosive atmosphere, increase the ventilation rate or efficiency so that the air-gas or air-dust concentration does not reach the lower explosive limit under any circumstances.
Humidify the atmosphere so as not to favor the appearance of electric charge
Avoid floors and insulating floor coverings
Make equipotential and earth all the conductive elements
Use inductive or electric eliminators
Use antistatic material or equipment
Staff clothing
Use anti-static shoes or clothing
Chemicals used
For example, replacing a flammable solvent with a non-flammable solvent or one with a higher flash point
Add certain flammable liquids to antistatic products to reduce their excessive resistivity
Means of production and handling
Replace insulating elements with electrostatic conductive elements
Work in a closed chamber and under an inert atmosphere (nitrogen)
If possible, limit the flow speed of liquids and powdery materials by increasing the section of the pipes which will be made without sharp bends or superfluous internal protrusions
Limit the height of the fall in gravity feed
Limit the pressure and friction on the guides and the drive rollers which will be slightly frosted (textiles, papers)
Use conveyor belts, belts and pipes made with conductive materials
In addition to these measures, work to raise employee awareness and training in the risks associated with static electricity must be carried out.