GROUP 6: Electric Current
Up to 25 V for the DC and 60 V for the AC
Extra-low voltage
Extra-low voltage: Up to 25 V for the DC and 60 V for the AC • Low voltage: Up to 1000 V for the DC and 1500 V for the AC • High voltage: Over 1000 V for the DC and 1500 V for the AC
Extra-low voltage: Low voltage: High voltage:
Protection against direct contact • Protection against indirect contact • Additional safety measures
Protection against electric shock requires the following three elements:
a direct current (DC) or an alternating current (AC)
The electric current can be transmitted as a __________.
Lightning Conductor
The first effective protective device, that was invented by Benjamin Franklin.
Application of residual current protective devices (additional safety measures):
Using switches with a low residual current response (ΔI ≤ 30 mA) improves efficiency against direct contact if the protective means are ineffective (Figure 11.6). This method cannot prevent the risk of electric shock due to direct contact, but can reduce its consequences; therefore, this should not be the only protective measure
No harm to the human body, however when the time exposure is longer, it might be dangerous.
When a person is subject to an electric shock of a voltage lower than 1 kV for a short time, what is the effect to the human body?
Protection against direct contact
involves the prevention of contact between the human body and the live part of the device.
Electric shock
is a shocking current flow through a human body when it comes into contact with two points of different electric potentials.
Direct contact
t can cause electric shock to a human; that is, touching a live part, which, by definition, operates normally under a voltage higher than the safe one relative to the reference potential.
Direct contact
usually happens when work safety requirements are not satisfied, for example if one does not follow the instructions, if the design or use of the device is improper, if the device has not been adapted to environmental hazards, if a damaged or incomplete device is used, and so on.
-Insulation of the live parts -Application of guards, enclosures, and barriers: -Application of obstacles, portable barriers, and placing live parts outside the reach of the operator -Application of residual current protective devices (additional safety measures)
, protection against electric shock can be executed using the following
TN-S system TN-C system TN-C-S system
3 ways arrangement of TN network system.
Exposed conductive part
A conductive part of electrical equipment, but not a part of the working equipment; for example, electrical machine enclosures, switchgear enclosures, motor enclosures, luminaires, and so on.
Extraneous conductive part:
A conductive part, not a part of electrical equipment and liable to introduce a potential, generally the earth potential; for example, all pipelines, building structure elements, belt conveyors, and so on
Live part
A conductor or conductive part intended to be energised in normal use and that conducts electric current; for example, socket pins, cable phase wires (also neutral conductor N), switchgear conductor bars, motor windings, electrical equipment, and so on
Application of electric separation in power supply
A receiver is actuated through a separating transformer or separating converter for a DC. When using this type of protective means, no electric shock can happen because there is a one-point contact with the live part. However, its application is strictly limited.
Direct contact:
The contact of persons or livestock with live parts that may result in electric shock.
Insulation of the live parts
Application of basic insulation protects against both intended and unintended contacts. Replacement of the element with a new one is required if the insulation capacity is lost or if there are mechanical damages.
Indirect contact:
The contact of persons with exposed or extraneous conductive parts, which could become live under fault conditions or other disturbances.
Periodic training of employees • Appointing only the persons with relevant permissions to perform all operations of electrical installations and devices • Ensuring that the equipment is used for what it was intended • Displaying tables and warning signs about the hazards of electric shock • Performing periodic inspection of nonmobile electrical devices at least once a month and in the following cases: • Before starting the device after changes have been made to its electrical and mechanical parts or after any repairs have been done • Before starting the device after a one-month (or longer) break in its operation • Before starting the device after it has been transferred • While reporting all the repairs and inspections in the device maintenance book
Good Practice: The following aspects of work organisation should also be emphasised;
Over 1000 V for the DC and 1500 V for the AC
High voltage
Indirect contact, which is not always accompanied by a current flow through a human body, may cause mechanical injuries due to falling from a height or dropping an object or burns due to contact with objects of a high temperature or with fire.
How can electric current affects humans indirectly?
In direct contact, current flows through the body and causes physiological changes, that is, changes in biological, chemical, and physical characteristics. This can cause the following symptoms: • Pain due to contractions of transverse muscles through which the current flows. The muscle contractions make it impossible to get out of the electric shock, and may cause falling or mechanical injuries (e.g. damage of joints). • Failure of blood circulation, breathing, sight, heat regulation, and equilibrioception • Burns to the epidermal tissue and internal organs due to the large amount of heat supplied to the body; this may cause necrosis or carbonisation of tissues • Syncope • Ventricular fibrillation, usually causing death
How can electric current affectss humans in direct ways?
TN-C-S system
In part of the electrical network, one conductor plays the roles of protective and neutral conductors, respectively
TN-C system
In the whole electrical network, one conductor plays the roles of protective and neutral conductors, respectively.
TN-S system:
In the whole electrical network, separate protective and neutral conductors are used.
TN Network System
In this network system each point is directly earthed and the exposed conductive parts are connected to it by protective conductors.
IT network System
In this network system, all live parts are insulated from the earth, one point is connected to the earth through impedance and the exposed conductive parts of the electrical system are earthed, independent of each other or together, or are connected to the system earthing.
TT network system
In this system, one point is earthed directly and the exposed conductive parts are connected with an earth electrode, independent of the network system earth electrode.
Extensive burns, even of the extremities or other parts of the human body, often causing death • Increase in the temperature of vitreous bodies and injury of the retina due to infrared radiation • Injuries to the cornea due to ultraviolet radiation • Melting of metal parts and insulation carried by a hot flowing gas on unprotected surfaces of the body and the cornea • Mechanical injuries due to the scattering of the damaged electrical device elements or falling due to a shock wave
Indirect contact: Combined electric shoch symptoms
Up to 1000 V for the DC and 1500 V for the AC
Low voltage
-Automatic disconnection of supply: - Protection by provision of class II equipment or by equivalent insulation: -Insulation of a workstation: -Application of electric separation in power supply: -Application of equipotential bonding not being earthed:
Measures of protection against indirect contact
The type of current (current pulse, DC, and AC of higher frequencies [15-100 Hz] can cause most dangerous consequences to humans) • Voltage and intensity of the electric shock current and the shock duration • The path through which the current flows into the body • Psychophysical condition of the injured person
Pathophysiological consequences of electric shock depend on a variety of factors:
TN (TN-C, TN-S, TN-C-S), TT, and IT
Power Supply using AC networks can be divided into three basic systems.
Insulation of a workstation:
Preventing simultaneous contact with parts that may have different potentials. The surroundings of a given device should be completely insulated and at the same time no earthed objects or protective conductors should be connected.
Under special conditions:
R < 1000 Ω relative to earth; the term 'special conditions' applies to the areas where the skin can be wet, such as open spaces, bathrooms, and showers; farm buildings; industrial halls of humidity higher than 75% at a temperature higher than 35°C or lower than −5°C; rooms with low-resistance floors; and some special environmental conditions such as swimming pools and interiors of metal containers, where the human body can be wet or in contact with a conductive floor over a large area
Under normal conditions
R ≥ 1000 Ω relative to earth; the term 'normal conditions' applies to the premises of houses, offices, schools, and so on.
Automatic disconnection of supply:
Short circuits are created through the protective conductors, connecting the exposed conductive parts with a neutral point or the earth (depending on the network structure), combined with protective means ensuring that the supply is disconnected. This task may be executed using overcurrent protection means or residual current protective devices.
Application of guards, enclosures, and barriers:
This includes placing live parts inside casings or fences that allow for at least IP2X protection level, or the use of guards that prevent contact with the energised elements. The casing and fences should be fixed as firmly as possible and made up of materials that ensure their functionality within the lifetime specified by the manufacturer. The disassembly of casings, fences, and guards should be difficult (e.g. require special tools) or be possible only after eliminating the hazards against which they protect.
Application of equipotential bonding not being earthed
This prevents dangerous touch voltage. All exposed and extraneous conductive parts are connected using unearthed local equipotential bonding.
Protection by provision of class II equipment or by equivalent insulation
This protection prevents dangerous energising of the exposed conductive parts in case there is basic insulation damage. Double insulation, reinforced insulation, or equivalent insulation is used. Class II equipment is commonly used as an additional safety measure, especially when dealing with moving equipment or hand tools such as power tools (
Application of obstacles, portable barriers, and placing live parts outside the reach of the operator:
This reduces the risk of electric shock only when contact with energised elements results from deliberate action. The means of protection can be disassembled relatively easily. It is very rare in solid designs, which are not moveable, but is used very often in maintenance and repair works executed in electrical installations.
• Testing the insulation resistance of power supply and protective cables • Testing the electric strength of insulation • Checking the effectiveness of protection against electric shock, which is determined based on the measurement results of the continuity of the protective bonding circuit
To ensure effective protection against electric shock, the parameters of the protective circuit must be checked on a regular basis for its entire life time. The check is comprised of the following:
1. Disconnect the proper electric circuit. 2. Open the proper switching devices. 3. Take the cutouts from the power supply circuit. 4. Cut out or tear the conductors from the power supply side using special tools. 5. Make a short circuit of conductors from the power supply side. 6. Take out the cutouts using special holders
To free the injured person from an electric current of voltage lower than 1 kV, the following should be done;
Use the plugs that fit into the sockets on the receivers. • Avoid direct connection of the conductors with the socket (without a plug). • Use only the devices for which the enclosures reveal at least an IP44 protection level. • Apply the separating transformers when using the devices of category I. • Carefully follow the manufacturer's recommendations concerning the use of the devices. • Try to avoid exposing the electrical equipment to rain or using it under explosive, humid, or damp environmental conditions. • Use special tools designed for industrial applications when a long-lasting continuous task needs to be executed. • Use the conductor only for what it was intended. • Do not move the device by grabbing its conductors or disconnect the plugsocket combination by pulling the conductor. • Protect the conductors against high temperatures, oils, lubricants, and sharp edges. • Cut off the power supply to a device that is not operating or on which replaceable equipment is mounted. • Make sure that the device is in the 'off' state before connecting it with a power supply. • Do not make any changes in the electrical system or in the design of electrical devices without professional help.
When operating electrically-powered devices, the following rules should be followed
1. Drag the injured person out of the energised device using special tools or objects made of dry wood or plastic. 2. Prevent the injured person from falling down, which can result from cutting off the power supply.
When the aforementioned actions cannot be taken (freeing the injured person from an electric shock), the following should be done:
indirect contact
contact with the exposed conductive elements (the elements of an electrical system or electrical equipment that are not live parts) or extraneous conductive parts (i.e. metal objects not connected to the electrical circuits).
Protection against indirect contact
is the prevention of electric shock when a person touches exposed conductive parts, which are energised due to insulation damage
Combined electric shock
k is when a discharge current flows through a human body and the arc breaks out very close to a human with no current flow.
Ensure that the safety requirements have been satisfied. • Ensure that the persons operating or maintaining the specified machines and devices have the relevant permissions. • Ensure that the relevant instructions on the machine and device operation are available. • Spread knowledge of the safe operation of electrical machines and devices and focus attention on initial and periodic employee training. • Perform a periodic medical examination of employees.
organisational safety measures
High-voltage systems
re used mainly for electrical energy transfer and power supply of high energy-consumption machines and devices.
