General principle of protection against electrical shocks in electrical installations
Protective measures against electric shocks are based on two well known dangers:
- Direct contact: contact with an active conductor, i.e. which is live with respect to the earth in normal circumstances. (see Fig. B11).
- Indirect contact: contact with a conductive part of an apparatus which is normally dead and earthed, but which has become live due to an internal insulation failure. (see Fig. B12).
- Touching the part with hand would cause a current to pass through the hand and both feet of the exposed person. The value of the current passing through the human body depends on:
- The level of the touch voltage generated by the fault current injected in the earth electrode (see Fig. B12)
- The resistance of the human body
- The value of additional resistances like shoes.
In Figure B13, the green curve shows the variation of the earth surface potential along the ground: it is the highest at the point where the fault current enters the ground, and declines with the distance. Therefore, the value of the touch voltage Ut is generally lower than the earth potential rise Ue.
On the left side, it shows the earth potential evolution without potential grading earth electrodes. On the right side, it describes how buried potential grading earth electrodes made of naked copper (S1,S2, Sn..) contribute to the reduction of the contact voltages (Ut, Us).
A third type of shock hazard is also shown in Figure B13, the "step- voltage" hazard (Us): the shock current enters by one foot and leaves by the other. This hazard exists in the proximity of MV and LV earth electrodes which are passing earth-fault currents. It is due to the potential gradients on the surface of the ground. Animals with a relatively long front-to-hind legs span are particularly sensitive to step-voltage hazards.
It clearly appears that the higher is the potential gradient without control (Ue), the higher are the levels of both touch voltage (Ut) and step voltage (Us).
Any presence of bonding conductors between all the metallic parts embedding concrete reinforcement contributes significantly to the reduction of contact voltages (touch, step).
In addition, surrounding the MV installation with any equipotential loop of buried naked copper contributes to a wider equipotential area.
Direct-contact protection or basic protection
There are four main principles of protection against direct contact hazards:
- By containing all live parts in housings made of insulating material or in metallic earthed cubicles.
- For MV switchgear, the IEC standard 62271-200 (Prefabricated Metal Enclosed switchgear and controlgear for voltages up to 52 kV) specifies a minimum Protection Index (IP coding) of IP2X to ensures the direct-contact protection. Furthermore, the metallic cubicles has to demonstrate an electrical continuity between all inside and outside metallic parts.
- By placing live parts out of reach. This principle is used in Air Insulated Substations "AIS" (see Fig. B15)
- By installations of barriers also used in AIS substations (see Fig. B14)
- By insulation. The best example of protection by insulation is the electrical LV and HV cables.
Indirect-contact protection or fault protection
As described above, a person touching the metal enclosure or the frame of an electrical apparatus affected by an internal failure of insulation is subject to an indirect contact.
Extensive studies have demonstrated that a current lower than 30 mA passing through the human body can be considered as not dangerous. It correspond to a touch voltage of about 50 V.
This means that the operation of installations may continue in presence of any phase to earth fault if the touch voltages can be maintained below 50 V. In all other situations where the expected touch voltages are above 50 V the interruption of the supply is mandatory. The higher the expected touch voltages are, the lower the interruption time must be. The maximum admissible interruption times, function of the expected touch voltages are specified by the IEC 60364 and IEC 61936 for LV and HV systems respectively.
Case of fault on L.V. system
Only the isolated neutral system (IT) allows to maintain touch voltages below 50 V and does not require the interruption of the supply in presence of phase to earth faults. Other two neutral systems (TT and TN) are always subjected to expected touch voltages above 50 V. In these cases the interruption of the voltage is mandatory. It is ensured within the time specified by the IEC 60364, either by the circuit breakers or the fuses protecting the electrical circuits. For more information concerning indirect contact in LV system, refer to chapter Protection against electric shocks and electrical fires.
Indirect-contact hazard in the case of a MV fault
In MV electrical systems, the expected touch voltages may reach values requiring interruption of the supply within much shorter times than the quickest opening time of the breakers. The principle of protection used for the LV systems cannot be applied as such for MV systems.
One possible solution for the protection of the persons it to create equipotential systems by means of bonding conductors interconnecting all the metallic parts of the installation: enclosures of switchgears, frames of electrical machines, steel structures, metallic floor pipes, etc. This disposition allows to maintain the touch voltages below the dangerous limit.
A more sophisticated approach concerning the protection of persons against indirect contact in MV and HV installations is developed in IEC 61936 and EN 50522. The method developed in these standards authorizes higher touch voltage limits justified by higher values of the human body resistance and additional resistances such as shoes and layer of crushed rock.