HV cables.
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. Only the isolated neutral system (IT) allows to maintain touch voltages bellow 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 F. 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 allow 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.
The electrical equipment and circuits in a substation must be protected in order to limit the damages due to abnormal currents and over voltages. All equipment installed in a power electrical system have standardized ratings for short-time withstand current and short duration power frequency voltage. The role of the protections is to ensure that these withstand limits can never be exceeded, therefore clearing the faults as fast as possible. In addition to this first requirement a system of protection must be selective. Selectivity or discrimination means that any fault must be cleared by the device of current interruption (circuit breaker or fuses) being the nearest to the fault, even if the fault is detected by other protections associated with other interruption devices. As an example for a short circuit occurring on the secondary side of a power transformer, only the circuit breaker installed on the secondary must trip. The circuit breaker installed on the primary side must remain closed. For a transformer protected with MV fuses, the fuses must not blow. They are typically two main devices able to interrupt fault currents, circuit breakers and fuses : The circuit breakers must be associated with a protection relay having three main functions: Measurement of the currents Detection of the faults Emission of a tripping order to the breaker
Schneider Electric - Electrical installation guide 2015