Protection against electric shocks - substation
From Electrical Installation Guide
Contents |
| Protection against electric shocks and overvoltages is closely related to the achievement of efficient (low resistance) earthing and effective application of the principles of equipotential environments. |
Protective measures against electric shock are based on two common dangers:
- Contact with an active conductor, i.e. which is live with respect to earth in normal circumstances. This is referred to as a “direct contact” hazard.
- Contact with a conductive part of an apparatus which is normally dead, but which has become live due to insulation failure in the apparatus. This is referred to as an “indirect contact” hazard.
It may be noted that a third type of shock hazard can exist in the proximity of MV or LV (or mixed) earth electrodes which are passing earth-fault currents. This hazard is due to potential gradients on the surface of the ground and is referred to as a “step-voltage” hazard; shock current enters one foot and leaves by the other foot, and is particular dangerous for four-legged animals. A variation of this danger, known as a “touch voltage” hazard can occur, for instance, when an earthed metallic part is situated in an area in which potential gradients exist.
Touching the part would cause current to pass through the hand and both feet.
Animals with a relatively long front-to-hind legs span are particularly sensitive to step-voltage hazards and cattle have been killed by the potential gradients caused by a low voltage (230/400 V) neutral earth electrode of insufficiently low resistance.
Potential-gradient problems of the kind mentioned above are not normally encountered in electrical installations of buildings, providing that equipotential conductors properly bond all exposed metal parts of equipment and all extraneous metal (i.e. not part of an electrical apparatus or the installation - for example structural steelwork, etc.) to the protective-earthing conductor.
Direct-contact protection or basic protection
The main form of protection against direct contact hazards is to contain all live parts in housings of insulating material or in metallic earthed housings, by placing out of reach (behind insulated barriers or at the top of poles) or by means of obstacles.
Where insulated live parts are housed in a metal envelope, for example transformers, electric motors and many domestic appliances, the metal envelope is connected to the installation protective earthing system.
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 which ensures the direct-contact protection. Furthermore, the metallic enclosure has to demonstrate an electrical continuity, then establishing a good segregation between inside and ouside of the enclosure. Proper grounding of the enclosure further participates to the electrical protection of the operators under normal operating conditions.
For LV appliances this is achieved through the third pin of a 3-pin plug and socket. Total or even partial failure of insulation to the metal, can raise the voltage of the envelope to a dangerous level (depending on the ratio of the resistance of the leakage path through the insulation, to the resistance from the metal envelope to earth).
Indirect-contact protection or fault protection
A person touching the metal envelope of an apparatus with a faulty insulation, as described above, is said to be making an indirect contact.
An indirect contact is characterized by the fact that a current path to earth exists (through the protective earthing (PE) conductor) in parallel with the shock current through the person concerned.
Case of fault on L.V. system
Extensive tests have shown that, providing the potential of the metal envelope is not greater than 50 V with respect to earth, or to any conductive material within reaching distance, no danger exists.
Indirect-contact hazard in the case of a MV fault
If the insulation failure in an apparatus is between a MV conductor and the metal envelope, it is not generally possible to limit the rise of voltage of the envelope to 50 V or less, simply by reducing the earthing resistance to a low value. The solution in this case is to create an equipotential situation, as described in Sub-clause “Earthing systems”.
Earth connection resistance
Insulation faults affecting the MV substation’s equipment (internal) or resulting from atmospheric overvoltages (external) may generate earth currents capable of causing physical injury or damage to equipment.
Preventive measures essentially consist of:
- Interconnecting all substation frames and connecting them to the earth bar
- Minimising earth resistance
