Electric injury can be explained as trauma caused by an electric current or electric arc.
Elektro traumatism characterized by such features: a defensive reaction appears only after voltage is applied across the human body, i.e. when an electric current is already flowing through the body; electric current acts not only in places of contact with the human body and on the path through the body, but also causes a reflex action, which manifests itself in violation of the normal functioning of the cardiovascular and nervous system, breathing and so on. Electric injury can be caused by direct contact with conducting parts, and with an injury by contact or step voltage, or through an electric arc.[7]
The human body is a conductor of electric current. However, different tissues of the body have an unequal current resistance. Effects which arise as a result of electric current to the human depends on many factors, namely:
the amplitude and frequency of current flow;
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the duration of its effects, the longer the duration of the current action on the body, the heavier the consequences;
the path, the current flowing through the brain and spinal cord, the zone of heart and respiratory constitute the greatest danger;
physical and psychological state of the person. A human body has a certain resistance; the resistance varies depending on the condition of the person.[7]
The effects of current exposure:
The current threshold for perception is less than 1 mA . A 1 mA current is not dangerous as such but the sensation might give people unintentional reactions with possibly dangerous consequences, such as falls.
If, what is called, the let go current level is exceeded the exposed person cannot control; the muscles to let go of energized equipment. This current level is in the range of 5 to 10 mA. It is painful and if the situation continues long enough the body resistance might decease, current level increase and the consequences become severe and even lethal.
Respiratory problems normally occur in the range of 20 to 40 ma, depending on current duration. If the exposure remains for several seconds, risk of injures is large.
Ventricular fibrillation is alive threatening condition that can only by the use of defibrillation. The threshold for ventricular fibrillation depends on current duration T and estimated according to :
100 Ifib mA
T (71) (72) The probability of ventricular fibrillation depends on the phase of the current and where in cardiac circle the body is exposed to the current. [7]
Nowadays network companies are obligated by law to ensure the customers' safety. In Finland SFS-6001 is the regulation for high voltage installations. Because of the high level
of dangers to the customers and possible damages in low voltage equipment during earth faults there were developed detailed safety requirements. The SFS-6001 provides contact
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voltage with respect to the earth fault current, which is presented in Fig. 13.
Fig. 14.The touch voltage respective to the current duration time.[8]
In the earth fault point can be observed an earthing voltage which is caused by the earth fault current. In order to estimate allowed voltage levels during the earth fault, the most dangerous for humans health situation should be chosen , in other words with the maximum earthing voltage. In such case, the earth fault current passes through the earthing of equipment in distribution network. The earthing voltage Um can be calculated by the next formula:
Um I Rf m, (73) where Rmis earthing resistance.[9]
Fig.15. Earthing voltage.[9,10]
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3 RELAY PROTECTION.
For the medium voltage distribution networks with isolated, compensated neutral or resistive grounding several principles to determine earth fault can be implemented:
Zero-sequence voltage measurements;
zero-sequence fundamental frequency current measurements;
Measurement and comparison of the zero-sequence power direction;
Measurement and comparison of harmonic components of the earth fault current in all feeders;
Measurements of transient currents and voltages at the reference time of the earth fault;
superposition of the alternative current with non-fundamental frequency[11]
General requirements for the earth fault protection.
1 Earth fault relay protection devices have to guarantee the fixation as permanent earth faults as intermittent earth faults through an arc.
2 In networks which are operating with isolated, compensated neutral or with high-resistive grounding, relay protection devices should also provide the fixation of short-period self-extinguished isolation rupture.
3 Relay protection devices should initiate tripping or a signal.
4 Relay protection device which is initiating a signal should selectively determine the damaged direction.
5 Relay protection devices which are initiating tripping should selectively determine the damaged network element.
6 Relay protection devices should initiating tripping with as low as practicable time delay:
1)all feeders, in networks which are operating with low-resistive grounding
2)In networks which are operating with isolated, compensated neutral or with high-resistive grounding:
electrical plants, where the tripping of the earth fault is necessary according to security requirements;
in generators, high-power electric motors or in other cases, when the expected damage because of the unexpected tripping of the damaged element is lower than the damage caused by the long time period under earth fault conditions;
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in all cases when the unexpected tripping of the damaged eleven does not inflict a loss.
7 Relay protection devices should initiate tripping with a time delay:
in case, when it should be done in purpose of selectivity;
in case, when the unexpected tripping is intolerable or cause a loss(for example due to the technological reasons or safety regulations )
8 Relay protection devices initiating tripping should continuously act in the earth fault steady-state.
9 Relay protection devices should initiate a signal in all cases apart from listed in items 6 and 7.[12,3]
3.3 Earth faults relay protections based on the usage of