1 Types system earthing
Before start speak about the types system earthing should know something about
three-phase voltage system, which almost all electric power produced and distributed. There are voltages L1-N, L2-N and L3-N.
Class III equipment
Equipment which protection against electric shock relies supply safety extra low
voltage (SELV) and which voltages higher than those SELV are not generated.2. The voltage exists between whichever conductor L1, L2, and neutral
point.
Class equipment
Equipment which protection against electric shock does not rely basic insulation only,
but which additional safety precautions such double insulation reinforced insulation are
provided. There are provisions for protective earthing (protective conductor). A
three phase voltage system composed three sinusoidal voltages agreeing frequency and
amplitude but differing phase.2.2.
The voltage exists the output the transformer.2 Protection automatic disconnection supply
The automatic disconnection the supply the occurrence fault intended prevent
dangerous touch voltage existing for such time considered dangerous.2 shows diagram secondary winding a
transformer three-phase voltage source. That called phase voltage.
There are voltages between conductors (so called phase conductors) L1-L2, L2-L3 and L3-
L1.2. There are three phase voltages the output the
transformer and the distribution system. This voltage not necessarily equal to
the voltage the distribution system but usually lower. (Disconnection
must done till the time given Table 4. These voltages are called line line voltages simply line voltages
Figure 5.
5.
5. The voltage spreads conductors from
the output the transformer into the distribution system (network) and through this system to
electrical appliances.4.) This protective measure
necessitates co-ordination type system earthing and the characteristics protective devices.2.FUNDAMENTAL RULES FOR PROTECTION AGAINST ELECTRIC SHOCK 37
installation such way that accessible conductive parts cannot become live the event a
failure the basic insulation. Figure 5.3 shows the course instantaneous phase and line voltages during the time of
.2. This time depends the voltage existing on
exposed conductive parts case failure insulation
