Introduction on equipment and personal protection

Personal protection

           There are safety regulations that we have to meet. The aim of such regulations is to minimise the risk of personal injuries due to different faults in the power system. 

Safety regulations often state:

*  Maximum allowed fault duration

*  Sensitivity of the protective relays

Every fault can be dangerous to person. In case of an arc in a switchgear there is a risk of burn injuries due to strong heat.  

The arc can also melt down metal that will be evaporated and very dangerous to inhale.
In case of a current flow through the body, the injuries are dependent on the duration of the current flow and the amplitude of the current.Others.

Personal protection

 

 

           A number of dangerous situations are regulated Step voltages must be kept within 100V. The problem arises in solidly earthed systems.  Voltage on towers for overhead lines must be below 100V, or 190V if cleared within 0.5 sec.Voltage on apparatuses such as e.g. motor enclosures.The requirements  are fulfilled with a correct dimensioning of the earthing system and by choosing a suitable protection system.In the case of a phase-to-earth fault, parts near the fault location will get a high voltage rise.This can be very hazardous to persons due to the fact that touchable parts can get dangerous voltage. Even in the case of high earth fault resistance and thus low fault current, the voltage rise can be dangerously high.In cases, with high resistive phase-to-earth faults, there is a risk that the fault is not cleared due to fault current levels, lower than the operation levels of the protective relays.This can happen for instance if a conductor is broken and falls to a high resistive surface. If a person touches thisconductor there will be a current flow through the body that can cause heart problems likely to lead to death.We can classify different current amplitude intervals, referring to the consequences:

·                    Currents smaller than 20 mA

·                    Currents from 20 to 40 mA

·                    Currents from 50 to 3 000 mA

·                   Currents larger than 3 000 mA

Sensitivity of the Protection System: In case of a phase-to-earth fault on a transmission line this fault can have a large resistance in the fault point. Even if the fault is low, due to high-fault resistance, the fault is very dangerous to person touching a part that has reached a high voltage as a consequence of the earth fault.