An insulation fault, irrespective of its cause, presents hazards for: human life; preservation of property; availability of electrical power; the above all depending on dependability.
Electric Shock of persons
A person (or animal) subjected to an electrical voltage is electrified. According to the gravity of the Electric Shock, this person may experience: discomfort; a muscular contraction; a burn;
c cardiac arrest (this is Electrocution)
Since protection of persons against the dangerous effects of electric current takes priority, Electric Shock is thus the first hazard to be considered. The current strength I -in value and time-, passing through the human body (in particular the heart) is the dangerous aspect.
In LV, the impedance value of the body (an important aspect of which is skin resistance) virtually changes only according to environment (dry and wet premises and damp premises). In each case, a safety voltage (maximum acceptable contact voltage for at least 5 s) has been defined: it is known as the conventional limit voltage UL in IEC 60479.
IEC 60364 paragraph 418.104.22.168 (and NF C 15-100) state that if there is a risk of contact voltage Uc exceeding voltage UL, the application time of the fault voltage must be limited by the use of protection devices
This hazard, when it occurs, can have dramatic consequences for both persons and property. A large number of fires are caused by important and localised temperature rises or an electric arc generated by an insulation fault.
The hazard increases as the fault current rises, and also depends on the risk of fire or explosion occurring in the premises.
Unavailability of electrical power
It is increasingly vital to master this hazard. In actual fact if the faulty part is automatically disconnected to eliminate the fault, the result is: a risk for persons, for example:
sudden absence of lighting, placing out of operation of equipment required for safety purposes; an economic risk due to production loss.
This risk must be mastered in particular in process industries, which are lengthy and costly to restart. Moreover, if the fault current is high: damage, in the installation or the loads, may be considerable and increase repair costs and times; circulation of high fault currents in the common mode (between network and earth) may also disturb sensitive equipment, in particular if these are part of a "low current" system geographically distributed with galvanic links.