COMBITEST422
EN - 106
12.9. VERIFY OF PROTECTION AGAINST INDIRECT CONTACTS IN IT SYSTEMS
In IT systems, the active parts must be isolated from the ground or be connected to earth
through an impedance of sufficiently high value. In case of a single earth fault, the first
fault current is weak, and therefore it is not necessary to interrupt the circuit. This
connection can be made to the neutral point of the system or to an artificial neutral point. If
there is no neutral point, connecttion can be made to earth through the impedance of a line
conductor. It is however necessary to take precautions to avoid the risk of harmful
physiological effects on people in contact with conductive parts simultaneously accessible
in case of a double earth fault.
Purpose of the test
Verifying that the impedance of the ground probe which the masses are connected to
satisfies the following relationship:
L
d
E
U
I
Z
*
where:
Z
E
= L-PE impedance of the ground probe to which the masses are connected
Id
= L-PE current of first fault (typically expressed in mA)
U
L
= Limit contact voltage 25V or 50V
Parts of the system to be checked
The earth system under operating conditions. The verification should be performed without
disconnecting the ground probes.
Kommenterede [S22]:
Questo paragrafo mi suona un po’
confuse, e non sono sicura di avere interpretato bene il testo, è meglio
verificarlo ulteriormente.
