Appendix A. EMC Standard Installation GuideVFD-ED
A-3-3 Earthing Systems
The international standard IEC60364 distinguishes three different earthing system categories, using
the two-letter codes TN, TT, IT.
• The first letter indicates the type of earthing for the power supply equipment (generator or
transformer).
T: One or more points of the power supply equipment are connected directly to the same earthing
point.
I: Either no point is connected to earth (isolated) or it is connected to earth via high impedance.
• The second letter indicates the connection between earth and the power supply equipment.
T: Connected directly to earth (This earthing point is separate from other earthing points in the
power supply system.)
• N: Connected to earth via the conductor that is provided by the power supply system
• The third and fourth letter indicate the location of the earth conductor.
S: Neutral and earth conductors are separate
C: Neutral and earth are combined into a single conductor
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