Practical Aspects Of AC Mains Systems

Monday, August 18, 2014

Various types of AC mains systems are used to supply power to electrical drives. They all affect the EMC characteristics of the system to various degrees. The five-wire TN-S system is the best in this regard, while the isolated IT system is the least desirable.

TN mains systems

There are two versions of this form of mains distribution system: TN-S and TN-C.


This is a five-wire system with separate neutral (N) and protective earth (PE) conductors. It thus provides the best EMC properties and avoids the transmission of interference.


This is a four-wire system with a common neutral and protective earth conductor throughout the entire system. Due to the combined neutral and protective earth conductor, a TN-C system does not have good EMC characteristics.

TT mains systems

This is a four-wire system with an earthed neutral conductor and individual earthing of the drive units. This system has good EMC characteristics if the earthing is implemented properly.

IT mains system

This is an isolated four-wire system with the neutral conductor either not earthed or earthed via an impedance. All EMC features of the frequency converter (filters, etc.) must be disabled when it is used in an IT mains system.

Article by Gregers Geilager