Network type TT
In the case of TT-systems, safety regulations mostly stipulate the use of an earth leakage circuit
breaker (e.l.c.b). In combination with frequency converters, type B residual current-operated
protective devices should be used. Atlas Copco advises customers to install the following earth
leakage detection device: BENDER RCMA 470LY with external current transformer: http://
www.bender.org/prod.html
Network type IT
In IT-network systems, it is permissible for the power supply to continue in the event of an initial
earth leakage fault of a motor phase. However, the customer must detect the initial earth leakage
fault. The fault must be indicated as an alarm. If this is not the case, the initial earth leakage fault
must trigger automatic disconnection of the power supply (EN 60204-1 par.: 6.3). However, the
high frequency capacitive leakage currents caused by frequency converters cannot be detected
by standard earth leakage detection systems. Atlas Copco recommends that customers install:
BENDER IRDH 275/435 http://www.bender.org/prod.html.
Atlas Copco strongly recommends that customers do not connect the neutral conductor from the
power supply to the compressor unit. The exposed conductive parts of the installation must be
earthed locally. (EN 60204-1 par.: 7.2.3)
Power distribution systems (TN, TT, IT)
According to IEC 60950, there are three main types of power distribution systems: TN, TT and IT.
The electrical installation instructions depend on the type of network of the customer's power
supply.
Network type TN (Terra Neutral) :
The TN power distribution system provides a direct earth to the exposed conductive parts of the
installation. The connection is made through the PE-conductor. Within the TN power distribution
system, there are three sub-types: TN-S, TN-C and TN-C-S, identified according to neutral and
protective earth conductors. Nearly 85 % to 90 % of industrial networks are TN type.
Network type TN-S (Terra Neutral-Separate) :
The TN-S power distribution system provides separate neutral and direct earth conductors.
A Power system earth
B Exposed conductive parts
Network type TN-C (Terra Neutral-Combined) :
Instruction book
50 AIF999999
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