1.6.2 Guidelines for Selection of Cables and Fuses in a Power Line Installation with FCD 300
It is assumed that the installation follows the Low Voltage Directive as stated in HD 384 and IEC 60364. This section can’t be used in explosive areas and
where fire hazard exists. In general cable dimension has to follow IEC 60364-5-523. If the installation is part of a machinery EN 60204-1 has to be
followed. Cables as mentioned under point 1, 2 and 3 in the figure has to be protected by an enclosure or conduit.
The following section numbers refer to the figure.
1.
The cable shall only be able to carry the maximum continuous current of the friction brake. By ground fault non-renewable protective circuit in
the FCD will interrupt the flow of current.
2.
If the IP 65 brake resistors recommended by Danfoss are used the cable will only be exposed to the continuous current of the brake resistor.
If the brake resistor becomes overheated it will disconnect itself. If another type or make of brake resistor, without any power limitation device,
is used, the maximum power must be equal to the rated power of the motor.
The current in Amps would be: I = 0.77/motor power, with motor power inserted in kW; [A=V/W]. The rated motor current comes fairly close
to the current in the cable to the brake resistor.
3.
The cables to encoders and thermistors are on PELV potential. The currents are in mA range and limited by the FCD. In order not to violate the
PELV protection of the control terminals of FCD the thermistor has to have reinforced insulation according to the PELV demands. For EMI purposes
the cables must have their own electrical shielding and if possible be kept separated from power cables.
4.
The cable is protected by the current limit function in the FCD. By ground faults and short circuit of low impedance the FCD will interrupt the
current.
5.
The current is limited by the downstream FCD. The CB makes the ground and short circuit protection. The impedance in the leads has to be so
low that the CB disconnects in 5 s by low impedance ground faults. (TN supply).
6.
If installation is on a machine (EN 60204-1) and the distance between the T connection and the FCD is less than 3 m, the cable can be downsized
to the current capacity that is needed for the down stream FCD.
7.
The trip current for the CB upstream must not be higher than the highest maximal prefuses for the smallest FCD downstream.
For EMC purposes cable # 2, 3 & 4 has to be shielded or placed in metal conduits.
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Illustration 1.20: Example of decentral cable dimensioning
VLT
®
Decentral FCD 300 Design Guide
1 The Decentral Concept
MG.90.S1.02 - VLT
®
is a registered Danfoss trademark
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