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Guidelines for selection of cables and fuses in a
power line installation with FCD and DMS products
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 have
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 HP (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 or DMS. 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 DMS*, and ground and
short circuit protected by the circuit breaker (CB).
6. The current is limited by the downstream FCD and
DMS*. 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).
7. If installation is on a machine (EN 60204-1) and
the distance between the T connection and the
FCD or DMS* is less than 10 ft (3 m), the cable
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can be downsized to the current capacity that
is needed for the down stream FCD.
8. The trip current for the CB upstream must not
be higher than the highest maximal prefuses for
the smallest FCD or DMS downstream.
See section Fuses for DMS coordination class.
For EMC purposes cable # 2, 3 & 4 has to be
shielded or placed in metal conduits.
* only extended versions
MG.90.F2.22 - VLT is a registered Danfoss trademark
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