Danfoss VLT FC51, Руководство по конструкции, Страница: 52 / 94

6.10 Earthing of Screened/Armoured
Control Cables
Control cables should be braided, screened/armoured, and
the screen must be connected with a cable clamp at both
ends to the metal cabinet of the unit.
Illustration 6.11
shows correct earthing examples.
PE
FC
PE
130BA051.11
PE PE
FC
PE PE
FC
100nF
PE
FC
69
FC
PE PE
PE
68
61
68
69
FC
PE
a
b
c
d
e
PLC etc.
PLC etc.
PLC etc.
PLC etc.
Equalizing cable
Min. 16mm 2
a Correct earthing
b Incorrect earthing
c Protection from potential between PLC and frequency
converter
d 50/60 Hz earth loops
e Cables for Serial Communication
Illustration 6.11 Earthing Examples
a. Correct grounding
Control cables and cables for serial communi-
cation are fitted with cable clamps at both ends
to ensure the best possible electrical contact.
b. Incorrect grounding
Do not use twisted cable ends (pigtails). They
increase the screen impedance at high
frequencies.
c. Protection from potential between PLC and
frequency converter
If the earth potential between the frequency
converter and the PLC (etc.) is different, electric
noise that disturbs the entire system is possible.
Fit an equalizing cable, next to the control cable.
Minimum cable cross-section: 16 mm
2
.
d.
For 50/60 Hz earth loops
Using long control cables sometimes causes
50/60 Hz ground loops. Connect one end of the
screen to earth via a 100 nF capacitor (keeping
leads short).
e. Cables for serial communication
Eliminate low-frequency noise currents between 2
frequency converters by connecting one end of
the screen to terminal 61. This terminal is
connected to ground via an internal RC link. Use
twisted-pair cables to reduce the differential
mode interference between the conductors.
6.11 Residual Current Device
Use RCD relays, multiple protective earthing or earthing as
extra protection, if local safety regulations are complied
with.
If an earth fault appears, a DC content may develop in the
faulty current.
If RCD relays are used, local regulations must be observed.
Relays must be suitable for protection of 3-phase
equipment with a bridge rectifier and for a brief discharge
on power-up see
chapter 3.4 Earth Leakage Current for
further information.
How to Install
VLT
®
Micro Drive FC 51 Design Guide
50 MG02K302 - Rev. 2013-12-03
6 6