VEGA VEGADIF 65 4 … 20 mA_HART - Manual (Page 34)

34
5 Connecting to power supply
VEGADIF 65 • 4 … 20 mA/HART
36128-EN-130417
On the instrument with cable entry ½ NPT and plastic housing there is
a metallic ½" threaded insert moulded into the plastic housing.
Caution:
No grease should be used when screwing the NPT cable gland or
steel tube into the threaded insert. Standard grease can contain
additives that corrode the connection between threaded insert and
housing. This would influence the stability of the connection and the
tightness of the housing.
If screened cable is necessary, connect the cable screen on both
ends to ground potential. In the sensor, the screen must be connected
directly to the internal ground terminal. The ground terminal on the
outside of the housing must be connected to the potential equalisa-
tion (low impedance).
If potential equalisation currents are expected, the connection on the
processing side must be made via a ceramic capacitor (e. g. 1 nF,
1500 V). The low-frequency potential equalisation currents are thus
suppressed, but the protective effect against high frequency interfer-
ence signals remains.
Warning:
Considerable potential differences exist inside galvanic plants as well
as vessels with cathodic corrosion protection. Very large equalisa-
tion currents can flow through the cable screen when the screen
is grounded on both ends. To avoid this, the cable screen must be
connected to ground potential only on one end (inside the switch-
ing cabinet) in such applications. The cable screen must not be
connected to the internal ground terminal in the sensor and the outer
ground terminal on the housing not to potential equalisation!
Information:
The metallic parts of the instrument (antenna, transmitter, concen-
tric tube, etc.) are conductively connected with the inner and outer
ground terminal on the housing. This connection exists either as a
direct metallic contact or via the shielding of the special connection
cable on instruments with external electronics. You can find specifica-
tions on the potential connections within the instrument in chapter
"Technical data".
Take note of the corresponding installation regulations for Ex applica-
tions. In particular, make sure that no potential equalisation currents
flow over the cable screen. In case of grounding on both sides this
can be achieved by the use of a capacitor or a separate potential
equalisation.
5.2 Connection procedure
Proceed as follows:
1. Unscrew the housing cover
2. If a display and adjustment module is installed, remove it by turn-
ing it to the left.
3. Loosen compression nut of the cable entry
Cable gland ½ NPT
Cable screening and
grounding
Single/Double chamber
housing