VEGA VEGAPULS 45 4 … 20 mA; HART compact sensor - Manual (Page 27)

VEGAPULS 42, 44 and 45 – 4 … 20 mA 27
Electrical connection
4 Electrical connection
4.1 Connection and connection
cable
Safety information
As a rule, do all connecting work in the com-
plete absence of line voltage. Always switch
off the power supply before you carry out
connecting work on the radar sensors. Pro-
tect yourself and the instruments, especially
when using sensors which do not operate
with low voltage.
Qualified personnel
Instruments which are not operated with
protective low voltage or DC voltage must
only be connected by qualified personnel.
Connection cable and screening
A standard two or four-wire cable (sensors
with separate supply) with max. 2.5 mm
2 wire
cross-section can be used for connection.
Make sure that the connection cables are
specified for the expected application condi-
tions in your systems. The cable must have
an outer diameter between 5 and 9 mm (1/2
to 1/3 inch), with NPT threads, 3.6 … 8.7 mm
(0.12 to 0.34 inch) and with threaded PG
cable entries, 5 … 10.5 mm. Otherwise, the
seal effect of the cable entry would not be
ensured.
Quite often, the "electromagnetic pollution"
caused by electronic actuators, energy ca-
bles and transmitting stations is so consider-
able that measures against the effects of
electrical and magnetic fields can be neces-
sary. This so-called "electromagnetic pollu-
tion" has increased considerably in the last
few years, caused e.g. by fast-cycle power
supply units and mobile phones, especially in
the high-frequency range. VEGAPULS radar
sensors take this into account. They are
insensitive to electromagnetic pollution.
In critical systems, the signal cables are the
source of the problem. The signal cables
often act as antennas that pick up interfering
signals. The 4 … 20 mA signal lines are af-
fected by earth equalisation currents and
especially by current peaks in the ms or µs
range (more so than digital signal lines). This
can be avoided with sophisticated wiring, of
which screening at both ends is a major
feature.
Circumspect system planning, however, will
take into account possible sources of interfer-
ence from electromagnetic pollution. Due to
the complex interrelationships, it may be
difficult to decide whether measures against
such interference should be taken, and if so,
which ones. And in fact, it is extremely difficult
to describe in theoretical terms the actual
forces at work, since the effects depend
greatly on the frequency of the interfering
magnetic fields: what is very effective for one
frequency can have completely opposite
effects for other frequencies.
Experience has shown that even some rela-
tively simple measures can protect the signal
current circuits against electromagnetic influ-
ence. As one of the more costly measures,
screening usually comes at the end of any
catalogue of preventive measures (interfer-
ence suppression).