Red Lion ILS User Manual
Page 2
Most electronic equipment designed for use in industrial environments has
a high degree of noise immunity and protection against damage. But even the
best can experience difficulties in operation if certain minimal considerations
are not adhered to when installing the equipment. When relay contacts are used
to switch inductive loads, such as auxiliary relays or solenoids, extremely large
voltage spikes can be generated when the relay contact opens, these voltage
spikes can cause pitting of the relay’s contacts, thereby reducing its usable life.
The internal functioning components of an electronic instrument operate on
a low DC voltage, generally 5 V, and respond to signals as low as 1 V or less.
In contrast, stray voltage spikes in excess of 100 V and some times thousands
of volts can be detected in the industrial environment. These voltage spikes can
be coupled from power lines that are powering equipment that contains S.C.R.
circuitry, or in other ways causes rapid load changes on the AC line. These
spikes can also be coupled from lines that are actuating AC or DC solenoids or
actuators. In other words, any wiring in an industrial application should be
considered a potential noise source.
How can these noise spikes get into the instrument? There are three major
ways that noise spikes can enter the instrument.
1. Noise can enter directly, via the AC power input. It is recommended that
electronic instruments be connected to a relatively clean source of power. If
this cannot be accomplished, there are means of suppressing noise or
isolating the instrument from the noise. These consist of everything from
simple inductive load suppressors (M.O.V.’s) to constant voltage isolation
transformers, depending on the severity of power line disturbance.
2. Noise can enter via the input leads. Here, there are two modes (See Fig. 1)
by which the noise can enter. Normal mode, which means the noise enters
on the input lead, with respect to the instrument common; and common
mode, which means the noise enters on both the input and the instrument
common with respect to earth ground (power line neutral). It is recommended
that sensor input and control input wiring not be run in the same conduit or
raceways with power lines or current carrying control lines. It is also
recommended that these lines be kept away from inductive loads such as
motors, solenoids, relays and contactors. For best results, it is recommended
that two-conductor shielded cable be used to connect these inputs. The
shield should be connected to the input common at the instrument only. In
addition, the input common should only be connected to machine ground
(earth) at one point, preferably a direct connection to the input common
terminal.
3. The third way noise can enter the instrument is via the output lines. This is
one of the most overlooked sources of trouble. When an output is driving an
inductive load, such as solenoids, contactors, or relays; a large noise spike,
several times the supply voltage, is generated every time the output is turned
INSTALLATION CONSIDERATIONS OF ELECTRONIC INSTRUMENTS &
CONTROLS, IN INDUSTRIAL ENVIRONMENTS
off. This noise spike, in addition to physically degrading the relay contact,
can radiate off the output lines and into more sensitive areas of the
instrument. The surest way to alleviate this situation is to suppress the noise
spike. It is best to do it at the noise source (See Fig. 2), to prevent noise
currents from flowing in the output lines. There are several ways to do this.
If it is a DC device, then either a diode or a M.O.V. (Metal Oxide Varistor)
can be placed across the device to suppress it. The greater the current load of
the device, the higher wattage diode required. If it is an AC load, then a
M.O.V. or capacitor and resistor in series can be used. It can be seen that the
output lines can be noise sources and as such should be kept away from the
instrument’s own input lines, as well as the input lines of other instruments.
In addition to the foregoing considerations, care should be taken when
connecting input and output returns to the instrument’s common. When separate
input and output commons are provided, they should not be mixed. When an
output device return is connected to an input common (See Fig. 4), the output
current will flow in the input common line. This will cause a noise voltage to
be present, which can affect the operation of the instrument.
In summary, it is much easier to eliminate problems when building up a
system than after it is installed.
LOAD
+V
IN COMM.
IN
SENSOR
OUT COMM.
NOISE
CURRENT
DC DEVICE
AC DEVICE
Figure 3
Figure 4
IMPROPER GROUND CONNECTIONS
INSTRUMENT
LOAD
MOV
-
+
DC
SUPPLY
RELAY
CONTACT
RELAY
CONTACT
INSTRUMENT
SUPPLY
DC
+
DIODE
-
LOAD
INSTRUMENT
LOAD
MOV
RELAY
CONTACT
RELAY
CONTACT
INSTRUMENT
SUPPLY
AC
R
LOAD
C
AC
SUPPLY
NOISE SUPPRESSED AT LOAD
NOISE SUPPRESSED AT INSTRUMENT
Figure 2
INSTRUMENT
OUT
COMM.
SUPPRESSION
DEVICE
NOISE
CURRENT
LOAD
INSTRUMENT
OUT
COMM.
SUPPRESSION
DEVICE
NOISE
CURRENT
LOAD
INSTRUMENT
INSTRUMENT
IN
COMM.
COMM.
IN
NOISE
SOURCE
NOISE
SOURCE
EARTH GROUND
AC NEUTRAL
Figure 1
NORMAL MODE
COMMON MODE