There are a number of items to consider when installing a
permanently mounted vibration monitoring system. One of the most critical
considerations is the selection and installation of the wire connecting the
vibration transducers mounted on a machine to the associated Monitoring
Instrument. Along with this wiring, there are a number of External Monitor
Connections that require consideration. All of this Monitor System
interconnection is often referred to as the "Field Wiring".
If the Field Wiring is not designed and installed in an
appropriate manner, Noise or Line Interference can be induced into the Vibration
Monitoring System. Since the induced noise is normally an alternating waveform,
the Monitoring Instrument will interpret this signal as false vibration. The
Vibration Monitoring System will then not function as a reliable and credible
protection or information system.
Low Level Signals
Vibration monitoring systems
utilize the low level (voltage) output signals available from vibration
transducers. These signals represent the actual vibration or motion of the
machine's shaft or bearing housing.
The following table provides a comparison of vibration
transducer signal levels expected from a machine running at 3600 RPM.
||Transducer Scale Factor
||1080 mV/in /sec
||100 mV/g 10 mV
As can be seen from this table, the signal level available from
these transducers is quite low. Base Line Noise especially in Power Plants, can
be as high as 200 mV if the Vibration Monitoring System is not properly
installed. Since a running speed of 3600 RPM is the same as 60 Hz, any noise
induced on the Vibration Monitoring System by a power source will be interpreted
as 1 times running speed vibration. These transducer signal levels are also
frequency dependent. When the machine's designed running speed is increased, in
general, the expected Displacement level will decrease, the expected
Acceleration level will increase, and the expected Velocity level will remain
Noise or Line
Interference can be induced in a Vibration Monitoring System in a number of
ways. However, there must first exist a source for the induced noise. There are
numerous noise sources available in an industrial or power generation plant:
- AC Power Transients
- Ground Differentials
- Switching Circuits
- High Voltage Circuits
- Improper Load Balance
Noise can be induced in a Vibration Monitoring System through
Electrostatic (Capacitive), Electromagnetic (Inductive) or Conductive Coupling
(Direct Connection). All noise will be induced in the monitoring system through
one or more of its external connections or Field Wiring.
The AC Power source for the Vibration Monitoring System or any
other Electronic Instrument needs to be a "CLEAN" source. This implies that the
source must be free of power surges and transient voltages. A power conditioning
device, such as a Sola Power Line Conditioner, can be installed to alleviate
these power source problems. For installations with a switchable power source,
an Uninterruptable Power Supply (UPS) will be required.
A Straight Blade 3-Wire Grounding Power Receptacle (Nema 5-15R)
is required for the power connection. This AC Power Source should have a
manually operated switch device, or circuit breaker, in line. A separately
sourced Service Power Receptacle should be located near the Vibration Monitoring
System for test equipment power. The Power Source should be checked for the
following voltage tolerances at 50/60 Hz.
110 VAC Instrument Power
|Neutral to Ground
|Line to Neutral
|Line to Ground
220 VAC Instrument Power
|Line-1 to Line-2
|Line-1 to Ground
|Line-2 to Ground
Monitor's Relay Contact Connections are used for annunciation or shutdown, these
driven circuits must be free of noise or voltage transients. If these circuits
present problems, Slave Relays should be utilized to isolate the Vibration
Monitoring System from this known noise or transient sources.
Contact Connections are used to initiate a Monitor's Startup feature, the drive
circuit (dry contact) must be free of noise or voltage transients. Again, if
these circuits present problems, Slave Relays should be utilized to isolate the
Vibration Monitoring System from this known noise or transient sources.
Transducer Instrument Wiring
Instrument Wire from the Vibration Transducer to its Monitor should be either a
twisted pair or triad cables depending on the Transducer's requirement. These
cables should be stranded, individually insulated, shielded, and overall jacket.
The shields or drain wires must be insulated or isolated from each other and the
conduit. The use of multi-conductor cable with a single shield is strongly
discouraged due to its susceptibility to induced noise and line
The gauge or thickness of the Instrument Wire is determined by
the distance between the Vibration Transducer and Monitor. Long lengths of
Instrument Wire acts as a low pass filter, and will attenuate high frequency
signals. This situation can be a problem when monitoring gear mesh frequencies,
blade passage, or roller element bearings with a high frequency
The following table offers a guideline to help select the
proper Instrument Wire gauge (AWG).
||Length of Cable Runs|
The following table is a partial list of Belden Cables that
should be used for the Instrument Wire. These part numbers can be cross
referenced to equivalent cables from other manufactures. These cables are
polyethylene insulated, twisted, with Beldfoil shield and drain wire, and PVC
|Belden Part Numbers|
|Pair Nom. O.D.
||Triad Nom. O.D.|
A color code convention should be used when installing the
Instrument Wire as outlined in the following list:
Red - Power
Black - Common
Clear - Signal
from the Vibration Monitor's Recorder Outputs to any Recording Device should be
a twisted pair cable. These cables should be stranded, individually insulated,
shielded, and overall jacket. Before installing these cables, the manuals for
both the Vibration Monitor and the Recording Device should be reviewed for
proper connections of Signal, Common, and Shield.
piece of hardware for installing Vibration Transducers are Junction Boxes. These
should be located at the machine for mounting the Non-Contacting Pickup's Signal
Sensors and interfacing terminal strips.
A Junction Box is also the transition point of Flexible Conduit
to the Vibration Transducer and Rigid Conduit to the Monitor. The Junction Boxes
should be installed close to the monitored point and within the length of the
Vibration Transducer's extension cable. The Junction Boxes should be mounted in
a convenient location for serviceability. They should not be mounted under
machine skirts or other inaccessible locations when the machine is running.
Following is a list of available CMCP NEMA 4X Junction
|Two Channel (2 Drivers)
|Four Channel (4 Drivers)
|Six Channel (6 Drivers)
|6"H x 6"W x 4"D
|8"H x 6"W x 4"D
|10"H x 8"W x 6"D
Following is a list of available Extension Cable Lengths for
location distance from the associated transducer.
|5mm Eddy Probe (meters)
||5 or 10|
|8mm Eddy Probe (meters)
||5, 10, or 15|
|CMCP1000 Accelerometer (meters)
||5, 10 or 20|
|CMCP1200 Accelerometer (feet)
||16, 32 or 64|
|CMCP420VT(T) 4-20 mA
||Max of 150'|
For a quality
Vibration Monitoring System installation, it is critical that conduit be
utilized on the Vibration Transducer and its associated Instrument Wiring. The
use of conduit greatly reduces the possibility of induced noise or line
interference on the signal path. The conduit system should be dedicated solely
to the Vibration Monitoring System, and no other wiring of any classification
should be in the same conduit.
Cable Trays, Wire Ways, or Instrument Trays are an unacceptable
alternative to dedicated conduit. This conduit must be routed as far as possible
away from any power cables. This is also the case when the Vibration Monitor is
installed in a cabinet. The Instrument Wire for the transducers must be
separated as much as possible from both Power and Relay Contact cables. All
conduit must be installed and grounded in compliance with the appropriate
Articles of the National Electrical Code, in effect at time of installation.
Instrument Wire Conduit and Power Cable Conduit parallel runs
should be avoided when possible. When parallel runs cannot be avoided, the
following spacing should be used.
|Length Of Run
This recommended spacing is based on a 500 Ampere Circuit and
can be adjusted proportionally for other loads. However, a minimum spacing of
(1) foot should be maintained. At conduit cross over locations, a minimum
spacing of one (1) foot should be maintained.
Rigid Metal Conduit
Rigid Metal Conduit (IMC) continuously
bonded made of ferrous (magnetic) material must be used between the Vibration
Monitoring system and the Junction Boxes located at the machine.
Flexible Metal Conduit
Coated Metal Conduit (Sealtite or Liquatite) continuously bonded may be used
from the Junction Box to the Vibration Transducer or machine entry point
provided it is made of ferrous (magnetic) material.
When installing Instrument Wire Conduit, the Conduit must not
be overfilled with Instrument Wires. As a rule, only 40% to 50% of the Conduit's
cross sectional area should be filled with Instrument Wires. This fill ratio
allows easier installation of the Instrument Wires with some future expansion
capability. Following is a cross reference table of recommended cables, conduit
size, and number of cables installed in the conduit.
|Number of Cables per Conduit|
"Single Point Grounding" scheme should be utilized when installing a Vibration
Monitoring System. This scheme of grounding means that all grounds are connected
or tied down at one location. It is highly recommended that for a Vibration
Monitoring System installation the Single Point Ground should be at the Monitor
not at the Machine. On a large machine or where multiple machines are being
monitored, substantial ground differentials (potentials) can be found between
All Instrument Wire shields must be grounded at one end of the
cable, and the other end left floating or not connected. The Instrument Wire
should be grounded at the Vibration Monitoring System. If the shield is not
grounded, the shield will become an antenna increasing induced noise on the
signal path. If the shield is grounded at both ends, it will allow ground
differential (potential) current (ground loop) to flow through the shield
seriously increasing induced noise and voltage transients.
- AC Instrument Power
- Proper Voltage
- Power Ground
- Common to Ground <5 Volts
- System Neutral to Ground = 0 Volts
- Junction Boxes Used and Accessible
- Solid Ferrous Metal Conduit (Monitor to J-Box)
- Flexible Conduit (J-Box to Transducer)
- Instrument Wire
- Individual Wires Used
- Insulated, Shielded, Twisted
- Proper Gauge
- Common Point Ground Scheme
- Shield Grounded One End Only