Control of “Noise” in Instrumentation Circuits

Precaution should be taken during design, engineering and installation to reduce the effects of noise. The type of signal transmitted by the sensor is related to its sensitivity to noise. The lower the voltage level and the higher the impedance of a circuit, the greater the circuits sensitivity to noise of all types. The following discussion describes the major types of “noise” and commonly accepted solutions for each.


 
 
Control of “Noise” in Instrumentation Circuits


 
Precaution should be taken during design, engineering and installation to reduce the effects of noise. The type of signal transmitted by the sensor is related to its sensitivity to noise. The lower the voltage level and the higher the impedance of a circuit, the greater the circuits sensitivity to noise of all types. The following discussion describes the major types of “noise” and commonly accepted solutions for each.


Common
Mode
A result of different ground potentials at each location in a process plant. Noise created by current flow between grounds. Occurs even with high common mode rejection when shields are improperly grounded. Is particularly critical with thermocouple extension wire circuits.

1. To protect against common mode noise pickup within the wire and cable, a shield circuit
should be grounded at the point which the instrument circuit is grounded and isolated from
all other grounds; i.e., with a grounded couple, ground the shield on the extension wire at
the couple. As the shield circuit is carried back to the control room through a junction box
and a multipair cable, connect the pair shield in the cable to the single pair which leads to
the couple without grounding the shield in the junction box or connecting it to any other
shield (on other pairs). The shield should not be grounded in the control room.

2. Ground all shields. An ungrounded shield will not provide noise protection.

3. Ground a shield at one point only.


Cross
Talk
Occurs with ac instrument signals, especially pulse-type signals where more than one circuit is carried in the same cable. It is the tendency for a signal to be coupled from one pair to another within the cable, resulting in noise being superimposed on a circuit. Cross talk noise may be eliminated by the use of cables with individually shielded, isolated pair shields. The pair shield protects against noise picked up from adjacent pairs, as well as reducing noise radiated by the pair it surrounds


Static
Caused by the electric field radiated by a voltage source being coupled capacitively into the instrument circuit. The best way of fighting static noise is to place the circuit inside a total coverage shield which isolates the pair of wires from outside influence. The grounded shield intercepts static interference and carries it off to ground. The shield must be grounded in order to reduce static noise; an ungrounded shield will not reduce noise.


Magnetic
Produced by currents flowing through conductors and pieces of electrical equipment such as motors, generators, etc. As the current flows through equipment, a magnetic field is radiated around the conductor. As this field passes through the space between the conductors in a circuit, a current is set up in the circuit to oppose the magnetic field (transformer action). This current causes a noise to be superimposed on the signal in the instrument circuit. The best way of compensating for this type of noise is to twist the wires in the instrument circuit. Twisting causes the noise to be cancelled in adjacent sections of the wire. This is the least expensive, most effective way of combatting magnetic noise.


 
       
Environment vs. Relative Noise Levels


Environment Relative Noise Level


Wiring located far from power lines, motors; motors less than 5 hp;
no induction heating, arcs, control or power relays nearby:
tank farms, material storage areas, light process plants, blending
operations, fall into this classification.
Low


Instrument wire run near medium sized motors, control relays:
the average process plant falls into this classification.
Medium


Electrolytic processes, large motors, generators, transformers,
induction heating, relay controls, power lines or control wire nearby:
heavy industry, metals, utilities, fall into this classification.
High
 
     



Class of Wire Single Pair Or Triad Multipair/Multi Triad Cable



I Twisted pair or triad, non-shielded Overall shield, individual pairs
or triads twisted but not shielded



II Twisted pair or triad, shielded Overall shield, individual pairs
or triads twisted but not shielded



III Twisted pair or triad, shielded Overall shield, individual pairs
or triads twisted and shielded



 
          Process Instrumentation Wire and Cable Selection Guide    
         





Signal Type Sensor Type Noise
Sensitive To
Noise
Environment
Level
Wire Class





DC, low level
<100 mV
Thermocouples Static
Magnetic
Common Mode
Low
Medium
High
Class III
Class III
Class III





DC, low level
<100 mV
Bridge circuits,
thermistors, RTD’s,
chromatographic ph,
magnetic flow meter
Static
Magnetic
Common Mode
Low
Medium
High
Class III
Class III
Class III





DC, medium
level 100 mV-5V
Analog computer
outputs
Static
Magnetic
Low
Medium
High
Class II
Class III
Class III





DC high level
75 V
Retransmission
potentiometers,
annunciators
alarms
Static
Magnetic
Low
Medium
High
Class II
Class II
Class II





AC low level
<1OO mV
Bridge circuits,
“carrier”
transducers
Static
Magnetic
Common Mode
Cross Talk
Low
Medium
High
Class III
Class III
Class III





AC medium level
1000 mV-5V
Turbine flow meters,
tachometers
Static
Magnetic
Cross Talk
Low
Medium
High
Class III
Class III
Class III





AC high level
75 V
Annunciator
pick-up circuits
Static
Magnetic
Cross Talk
Low
Medium
High
Class III
Class III
Class III





Current Systems 1-5 mA
4-20 mA
10-50 mA
Force balance.
P/I transducers,
differential pressure
flow meters
Magnetic Low
Medium
High
Class I
Class II
Class III