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Noise Reduction

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 rounded 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

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.

Relative Noise Level

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

I

Single Pair Or Triad

Twisted pair or triad, non-shielded

Multipair/Multi Triad Cable

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

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 Common Mode

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

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