Industrial Automation

How to properly ground an Instrumentation System to reduce noise?

  • Most instrumentation systems in process industries consist of analog signal measurement where noise is an eminent component. 
  • Basically, in major instrumentation systems Analog instrumentation signals are used to control.
  • These analog instrumentation signals are very sensitive to various types of noise.
  • These noise signals may damage the control signals being transmitted for process control.

What is Noise?

  • In a given electronic system, the term Noise is a collection of unconstrained oscillations in the current and in voltage signals     . 
  • Noise is a natural result of the aimless motion of free negatively charged electrons and ions in any conductor for a temperature not equal to absolute zero on the Kelvin scale.
  • In a communication channel, the term Noise is defined as aimless disturbance of a useful information signal. 
  • Noise is a summation of disturbance or unwanted signals from natural or man-made sources
  • Noise also emerges from the thermal motion of the electrons, and also from the quantized nature of the electric charge.
  • In digital communication and information theory, noise refers to random variations in digital data or signals that can result from channel imperfections, signal degradation, or errors introduced during transmission. This can lead to data corruption or loss.
  • In the context of measurements and instrumentation, noise refers to any unwanted and random variations in a measured signal that can reduce the accuracy and precision of measurements. It can be caused by factors like sensor limitations, electronic components, or environmental interference.
  • In telecommunications and information theory, noise refers to any unwanted signal or interference that degrades the quality of a transmitted message. This can include channel noise, background noise, or interference from other sources.

Sources of Noise

  1. Capacitive coupling
  2. Inductive coupling
  3. Ground Loops noise
  4. Impedance Coupling (or Conductance Coupling)

Outline of Signal Grounding in electronics and instrumentation systems for Noise Reduction

  • The National Electrical Code (NEC) permits an isolated equipment grounding conductor to issue a noise-free, zero-volt reference in microprocessor-based electronic systems.
  • The Isolated Equipment Grounding Conductor (I-EGC) originates from the main bonding jumper or neutral-ground junction point or an individual derived source to isolated containers
  • The Isolated Equipment Grounding Conductor (I-EGC) green cable with yellow stripes is installed with the Phase, Neutral, and Ground.
  • Green cabled conductors located in the same channel pass through subpanels without linking to the ground bus. 
  • The equipment control room receives power signals from a dedicated isolation transformer, but for common-mode noise attenuation, this isolation transformer should be shielded properly.
  • The primary function of the Isolated Equipment Grounding (IEG) Conductor is to issue a distinct energy source at the nearest point to the electronic equipment and isolate it from remaining energy sources.
  • The transformer’s neutral-ground bond works as a single point source for every earth connection located in an electronic equipment room.

A Practical Arrangement to Reduce Common-mode Noise

How to properly ground an Instrumentation System to reduce noise? 1
  • The figure shown above represents a practical arrangement to supply power to plug-in electronic equipment such as a set of Personal Computers & peripherals that uses the principle of Isolated Ground IG.
  • This arrangement furnishes an acceptable common-mode noise rejection level.
  • A shielded isolation transformer is used as a power source placed near the panel board to minimize the required length of cable, & noise.
  • The transformer’s shield refuses a lot of high-frequency noise.
  • According to the National Electrical Code (NEC), the isolation transformer behaves as an individual-derived system unit.
  • But in this, the neutral signal is not conveyed from the input to the output demanding a neutral-to-earth bond at lower voltage. 
  • This neutral-to-earth bond gives zero-volt reference to the electronic equipment.
  • The isolated EGC runs directly from the isolated ground-type container to excite electronic equipment to the isolation transformer is a neutral-to-earth bond through the channel. 
  • The EGC may also run via subpanels without linking to the earth bus of the equipment metal frame. 
  • The metal conduit carries the isolated EGC to the enclosure at both ends. 
  • The NEC requires all cables like Phase, Neutral, and Isolated EGC arising at the secondary winding the green wire is used to run in the same channel.
  • This arrangement minimizes difficulties in a connection between the peripheral device and the personal computers’ grounding system via the data wire’s shield.

Important Tips to Reduce Noise in Instrumentation Systems

  1. Shielded Cables
  • The right use of shielded cables minimizes common-mode electrostatic noise. 
  • When a shield gets surrounded by a signal wire then the signal cable gets coupled capacitive only to the shield but not to conductors outside the shield.
  • Now, these function      in both ways, to keep electrostatic noise away from conductors within a shield cable but can also be used to keep electrostatic noise contained within the shield.
  • The shield must be properly terminated while using shielded cables.
  • Usually, for the signals within a shield, only one terminal of the shielded cable is terminated at a zero-signal reference potential.
  • The shields can be terminated at both ends by ensuring no difference in potential between either end of the shield.
  • In case there is a difference in potential between either end of the shield a ground loop gets induced.
  1. Twisted Pair Cables
  • An application of twisted pair cable eliminates normal mode noise as a pair of conductors is equally exposed to an electromagnetic noise source.
  • A twisted pair cable reduces the magnetic field developed by the current in the conductors. 
  • The magnetic fields for the conductor in a pair get canceled out of each other because each and every conductor in a pair conveys an equal current in the same direction but not in opposite directions.
  1. Signal Isolation
  • Some systems require several ground planes due to the system’s physical size to isolate noisy devices from the system. 
  • It is essential to verify that the signal cabling must not generate a return path between these two ground planes of different potentials proportional to ground generating a ground loop.
  • To develop the potential to create a ground loop Isolation is provided for all instruments 
  • Control systems such as PLC modules or DCS modules having on-board isolation or an external device must only be used.
  • Verify that each and every device can grip the potential difference between the two ground planes
  • The isolator has enough bandwidth to isolate analog signals.
  1. Differential Measurements
  • Differential measurements eradicate any common mode noise disturbances, & take advantage of twisted pair cabling
  • Generally, designing an instrumentation system using differential measurements is the best option 
  • In the differential amplifier, the Common Mode Rejection Ratio (CMRR) determines the introduced error rate by common-mode voltages.
  • This Common Mode Rejection Ratio (CMRR) varies with change in frequency.
  • The Common Mode Rejection Ratio (CMRR) for instrumentation amplifiers ranges between 70 dB to 120 dB at 60 Hz.
  • The selection of cable with actual impedance is essential to deal with high-speed differential digital signals for transmitting the signal with maximum power such as RS-485, & Ethernet.
  • A cable with an appropriate impedance causes the square wave shoulders to round off and maximize the probability of errors during signal transmission.
  1. Grounding
  • During the designing and installation phase, Establishing accurate grounding &      bonding is essential to minimize or eliminate unwanted noise issues in the Data Acquisition system. 
  • The ground plane also reduces and confirms that all circuits within a system have equal potential to compare various voltage signals.
  • In most signals this ground plane exists as infinite ground potential, here the potential for all frequencies is equal on each and every point on its surface.
  • Grounding establishes a low impedance ground fault remitting path to the power source required to energize the overcurrent protection device.
  • To achieve grounding of all components, the cables and other adornments used to obtain data are effectively affixed to the ground plane and finally to the system earth ground to impart a return path for any ground faults. 
  • Grounding imparts a zero potential reference to reduce electrical noise from High Radio Frequency sources.
  1. Wire Routing
  • Wire routing should be contemplated to retain high voltage power, & motor wiring noise from influencing low voltage I/O & signal wiring.
  • For better signal integration, Separating and partitioning signal cables from AC power cables from a Variable Frequency Drive (VFD) or servo drive is essential.
  • This partitioning can be achieved by segregating low-voltage cables from high-voltage cables via free air, or by using individual cable trays or channels.
  • For this partitioning, we can use cable tray dividers, steel divider panels, and special ducts precisely intended for this purpose. 
  • Signal or AC cables must be crossed at right angles, here parallel runs, & overlapping runs of mixed-category cables must be minimized.
  1. Anti-Aliasing Filters
  • To determine the sampling rate for a given control system, 
  • The sampling rate must satisfy the Nyquist sampling theorem, such that the sampling rate must be 2X times the maximum rated frequency.
  • Aliasing can be prevented by ensuring that the signal does not contain any frequencies above 50% of the sampling rate.
  • Anti-aliasing filters can be used in software by using oversampling or on the front end of the control system modules.
  • These anti-aliasing filters minimize the possibility of aliasing and eliminate the noisy signals of frequencies having cutoff frequency more than anti-aliasing filters.
  1. Temperature and Humidity Control
  • Maintain a stable environment for your instrumentation system, as temperature and humidity variations can introduce noise and affect measurements.
  1. Proper Signal Grounding for Sensors 
  • Pay special attention to grounding for sensors, as they are often the most sensitive part of an instrumentation system. 
  • Consult sensor datasheets for grounding recommendations.
  1. Proper Power Conditioning
  • Ensure a stable and clean power supply for your instrumentation system. Use power conditioning equipment, like voltage regulators and surge protectors, to mitigate power-related noise.
  1. Consider Environmental Factors
  • In some cases, external environmental factors like nearby machinery or radio transmissions can introduce noise. 
  • Consider these factors when designing and locating your instrumentation system.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

Related Articles

Back to top button