How to Ground a Process Transmitter For Pressure, Flow, Level & Temperature Measurement

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Improper grounding of a pressure, flow, level or  temperature transmitter could compromise the successful  operation of the device. Most transmitters will operate with the current signal loop either floating (ungrounded) or grounded.

Ungrounded or Floating Process Transmitters
In a floating system, the extra noise affects many types of readout devices. If the signal appears noisy or erratic, grounding the current signal loop at a single point may solve the problem. The best place to ground the loop is at the negative terminal of the power supply. Do not ground the current signal loop at more than one point.

Grounded Process Transmitters
Because of noise, the majority of process transmitters – pressure, temperature, level, flow etc – are required to be grounded for good operation. A transmitter is said to be properly grounded when the signal wiring and transmitter case are grounded.

Transmitter Signal Wiring
The signal wiring of a process transmitter should be grounded at ONLY one point on the signal loop or may be left ungrounded as in floating systems. The negative terminal of the power supply is a recommended grounding point.

For good grounding practice, do not run signal wiring in conduit or open trays with power wiring or near heavy electrical equipment. See How to Reduce EMI in control Loops and Instrumentation Systems .

It is important that the instrument cable shield at the transmitter be:

•  Trimmed and insulated from touching the transmitter housing or casing.
• Connected to the next shield if signal cable is routed through a junction box
•  Connected to a good earth ground at the power supply end preferably at the negative terminal

The above grounding practice is very common in most plants however it pays to use the grounding options recommended by the facility for the specific transmitter type.

Transmitter Case
The transmitter case must be grounded in accordance with national and local electrical codes. The most effective transmitter case grounding method is a direct connection to earth ground with minimal impedance. The transmitter case may also be grounded through the process or conduit connections however this may not provide sufficient ground continuity.

Inside the field terminal side of most transmitters’ electronics housing, you will find the internal ground screw required to ground the transmitter case.  This screw is identified by the ground symbol below:

Transient Protection for Process Transmitters
Good transmitters are generally protected by integral galvanic isolation from potential damage from high voltage induced by welders, motor start¬ers, lightning strikes, switchgear and inadvertent exposure to power lines up to 500 to 700 VAC.

However, lightning strikes and other induced tran¬sient over voltage events can cause spikes and surges at much higher voltage levels. For this reason, additional protection for receiving devices may be a wise investment for installation areas prone to transient over voltages.

Many transmitters offer transient suppression options that can be integrally mounted onto the terminal strip within the housing. For other transmitters an external protection device may be used. External field mounted suppressors may not carry agency certification for explosion proof applications. A transmitter with an integral style suppressor should be the first choice for high risk applications.

How to Reduce Electromagnetic Interference (EMI) in Control Loops and Instrumentation Systems.

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What is Electromagnetic Interference (EMI)?
Electromagnetic Interference (EMI) means the presence of undesirable interference voltages in a control loop or instrumentation system.  These undesirable voltages are generated by time changing external electric or magnetic fields emanating from such sources as:

• Electric motors

• Transformers

• Power lines

• Thyristors used in certain electronic devices

• High frequency radiation

• Leakage currents due to damaged electrical heaters

• Ground loops in poorly earthed instrumentation systems

Electromagnetic Compatibility (EMC) of Instruments
The ability of an instrument or instrumentation system to withstand or suppress interference is defined as Electromagnetic Compatibility (EMC)

How to Reduce EMI in Measurement and Instrumentation systems
There are a variety of methods that have proven to be successful in combating electromagnetic interference in measurement and instrumentation systems. Some of these methods are briefly discussed below.

EMI Due to Electrical Alternating Current (AC) Fields
The interference due to electrical AC fields can be reduced by adequately shielding the connection wires.

Magnetic Induced EMI
The effect of magnetic induced interference is difficult to reduce using conventional shielding methods. One practical solution that has worked very well is to space the measurement circuit from the EMI sources as far apart as possible. Where interference is still possible after spacing as far as possible, the measurement connection leads should be very close to each other and in parallel if possible.

Twisted pairs or coaxial cables provide good protection against alternating current (AC) magnetic fields interference in instrumentation circuits by shortening the interference sensitive signal path and transmitting the signal over the remaining path by using the milliamps (mA) output signals from a very good transmitter.

EMI Due to Ground Loops
Ground loops are introduced in measurement system due to poor earthing practice. They are caused by currents flowing as a result of the differing ground potentials in a measurement circuit. Ground loops can effectively be suppressed by using grounded metallic sheath for the instrumentation measurement circuit.

Ground Loops and Impedance Coupling: Causes and Reduction

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A ground loop is an undesirable current path in an electrical circuit. Ground loops occur whenever the ground conductor of an electrical system is connected to the ground plane at multiple points. Not only can ground loops induce noise in instrument signal cables, but in severe cases it can even overheat the instrument signal cable and thus present a fire hazard!

The phenomenon of ground loops is illustrated in the schematic diagram below:

There are several causes of ground loops in any instrumentation installation.

Some of them are itemized below:

Inductive Coupling in Analog Instrumentation and How to Reduce It

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When a wire carries an electrical current it produces a magnetic field; if this wire is in the vicinity of another wire also carrying electrical current or signal, the magnetic field they produce interact with one another resulting in noise voltage being induced in the wires. This is the principle through which inductive coupling takes place in instrumentation signal cable wiring

As we already know, Inductance is a property intrinsic to any conductor, whereby energy is stored in the magnetic field formed by current through the wire. Mutual inductance existing between parallel wires forms a “bridge” whereby an AC current through one wire is able to induce an AC voltage along the length of another wire. This become even more pronounced if we have power cables and instrument signal cables going through the same duct or conduit.

Ways to Reduce Capacitve Coupling in instrumentation signals

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If sets of wires lie too close to one another, electrical signals between the wires tend to couple or interfere with one another thereby introducing noise into the analog signal circuitry and corrupting the signals in the process. This can be especially detrimental when the coupling or interference occurs between AC power conductors and low-level instrument signal wiring such as thermocouples or pH sensor cables.

Capacitance is a property intrinsic to any pair of conductors separated by a dielectric (an insulating substance), whereby energy is stored in the electric field formed by voltage between the wires.

Sources of Noise in Analog Instrumentation Signals

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Noise

Many instrumentation systems involve the measurement of analog signals in which noise can be a prominent component. Analog instrumentation signals are commonly used for control purposes in most instrumentation facilities. These analog signals are very susceptible to various forms of noise which if not checked could corrupt the signals being transmitted for control purposes. The obvious result would be poorly controlled  and dangerous systems with very low signal integrity that could potentially be hazardous.