Last updated on July 28th, 2026 at 02:43 pm
Improper instrumentation grounding and shielding can cause measurement errors and degrade reliability in instrumentation and control systems. An understanding of these factors is essential for both system design and installation.
How Many Grounds?
The term ground is used in several ways. It can be a direct physical connection to the Earth, a common return path for electric current, or a reference point in an electrical circuit from which measurements are made. The IEC defines 5 symbols to distinguish the various grounds. 5017 is the general symbol for an earth ground terminal. 5018 identifies a noiseless or clean earth ground terminal specially designed to avoid causing malfunction of equipment. 5019 designates a protective earth (safety) ground terminal to be connected to an externally earthed conductor (e.g. green or green/yellow wire) or a terminal that is connected to an earthed conductor. 5020 is the symbol for a frame or chassis terminal. 5021 identifies an equipotential or common connection point.
Many legacy systems are documented using three basic ground symbols. No distinction is made between protective and clean earth connections. Common ground is identified with a simple triangle. However, performance issues in measurement and control applications often require the use of more than one common ground. These are identified by a letter inside the triangle, such as A for analog and D for digital, or S for signal and P for power.
The need for separate signal and power grounds is illustrated by this example. A measurement system, transducer and dc power supply are all referenced to a common ground. It may be represented schematically in the left diagram. A single-point (star) grounding scheme is often recommended to eliminate ground loops, as shown in the second diagram. However, this configuration can cause a measurement error due to voltage drop in the lead shared by the transducer and power supply commons (red). If the measurement system draws 1 amp, the transducer has a 0-1 V output and the shared lead is 1m of #18AWG wire, the 21 mV drop in the lead creates a 2% measurement offset. If current drawn by the system varies (e.g. due to a relay closure or communication port traffic), the offset, and therefore the error, changes and cannot be nullified by an offset correction. Depending on the frequency of the current change, it may appear to the measurement system as a noisy or drifting transducer output. Ideally, the measurement system has a 4th terminal for power common and the three elements are wired using separate power and signal grounds. If not, the common connection from the power supply should be made as close as possible to the measurement system’s common terminal to minimize the I2R drop in the shared lead.
Another way to avoid this type of error is to use transducers with a current output. 4 to 20 mA is a popular choice. Any voltage drop in the common lead adds to the burden that the transducer output must drive, but does not affect the measurement accuracy.
What about Ground Loops?
Ground potential difference of several volts can develop between grounded electrical devices only a few feet apart. For small systems operating at low frequencies, ground loop effects can be eliminated by using a single ground point. This is not achievable in large systems. With many devices referenced to ground within a typical industrial facility, multiple ground points are inevitable. The addition of signal interconnection cables to a system where equipment enclosures are already grounded also can create ground loops. Unfortunately, these loops may adversely affect system performance.
A ground loop is created when two points of a circuit are intended to have the same ground reference potential but instead have a potential between them. This is typically caused when enough current (dc or ac) is flowing in the connection between the two ground points to produce a voltage drop. In measurement and control applications, ground loops can cause unstable readings, data communication errors and control malfunctions. Lowering the impedance between multiple ground points reduces the ground loop effect. For example, a CPU board in a computer typically uses one layer as an equipotential digital ground plane. In a cabinet, a heavy bus bar may be utilized as a low-impedance power common or earth ground. However, low impedance between ground points is difficult to achieve in systems with multiple equipment housings or elements widely separated in an industrial plant. A ground loop on earth connections is shown in the diagram. Ground loops can also occur between multiple signal grounds or chassis grounds.
When a low impedance ground cannot be achieved, one method to eliminate a ground loop is to place a signal isolator in the path between two system blocks. This device provides galvanic separation between input and output by inserting an optical or magnetic barrier that breaks the ground loop. The signal of interest is passed across the barrier while the circulating current is blocked. Care must be taken in system design to ensure that power supplies to the system blocks do not provide an alternate ground loop path. If externally powered, a signal isolator typically provides three-way galvanic isolation: input to output, input to power, and output to power. The isolation function can also be combined with a transducer function to simplify transmitting a signal across the barrier. For example, an isolated temperature transmitter converts the non-linear mV signal from a thermocouple into an isolated, linear temperature output.
Does Shielding Help?
A shield is a conductive or magnetic barrier that blocks electromagnetic interference (EMI) from entering or exiting a cable or device. Shielding cannot be used to reduce the dc current flowing in a ground loop. It also does not affect conducted ac currents, regardless of frequency. Shielding can reduce induced ac currents if properly configured. For low-frequency signals (<1MHz), the standard rule is to earth ground the shield at only one end (typically at the power source or controller) and leave the other end floating. This prevents a closed circuit (ground loop) from forming through the shield. If a cable’s shield is grounded at both ends, any small voltage difference between those two grounding points will cause an electric current to flow through the shield. These circulating currents add noise to the signal. For systems where the shield needs to be connected at both ends (e.g. digital communication cables), a capacitor is sometimes used at one end. This blocks direct and low-frequency currents (preventing ground loops) while still allowing high-frequency interference to pass to the ground.
Frequently Asked Questions
What other methods suppress high-frequency ground loops?
Single-point grounding is effective in systems monitoring dc and low-frequency signals (e.g. line frequency & harmonics). At higher frequencies, a single-point ground scheme is not sufficient. As frequency increases, the inductance of each ground wire becomes significant. Short wires to a ground plane are required to reduce ground loop effects. Circulating currents induced in signal circuits can be reduced by minimizing the loop area (e.g. by using twisted pair wiring). Data transmission circuits, such as RS-485, also recommend inserting a series resistor to reduce the effect of any difference in signal ground potential between transmitter and receiver.
Should the common ground always be connected to earth ground?
In many small systems, the earth, chassis and common grounds are tied together at a single point. For safety, chassis ground is usually connected to earth ground. Depending on the equipment, circuit configuration and operating voltages, the common ground may be connected to the chassis/earth or isolated from it (floating).
How are protective and clean earth grounds used?
In the US, the National Electrical Code (NEC) describes requirements for safe grounding of electrical equipment in various locations. Generally, NEC (and IEEE Std 142) requires all exposed metal in electrical or electronic systems to be connected to earth and specifies construction methods that ensure the ground path is adequate to carry the expected fault
current. Industrial systems may include motors, motor drives and power devices that inject conducted noise onto earth through their protective ground. This noise may be of sufficient magnitude and frequency to disturb instrumentation and measurement equipment. To avoid this interference, the noiseless earth connection establishes a parallel path to the earth electrode for these devices. IEEE Std 1100 has more information on powering sensitive electronic equipment.
Summary
When designing measurement and control systems:
– Structure the earth, chassis and signal grounds to meet safety and performance requirements.
– Avoid or mitigate ground loops that cause circulating currents.
– Use shielding to reduce EMI.
Industrial systems often include multiple grounds that can affect instrumentation and measurement equipment. Careful design and installation of grounds and shields are essential to ensure safe, accurate, and reliable system operation. At Weschler Instruments, our focus is Measurement and Control products for industrial applications. Contact us for assistance in selecting the meters, sensors, transducers and signal conditioners for your instrumentation system.
