Grounding and Shielding Tips

Introduction:

When designing electronic equipment or systems, it's important to plan ahead. Be sure to plan a method of noise abatement before designing your machine, rather than trying to problem-solve EMI issues once the machine is built. Take some time to understand the coupling mechanisms of electrical noise that create electromagnetic interference in your system. To avoid electrical noise issues, all aspects of the machine design must be taken into consideration.

Attempts at noise abatement often have unexpected, even counterintuitive, results. Especially when more than one source of noise exists in a system, proper shielding may be more difficult to achieve, and initial fixes may not be as effective as expected. Accurately understanding the coupling mechanisms leading to the noise problems within the system will lead to better and more predictable results. For this reason, it is critical that motion control systems are installed and wired by professionals with a deep understanding of electromagnetic principles.

Grounding:

Ideally, machines and systems will be designed with the best grounding methods planned in advance. Grounding is the connection to the grounding-electrode system to place equipment at earth ground potential. Proper grounding will go a long way toward ensuring the safety of operators and system components.

Grounding is important for two reasons:

1. To prevent hazards to personnel in case of a breakdown between current electrical components and the exposed metal surfaces.
2. To minimize the effects of electrical noise on the control system.

There are different methods of grounding. In single-point grounding, multiple pieces of equipment are connected to a single ground. In multi-point grounding, each piece of equipment has its own ground connection. Whatever the method, the goal of grounding is always to achieve the lowest impedance possible for each grounding connection.

Single-point grounding is typically only used when the length of the ground wire is less than 1/20 the wavelength of the operating signal; otherwise, use multi-point grounding, especially when operating frequency is high (above 30 MHz).

There are two types of single-point grounding: series and parallel. For low-frequency circuits, it is better to use parallel single-point grounding, as series single-point grounding generates common ground coupling.

Grounding Tips:

Local codes and ordinances dictate which bonding and grounding methods are permissible in a given region. For U.S. installations, the National Electrical Code (NEC) will provide the requirements for safe bonding and grounding, such as information about the size and types of conductors, and methods of safely grounding electrical components.

Perform an inspection of the plant or factory's grounding and power systems prior to installation of any motion control systems, automated machine tools, or similar equipment. A Dranetz line analyzer is a trustworthy tool for this pre-installation inspection.

A utility ground, such as a cold water pipe or the metal frame of a building, is typically an adequate ground for safety purposes, but is likely insufficient for minimizing the effects of electrical noise.

A separate earth ground should always be used to ground a computer-controlled machine tool, or motion control system that uses drives, controllers, PLCs and/or HMIs. Connect only one such machine or system per earth ground. The earth ground may consist of a driven rod, driven pipe, buried plate, or any such device approved for this purpose.

The dimensions of the earth ground rod should be determined by the length required to reach the water or moisture table in the subsoil. Preferably, earth rods – also known as ground rods – should be located where saltwater can periodically be poured down the side of the rod. Keep earth rods out of oily areas.

The cable connecting the control panel's ground point to the earth rod should be continuous, as short as practical, and at least as large as the conductors used to connect the electrical power to the machine tool or process line.

Power line disturbances should not exceed + or - 15% of the machine's, or motion control components', specified power requirements.

Shielding:

Shielding describes the practice of reducing EMI emissions and noise coupling in electrical systems and components using barriers made of conductive or magnetic materials. Shielding can involve either the containment of EMI emissions from the source, or the shielding of critical circuits to prevent noise from leaking in.

Start by identifying the source of the noise, the receiver, and the coupling medium. Different shielding techniques will be needed depending on the noise source, the coupling channel, and the receiver. Inaccurate identification of these factors will only complicate things further.

Designers must understand the ground system and make all connections correctly. Shielding measures may confine EMI emissions, but captured noise can cause issues if its return path to ground is not planned carefully.

1. Know how the captured noise current will return to ground.
2. The return path should be as short as possible to minimize inductance.

Noise that results from an electric field requires the use of electrostatic shields.

• Connect electrostatic shields to the reference potential of any circuitry contained within the shield.
• If the signal is earthed or grounded, the shield must also be earthed or grounded.
• Each signal within a system should have its own shield, with no connections to other shields in the system, unless they share a common reference potential.
• Do not connect both ends of the shield to ground, as this will cause a shield current. Shield current will induce a noise voltage in the center conductor via magnetic coupling.
• Grounding cable shields at one end is standard advice for avoiding ground loops, but it is not apt advice for every scenario. For example, a cable routed over a long distance may cross different ground potentials, creating a noise current flow in the shield. This situation would require high-frequency shielding, in which the shield is grounded at both ends. When grounded at both ends, avoid using the shield as a return path.
• Follow the manufacturer's wiring diagram closely.
• Do not use a "pigtail" to connect the shield, as this will render the shield ineffective. Connect the shield on all 360 degrees of the cable.

Noise resulting from a magnetic field will induce voltages in a conductor or circuit. Magnetically coupled noise is more difficult to shield against, because it can penetrate conducting materials.

• At low frequencies, minimize the strength of the magnetic field, minimize the receiver loop area, and minimize coupling by optimizing wiring geometries.
• Locate the receiving circuits as far as possible from the source of the magnetic field.
• Avoid running wires parallel to the magnetic field, and instead, cross the field in right angles.
• Shield the magnetic field with a material appropriate for the frequency and field strength. Conductive coatings, plastics, and other shielding materials may not perform as expected. Different materials will be more or less effective at shielding different types of interference – whether plane wave, electric field, or magnetic field. System installation should be performed by a professional with a fundamental understanding of electromagnetic principles.
• An all-metal enclosure is not always sufficient to shield equipment from EMI. Cables entering or exiting the enclosure may radiate noise, and the seams and apertures in the enclosure may make the enclosure leaky. Be sure to limit the size of enclosure seams, taking particular care to avoid openings around the size of the half-wavelength of the problem frequency.
• Use a twisted pair of wires for conductors carrying the high-level current that is the source of the magnetic field.
• The voltage induced by magnetic coupling can be reduced by reducing the area of the receiver loop.
• Reduce the loop's area by decreasing the length of conductors, or reducing the distance between them.

Power-line Filters

Power-line filters often don't perform as expected. One reason for unexpected results may be a lack of understanding about datasheet specifications. Many commercially-available power-line filters are made with a common-mode (CM) inductor, and a capacitor for differential-mode (DM) noise, so that only the capacitor is effective against DM noise, and only the inductor is effective against CM noise. It is important to determine if the level of noise mitigation specified in the datasheet applies to CM noise or DM noise, and to further understand how that applies to your system.

Additionally, a power-line filter is unlikely to perform the same when tested alone with a power supply vs. when mounted in a system enclosure. Both the filter components and the input connections to the filter are susceptible to noise if either the power-line filter or input power lines are unshielded. This is especially true if the filter components are mounted near noise sources, or if the input power to the system is routed far from the load.

To avoid power-line filter surprises, filter testing should be customized for the EMI test source impedance, using the real switching power supply to be used with the product, at the expected current. To ensure power-line filters meet EMC (electromagnetic compatibility) requirements, they should be shielded and mounted on the enclosure wall, with the input power connector mounted to the filter enclosure. Any filter with a connector should be mounted inside the enclosure with the connector facing out. If this is not possible, mount a shielded filter as close as possible to the input power connector, and use shielded wires to connect the filter and connector.

EMI Troubleshooting Example

Consider a scenario in which a power output connection is magnetically cross-coupled to the data line from a servo controller. In this case, high speed noise would be coupled into the data line, causing an EMI problem. The data line must be single‐ended or the differential output will reject the noise.

The best method for noise abatement in such a scenario is to avoid cross-coupling by taking both the noise source and the receiver into consideration. The power output and the controller data line should be laid out so that individual conductors are surrounded by magnetic shields – one for power and one for ground – grounded on both ends. Shielding both the emitter and the receiver is recommended if the cable cannot be relocated.

It is best that the grounds are near the inputs and outputs at the ends of the signal. If this cannot be done, a braided shield grounded at one end is recommended over no shield at all. Because servo controllers emit powerful PWM signals, un-terminated shields should not be used in a case like this.

It is necessary to ground the magnetic shield. However, low frequency can sometimes result in “ground loops,” currents generated between two ground points in single‐ended signals. A common solution that does not result in the redesign of a system is using a multi‐shield connector. The outer shield is grounded at both ends, and the inner shield is ground-referenced at the source of the signal.

Installation Questions?

If you have any questions about grounding or shielding your motion control system not answered in this guide, our application engineers are ready to offer their expertise!