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Communication Grounding System: Best Practices

08.23.2013

Authored by:

Khosrow Haratian

CCG Director of Technology Planning

With respect to communications systems, here narrowly defined as computer network system, grounding became a concern when IEEE 802.5 (Token Ring) and 802.3 (10Base-5 and 10Base-2) were being deployed in 1980s.  They required shielded cables (STP and coaxial, respectively).  However, by the advent and wide spread utilization of Ethernet over twisted pair (802.3i, 802.3u, etc.) in which shielding was not required, grounding was relegated to rack, cabinets, and pathways as matter of routine following the established standards. 

However, to satisfy the ever-increasing desire for higher transmission speed, two media take the center stage, optical fiber and shielded cables.  Optical cables are immune to EMI, but when shielded cables are deployed, the grounding once again becomes important. 

Although there are published standards for grounding, it is important for the communications system designer to have a comprehensive understanding of the grounding system, its purpose, and appropriate applications, since published standards do not apply uniformly to all situations.  For example, the grounding requirement of a low frequency audio system differs from that of hi-frequency video systems, and if they are deployed in the same cabinet, the “standardized” approach may not be appropriate.

The design of grounding systems is the purview of the electrical engineer, as part of larger electrical system.  Communications grounding is a subset of the overall design, and it is not unusual that its particular requirements go unnoticed.  At CCG, the coordination efforts between the two departments, Electrical Engineering and Communications Engineering integrates these two grounding requirements into a single design.  This effort, however, is not limited to grounding, it includes all low-voltage and system communications requirements of the project, e.g., controls, lighting, security, etc.

A Short History

Initially, grounding requirements were developed as a mean of protection against electrical hazards, by providing a low impedance path to earth without any regard to Electro-Magnetic Interference (EMI) problems.  However, after the introduction of electronic equipment, widespread grounding problems aroused.  This was because equipment and system ground provided an environment for unwanted EMI coupling.  To avoid EMI problems, it was realized that the grounding system must be designed with the same level of due diligence as other segments of the system or facility.  Unlike the power system, grounding in electronic devices may perform various functions at the same time, e.g., signal return, antenna, EMI suppuration, as well as safety. 

Characteristics of Grounding Systems

In an ideal condition, a grounding system provides a zero-impedance pathway.  Under this condition, the returning ground currents of differing circuits, equipment, or systems flowing on the ideal ground will not cause undesired voltages between the circuits or equipment.  However, in the real world, grounding paths are not zero-impedance and interference problems do occur.  This is because ground paths are often assumed to be in an ideal condition and actual grounding characteristics and conditions are not considered.  It must be noted that often the ideal ground condition is a valid assumption since, from the standpoint of system design, the impedance at power or signal frequencies is insignificant and has very little or no impact on circuit or equipment performance.  However, to avoid the EMI problem the actual ground characteristics must be considered.

Impedance Characteristics

Impedance can be thought of as friction in an electrical system.  It is a function of resistance, capacitance, and inductance.  Grounding conductors (segments) like any other electrical conductor, have resistance, capacitance, and inductance properties.  The resistance of a ground path conductor (like any other conductor) is a function of the specific electrical resistivity of the material, its length, and its cross-sectional area.  The capacitance of a conductor is function of its geometric shape, distance to other conductors, and the characteristics of its dielectric sheathing.  The inductance of a conductor is a function of its size, shape, length, and, the relative magnetic permeability of the conductor’s immediate surroundings. 

Ground-Related Interference

Interference is any unwanted electrical or electromagnetic disturbance that tends to disrupt the reception of desired signals or produces undesirable responses in a circuit or a system.  Interference sources can be either internal or external to the system; it can be from natural or man-made sources.  The proper operation of any system or facility is based upon the frequencies and amplitudes of the utilized signals and the potential interfering radiation emissions.  If the frequency of an undesired signal is within the operating frequency range of a circuit, the circuit may respond to the undesired signal.  The amplitude and frequency of the unwanted signal relative to that of the desired signal determines the severity of the interference.  In general, common-ground impedance or EMI couplings are the culprit for causing interference in the grounding systems.

  • Common-ground impedance coupling: It is caused by two circuits that share a common ground (return path).  The impedance of the shared conductor determines the amount of  interference coupling.  It must be noted that the interference current can be an operational current or one that occurs by lightning, load variation, electrical faults, transient power, etc. It is also should be noted that the coupling can still occur even if the ground wire is not utilized for signal return.

  • EMI coupling: Electromagnetic (EM) radiation can induce unwanted current into the grounding system when the ground acts as an antenna.  The resistance or impedance of the conductor is not a significant factor since the induced EMI voltage is primarily related to the function of the operative factors of the EMI, namely field strength, voltage, current, shape of the ground loop, and the frequency of the EMI signal. It should be note a ground loop is a common accordance in both types of couplings.


Choosing the right Grounding Scheme

The grounding system plays a very important role in communications systems.  However, due to improper design, grounding is often the cause of many of the system level EMI problems.  Grounding system configuration can be single point connection, multi-point connection, floating (isolated), or a combination of them (hybrid).  Each configuration has its advantages and disadvantages.  The grounding scheme has to be chosen based upon the characters and requirements of system it serves.  The next installment will discuss the differing grounding schemes.

To download, please click CCG Insight_ Communication Grounding 2013-08-23.pdf.