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Ground Skew

Ground skew occurs when a surge of transient energy is applied to a power line to which two or more pieces of networked equipment are attached. The ground skew voltage is the instantaneous voltage difference between any two pieces of equipment. This voltage develops when transient ground currents occur and the impedances of the equipment and power wiring are not identical at all frequencies (illustrated in Exhibit 1-6-12).


Exhibit 1-6-12.  Ground Skew in Electrical Power Line

In a local network segment that connect several pieces of equipment even slight length differences in the wires between the point of origin of the surge and the equipment will cause a voltage skew between the equipment grounds. This voltage stress shows up across the data communications cabling. In severe cases it can damage network adapter cards. Ground skew related voltage stress is aggravated by the widespread use of individual surge suppressors. At present, the only way to avoid ground skew problems is to use electrically isolated data communications connections.

The Physical Model: Typical Building Communications Cabling

Communications lines between two distant elements of a network can complete an electrical connection between two buildings’ electrical grounds. This circuit is called a ground loop and is susceptible to transient ground skew or continuous ground offset.

These occur when different ground locations are at different electrical potentials (either continuously or momentarily) because of wiring problems or transients. The continuous or momentary potentials can cause continuous or momentary currents in the communications lines that are unrelated to the communications signal. Whether this presents a real problem or not depends on the communications topology, a site’s characteristics, and the design of the power treatment devices used throughout the network.

Data Communications Topologies

Communications topologies can be classified as balanced or unbalanced. If the ground connection is used as a return path or if either signal wire connects to ground the system is unbalanced. The two arrangements are illustrated in Exhibit 1-6-13.


Exhibit 1-6-13.  Balanced and Unbalanced Communications Topologies

Ground skew and ground loops are typically not a problem in networks connected with a properly isolated, balanced communications topology. Telephone circuits are an example of balanced communications lines. Balanced communications topologies use a pair of wires for each signal and these are not connected to earth ground. No ground loops occur in networks that use a balanced topology as long as adequate isolation is provided by the network interface card or an external balun.

Standards for Ethernet LANs call for significant levels of AC and DC isolation (2,250 V DC and 1,500 V AC). This is accomplished either through optical isolation or by using baluns. but even balanced communications topologies can absorb transients by mans of external coupling if the electrical isolation is inadequate relative to the magnitude of the surge voltages. For example, in installations that use unshielded twisted pair to connect buildings in a campus network, lightning induced transients can break down the isolation barrier and damage devices on either or both ends of the cable segment.

In balanced communications topologies, ground skew concerns are limited to control of transients. However, for shielded coaxial cables typically used for Ethernet backbones care must be taken to ground only one end of the shield, as called for in the guidelines for this type of cable. If both ends of the shield are tied to grounds of different potentials, the shield completes the ground loop resulting in susceptibility to ground offset and ground skew, which can overheat the cable segment or interfere with communications traffic.

For LANs using RS-232 or other unbalanced communications links (or for certain exceptions to an otherwise balanced system), an awareness of ground problems is invaluable. In unbalanced networks, differences in electrical potential between multiple ground connections are almost always a problem. In these environments, specific installation techniques recommended by the system vendor must be carefully followed.

Simple tests for the diagnosis of electrical power for network installations are described in Exhibit 1-6-14.


Exhibit 1-6-14.  Power Quality Tests for Network Installations


Exhibit 1-6-14.  (Continued)

IMPROVING BUILDING WIRING SYSTEMS

When it comes to designing for maximum reliability of network devices, about all that can be properly asked of building wiring systems is that they be installed and maintained in accordance with the requirements of the National Electrical Code. The managers of well-managed facilities regularly inspect the building branch circuit connections, including those at wall receptacles, junction boxes, and distribution panels. Unfortunately such well-managed facilities are rare and inspections of electrical systems are infrequent.

Because the integrity of an existing building wiring system may be uncertain, a special branch circuit methodology known as dedicated isolated ground (I/G) is often recommended or required by better system integrators as part of a standard site preparation procedure. Most often, this method entails remaking critical branch circuits prior to system installation.

Isolated Ground Circuits

To more properly address the problem of interference from conducted electrical noise on building branch circuits, safe exceptions were added to the National Electrical Code that are intended to provide a ground-noise reduction method for computer system installations. These I/G circuits are usually identified by special orange colored receptacles.

Contractors and electricians sometimes misunderstand the special installation and connection techniques for I/G circuits. And even when they are installed correctly, usually at a premium in cost, I/G circuits are rarely any more free of electrical noise than a properly executed standard branch circuit.

The IEEE Green Book (IEEE Standard 142-1991) is a thorough, up-to-date reference resource on accepted and recommended grounding practices for industrial and commercial power systems.


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