Insure Network PERFORMANCE with Proper Grounding

When designing a network system, it is important to include proper power and grounding specifications in the design. Far too often, the network designer gives little concern to the power and grounding infrastructure of the facility and focuses only on the network requirements.

03/01/2001


When designing a network system, it is important to include proper power and grounding specifications in the design. Far too often, the network designer gives little concern to the power and grounding infrastructure of the facility and focuses only on the network requirements.

Many designers specify that the installation shall comply with industry standards-National Fire Protection Association (NFPA) 70, Electrical Industry Association/Telecommunications Industry Association (EIA/TIA) 607, Institute of Electric and Electronic Engineers (IEEE) 1100, Fips Pub 195-but they do not go into specific detail as to design and implementation. It is assumed that by simply referencing these standards the project will meet the intent of specifications.

Perhaps the assumption is that the electrical contractor will take care of the electrical power grounding and all the designer needs to worry about is the telecommunications grounding. Such assumptions, however, may result in the network failing to meet performance and reliability expectations, because the actual installers may not be familiar with these standards or properly trained to perform the work.

Power and grounding can have a direct effect on the reliability and performance of a network system by creating problems such as network collisions, broadcast storms, log-in problems and network downtime.

A Network at Sea

A communications network is a common link to multiple systems of a facility. Within a typical network there are connections to the electrical power, telecommunications lines, cable television (cable modems or CATV video cards) and various ground systems. Even larger networks might typically interface with security systems, video equipment, elevators, building controls, uninterruptible-power supply (UPS) systems, power and battery monitors, generators and other essential systems.

While the network can serve multiple functions from a design standpoint, this integrated design can bring disaster if the various systems are at different ground potentials. Any change in ground potential can generate large voltages within the network cabling, damaging communications ports and network interface cards.

To better understand this ground potential problem, consider this old but useful analogy: Imagine the parts of a network as individual boats floating on a calm lake. The boats are tethered together with ropes. People in the boats are asked to throw a ball from boat to boat. As long as the lake is calm, every boat is at the same level, and the people can successfully pass the ball from boat to boat.

But what if a large wave causes the boats to bob? Not only are there problems passing the ball but also the tether lines are being pulled and strained and the connection points are damaged. Since one cannot control the waves-just as one cannot control lightning-the boats are placed on the deck of a larger ship. Even if the larger ship pitches and rolls with the waves, the boats maintain the same potential to each other.

Without an effective ground system, every part of a network can be at different ground potentials, moving up and down in reference to each other all the time. Under normal circumstances a few intermittent and random network problems might occur, but a power surge or lightning storm results in damaged hub ports, routers and network components. Network design can avoid these problems by incorporating a common ground reference for all systems.

Proper Grounding

What is the function of a proper ground system? NFPA 70-the National Electric Code (NEC)-covers the safety aspects of grounding and bonding. NEC Article 250-2 summarizes the requirements for grounding and bonding for power and communications systems:

  • An earth-ground system will limit the voltage imposed by lightning, line surges or unintentional contact with higher voltage lines.

 

  • A ground system will limit voltage to ground of equipment and materials.

 

  • All conductive materials and equipment will be bonded to the supply-system ground to limit potential and provide an effective path for fault current.

 

  • The ground system shall be capable of safely conducting fault current to facilitate operation of overcurrent devices.

 

  • The earth shall not be used as the sole equipment grounding conductor or fault-current path.

EIA/TIA-607 complements NFPA 70 and addresses the functional and performance aspects of telecommunications grounding. IEEE 1100 provides a consensus of grounding recommendations, including high-frequency noise control. A design should comply with all of these requirements to have a proper ground system for a network. The following are some assumptions that designers make, and what can go wrong as a result.

Meeting Code Not Enough

Assumption 1. The grounding must be OK; after all, the building passed local codes. This is an assumption that is usually wrong. As mentioned, the NEC is a minimum safety code. It is not the intent of the NEC to make equipment work. A site that meets the code may still have grounding problems. The author's field experience is that 90 percent of facilities surveyed do not have adequate grounding systems and fail to meet the NEC requirements for grounding and bonding.

Consequently, when a site survey is performed, the first thing to look at is the grounding. Typical problems include: improper grounding and bonding of the main electrical service; no equipment-ground conductors; stray-neutral current problems; multiple neutral-ground; and improperly installed isolated-ground (IG)-type receptacles.

The designer of a system retrofit must find out where the main electrical service is grounded and look for loose or corroded connections, undersized conductors and improper bonding. She should take current readings of the ground system including electrode conductors, bonding jumpers and equipment grounds. If a 180-Hz current is measured in the ground system, it is likely that the system has a "stray neutral" current condition caused by multiple bonding of the neutral conductors. This is a common problem, where neutral current is circulating in the facility structure and ground systems causing ambient electrical noise. In some facilities, stray-neutral current problems have been so severe that the client had to electrically isolate each wiring closet by installing fiber-optic backbone cabling.

Directing Lightning

Assumption 2. Building occupants can solve grounding issues by installing their own computer- and network-ground systems.

One finds this condition all the time. When one looks under the raised floor of a computer room, he finds ground rods driven under the floor-even in new technology facilities.

The problem with this is simply that if there is a better ground system in the middle of the data center than at the main service, a lightning surge will go to the data center. Does anyone really want to direct lightning and surge energy into the most critical part of the facility?

A power survey was performed at a site where the client had experienced damage to its computer and network systems from lightning storms. The client had installed a UPS system, transient-voltage surge suppressor (TVSS) and power conditioning but still experienced damage. Multiple ground rods installed under the computer-room floor and an inadequate grounding electrode system at the main service resulted in the computer and network systems being part of the surge path as the lightning sought a ground source.

Assumption 3. The telecommunications system is properly grounded. Deregulation of the communications industry has often led to installations by untrained workers who do not understand grounding and bonding requirements.

As a result, it is very common to find the telecommunications system bonded to a grounding-electrode system that is separate from the main electrical ground system. This problem creates a ground-loop condition where lightning and power surges can bypass TVSS and UPS protection and severely damage a network system. NEC 250-92(b) addresses this issue in more detail. Figure 1 shows a suggested way of designing an effective common-ground system for the facility.

Inadequate Isolation

Assumption 4. The network is not ground-reference sensitive. The validity of this assumption depends on the type of communications system used. Balanced wiring systems such as twisted-pair Ethernet are, in fact, less sensitive to ground potential problems. Balanced wiring has conductors that carry voltages of opposite polarities and equal magnitude with respect to ground. The conductors are twisted to maintain balance over a distance. Ethernet network components such as hub ports and network cards provide electrical isolation-sometimes called galvanic isolation-of 500 to 1,500 volts (V) root mean square (RMS).

However, it is wrong to assume that network-component-provided isolation is adequate for a network system. First, the equipment in use may not comply with the standards. Second, the voltage-isolation requirements are for 60-Hz waveforms, not high-frequency events such as lightning and power surges. Finally, the isolation standards are for short time periods, usually on the order of one minute.

Lightning-induced voltage transients up to 6 kilovolts (kV) can be present on phase conductors of common 120-volt branch circuits. Direct lightning strikes-though rare-can generate voltage differences of 1,000 V per floor in multistory buildings and voltages of 10 kV and higher within large single-story structures. These types of voltage transients can result in ground-potential voltages that exceed the withstand ratings of most network equipment, causing failures and weakening of components.

Unbalanced wiring uses transmission lines in which voltages on the conductors are unequal with respect to ground. Generally, one of the conductors is connected to a ground point. An example of an unbalanced line is a coaxial cable and RS232. Unbalanced communications are very susceptible to grounding problems and require special consideration.

Many of the routers and smart hubs have RS232 ports for diagnostic and programming functions. The designer must make sure that any equipment-such as laptop computers-plugged into such ports has a common ground reference. Network equipment rooms and closets must be provided with properly grounded, dedicated power outlets for test and monitoring equipment.

Assumption 5. The outside plant communications cables are properly protected. One must ask, "Protected by what?" When designing for an existing building, it is very likely that the telecommunications-network protectors are of the older carbon-block or gas-tube surge protection technology. It is a wise investment to upgrade the network protection to newer hybrid surge-protection technology.

When designing for a campus environment, the designer should follow NEC 800-30 and specify surge protection every time copper cable leaves and enters each building. Select a protector that is matched to the network's signal voltage and frequency. Make sure the protectors are properly installed and grounded.

Assumption 6. All that is needed is to tell the electrical contractor to install "dedicated outlets" for the network equipment. This is a bad assumption, because the term "dedicated" means different things to different contractors. The worst scenario is where the electrical contractor installs orange IG-type receptacles and grounds them to a "clean" ground. The result is that the "clean" ground usually ends up being a separate ground system-creating a ground potential problem.

Many times the grounding connections of IG receptacles are connected to a ground rod that is isolated from the rest of the building. This arrangement is a violation of NEC 250-2 and can create a lethal shock condition as well as cause many problems with the network equipment.

A proper dedicated branch circuit should have individual-phase, neutral and ground conductors. IEEE-1100, Section 9.5 recommends that the voltage drop for circuits serving sensitive loads not exceed 1 percent of line voltage-thereby limiting neutral-ground voltages to 0.6 volts.

Running long branch circuits from a central UPS system to wiring closets may cause significant voltage drop and distortion problems. To minimize these issues and maintain lower grounding potential, the branch-circuit conductors can be upsized or isolation transformers can be installed. Figure 2 shows a suggested way of grounding and bonding power transformers for IG circuits.

Networks can vary from large systems of thousands of nodes to small home-office systems. Whether large or small, proper grounding and bonding is essential to any sized system. The network designer should make sure the facility is grounded per recommended standards and that it has proper surge protection for the telecomm and power systems. But the most important part of the designer's job is to inspect the work-during installation and after completion.

From Pure Power, Spring 2001





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