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Tech Tips Suggestions for Grounding a Computer Room by Jeff Jowett Megger I n a previous article, we described the concept of the single point ground as essential to the proper grounding of a computer room in order to eliminate voltage gradients and stray currents that can damage equipment and disrupt operations. Here we will expand with some suggestions for effectively tying equipment into such a system.1 The goal is to establish and maintain an equipotential plane for all equipment in order to avoid voltage differences. This begins with a good neutral-to-ground bond at the power treatment device or in the secondary of the transformer feeding the computer room. The isolated transformer establishes a separately derived system that keeps the computer room free of noise existing in large building systems. However, the grounded system protecting the computer room should be interconnected with the building ground in order to maintain an equipotential plane. It is often felt that an isolated computer ground will keep the system separate from general building noise. But, in fact, all this does is establish a potential gradient that attracts noise (ground currents) into the computer room system. The isolated transformer should be as near the computer as possible. This is because the impedance of long conductors can establish potential differences that produce electrical noise and interference sufficient to disrupt the smooth operation of the computer. Conductor sizing is important in further reducing or eliminating this problem. Logic circuits of computers, which began operating at comparatively enormous levels around 50 V, have by development been progressively reduced to more and more efficient and tighter voltage “windows”: 12, 5, 1.8, and 1.2 volts. Speed of operation increased apace. With these increasingly demanding parameters, the leading edge of a noise-generated voltage rise can readily appear like a logic signal. Enough noise- generated voltage and the computer becomes a random number generator. The larger the gauge of the isolated grounding conductor, the lower the impedance of the return, in turn reducing noise. The recommendation is the grounding conductor be sized equivalent to the phase conductors, or at a minimum #8 AWG. If the color green is used for the safety ground conductor, then green with yellow stripes is used for the isolated grounding conductor. The latter must be extended with the circuit conductors, neutral and safety grounding conductor and go through the panel or reference control panels. It is not connected to conduit or secondary distribution panels through which it passes, and terminates on an isolated bus bar, main isolated ground cable or receptacle, and at the single point ground of the power source. www.netaworld.org Balancing a multiphase system will reduce neutral current, but this is not possible for single-phase circuits. In such instances, neutral sizing is critical. A convenient source of information is Table 9 in the National Electrical Code® (NEC®). For a typical acceptance value of no more than 2 Vpp, a convenient formula is: R = 1000 / (Icb x Lm) Where: R = resistance in ohms per 1000 ft Lm = max length in ft Icb = current capacity From this calculation and the data in Table 9, the appropriate AWG of copper or aluminum can be selected. It is important to note the difference with the goals of the National Electrical Code. The NEC is concerned with safety…protection from personnel hazards like fire and electrocution…more than with performance. A grounded system utilizing metallic conduit and panels is fine from the standpoint of safety and may well meet Code but at the same time be unsatisfactory in promoting system performance. A good way to monitor the equipotential balance of the system is to check the voltage between neutral Spring 2009 NETA WORLD and ground. There should be no current flowing in the grounding conductor. Ideally, the grounding conductor terminates at some insulation barrier, say, at a motor housing that is isolated from circuitry by insulation. If there is some breakdown of this protection, then current flows. If the system is carrying ground current during normal operation, a voltage is generated between neutral and the grounding conductor which represents a voltage drop in the neutral. To check correct bonding, with no current flow in the grounding conductor, a voltage measurement between the neutral and grounding conductors at, say, a subpanel or receptacle, will indicate generated voltage from that point through the neutral-ground bond at the main distribution panel. A well maintained system with all connections offering minimal resistance will hold this to a minimum. Voltage should be the product of current times the resistance of the neutral conductor. Any other resistive connection between the neutral-ground bond and the point of measurement will add to this. As an example, 100 feet of #12 copper, from a table in the NEC, would be expected to have an impedance of 0.17 ohm. If the neutral carried 5 amperes, the voltage would be expected to be 0.85 Vrms. If an actual measurement proved to be more than 2 V peak-to-peak, there could be a loose or high resistance connection in the neutral. If unusually low, neutral and ground must be touching at some point short of the neutral-ground bond. Manufacturers of electronic equipment will specify the maximum neutral-ground voltage allowable, and the system must be so designed, with the neutral sized accordingly, to assure a voltage drop within this allowance. This can be accomplished by increasing the size of the neutral in order to reduce impedance or by decreasing the length of the circuit to footage of acceptable impedance. The safety ground protecting the building at large can be a source of ground loops and the flow of noise currents, but separating these connections at the computer room creates voltage gradients, as has been discussed. This conflict can be resolved by the installation of a floor that serves as a ground plane. A thin metal plate has low surface impedance but lacks strength and is difficult to connect. A successful alternative is an interconnected ground grid used as support for the floor and as ground plane. The ceramic floor tiles can fit within the squares of the grid while permitting access underneath. The structural supports that constitute the grid are arranged in the form of two-foot squares. Such a structure proves adequate to noise levels up to 30 megahertz.1 Above this frequency, noise signals can be filtered or ignored. The various members of the structure are bonded at less than one milliohm, in order to serve as would a very thin plate at low frequencies. Such connections must be sealed against oxidation, maintained under pressure, and able to withstand the weight of the load on the floor and vibrations. The floor is a lightly conductive material (typical value: 109 ohms/square1) which is connected to the grid. It serves to drain static charges and impede charges of personnel on the floor. Signal cables under the floor run near the surface NETA WORLD Spring 2009 of the ground plane in order to reduce noise. Cables that leave the floor area should follow a properly bonded extension of the ground plane. Lightning protection in the form of a ground ring was described in the previous article on single-point grounding. The installation is energized with its own dedicated transformer, typically 208Y/120 V. This can be an isolation transformer providing only common mode attenuation, or can have a Faraday shield with constant voltage or a line conditioner to provide regulation and attenuation. This constitutes a separately derived system that must be connected to ground in accordance with Section 250-30 of the NEC. The bonding jumper from neutral to ground must not be sized smaller than the grounding electrode conductor, as specified in Table 250-66 of the NEC. If an uninterruptible power source (UPS) or standby power supply (SPS) is installed, the transformer must be on the load side. Power distribution units (PDUs) can also be used. Multiple PDUs can be connected at the output of a UPS in order to service multiple computer rooms or other equipment. As can now be easily recognized, computer room grounding must studiously be applied while avoiding haphazard or merely generic wiring practices. Levels of noise that would ride through much electrical equipment will adversely affect computer room operation. Only close adherence to a specific set of proven practices will assure smooth operation. 1 Source of recommended practices: Grounding for Electrical Distribution Systems, ALLTEC® Corp., Cohasset, MA Jeffrey R. Jowett is Senior Applications Engineer for Megger in Valley Forge, Pennsylvania, serving the manufacturing lines of Biddle®, Megger®, and Multi-Amp® for electrical test and measurement instrumentation. He holds a BS in Biology and Chemistry from Ursinus College. He was employed for 22 years with James G. Biddle Co. which became Biddle Instruments and is now Megger. www.netaworld.org