A step-by-step look at a highly effective grounding design for an information technology area that also fully complies with the NEC.
The Computer and Business Equipment Manufacturers Association (CBEMA) has just published a "white paper" on power quality, and it states that 75% of the problems with perceived power quality are actually grounding problems. Not only must the grounding system be compatible with signal quality within the data processing equipment, but it must also be safe. The rules in the NEC on grounding have evolved over its 100-year history for good reasons, and nothing in the information technology business has repealed the laws of physics on which those rules are based. Happily, you can design a system that provides excellent power for the equipment and will safely and promptly clear hazardous faults if they occur.
A proper grounding system is the essential foundation for other refinements that enhance power quality through reductions in harmonics, etc. Without such a foundation, any sophisticated attempts to improve power quality will likely prove to be transient, perhaps lasting only until the next thunderstorm.
We want to create, as perfectly as practicable, an equipotential grounding structure for information technology equipment (ITE), which is the new official name for electronic data processing (EDP) equipment. The ITE system should be relatively immune to the usual disturbing effects of either lightning or electrical noise, whether in the normal (transverse) mode or common mode (to ground). The system described here [ILLUSTRATION FOR FIG. 1 OMITTED] can usually be retrofitted into an existing installation, and will work with either 60 (or 50) Hz or 415 Hz systems.
The equipotential plane is the key to this system. You usually can't isolate a system from outside events. Through capacitive or inductive coupling, or through inadvertent contact with conductive building elements, or both, a major disturbance on the outside power system will almost always influence a sensitive system. We think it is far better to assume that the disturbance will reach the equipment.
We intend this design as one that is arranged so, in effect, the equipment won't notice the disturbance when it gets there. We do this by minimizing potential differences. If there is almost no potential difference, then there will be almost no current. To the extent there is no current from outside influences, then there will be no effect on the logic circuits in the equipment.
We understand that this design goes beyond NEC requirements. The mission of the NEC, per Sec. 90-1 (b) is not to ensure that an ITE system will work. It is to ensure that when wired, an ITE system won't cause a fire or electrocute someone. The design presented here should allow the ITE system to work properly, and to ensure that its operation will be safe.
The grounding electrode system
In order to distribute and equalize any current flow into the earth as much as possible, the grounding electrode system should surround the building. A ground ring [per Sec. 250-81(d)], is a good way to begin. This must completely encircle the building at least 2 1/2 ft below grade, and must be bare and no smaller than No. 2 AWG copper. If there are steel columns, then these must be bonded to the ground ring as shown in the drawing. Remember, grounded structural steel is a qualified electrode in Sec. 250-81(b), and Sec. 250-81 requires all elements of the grounding electrode system to be bonded together.
For best performance, connect each of the exterior columns to the ground ring. In fact, all vertical columns should be bonded below the soil line into a grid, although this would be difficult to retrofit. On new construction, however, this will minimize circulating currents through the vertical members and the horizontal members up on the next level.
The standard 2 x 2-ft raised floor will work well, but it must be the type that bolts rigidly together at each intersecting point. If a rigid-grid system can't be used, then you will need to create a substitute for grounding purposes. You should use No. 4 AWG copper, also on a 2 x 2-ft pattern, bonded to each support post and to each other at each intersection. Note that at the frequencies likely to affect the equipment, most of the current only flows over the outer skin of the conductor. Therefore, we are designing the size of the conductor for mechanical stability, not conductivity.
Sec. 250-92(a) sets No. 4 AWG as the smallest grounding electrode conductor (which these conductors are not) that can be run without following the building surface. The preformed grids using copper strapping work even better than No. 4 because they have a greater surface area (more "skin") and therefore lower impedance at high frequencies.
The grid will act as a broad-band ground-plane beneath the ITE system, and it will aid in equalizing potentials between various ITE devices in reference to each other, and in reference to "ground" and also to the facility itself (i.e., building steel, etc.) The resultant ground plane should be effective at 60 Hz and throughout most of the high-frequency radio spectrum (i.e., to about 30 MHz) without supplemental bonding of the ITE devices being required.
To be fully effective, all equipment should be bonded to this grid with at least two bonding straps, also preferably of rectangular cross section.
The 2-ft spacing is to assure that the grid won't become resonant with a high frequency signal, turning it into an antenna for unwanted electronic noise. This will happen if the conductor is a significant fraction of the wavelength of the signal received, and this must be avoided. The wavelength is the speed of light (3 x [10.sup.8] m/sec) divided by the frequency. At 30 MHz, that turns out to be about 10 m, or about 33 ft. Even if the room is that long or longer, the 2-ft bonding intervals create a collection of shorted-turn cells that form a two-dimensional network of impedances for all current flowing in the grid. As noted (see box above), due to their length, the shorted-turn cells are not normally self-resonant at any frequency of concern for commercial grade ITE.
Bond everything and more
The next step is to make sure everything rises and falls together in the event of a power system disturbance, from whatever source. Begin by drawing an electrical fence around the area using No. 2 AWG (or larger) copper, which is the same size as the minimum for the ground ring. This conductor must encircle the entire perimeter of the raised floor. It must be bonded to each pedestal it passes, and to each building column wherever possible. The conductor can also be strap copper, which as noted for the grid will perform even better at high frequencies. Manufacturers of exothermic welding apparatus have special molds to facilitate reliable connections using this material.
If the room is large with internal columns, then pick up those internal columns by using them as points of reference to subdivide the area. Go column to column from the perimeter bonding conductor with additional runs of No. 2. As in the case of the perimeter bonding conductor, connect these conductors to each other wherever they intersect, and also to each support pedestal they pass or approach under the floor.
These steps minimize the ground planes's impedance at its operating frequencies, and also keep the impedance within the grid both low and equally distributed. At power frequencies any current entering the grid will spread out geometrically at each grid junction, dramatically reducing current density and thus magnetic fields at any single point. At high frequencies, the entire grid will act like one plate of a "spatial capacitor," and a traveling wave injected at one point will thin out, again reducing the effects on equipment in the room.
In addition, this provides a ready bonding point for any conductor, whether in the form of a wire or in the form of conduit or pipe, that will enter or leave the ITE area. The grid won't work, however, unless you use it for everything in the room:
* Bond the device box enclosing any receptacle connected to the wiring system for the building at large to the grid. Note that such devices must not be used to supply any equipment that is logically or electronically interconnected into the ITE system. We think you should provide suitable labeling for these devices so others will be aware of this restriction. ITE loads should only be served by designated data-processing power supply equipment.
* Bond any electrical, alarm or control panel, whether wall or column mounted, to the ground-plane established previously. Also, bond all environmental support or control (HVAC) equipment located within the ITE room to the ground plane as well. This includes any metallic floor-level plumbing and any conductive fire-suppression plumbing, whether water, C[O.sub.2], or halon.
* Bond any conductive electrical raceway (or cable assembly at a terminating enclosure) to the perimeter bonding conductor as close as practicable to the point where it enters the area. Vertical penetrations must be bonded to one of the transverse bonding conductors, or to a pedestal of a rigid-grid system. There must be no exceptions to this. Note, however, that the usual loosely jointed jack-screws should not be used as a bonding point.
The power supply
The drawing [ILLUSTRATION FOR FIG. 2 OMITTED] shows a typical power center for information technology equipment. The branch circuit supplying this equipment must meet all Code rules for equipment grounding. The equipment grounding conductor must run with or enclose the circuit conductors, per Sec. 250-91(b), no exceptions. For reliability and to reduce potential differences, a separate equipment grounding conductor of equal size to the line conductors should be run.
This conductor must run in parallel with the raceway, bonded to it at both ends. No matter what the equipment supplier may say, never, ever, for any reason, try to ground one of these systems through anything other than the normal equipment grounding return path that runs with the supply conductors. Any time fault return current in an AC system is forced to flow through a separate path than the supply conductors, the impedance increases dramatically, with the likely result that the fault won't clear as it should. In addition, such "isolated" or "special" grounding systems usually are counterproductive, inserting additional common-mode disturbances into the ITE system.
This brings us to Sec. 645-15, which frequently applies to this type of equipment. Much has been written, hysterical in tone, about how we all may be in immediate danger of electrocution because of this section. The second sentence reads as follows: "Power systems derived within listed...equipment that supply...systems through receptacles or cable assemblies supplied as part of this equipment shall not be considered separately derived for the purpose of applying Section 250-5(d)." It is worth looking carefully at what this sentence does not say.
It does not say that the systems are not separately derived. They are, and as such, they are subject to the rules of the Code, per the definition of premises wiring in Article 100. That definition was changed in the 1990 Code cycle to ensure that the Code would apply to those systems. The sentence merely says in effect that a particular requirement in the Code [Sec. 250-5(d)] will not apply. The rule does not apply to general branch circuit wiring, only to cables and receptacles tested and evaluated as part of the unit.
Finally, the requirements of Sec. 250-51 for an effective ground path are not waived. The [X.sub.o] terminal will be bonded to the equipment ground, unless the equipment is double insulated, or unless a qualified testing laboratory has implicitly evaluated the ability of a fault to be cleared by the overcurrent protective device protecting the power supply. Although the syntax of this sentence could be improved, it is not any immediate hazard. The first FPN, although incorrectly worded (equipment may provide grounding and bonding connections, not grounding and bonding "requirements"), does indicate that the product standards are safe.
Even with that said, the power center must be grounded to the ground-plane for the raised floor. If Sec. 645-15 applies, then Sec. 250-26(c) does not apply per se, and a grounding electrode conductor (running without splice to a Part H electrode in Article 250, etc.) won't be required by the Code. However, in terms of design, the equipment grounding system that originates from this equipment must be at an equal potential to that of the other equipment in the room, or all the effort (and expense) to properly bond the information technology area will be for nothing. We aren't saying to ground the power center to the grid in lieu of the traditional grounding return path; we're saying to make the connection to the grid in addition to the normal grounding return path, as allowed by the Code in Sec 250-91(c).
The use of insulated ground (the Code word is "isolated" but as noted we question if such grounds are truly isolated) receptacles [covered in Sec. 250-74, Exception No. 4 and Sec. 410-56(c)] needs to be carefully thought out on the load side of the power center. Although they will help make the power center a single grounding point for the system, that branch circuit isolation may or may not be useful when the entire room has been bonded as discussed. The safest course is to consider them on a case-by-case basis, and experiment accordingly.
Grounding cable shields, etc.
If shielded twisted-pair (STP) cables are used, the shield helps control interference. Digital signals consist of rapid pulses from near zero to 5V (or less on some newer systems) with a specified repetition rate (9600 Hz, 28.8 kHz, etc.) As these signals degrade, the sharp corners of the original square wave become rounded and less distinct. Gradually undetected transitions occur, each one an error.
The outer shield isn't part of the communications circuit, and grounding tricks are common. Floating the shield isn't helpful, but often only one end of the shield will be grounded. The idea is to provide capacitive shielding from interference by shunting the noise back instead of having it reach the twisted pair.
Note, however, that if these cables run outside the building so as to be subject to lightning exposure or higher voltage crosses, Sec. 725-54(c) applies, which then incorporates numerous rules in Article 800, including Sec. 800-12 on protectors and Sec. 800-33 on cable grounding. This is also long-standing Bell System practice. Sec. 800-33 is frequently misread because the insulating joint permitted in that section only allows the sheath to be ungrounded inside the building. It does not waive the protector rule. The building must be protected against hazardous voltages that could otherwise enter over a conductive sheath.
The protector is a form of arrester and is not solidly grounded until it closes on a surge, typically at about 60V and then maintaining itself closed down to about 15V. Since information technology signaling voltages usually don't run much over 5V, you could still effectively ask for single-point shield grounding with this procedure.
However, in terms of design, there will be no H field protection unless the cable shield is grounded at both ends, and coupled E field noise is only attenuated at the grounded end. Cables have two ends; which end gets the grounding connection and which end floats when they connect two offices? Think about a bidirectional data cable; now which end gets the grounding connection? If circulating currents are a problem, consider solidly grounding one end and grounding the other end through a capacitor. This will block the DC and low frequency current while retaining an effective shield at high frequencies.
Lightning and surge protection must be included in an adequate grounding design for these areas. Surge protection must be provided at the service and at all intervening levels of the distribution. Each surge arrester must be bonded to its distribution panelboard or switchboard enclosure, and the grounding return path must be secure, all the way back to the source.
The individual protective devices must be connected to ground through a conductor that is no longer than necessary, and that avoids unnecessary bends, per Sec. 280-12. The perimeter bonding conductor is usually the best reference point for these devices. Never coil the leads, or bend them at sharp right angles. The lightning waveform, although DC, isn't smooth like a battery. Instead it approximates a 100 kHz radio wave, and a conductor with loops and right angles introduces unnecessary impedance. In addition, once it interacts with the impedance of metallic objects in the building, it becomes partially AC in character. Given the levels of current, particularly from a lightning strike, a very small increase in impedance can lead to thousands of volts on the system.
Within the information technology area, the branch circuit supplying each power center must have surge protection at a junction box located under the floor nearby. The surge protection and the junction box must be securely bonded to the grid structure [ILLUSTRATION FOR FIG. 1 OMITTED], in at least two places for reliability and low impedance. In addition to the location shown in Fig. 1, the protection could be located on the wall at the point of entry, which is often even better. The worst location is right in the PDU.
The typical telecommunications surge protector needs to be supplemented by a high-performance one using solid-state shunt elements that operate just above the permitted signal's maximum voltage. The two types are cascaded. The signal reference grid is a good ground reference for both.
The result of following this design will be the creation of a large number of low inductance/resistance parallel paths for noise, fault, and lightning currents to flow in. For purposes of equalization, providing these defined paths is superior to allowing these undesirable currents to "find" their equalizing paths in a random fashion through the ITE system via interconnecting cables, telecommunication lines, and similar paths. We also strongly recommend using the IEEE Emerald Book, which is an invaluable reference for grounding, bonding, and providing surge protection for electronic signaling circuits.
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