Major power disturbances that originate outside of a building will almost always cause capacitive or inductive coupling, inadvertent contact with conductive building elements, or both, and that can disrupt or shut down a sensitive information technology equipment (ITE) system. To help protect ITE you must assume that every disturbance is a direct threat, and an equipotential grounding structure is your best bet for averting disaster.
Try as you might, you can't completely isolate a system from outside events. But with a properly designed signal reference grid (SRG), the equipment won't feel the effects of the disturbance when it does get there. An SRG is the key component of an equipotential grounding structure, and it protects ITE from power disturbances by minimizing potential differences in the system. If there's almost no potential difference, then there will be almost no current flow in the system. And if there's no current flow from outside influences, then there will be no effect on the logic circuits in the equipment. In effect, your ITE system will 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).
Code compliance is also important. In addition to being compatible with the required signal quality within the ITE area, the electrical grounding system that the SRG ties into 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have evolved over its 100-year history for good reasons, and nothing in the information technology business has repealed the laws of physics on which those rules are based.
Almost 10 years ago, the late Warren Lewis, a true pioneer in the power quality industry, and Fred Hartwell, a nationally recognized NEC expert, came up with a truly Code-compliant design that provides excellent protection for ITE without affecting the safe and prompt clearing of hazardous faults if they occur (Click here to see Figure). You can retrofit the design into an existing installation and use it with either 60 (or 50) Hz or 415 Hz systems.
Let's look at the six key steps that go into creating this Code-compliant design and the relevant 2005 NEC installation requirements associated with it.
Distribute and equalize any current flow into the earth. To do this, you should install a grounding electrode system so it surrounds the building, as in a ground ring [250.52(A)(4)]. It must be bare, no smaller than 2 AWG copper, and buried at least 2½ feet below grade. You should bond any steel columns within the building to the ground ring. Remember, grounded structural steel is a qualified electrode per 250.52(A)(2), and 250.50 requires you to bond all elements of the grounding electrode system together.
For optimum performance, you should connect each of the exterior columns to the ground ring. In fact, you should bond all vertical columns below the soil line into a grid. Obviously, this would be a difficult process in a retrofit, but on new construction, it would be relatively easy and beneficial because it will minimize circulating currents through the vertical members and the horizontal members up on the next level.
Create a broadband ground plane. This ground plane, which equates to a 2-foot by 2-foot grid upon which the ITE is placed, helps to equalize potentials between various ITE devices in reference to each other, to “ground,” and to the building, such as structural steel (Sidebar below). The resultant ground plane will be effective at 60 Hz and throughout most of the high-frequency radio spectrum (to about 30 MHz), even if you don't install supplemental bonding for the ITE devices.
The standard 2-foot by 2-foot raised floor will work well, but make sure the pieces bolt together rigidly at each intersecting point. If a rigid-grid system isn't used, then you'll need to create a substitute for grounding purposes by using 4 AWG copper, also on a 2-foot by 2-foot pattern, bonded to each support post and to each other at each intersection.
In 250.64(B), the NEC sets 4 AWG as the smallest size of grounding electrode conductor (which these conductors are not) that you can run without following the building surface. At the frequencies likely to affect the ITE equipment, most of the current only flows over the outer skin of the conductor. So the chosen size of the conductor is really for mechanical stability, not conductivity.
The preformed grids that use 0.010-inch by 4-inch copper strapping work even better than 4 AWG conductors because the grids have a greater surface area (more “skin”) and therefore lower impedance at high frequencies.
To be fully effective, you should bond all equipment to the grid with at least two low-impedance equipment bonding straps, also preferably of rectangular cross section.
Bond everything in sight. You basically want to make sure everything rises and falls together in the event of a power system disturbance, from whatever source. Begin by installing an “electrical fence” around the area, in the form of a 2 AWG (or larger) copper conductor, which is the same size as the minimum for the ground ring. Make sure this conductor circles the entire perimeter of the raised floor and that you bond it to each pedestal and building column it passes, wherever possible. This “electrical fence” conductor also provides a ready bonding point for any conductive path, such as wire, conduit, or pipe that enters or leaves the ITE area.
If the room is large with internal columns, make sure you 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 2 AWG. 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.
You can also use strap copper, which, as noted for the grid, performs even better at high frequencies. Manufacturers of exothermic welding products have strip-to-strip, strip-to-pedestal, and strip-to-bonding strap molds so that you can make reliable connections using this material.
Your system must adhere to the following requirements:
Bond device boxes connected to the building wiring system to the grid. (Note: You can't use such devices to supply any equipment logically or electronically interconnected into the ITE system. To ensure this, provide suitable labeling for these devices so others will be aware of this restriction. Remember, serve ITE loads only by designated data-processing power supply equipment.)
Bond any electrical, alarm, or control panel — whether wall- or column-mounted — to the ground plane. All environmental support or control (HVAC) equipment located within the ITE room should be bonded to the ground plane as well. This includes any metallic floor-level plumbing and any conductive water, CO2, or Halon fire-suppression plumbing.
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. Make sure you bond vertical penetrations to one of the transverse bonding conductors, or to a pedestal of a rigid-grid system. There must be no exceptions to this. Be sure not to use the usual loosely jointed jackscrews as a bonding point.
Installing the ITE power supply. The branch circuits that supply ITE must meet all Code rules for equipment grounding. Basically, you must run the equipment-grounding conductor with or enclosed with the circuit conductors, per 250.118 and 300.3(B). For reliability and to reduce potential differences, you should run a separate equipment-grounding conductor of equal size to the line conductors. This conductor must run in parallel with the raceway, bonded to it at both ends.
No matter what the equipment supplier may say, do not 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 path other than the one used by supply conductors, the impedance increases dramatically and it's likely 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.
In this situation, the NEC doesn't waive requirements for an effective ground path, meaning you must bond the X0 terminal of the power center to the equipment ground unless the equipment is double insulated or a qualified testing laboratory has implicitly evaluated the ability of a fault to be cleared by the overcurrent protective device that protects the power supply. You must also ground the power center to the ground-plane for the raised floor (Sidebar below).
If the requirements of 645.15 apply to the power center, then 250.30(A)(3) doesn't apply, so you won't need to run a grounding electrode conductor. However, don't ground the power center only to the grid in lieu of the traditional grounding return path. Instead, make the connection to the grid in addition to the normal grounding return path, as allowed by the NEC in 250.54.
In terms of design, the equipment grounding system that originates from this power center must be at an equal potential to that of the other equipment in the room, or you've wasted all the effort and expense to properly bond the ITE area.
Ground those cable shields. The shielding in shielded twisted-pair (STP) cables is used in ITE environments because it provides capacitive shielding from interference by shunting the noise back, instead of letting it reach the twisted pair. If these cables run outside the building and are therefore exposed to lightning or higher voltage crosses, the requirements of 725.57 apply, which then incorporates numerous rules in Art. 800, including 800.100 on protectors and 800.93 on cable grounding. This was also a long-standing Bell System design practice.
Many designers/installers frequently misunderstand 800.93 and think it waives the protector rule. However, the insulating joint permitted in the section only allows the sheath to be ungrounded inside the building. You must protect the building against hazardous voltages that could otherwise enter over a conductive sheath.
The protector is a form of arrester and isn't solidly grounded until it closes on a surge, typically at about 60V and then maintaining itself closed down to about 15V. Since IT signaling voltages usually don't run much over 5V, you could still effectively ask for single-point shield grounding with this procedure.
Provide surge protection. You must include lightning and surge protection in an adequate grounding design for ITE areas, including surge protection at the service and all intervening levels of the distribution system. Make sure you bond each surge arrester to its distribution panelboard or switchboard enclosure. Also, make sure the grounding return path is secure all the way back to the source.
Per 280.12, it's important that you keep the conductor connecting the surge protection devices to ground as short as possible and free of any unnecessary bends. The perimeter-bonding conductor is usually the best reference point for these devices. Never coil the leads or bend them at sharp right angles.
Although it's DC, a lightning waveform isn't smooth like a battery's waveform. Instead, it's very similar to a 100 kHz radiowave, so a conductor with loops and right angles introduces unnecessary impedance. In addition, once it interacts with the impedance of metallic objects in the building, the waveform 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 IT area, the branch circuit that supplies each power center must have surge protection at a junction box located under the floor nearby, with the box securely bonded to the grid structure in at least two places for reliability and low impedance. In addition to this location, you should install protection on the wall at the point of entry, which is often even a better location. The worst location is right in the power distribution unit.
Make sure you supplement the typical telecommunications surge protector with a high-performance one that uses solid-state shunt elements that operate just above the permitted signal's maximum voltage. Basically, you cascade the two types. The signal reference grid is a good ground reference for both.
This design will create a large number of low inductance/resistance parallel paths in which noise, fault, and lightning currents can flow, but providing these defined paths is much better than allowing these undesirable currents to “find” their equalizing paths in a random fashion through the ITE system via interconnecting cables, telecommunications lines, and similar paths.
An invaluable reference for grounding, bonding, and providing surge protection for electronic signaling circuits is the IEEE Emerald Book.
Editor's Note: The text for this article is an adaptation of an article that first appeared in the February 1996 issue of EC&M, authored by then staff members Warren Lewis (deceased) and Fred Hartwell. This version includes references to the most current version of the NEC and makes mention of the latest technology available in the industry today.
Sidebar: Grid Spacing Design is More Than Structural
The 2-foot by 2-foot grid spacing also ensures that the grid won't become resonant with a high-frequency signal and turn into an antenna for unwanted electronic noise. This condition will occur if the conductor is a significant fraction of the wavelength of the signal received. Therefore, you must avoid this situation.
The wavelength is the speed of light (3 × 108 meters per second) divided by the frequency. At 30 MHz, the wavelength turns out to be about 10 meters, or about 33 feet.
Even if the room is that long or longer, the 2-foot bonding intervals create a collection of shorted-turn cells that form a two-dimensional network of impedances for all current flowing in the grid. Due to their length, the shorted-turn cells aren't normally self-resonant at any frequency of concern for commercial-grade ITE equipment.
Sidebar: NEC Grounding and the SRG
To prevent electrical noise from affecting ITE equipment and other equipment that contains solid-state devices, you must have two entirely different and separate grounding systems.
Obviously, you must ground the power distribution system per the NEC and all applicable local codes for safety. But you also must protect the ITE equipment and enclosures from high-frequency electrical noise. This is why you need an SRG.
This high-frequency protection is in no way related to system grounding. In fact, the high-frequency reference will work whether or not it's bonded to the power distribution grounding system. Since the former consists of exposed, metallic, noncurrent-carrying parts that could accidentally become energized, you must bond it to the latter system.