Your choice of SRG depends on knowing what's legitimately true and patently false about grounding, bonding, and electrical noise mitigation.
Proper bonding and grounding is based on science, not snake oil. While there is an abundance of in-vogue Computer grounding information, some of the "facts" are unsubstantiated and can cause unnecessary confusion and expense. To set the record straight, let's discuss the real facts and debunk the prevalent myths.
Facts and myths
To help you design an effective computer grounding system, you must understand and appreciate the following.
* Electric current, with no exceptions, always returns to its source. It does not run into earth ground unless that is the lowest impedance back to the source.
* Transient noise current enters the grounding conductors, possibly creating errors in transmission of data between interconnected equipment.
* It's possible to reduce the effects of this noise with properly designed ground planes, which are usually in the form of grids called signal reference grids (SRGs).
* An SRG's bandwidth, where it must have a low impedance, is known and is not related to the bandwidth of low level (5V or less) signals or data.
* Data integrity can be protected by SRGs, if they are properly utilized.
[TABULAR DATA FOR TABLE 1 OMITTED]
In our discussion here, we'll debunk the following myths. Myths:
* The bandwidth of the electrical noise is the same as the signal bandwidth.
* Bonding strap resonance can seriously affect digital systems operations.
* Loose or casual connections are "good enough."
* Access floor pedestal connections are important in high frequency grounding.
* You must have a "single point ground."
* Larger conductors are significantly more effective in minimizing electrical noise than small conductors.
* Good DC or 60 Hz earth ground is of critical importance for noise control.
Control of electrical noise
Noise caused by transients in the power and grounding system of sensitive electronic equipment can interact with signals on interconnecting data cables. It can be minimized with proper use of components such as a transient voltage surge suppressor (TVSS), an isolation transformer, a filter, and proper wiring techniques.
There are two reasons why noise gets into the ground system. First, the inductive reactance of the typical ground system ("green wire" ground) is much higher than its resistance at the frequencies we are concerned with here. (See Table 1.) This means that all references to "ground resistance" are not a valid measure of required grounding effectiveness. For example, the frequent requirement of a "one ohm ground" for communications or computer sites, in fact, may be counterproductive. This may be the case if the connection to this point is through conductors having high impedance at frequencies that can disrupt sensitive electronic equipment. Digital equipment are sensitive to virtually all frequencies from "DC to daylight."
Second, disruptive transients are often caused by changes in load current in large systems; however, other load changes, such as capacitor bank switching or even remote lightning, can cause significant transients. For power equipment such as transformers, motors, and lighting circuits, this is usually not a problem because the transient voltages are usually of very short duration and low amplitude relative to the equipment operating voltage.
For data and communications circuits, however, the ground voltage may be zero at one location but may be much different at another location. This difference in potential may look like a signal to the sensitive electronic circuits involved and, as a result, may create false information.
The equipment ground system (green wire ground) is required by the NEC to assure personal safety. The Code does not address reliable equipment operation.
SRGs, on the other hand, do provide an improved degree of operating reliability. They are a "grounding/bonding" system in addition to the safety ground required by the Code. (Please note: nothing we discuss here should be interpreted as being in conflict with the Code.)
Noise sources that threaten system performance
The most severe transients on power systems have been well documented and are described in ANSI/IEEE C62.41-1980, Guide for Surge Voltages in Low-Voltage AC Power Circuits. We can accurately determine the frequency spectrum of any non-sinusoidal wave and we can verify if conditions that are detrimental to sensitive electronic equipment exist. Based on this information, we can then take effective action to protect the equipment.
If we treat conducted noise as a circuit problem, then bonding and grounding are part of the solution. (Note, we are concerned With conducted transients including those that are capacitively and magnetically coupled into the conductors.)
SRG performance requirements
A total SRG system must do the following to be effective.
* Function as a broadband equipotential plane for transient noise.
* Provide a lower impedance ground reference than signal conductors and shields.
* Maximize capacitance from data cables to a zero voltage reference.
* Assure low surge impedance to transients.
* Reduce the magnitude of common mode noise on data cables. [ILLUSTRATION FOR FIGURE 1 OMITTED]
* Guarantee predictable performance not subject to long term degradation.
Some examples of components, circuits, and systems known to perform reliably in noisy environments are discussed below.
Noise in integrated and printed circuits
Computer data and other signals are not a source of noise. If they were, then the circuitry inside of an enclosure would never function correctly. Integrated circuits often have conductor spacings of 1 to 2 microns (40 to 80 millionths of an in.). This small spacing provides a wonderful opportunity for crosstalk and coupling between conductors; yet, the circuits work reliably, unless acted upon by external noise.
One reason for this successful performance is that a built-in signal reference plane covers most of the chip area. This is the common (or zero voltage) signal reference for the data.
The same technique is used at the printed circuit board level, where a signal reference plane is one of the layers. This ground plane is separated from signal conductors by a few thousandths of an inch. We've known for many years that noise sensitive conductors must be as dose as possible to the ground plane.
Conclusion. To be effective, the SRG ground plane must reduce noise according to the noise threat's spectrum, not the signal's spectrum.
The worst transient threat: the ring wave
Of the various waveforms described in the C62.41-1980 standard, the ring wave is the most likely transient in most facilities. A typical ring wave is a 6kV peak voltage that oscillates at 100 kHz, has a decrement due to resistance in the power circuit, and is the result of distributed series inductance and shunt capacitance of branch circuit wiring interacting with a transient. (Considerations of other waveforms would not cause significant change in any of the following conclusions; this includes the various waveforms listed in IEC Standards.)
If there is more than a 6kV peak amplitude on branch circuits, flashovers may occur within panelboards and other wiring devices. As such, you should use TVSSs to limit the magnitude of these transients. Below 6kV, the common mode voltages on data cables can be reduced by an SRG.
Regardless, an SRG must provide a sufficiently low impedance to the transients. Because these transients are not related to the bandwidth of digital systems, the SRG design only has to consider real-life transient threats to proper operation.
Results of SRG impedance studies
At frequencies above 100 to 200 Hz, inductive reactance is the major factor in determining the impedance of conductors. Resistance, including skin effect resistance, is not a major factor.
The total inductance is the sum of self-inductance (determined by conductor size, shape, and length) plus or minus mutual inductance (determined by spacing and orientation) of nearby conductive objects. For current traveling in the same direction, the mutual inductance adds to the self-inductance.
For an SRG, total inductance ([L.sub.T]) can be expressed as follows.
[L.sub.T] = [L.sub.S] (Self inductance) + Summation of [L.sub.M] of all elements in grid
where [L.sub.S] = self inductance
[L.sub.M] = mutual inductance
Fig. 2, shown on page 74, illustrates the effects of self-inductance and mutual inductance on the total inductance of grids of varying sizes. The mathematical model agrees closely with experimental results.
In a grid, the mutual inductance is a complex function of grid spacing and orientation. Mutual inductance increases the impedance across a grid. The worst case is diagonally across the grid, which is assumed in our discussion here. As grid spacing decreases, total inductance decreases but at a slower rate as the floor size increases. The most important factor in grid design is to minimize [L.sub.S].
Table 2 (see page 74) compares impedance between commonly used grounding conductors. Here we see that two parallel No. 6 AWG conductors provide lower inductanc than one 4/0 AWG conductor, provided they are separated enough to minimize mutual inductance. Also note that a 2-in. wide flat strip, with only 14% of the amount of copper, also provides even lower inductance than either the 4/0 AWG or 210 AWG conductors. It's obvious that oversizing conductors is not the way to go.
Types of SRGs
There are different types of SRGs.
Access floor stringers. The bolted stringers of an access floor can be used as an SRG. There are pros and cons to this approach.
* Con: The inductance of steal stringers is generally higher than copper strips, especially at frequencies below 1 MHz. [ILLUSTRATION FOR FIGURE 3 OMITTED]
* Con: Bolted connections are known to loosen over a period of time and lose electrical continuity. These connections are structural connections and are not tested or guaranteed as an effective connection for low current/voltage noise. Retightening exposes new surfaces between mating members to provide a renewed path for low level noise. Periodic maintenance is necessary and recommended.
* Con: Vertical spacing between cables and stringers is relatively large, thus limiting the ability to minimize common mode noise.
* Pro: Bonding straps to equipment are shorter.
Prefabricated mesh. Another type of SRG is a prefabricated mesh, made of solid round wire with brazed crossovers, that is usually buried in the concrete of the structural floor. Equipment connections are made to ground plates, which are flush with the floor and connected to the mesh. While prefabricated mesh is very effective, this type of SRG is usually a more expensive solution.
Flat conductors. Flat conductors with a large surface area have lower inductance than other conductors. Increasing conductor thickness only adds cost, although conductors thinner than 0.0159 in. (26 gauge) may be prone to damage during normal installation. This is an important consideration when the SRG lies on the structural floor (recommended where possible) or is suspended over the cables (second choice). The SRG must be able to withstand the possible pulling forces of cables being added or removed. The suspended SRG (whether wire or flat strip) is somewhat more prone to damage and is not recommended except in retrofit applications. Another benefit of flat strip construction is that, properly made, it lays flat on the floor and is less likely to snag or dam age cables during installation or removal.
Table 3 (see page 76) lists the construction, relative costs, and effectiveness of various SRGs.
As an example of better performance, the relative inductance of a No. 2 AWG SRG on 24-in. centers compared to an SRG on 48-in. centers connected to pedestals using No. 6 AWG was calculated. The inductance of the No. 2 conductor on 24-in. centers was 28% higher than the 48-in. grid using flat strip. This calculation assumes that all No. 2 conductor cross-overconnections were conducting as they would with welded or brazed crossovers.
Conclusion. Broad, thin strips are the most effective choice of SRG conductor.
Assuring low surge impedance within a ground plane
Surge impedance can be expressed by the following equation.
[Z.sub.o] = [square root of (L/C)]
where [Z.sub.o] = characteristic impedance
L = total inductance between any two points
C = capacitance to ground between the same two points.
You would need a low surge impedance to minimize the voltage drop along the interconnecting cables as well as between the cables and all points on the ground plane. Therefore, a low value of L in an SRG and a high value of C between the SRG and cables is desirable.
To minimize L, the SRG provides many low impedance current paths in parallel. To maximize C, the spacing between the cables and the SRG must be kept as small as possible. It doesn't matter if the SRG is above or below the cables, as long as the distance is small. Laying the cables on top of the SRG and using random lay is the best method and least costly to install.
Cables laying one on top of another have shields that are closely coupled; this helps increase capacitance to the SRG. Impedance (mainly capacitive reactance) to the stringers above or reinforcing steel below is always very high because of the separations involved.
There's another way of looking at minimizing coupling of noise into the system. The following excerpt from the book Grounding and Shielding in Facilities by Morrison and Lewis (John Wiley & Sons, New York, 1990) is especially noteworthy.
The primary function of a ground plane under an electronic facility is to reduce common-mode coupling into signal paths. The interference can arise from any nearby interconnecting cables or from external radiation. Common-mode coupling causes interfering currents to flow in the same direction on all conductors in a cable bundle. The extent of common mode coupling is dependent on the open area between conductors and the ground plane. This coupling is minimum when the cable run is very close to the ground plane.
In other words, the best way to minimize noise in cables is to lay them directly on the SRG. Even a few inches of spacing between the cables and the SRG will reduce its effectiveness.
Conclusion. Keep cables very close to the SRG.
We've been warned of resonance problems in grounding and bonding conductors. Resonance may occur at a single frequency or at a very narrow band of frequencies. At 1 MHz, the one quarter wavelength resonant length of a conductor in free space is 75 m (246 ft.); also, the peak amplitude of the ring wave is about 100 microvolts and decreases rapidly at frequencies above this value. If the equipment being protected is sensitive to voltages of this magnitude, then you must consider the effects of resonance. Note that the amplitude of a single frequency at 1 MHz or any other single frequency above this value is too low to be a threat to the proper operation of digital systems working from a 5V power supply. At 10 MHz and above, this voltage "threat" is so low that it can usually be neglected.
Conclusion. Grounding/bonding conductor resonance is not a concern in digital systems.
Raised floor SRGs
A few years ago, SRG studies were performed at a laboratory equipped to test large assemblies. The test program concentrated on evaluating the inductive reactance of various approaches to SRG design. At that time, the use of raised floor bolted stringers was thought to be sufficient for effective performance. Cost wise, the raised floor SRG was "free" because in most cases, a bolted stringer floor was required for the sped tic installation at hand. Test results are summarized below.
* The inductance of the bolted stringer floor was about 20% to 25% higher than the flat strip copper SRG.
* The raised floor bolts had to be over-torqued to get the above measurements. (Overtorqued bolts loosen over time.)
* Loosening of some bolts made no significant difference in performance, while the loosening of other bolts (near the signal injection or take-off point) made the floor behave as a much higher impedance circuit. This is a major concern because the path of the noise source cannot be known beforehand. Therefore, all inter-connections must remain highly conductive for the raised floor to be considered as a reliable ground plane.
While electrostatic discharge (ESD) is not a primary topic of our discussion here, an SRG can be used effectively to dissipate it. An SRG can be useful in conducting charges on the access floor surface to a common reference plane. In fact, testing has shown that connecting floor pedestals to an SRG can be very effective for this purpose. One connection for every 144 sq ft of floor should be adequate.
Low inductance bonding straps
A bonding conductor is very important. Not only must it be the lowest possible impedance, but terminations at both its ends must be of the highest quality to assure a long lived, low resistance connection. Due to their relatively small size in comparison to the length of conductor, connections are primarily resistive and exhibit very low inductive reactance.
Keep the length of bonding straps as short as possible to minimize inductive reactance. In practice, bonding straps using the same broad flat strip used in the SRG have worked well. Flexible braid can also be used; it's more flexible than solid straps but has a higher impedance for the same ampacity as well as a higher cost.
Where very long straps (perhaps 1 1/2 m or longer) are required, you might consider using two straps of unequal lengths (30% difference usually recommended) to minimize any possible resonance effects. However, you must maintain maximum separation between these straps to reduce mutual inductance. (Straps close to each other exhibit about the same impedance as one strap; wide separation reduces total impedance to almost 50% of a single strap.)
Conclusion. Keep bonding straps broad and short if possible.
Bonding the SRG to building steel
In small areas having a single power source and a small equipment configuration, there's probably no need for an SRG. In large areas, however, where interconnected equipment is concentrated and especially where there's more than one panelboard or computer power supply [ILLUSTRATION FOR FIGURE 4 OMITTED], you should provide an SRG. In this case, you must decide on whether to bond the SRG to building steel (every available column) or to a common ground point. (A common ground point is often called a single point ground; this term is misleading because, except at DC, distributed inductance and capacitance prevent a true single point ground return.)
A single point ground that is effective over a large area is not possible.
An SRG, on the other hand, can provide noise reduction for a large area of interconnected equipment and can be connected to building steel or another return path to earth at a common point, or at many points. Either approach provides protection from noise and meets NEC requirements for a safety ground effective at power line frequencies.
A common mistake by designers and installers is to have the SRG connected to building steel at two or three points. You should either connect the SRG to an earth return at only one point, or connect it at as many points as possible. If the SRG is connected to building steel at two points, then a well defined path for lightning or other transients exists right under the computer floor. Because a single point ground must be maintained to assure its continuing integrity, the usual recommendation is to connect the SRG to all available building steel.
Conclusion. Multipoint bonding of an SRG to all available building steel is usually best.
Special note: when making the SRG and ground connections as shown in Fig. 6, you must adhere to the following guidelines.
* NEC and local codes must be followed.
* All equipment must be bonded to the SRG using low impedance risers. Never connect to the strip closest to the outside wall.
* Every 6th floor pedestal in each direction must be connected to the SRG with a No. 6 AWG concentric copper conductor.
[TABULAR DATA FOR TABLE 4 OMITTED]
* All columns, conduits, water pipes, etc. entering the computer room must be bonded to the SRG (at each side of the room if these are horizontal).
* Power distribution panels and centers should be mounted directly to the building steel or bonded to it by a short length of grounding conductor equal to the "green wire ground", but at least a No. 4 AWG copper.
* The grounding wire inside any enclosure or panel supplying AC power to the computer must be bonded to its enclosure.
Richard E. Singer is Applications Engineering Manager, ERICO, Inc., Solon, Ohio.