Applying SPG for an ITE complex is eased by understanding four standardized (yet simple) variations of the IBN
In Part 3 of this article (September 2007), we described how single-point grounding (SPG) is not nearly as straightforward as multipoint grounding systems. Using this improved understanding of SPG and equipotential systems, we can now take a look at variations of isolated (insulated) bonding networks (IBNs), including star, mesh, and sparse-mesh types — and discuss their connection to the T(M)GB.
Isolated (insulated) bonding network
SPG for an IBN must be derived from the common bonding network (CBN). IEEE Std 1100-2005, “The Emerald Book,” points out that although the internationally derived term is ‘isolated bonding network’ (IBN), the BN is not isolated from the building's telecommunications grounding and bonding infrastructure (described in Part 1 of this article — July 2007). An IBN is insulated so as to maintain a grounding connection only at a controlled physical location.
From Part 2 of this article (August 2007), we noted from a reference “the purpose of a BN is to reduce the magnitude of the transfer function by controlling the design of how the BN is attached to the CBN.” The IBN attachment method employed is blocking — isolated to effectively one galvanic connection. Note that this connection is at the “systems” level.
It's important to recognize that the IBN is harmonized with international and national standards. The Table (click here to see Table) provides identification and a brief description of four variations of IBNs. The distinction of these four variations allows all interested parties to readily identify the variation(s) of IBNs addressed at a given location.
Variations of isolated (insulated) bonding networks (IBNs)
The IBN generic concept is illustrated in Fig. 1. For simplicity, the DC power system is not shown. For details on IBN power and grounding, see “The Emerald Book,” Chapter 9.
From Fig. 1, the following items should be understood:
ITE and other metal structures declared as part of the IBN must be sufficiently insulated (for example, at 10kV) from the CBN so as to be grounded only by the grounding wiring to the single point connection bar (SPCB).
The SPCB is shown with transparency to illustrate that it is connected to the CBN and T(M)GB (which is part of the CBN).
IBNs are historically DC powered, but the concept remains intact even where the IBN is totally powered from AC.
Grounding conductors (i.e., DC equipment grounding conductor (DCEG), AC equipment grounding conductor (ACEG), shields of metallic links such as coax, etc.) entering into the IBN must enter within the designated area of the single-point connection window (SPCW), be referenced to the SPCB, and be insulated from the CBN from there forward. Note: The SPCW has historical reference to the “ground window,” a term not recommended by “The Emerald Book.”
The SPCB must be of restricted size to control common impedance across the bar during transients.
An IBN is typically more explicit and visible in a restricted access area, especially for telecommunications service provider (TSP) DC-powered equipment, such as a dedicated equipment room. Compare this to a typical office area in a commercial building where the SPG may be undeclared, but still provided by the IGR circuit (Sparse CBN) or otherwise by AC circuits as previously described. The recent trend is for TSPs to deploy ITE into a CBN, citing the IBN as too maintenance-intensive for the grounding. In order to accomplish this, the TSPs typically require the ITE to meet stringent testing requirements specified in Telcordia document GR-1089-CORE-2004. However, there are still TSPs and other users holding on to the IBN concept because it is tried and true for blocking transient currents into the ITE.
Interestingly, the IBN concept is given significant attention at IET's 2007 tutorial workshop in London on Earthing & Bonding Techniques for Electrical Installations. Indeed, the IBN concept is still viable, even for regions entrenched to EN 50310 and Mesh-BNs. A great benefit of the IBN is the inherent ability to measure and monitor AC and DC currents on the SPG wiring. The measurement results readily lead to identifying wiring errors, insulation breakdown, and defective ITE. Compare this testing feature to the difficulty in trying to locate defective ITE in a CBN. Interestingly, the “intensive-maintenance” argument can also be brought to bear on the NEC requirement for SPG of the power system neutral. Inadvertent multi-grounding of the neutral downstream from the system grounding point is a commonly recognized finding during site grounding evaluations. However, don't expect the NFPA to soon forego that requirement due to maintenance issues.
Testing the IBN for integrity involves measuring the isolation (insulation) resistance. For detailed information on measuring the IBN, see “The Emerald Book” and Telcordia GR-295-CORE-2004. Continuous monitoring (with alarm function recommended) for leakage DC and stray AC at strategic SPG locations is recommended. The net effect is that if leakage current can flow on the grounding system, so can lightning and surge currents.
The IBN by itself does not ensure ITE will meet electromagnetic compatibility (EMC) requirements or objectives. ITE having a regulatory mark (such as CE) does not ensure its electromagnetic immunity when placed into the IBN. The ITE, IBN, and the CBN perform as a system, and you must coordinate the desired immunity margin to accomplish EMC. Supplemental grounding and bonding provided by the CBN and the IBN are key factors in achieving acceptable EMC.
Star isolated bonding network
An SIBN is equivalent to the IBN except the grounding conductors within the ITE block are specifically arranged into a star or radial pattern (Fig. 2). Advantages can include:
An increased ability to monitor and measure the grounding conductor to a specific ITE unit.
Reduction of magnetic energy induced into the ITE due to absence of ground loops within the block.
Mesh isolated bonding network
An MIBN is equivalent to the IBN except the grounding conductors within the ITE block are specifically arranged into a mesh pattern (Fig. 3 on page 38). The density of the mesh is determined by the manufacturer, user, or both. The mesh is typically designed into the ITE block by means of metal racks, cable tray, raceway, metal sheets, etc. Although difficult and rarely performed, the MIBN can be arranged to include an insulated version of a mesh common bonding network (MCBN) installation under the raised floor, at the floor level (metal structure of the raised floor), or above the cabinet or rack (i.e., superstructure). The MCBN was described in Part 2 of this article (August 2007). Effectively, the insulated version of the MCBN is incorporated into the MIBN and must follow IBN grounding rules. You can also describe such structures as insulated bonding mats. Advantages of an MIBN can include:
Approximation to a reference plane whereby utilized single-ended circuits or low common-mode rejection ratio (CMRR) balanced circuits are made less susceptible to common-mode currents flowing between ITE units. Note that such circuits are atypical for modern data centers and telecommunications facilities.
Increased electromagnetic shielding for the ITE block even though the block is IBN. This may become important where the ITE block is located near high power RF sources.
The ability to bond the DC power circuit return conductor (which is system grounded) to the MIBN at multiple locations within the ITE block. Note that this practice, although required by ETSI ETS 300 253 for European telecommunications facilities, is not recommended by “The Emerald Book.”
Sparse mesh isolated bonding network
An S-MIBN is equivalent to the IBN except the grounding conductors within the ITE block are specifically arranged into a mesh pattern (Fig. 4). The density of the mesh is not a major concern and is determined by the manufacturer, user, or both. Advantages can include:
Easy configuration of ITE units within the ITE block.
Some limited ability to monitor and measure the grounding conductor to a specific ITE unit.
Supplemental grounding for SPG circuits
We previously raised the question regarding the value of supplementing an SPG path. The following statements are useful in making such a determination.
Where inductance of the existing wiring path is desired to be lowered, a supplemental path is useful.
Where reliability of the existing wiring path is paramount, a supplemental path is useful.
Where an IBN configuration is utilized, supplemental paths within the ITE block may be encouraged, especially for mesh variations.
For general/typical AC branch circuits that are intentionally (IGR circuit) or unintentionally performing as a SPG path, a supplemental path is of little added value.
Supplemental grounding and bonding are more understandable when divided into segments addressing the telecommunications grounding and bonding infrastructure, ITE multipoint bonding networks, and ITE single-point bonding networks. It's important to be able to recognize the grounding and bonding topology actually applied to the ITE, whether AC or DC powered. By identifying against a standardized industry term, all parties involved can cogently analyze the grounding and bonding arrangement. In addition, there are recognized differences in North America and European approaches to supplemental grounding and bonding.
However, there is significant harmonization of both approaches to international standards. As a single source reference, “The Emerald Book” provides significant guidance on grounding and bonding electronic equipment and is a recommended practice. ITE should use supplemental grounding and bonding for improving performance and safety, especially for the more common multipoint grounding arrangements, such as a MCBN. In some instances, supplemental grounding and bonding for a single-point grounding arrangement is useful and can be accommodated. Complications arising from grounding of multi-powered ITE are beyond the scope of this article.
Bush is director of research — power & grounding for Panduit in Tinley Park, Ill.