Appendix_K_-_SRG_Specification.pdf

DISA Standard 360-95-3, Attachment 26-44-00 SRG Design Options

The high operating frequencies of modem computer equipment require that computer room products be supported both by a good equipment-grounding system and an effective signal reference grid. When there is no effective signal reference grid, points of excessive impedance and excessive voltage often develop between equipment enclosures. These can cause momentary losses of data integrity between critical system components.

An effective signal reference grid creates multiple, parallel, low impedance conducting paths between the various equipment enclosures within the computer room. When the impedance of any one ground-conducting path is too high because of full or partial resonance, other ground-conducting paths of lower impedance are available. The signal reference grid equalizes the impedance levels throughout the computer-room floor structure, establishing an equipotential reference point for all computer-room equipment installed on and connected to it. This design greatly minimizes the impact of any individual interference point within the floor.

One of the following approaches shall be used to create a signal reference grid:

1. Construct a new raised-floor structure connected to a separate grid of copper straps that are bonded together and laid on the subfloor. This option, which creates a superior signal reference, is described in Option 1: New Raised Floor with Subfloor Reference Grid. (See also Figure 1-1).

2. Upgrade an existing raised-floor structure by attaching a new signal reference grid to the existing floor pedestals, so that it is suspended just under the raised floor. This option, which requires working around existing equipment, is described in Option 2:

Upgrade Existing Raised Floor Structure.

Reference Grounding Bond Grid

In addition to the Signal Reference Grid a grounding bond system as shown on the SRG Bonding detail (Figure 1-2) shall be installed to provide a solid reference system for the SRG system. The grounding bond system shall consist of a dedicated connection to a dedicated isolated SRG Ground bus bar which is connected to the building earth ground system and to the building steel.

In addition, each SRG subsystem is to have a 4/0 AWG bare copper perimeter ground conductor which is connected back to the dedicated SRG ground bus bar. The reference for this is the Multi Point Ground Configuration Per Mil Handbook 419A Vol 1 Paragraph 5.3.3

The best Signal Reference Grid design is to incorporate a separate signal reference grid located under the raised floor. All equipment, as well as the raised floor structure, shall be connected to that grid.

FIGURE 1-1 SRG GENERAL DETAIL

FIGURE 1-2 SRG BONDING DETAIL

Option 1: New Raised Floor with Subfloor Reference Grid

The optimum signal reference system for a computer room is a bolt-down raised-floor that has its undersurface connected to a separate signal reference grid of continuous 2”x26 ga copper straps on 2’ centers, perpendicular to each other secured to the subfloor. This arrangement integrates the benefits of having a metallic barrier under the computer equipment and of having a n effective signal reference grid that creates multiple, parallel, ground-conducting paths between the various equipment enclosures.

To construct the integrated raised floor with subfloor signal reference grid, perform these steps:

1. Work with the contractor to plan the exact layout of all computer-room equipment (for example, metal power conduits or raceways, equipment cables, floor pedestals, PDU’s, CRAH u n i t s , computer equipment, PDU stands, CRAH stands, and so on).

2. Use tape or paint to mark the intended locations of the floor pedestals on the computer-room subfloor.

3. Attach the floor pedestals to the subfloor.

4. Prior to installing any other equipment install continuous 2-inch-wide, 26-gauge copper straps on the subfloor as shown in Figure 1-1. Securely bond the copper straps together at their intersections. The minimum acceptable grid pattern for these copper straps is a 2-foot by 2-foot square (that is, one copper strap installed parallel and perpendicular with each row of horizontal stringers). Around the perimeter install a 4/0 stranded copper loop and connect the 2” wide straps to the loop on not less than 6’ centers.

5. Install all equipment that is mounted on the subfloor including water pipes and cable trays. If metal raceways or rigid metal conduits are being used to distribute power conductors throughout the computer room, install them at this time. Flexible metal conduits (LFMC), if used, can be installed later in this procedure. Note that LFMC are not required to be connected to the SRG.

6. Install/connect to existing grounding system within the building as shown in SRG Bonding detail

Figure 1-2.

7. Install signal reference grounding conductors at these locations:

• Install a #6 AWG bare copper conductor attached to the SRG to all ducts, water pipes, metal conduits, cable trays, and metal raceways that enter the computer room. These items, not shown in Figure 1-1, should be bonded to the perimeter loop conductor at each end of the computer room.

• Install or connect to existing SRG Bonding system per Figure 1-2.

• Bond every 3rd raised-floor pedestal (ie 6’ centers) as shown in Figure 1-1 to the SRG with a #6

AWG concentric copper conductor.

8. Install the horizontal stringers onto the stanchions at the tops of the floor pedestals. Screw a metal bolt with a metallic washer down through the hole at both ends of each horizontal stringer until the horizontal stringer is firmly bolted onto the stanchion (see Figure 1-3).

FIGURE 1-3 Bolted connections at floor pedestals

9. If flexible metal conduits are being used to distribute power conductors throughout the computer room, have the contractor install them at this time. Also install cable tray and bond to SRG.

10. Install the floor tiles where they are needed to support the computer-room equipment.

11. Install the computer-room equipment, including equipment enclosures/racks, PDU’s and CRAH units

(if they have not previously been mounted on the subfloor), on top of the floor tiles.

12. Install bonding conductors between each system or peripheral enclosure and the SRG. Perform one of these two procedures:

• Connect a copper strap, 2”x26 ga. copper conductor riser like that shown in Figure 1-4 between each equipment enclosure chassis and the nearest subfl oor grid strap.

• Or connect a flexible #6 AWG concentric copper conductor on oppos i te s ides of the enclosure wi th a UL l is ted pedes ta l c lamp to the nearest floor pedestal.

• Or use a 1” braided copper strap to connect to opposite sides of the enclosure.

Figure 1-4 SRG to Equipment connection

13. Install signal reference grounding conductors at these locations (see Figure 1-1 ):

a. Install a concentric copper conductor (minimum size of #4 AWG or as required to meet the requirements of NFPA 70) between the XO terminal of each PDU transformer or stand-alone isolation transformer and the earth-grounding system, such as a nearby vertical steel column or grounding plate. The conductor must be connected to the steel column by an exothermic bond weld. In addition connect the enclosure to the SRG using either a #6 bare copper conductor or a 2”x 26 ga copper conductor.

14. Before installing the remaining floor tiles, use an ohmmeter to verify good continuity in the new signal reference grid. If the resistance measured between any intersection on the grid and any enclosure grounding conductor exceeds 1 ohm, recheck all connections (between floor pedestals, SRG straps, and equipment enclosures) to ensure that they are secure. Provide documentation of all testing to certify that all connections have been verified to provide a complete SRG system.

15. When the measured continuity of the SRG is acceptable, install all remaining floor tiles.

NOTE: THIS OPTION SHOULD BE USED FOR ALL NEW RAISED FLOOR AREAS AND ALL AREAS

WHERE THE RAISED FLOOR IS BEING TOTALLY REPLACED.

To Equipment

Low-Impedance Copper

Signal Reference Grid (2-lnch-Wide Copper

Option 2 : Upgrade Existing Raised-Floor Structure

Computer rooms containing raised-floor structures that did not have a SRG system installed must be upgraded to have a continuous SRG system.

A raised-floor structure in a computer room can be upgraded to perform as an effective signal reference grid by following this procedure:

WARNING: Before you remove any floor tiles, make certain that the metallic subsurface of the tiles have not been galvanized by an electroplating process. Such a material generates particles that become airborne with normal handling and become a hazard to electronic equipment. Galvanized surfaces that have been prepared by a hot-dipped process do not have this problem.

1. Remove as many floor tiles as possible to gain unobstructed access to the raised- floor pedestals and horizontal stringers. If floor tile cannot be removed because of equipment or cable placement, access its support structures from an adjacent floor- tile opening.

2. To provide a continuous SRG system on an existing floor use one of the following methods:

a. Two-inch-wide copper straps (#26 AWG), which are secured to the floor pedestals by U L l i s t e d p e d e s t a l mounting clamps (see Figure 2 - 2 )

b. One-inch-wide flat metal braids with copper subcenters, which are mounted to the floor pedestals with steel pipe clamps (see Figure 2-3)

c. #6 AWG bare copper conductors routed on 2’ centers each direction and bonded to the floor pedestals with UL listed clamps (see photo Figure 2-1)

Note. The 2-inch-wide copper straps ensure a better signal reference grid than the 1-inch- wide metal braids or the #6 conductors. The wider straps provide a larger conductive surface, so higher frequencies f o r example, 20 MHz can be carried with minimal self-inductance

Note. If the raised-floor pedestals are laminated with non conductive material, you must scrape away the laminate material to expose metal where the straps are clamped to the pedestals.

If you are using I -inch-wide flat metal braids, install one long braid (or an interconnecting series of shorter braids) continuously along rows of floor pedestals (see Figure 2-4). To ensure optimum continuity and reliability, use a UL listed bonding clamp to connect the braid to the top of each floor pedestal, just below the stanchion. This design should still leave ample clearance on the subfloor for pipes, conduits or raceways, and cables.

To create a higher frequency of resonance and optimize the effectiveness of the signal reference grid, the braids should be installed in the smallest grid pattern possible. Observe these guidelines:

• If possible, install the braids underneath every row of horizontal stringers to create a series of 2-foot by 2-foot grids (as shown in Figure 2-4).

3. If a grid of 1 -inch-wide metal braids has been installed, connect a flexible, laminated copper ground strap between each equipment enclosure and the nearest floor pedestal (see Figure 2-6).

Bolt the end of the strap that has approximately 0.5 inch (1.3 cm) of exposed tin plating to a mounting screw inside the enclosure. Route the ground strap out through the bottom of the enclosure door and down the edge of the floor tile. Secure the end of the ground strap that has 4 to 8 inches (10 to 20 cm) of exposed tin plating to the nearest floor pedestal, using a steel pipe clamp, which provides the optimum connection and durability. UL listed binding clamps can also be used if they are securely clamped and tightened to the pedestals.

FIGURE 2-1 #6 BARE COPPER SRG

Figure 2-2. Grid of 2-Inch-Wide Copper Straps

2-lnch-

Figure 2-3. Grid of 1-Incb-Wide Flat Metal Braids

Signal Reference Grid (1-lnch-Wide Flat Metal Braid)

Figure 2 -4 . Grid Pattern for Metal Braid

Horizontal

Figure 2-5. Low-Impedance Copper Risers

To Equipment

Low-Impedance Copper

Signal Reference Grid (2-lnch-Wide Copper

Figure 2-6. Enclosure Ground Connections

4. Continue the SRG to the perimeter of the raised floor area.

5. Connect the new SRG to a grounding Grid per Detail 1-2 with a perimeter conductor using a 4/0

AWG bare conductor, and a 600 KCMIL conductor back to the dedicated SRG ground bus bar and the building steel and a 750 KCMIL conductor to the building counterpoise or earth ground system.

6. Note that all transformers for PDU’s are to be connected to an independent grounding system that connects back to the building grounding system. The enclosures should be connected to the SRG.

7. Before replacing the floor tiles, use an ohmmeter to verify good continuity in the upgraded raised-floor structure. If the resistance measured between any floor pedestal and any enclosure grounding conductor exceeds I ohm, recheck all connections (between floor pedestals, straps or braids, and equipment enclosures) to ensure that they are secure.

8. When the measured continuity of the raised-floor structure is acceptable, replace the floor tiles that were removed in Step 1.

9. Reference MIL-HNDBK-419A, MIL-STD-188-1248, and NFPA 70 for additional requirements.

EMI Gasket

Front Door or Rear Door

Equipment Enclosure

Ground Strap Bolted to enclosure on Opposite sides.

Horizontal Stringer Floor Tile

UL listed SRG floor pedestal clamp

Floor Pedestal

Subfloor

NOTE: This standard is generally designed for installation of a SRG below a raised floor either existing or new. Should there be a facility that is not on a raised floor a SRG can be created above the ceiling with #6 AWG conductors on a 2’x2’ grid, supported off of the ceiling or deck. Exact details will have to be decided based on the construction of the room.