Appendix B.pdf

Appendix B

UNCLASSIFIED – FOR OFFICIAL USE ONLY

MINIMUM TECHNICAL SPECIFICATION FOR

SIGNAL REFERENCE GRID FOR GROUNDING

FY21 MEC REAR DOOR HEAT EXCHANGER ATTACHMENT C

EFFECTIVE DATE: 12 JUL 2019

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1. PURPOSE

This standard details the requirements Signal Reference Grid (SRG) for grounding. All DISA data centers are required to utilize SRG systems to prevent excessive impedance and excessive voltage from developing between equipment enclosures.

2. SCOPE

IT equipment racks installed as part of this project shall be connected to the existing SRG in accordance with this standard.

3. REFERENCES

The publications listed below form a part of this specification to the extent referenced. The publications are referred within the text by the basic designation only. The current revision of the reference, including all issued updates, must be followed.

NATIONAL FIRE PROTECTION AGENCY (NFPA)

NFPA 70 National Electric Code

U.S. DEPARTMENT OF DEFENSE (DOD)

MIL-HDBK-419 Grounding: Bonding, and Shielding for Electronic Equipments and Facilities; Volume 1 (Basic Theory) and Volume 2 (Application)

MIL-STD-188/124 Grounding, Bonding and Shielding

3.1. MODIFICATION OF REFERENCES

The advisory provisions are mandatory. Substitute "must" for "should" wherever it appears and interpret all references to the "authority having jurisdiction" and "owner" to mean the Contracting Officer and DISA.

4. DEFINITIONS

AWG. American Wire Gauge

CRAC. Computer Room Air Conditioning

CRAH. Computer Room Air Handling

PDU. Power Distribution Unit.

DISA. Defense Information Systems Agency, Facilities Engineering Denver.

SRG. Signal Reference Grid.

5. REQUIREMENTS

5.1. EXISTING CONDITIONS

Connect the IT racks installed as part of the project to the existing SRG, matching the existing configuration.

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5.2. REFERENCE GROUNDING BOND GRID

As a basis for the SRG, a reference bonding grid is installed in accordance with Figure 2 and MIL-HDBK-419 Volume 1, paragraph 5.3.3.

Figure 1: Reference Grounding Bond Grid

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5.3. SRG CONFIGURATION

The layout of the SRG is based on the exact layout of all computer room equipment (i.e., metal power conduits or raceways, equipment cables, floor pedestals, computer equipment). The access-floor itself is not used to create the SRG. The SRG is located under the access-floor, and connected to all equipment as well as the metallic undersurface of the access-floor, configured in 2 foot by 2 foot (maximum) grids in accordance with the following:

5.3.1. Secure SRG to Subfloor (New Access-Floor Installations)

This configuration is used for all new access-floors where equipment is not physically in the computer room at the start of the project. The grid arrangement is configured in accordance with the following:

a. When the SRG is installed before equipment is physically in the computer room, continuous two-inch-wide copper straps (#26 AWG) are placed in a 2 foot by 2 foot grid (maximum), bonded at every intersection (see Figure 2).

Figure 2: Integrated Access-Floor with Copper Strap SRG

When the SRG is installed after equipment is physically in the computer room, the SRG is connected to the access floor pedestals using UL listed pedestal bonding clamps (see Figure 3).

2 Feet

2 Feet

Signal Reference Grid Mounted on Floor Pedestals

Figure 3: Grid Pattern Secured to Floor Pedestals

b. The SRG is bonded to the 4/0 copper perimeter loop at not less than 6 foot centers.

c. SRG Grounding Bond system is installed in accordance with Figure 1.

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d. Signal reference grounding conductors are at the following locations:

(1) SRG is bonded to all ducts, metal pipes, metal conduits, and cable trays entering the computer room using a #6 AWG (minimum) bare copper conductor.

(2) SRG is connected to the existing SRG bonding system (See Figure 1).

(3) SRG is bonded every 3rd access-floor pedestal with a #6 AWG concentric copper conductor (See Figure 2).

e. Bond conductors between each system or peripheral enclosure (except PDUs, transformers, humidifiers, CRAC or CRAH units) to the SRG by in accordance with one of the following:

(1) For grids of copper straps, perform one of the following:

(a) Connect a copper strap, 2 inch, 26-gauge copper conductor riser (see Figure 2 and Figure 4) between each enclosure chassis and the nearest subfloor grid strap. (preferred method)

To Equipment Enclosure

Low‐Impedance Copper Riser

SRG

(2‐inch wide

Copper Straps)

Figure 4: Low-Impedance Copper Risers

(b) Connect a flexible #6 AWG concentric copper conductor on opposite sides of the enclosure with a UL listed pedestal clamp to the nearest floor pedestal.

(2) For grids of metal braids or bare conductors, connect a flexible, laminated copper ground strap between each equipment enclosure and the nearest floor pedestal (see Figure 5). Bolt the end of the strap that has approximately 1/2 inch 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 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.

Equipment

Enclosure

Ground Strap Bolted to Enclosure

Horizontal Stringer Floor Tile

Conductive Pipe Clamp

(or Hose Clamp)

Floor

Front or Rear Door

Ground Strap

Bolted to Enclosure on Opposite Sides

Horizontal Stringer Floor Tile

UL Listed Conductive Clamp

Equipment Enclosure

Floor

Pedestal

Floor Pedestal

Subfloor

Figure 5: Enclosure Ground Connections for Metal Braids or Bare Conductors

(3) Connect a 1 inch braided copper strap to opposite sides of the enclosure

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