C - Signal Reference Grid for Grounding.pdf

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MINIMUM TECHNICAL SPECIFICATION FOR

SIGNAL REFERENCE GRID FOR GROUNDING

DATA CENTER COLUMBUS – APPENDIX C

EFFECTIVE DATE: 12 JUL 2019

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1. PURPOSE

This specification 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

Provide a complete SRG in accordance with applicable codes and standards by installation of a complete new system or by modifying an existing SRG.

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.

DEFENSE INFORMATION SYSTEMS AGENCY (DISA)

DISA Facilities Engineering Circular

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.

SIGNAL REFERENCE GRID FOR GROUNDING

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5. REQUIREMENTS

5.1. EXISTING CONDITIONS

When modifying an existing SRG, update to meet the minimum requirements of this standard. When the installed SRG does not require replacement of components, the modification to the SRG typically uses the same configuration. For computer rooms with access-floors, ensure the floor tile metallic underside is not galvanized by an electroplating process. Galvanized by a hot-dipped process is acceptable.

5.2. REFERENCE GROUNDING BOND GRID

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

Figure 1: Reference Grounding Bond Grid

SIGNAL REFERENCE GRID FOR GROUNDING

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5.3. SRG CONFIGURATION

Plan the layout of the SRG based on the exact layout of all computer room equipment (i.e., metal power conduits or raceways, equipment cables, floor pedestals, computer equipment). Use of an access-floor itself to create the SRG is not acceptable. The incorporation of a separate SRG, located under the access-floor, and connected to all equipment as well as the metallic undersurface of the access-floor is required.

Configure the SRG in 2 foot by 2 foot (maximum) grids in accordance with one of the following:

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

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

a. The 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). Note, the copper strap grid is typically installed after the floor pedestals but before other equipment.

Figure 2: Integrated Access-Floor with Copper Strap SRG

b. Bond the copper straps to the 4/0 copper perimeter loop at not less than 6 foot centers.

c. Install the SRG Grounding Bond system in accordance with Figure 1.

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

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

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

(3) Bond every 3rd access-floor pedestal (minimum, 6 foot centers) to the SRG with a #6 AWG concentric copper conductor (See Figure 2).

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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) Connect a copper strap, 2 inch, 26-gauge copper conductor riser (see Figure 2 and Figure 3) 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 3: Low-Impedance Copper Risers

(2) 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.

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

f. When the SRG is installed, bonded and grounded, ensure the continuity of the SRG by measuring the resistance. If the resistance exceeds 1 ohm, all connections must be rechecked (between floor pedestals, SRG straps, and equipment enclosures) to ensure the connections are secure.

(1) Record all testing to certify connections are verified and less than 1 ohm.

(2) Once acceptable continuity is confirmed, the remaining floor tiles may be installed.

5.3.2. Secure SRG to Floor Pedestals (Existing Access-Floor)

Connection of the SRG to the Floor Pedestals is permitted for computer rooms where the access-floor and equipment are physically in the computer room at the start of the project. This configuration is used for a new SRG or when modifying an existing SRG. Configure a grid arrangement (see Figure 4) in accordance with the following:

2 Feet

2 Feet

Signal Reference Grid Mounted on Floor Pedestals

Figure 4: Grid Pattern Secured to Floor Pedestals

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a. Mount the SRG to the bare metal just below the stanchions on each floor pedestals using UL listed pedestal bonding clamps and one of the following:

(1) Two-inch-wide #26 AWG continuous copper straps (see Figure 5, preferred method)

2-lnch-

Support

Figure 5: Mounting for Copper Straps

(2) One-inch-wide continuous flat metal braids with copper core (see Figure 6).

(3) #6 AWG continuous bare copper conductors (see Figure 6).

Stanchion

Horizontal Stringer

Subfloor

Floor Pedestal

Conductive Pedestal Bonding Clamp

Flat Metal Braid with Copper Core or

Bare Copper Conductors

Figure 6: Mounting for Flat Metal Braids or Bare Copper Conductors

b. Bond the copper straps or bare copper to the 4/0 copper perimeter loop at not less than 6 foot centers.

c. Install the SRG Grounding Bond system in accordance with Figure 1.

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

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

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

(3) Bond every 3rd access-floor pedestal (maximum 6 foot on centers) to the SRG with a #6 AWG concentric copper conductor (See Figure 2).

e. Install grounding conductors between each equipment enclosure (except PDUs, transformers, CRAC or CRAH units) and the SRG.

(1) For grids of copper straps, connect either a #6 bare copper conductor or a 2-inch-wide 26 gauge low impedance copper riser (preferred) between each equipment enclosure chassis and nearest copper strap of the SRG (see Figure 1).

(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 7). 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

SIGNAL REFERENCE GRID FOR GROUNDING

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Front or Rear Door

Ground Strap

Bolted to E on Opposit

Equipment En

Figure 7: Enclosure Ground Connections for Metal Braids or Bare Conductors

f. When the SRG is installed, bonded and grounded, ensure the continuity of the SRG by measuring the resistance. If the resistance exceeds 1 ohm, all connections must be rechecked (between floor pedestals, SRG straps, and equipment enclosures) to ensure the connections are secure.

(1) Record all testing to certify connections are verified and less than 1 ohm.

(2) Once acceptable continuity is confirmed, the remaining floor tiles may be installed.

(3) Submit testing report.

5.3.3. Configuration for SRG in Computer Rooms without Access-Floors

Configuration to be developed.

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