Appendix 9 - 26-04 - Signal Reference Grid for Grounding.docx

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Minimum Technical Specification for Signal Reference Grid for Grounding DISA Facilities Engineering Circular Attachment 26-04

Effective Date: 25 February 2020

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.

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.

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)

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.

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.

Requirements

EXISTING CONDITIONS

When modifying an existing SRG, update the SRG 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.

REFERENCE GROUNDING BOND GRID

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

Figure 1: Reference Grounding Bond Grid

The SRGs of different computer rooms are only bonded together when the rooms are on the same floor. All other connections as shown in Figure 1 apply to all SRGs in the building.

SRG CONFIGURATION for Computer Rooms with Access Floors 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:

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:

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

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

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

Additional signal reference grounding conductors are required at the following locations:

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

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

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:

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)

Figure 3: Low-Impedance Copper Risers

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.

Connect a 1 inch braided copper strap to opposite sides of the enclosure 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.

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

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

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:

Figure 4: Grid Pattern Secured to Floor Pedestals

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:

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

Figure 5: Mounting for Copper Straps

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

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

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

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

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

Additional signal reference grounding conductors are required at the following locations:

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

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

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

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

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).

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 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 bonding clamps can also be used if they are securely clamped and tightened to the pedestals.

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

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.

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

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

SRG Configuration for Computer Rooms without Access Floors Configuration to be developed.

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