Appendix C - Signal Reference Grid for Grounding.pdf

UNCLASSIFIED – FOR OFFICIAL USE ONLY

MINIMUM TECHNICAL SPECIFICATION FOR

SIGNAL REFERENCE GRID FOR GROUNDING

DATA CENTER MECHANICSBURG – APPENDIX C

EFFECTIVE DATE: 22 JULY 2020

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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 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.

5.2. REFERENCE GROUNDING BOND GRID

As a basis for the SRG, a reference bonding grid shall be installed in accordance with Figure 1 and

MIL-HDBK-419.

Bu ild in g

St ee l

Earth Electrode Subsystem/Counterpoise

IT Cable Tray Above Access Floor

First Floor Computer Room (1), with Access Floor

Access Floor

Pedestals

IT

Equip.

Racks

IT Cable Tray Above Access Floor

Access Floor

Pedestals

IT

Equip.

Racks

Dedicated ground bus bar for SRG use only.

Isolated and prominently labeled

Isolated, Insulated Conductors, 600 KCMIL (typ)

Isolated, Insulated

Conductors 4/0 (typ). Bond

SRGs together when on the same floor

Perimeter 4/0 Bare Copper Conductor

750 KCMIL (typ)

Isolated, Insulated Conductors, 600 KCMIL (typ)

Isolated, Insulated

Conductors 4/0

Isolated, Insulated

Conductors 4/0

First Floor Computer Room (2), with Access Floor

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.

SIGNAL REFERENCE GRID FOR GROUNDING

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5.3. 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.

Figure 2: Integrated Access Floor with Copper Strap SRG

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

5.3.1. 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. Configure a grid arrangement (see Figure 3) in accordance with the following:

2 Feet

2 Feet

Signal Reference Grid Mounted on Floor Pedestals

Figure 3: Grid Pattern Secured to Floor Pedestals

SIGNAL REFERENCE GRID FOR GROUNDING

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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 4, preferred method)

2-lnch-

Support

Figure 4: Mounting for Copper Straps

(2) One-inch-wide continuous flat metal braids with copper core (see Figure 5).

(3) #6 AWG continuous bare copper conductors (see Figure 5).

Stanchion

Horizontal Stringer

Subfloor

Floor Pedestal

Conductive Pedestal Bonding Clamp

Flat Metal Braid with Copper Core or

Bare Copper Conductors

Figure 5: Mounting for Flat Metal Braids or Bare Copper Conductors

b. Bond the SRG 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. Additional signal reference grounding conductors are required at the following locations:

(1) 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.

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

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

e. Install grounding conductors between each equipment enclosure (except PDUs, transformers, CRAC or CRAH units) and the SRG using a #6 AWG bare copper conductor with the length as short as possible, not to exceed 24 inches.

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.

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