Appendix L - HC102812R0067.pdf

APPENDIX L

Excerpt from CSI 360-95-3, Facilities Standards

26 44 00 Signal Reference Ground (SRG)

Signal reference ground (SRG) system shall be installed in all DISA datacenters. This is a method of keeping the shields of cables and chassis of IT equipment racks at the same zero potential as the derived system and of reducing impulse noise currents in IT equipment. The

SRG shall be bonded to the building’s earth ground electrode(s). The purpose of the SRG is to provide an equipotential plane multi-point ground system per Volume 1, Chapter 5 of

MILHBK419A. Existing facilities without an SRG shall be retrofitted with one.

SRG Designs ITIC Grounding Guidelines

Building Steel

Building steel shall be bonded together and the main vertical steel columns shall be earth grounded.

Raised Floor Signal Reference Grid (SRG)

The use of the raised floor supporting structure is permissible to use as an SRG provided: a bolt down stringer system is installed, and; the stringer system is plated with tin or zinc to allow low resistance pressure connections to be made. An SRG made of (minimum) #4

AWG bare copper wire shall be installed below the access floor. The SRG shall be bonded on two-foot centers and cover the full area beneath the raised floor.

Perimeter Conductor

A perimeter conductor (bonding conductor), sized no less than a class-A, #4 AWG, concentric stranded copper conductor shall be installed at the perimeter of the computer room, beneath the raised floor. The perimeter conductor shall be copper. The conductor shall be bonded to each raised floor pedestal and building steel column where possible.

Peripheral Electrical Panels

All electrical panels within the computer room shall be bonded to the SRG.

HVAC Ground

All HVAC equipment within the computer room shall be bonded to the SRG.

Pipes and Conduit

All metallic pipes and conduct entering the computer room shall be bonded to the SRG at the point of entry into the computer room. Cable trays beneath the raised floor shall also be bonded to the SRG.

IT Equipment

All IT equipment racks, zone distribution cabinets, stand-alone frames, mainframe bodies, and similar shall be bonded to the SRG with flat braided copper straps to allow equipment installed in each enclosure to have a common ground reference. Nonconductive rack rails

(used for mounting equipment inside the racks) inside of equipment racks shall be installed and/or retrofitted with conductive rails or covers which are bonded (electrically conductive low-impedance path) to the SRG through the rack bonding lug. Painted or powder-coated rack rail systems are not considered electrically conductive without bonding straps or other direct electrical connection.

Legacy “strings” of adjacent racks, up to six (6) standard racks in width, when properly bonded together may act as single enclosure and may remain bonded to the SRG by only a single or double connection at one or both ends of the string. Care needs to be taken to ensure that no “ground loops” are created in the SRG system, as these can couple signals between systems and disallow direct high-frequency paths to ground.

All new equipment installations, including of individual server and other IT equipment in existing racks, shall specifically require that signal reference grounding be installed and tested in affected equipment enclosures. Creation of “strings” of racks in new installations is impermissible due to the high equipment turnover expectations, which could lead to portions of strings not being properly grounded to the SRG. All individual racks shall be bonded individually to the SRG, and all racks shall have OEM or after-market grounding bars integrated into their vertical equipment distribution channels. This may be by conductive

(not painted or otherwise coated) equipment mounting rails, by a copper bar with bonding conductors extended to individual server equipment mounting screw points on the mounting rails, or similar. Each piece of mounted IT equipment shall have a direct, low-resistance pathway to the SRG through this grounding and bonding system. These installations shall require a grounding resistance test between the equipment grounding bar and the SRG following installation to confirm that the signal reference ground meets minimum resistance standards.

Equipment (already installed) without a proper signal ground connection shall be retrofitted with the required grounding connection equipment as rapidly as possible. These installations shall require a grounding resistance test between the equipment grounding bar and the SRG following installation to confirm that the signal reference ground meets minimum resistance standards.

Connection of IT equipment to the SRG means that there will be two grounding conductor connections at the equipment; the required power grounding conductor, and the supplemental

SRG grounding conductor.

Connections

The grid grounding bonds shall be connected by exothermic welding, listed pressure connectors, and/or listed clamps. Connection devices or fittings depending solely on solder or any unlisted attaching device shall not be used.

Connections to IT equipment shall be verified and made as needed any time a component is added, moved, or removed. The scope of any installation project shall require connection of the equipment to the SRG. IT equipment shall not be considered properly installed or given

CD521 authorization to operate unless the equipment is properly bonded to the SRG.

Coatings

Nonconductive coatings such as paint, lacquer, and enamel shall not be used on the grid grounding system.

NOTE: The signal reference grid is not a substitute for any safety ground required by the NEC.

For further information on signal reference grids, see FIPS Publication 94 and Military

Handbook 419A.