6.14 Computer Center Power Distribution
In some GSA buildings the power distribution system for computer centers will be designed by the tenant agency. In that case utility rough-in should be provided under the construction contract. If distribution is to be provided under the building contract, it should be designed according to the criteria in this section.
Power Distribution Units (PDU’s). PDU’s with internal or remote isolation transformers and output panel boards should be provided in all computer centers.
Non-linear Loads. Non-linear loads generate harmonic currents that are reflected into the neutral service conductors. Engineers should exercise caution when designing circuits and selecting equipment to serve non-linear loads, such as automated data processing equipment in computer centers. It is recommended to size neutrals at twice the size of the phase conductor. PDU’s with internal or remote isolation transformers should also be derated for non-linear loads. The transformer rating must take the increased neutral size into account.
Computer Center Grounding. To prevent electrical noise from affecting computer system operation, a low-frequency power system grounding and a high-frequency signal reference grounding system should be provided. The design of the computer room grounding system should be discussed with the computer center staff.
Low Frequency Power System Grounding. The primary concern is to provide a safe, low-frequency, single point grounding system which complies with Article 250 of the National Electrical Code. The single point ground must be established to ground the isolation transformer or its associated main service distribution panel.
A grounding conductor should be run from the PDU isolation transformer to the nearest effective earth grounding electrode as defined in the NEC. All circuits serving Automated Data Processing (ADP) equipment from a PDU should have grounding conductors equal in size to the phase conductors.
High Frequency Power System Grounding. In addition to the low-frequency power system grounding, a high-frequency signal reference grounding system for radio frequency noise is required (with the two systems bonded together at one point). A grid made up of 600 mm (2 foot) squares will provide an effective signal reference grounding system. The raised floor grid may be used if it has mechanically bolted stringers. Alternatively a grid can be constructed by laying a 600 mm mesh (2-foot squares) of braided copper strap or 1.3 mm (16 gauge, 0.051 inch) by 50 mm (2-inch) copper strip directly on the structural floor below the raised access floor. Data processing equipment should be connected to the reference grid by the most direct route with a braided copper strap.
Common Mode Noise Reduction. The reduction of common mode noise is particularly important for the proper operation of computer-based, distributed microprocessor-based systems, i.e., building automation systems, electronic security systems, card access control systems, and local area networks.
U.S. Census Bureau, Bowie, MD
The following guidelines should be considered to reduce common mode noise:
- Avoid running unshielded metallic signal or data lines parallel to power feeders.
- Where metallic signal or data lines must be routed in noise prone environments, use shielded cables or install wiring in ferrous metal conduit or enclosed cable trays.
- Locate metallic signal or data lines and equipment at a safe distance from arc- producing equipment such as line voltage regulators, transformers, battery chargers, motors, generators, and switching devices.
- Provide isolation transformers, electronic power distribution panel boards or power conditioners to serve critical electronics equipment loads.
- Provide isolated grounding service on dedicated circuits to critical data terminating or communicating equipment.
- Replace metallic data and signal conductors with fiber optic cables where practical.