Chapter 2
General System and Facility Guidelines
Electrical Factors
33
Dual Power Source Grounding
When dual power sources are utilized, strong consideration should be given to measure voltage potentials.
The use of dual power might create an electrical potential that can be hazardous to personnel and might
cause performance issues for the equipment.
Dual power sources might originate from two different transformers or two different UPS devices. Voltage
potentials from ground pin to ground pin of these sources should be measured and verified to be at or near 0.0
volts. Voltage levels that deviate or are measured above 3.0 volts should be further investigated. Increased
voltages might be hazardous to personnel, and should be further investigated.
Cabinet Performance Grounding (High Frequency Ground)
Signal interconnects between system cabinets require high frequency ground return paths. Connect all
cabinets to site ground.
NOTE In some cases power distribution system green (green/yellow) wire ground conductors are too
long and inductive to provide adequate high frequency ground return paths. Therefore, a
ground strap (customer-supplied) should be used for connecting the system cabinet to the
site-grounding grid (customer-supplied). When connecting this ground, ensure that the raised
floor is properly grounded for high frequency.
Power panels located in close proximity to the computer equipment should also be connected to site grounding
grid. Methods of providing a sufficiently high frequency ground grid are described in the next sections.
Raised Floor “High Frequency Noise” Grounding
If a raised floor system is used, install a complete signal-grounding grid for maintaining equal potential over
a broad band of frequencies. The grounding grid should be connected to the equipment cabinet and electrical
service entrance ground at multiple connection points using a minimum #6 AWG (16mm
2
) wire ground
conductor. Figure 2-1 illustrates a metallic strip grounding system.
NOTE Regardless of the grounding connection method used, the raised floor should be grounded as an
absolute safety minimum.
Hewlett-Packard recommends the following approaches:
• Excellent - Add a grounding grid to the subfloor. The grounding grid should be made of copper strips
mounted to the subfloor. The strips should be 0.032 in. (0.08 cm) thick and a minimum of 3.0 in. (8.0 cm)
wide. Connect each pedestal to four strips using 1/4 in. (6.0 mm) bolts tightened to the manufacturer’s
torque recommendation.
• Better - A grounded #6 AWG minimum copper wire grid mechanically clamped to floor pedestals and
properly bonded to the building/site ground.
• Good - Use the raised floor structure as a ground grid. In this case, the floor must be designed as a ground
grid with bolted down stringers and corrosion resistive plating (to provide low resistance and attachment
points for connection to service entrance ground and HP server equipment). The use of conductive floor
tiles with this style of grid further enhances ground performance. The structure needs to be mechanically
bonded to a known good ground point.
