Power quality

The quality of electrical power significantly impacts the performance of sensitive electronic equipment. These guidelines ensure that quality electrical power is provided to your data center.

IBM® equipment can tolerate some power disturbances or transients. However, large disturbances can cause equipment power failures or errors. Transients can come into the site on the power utility company lines but are often caused by electrical equipment installed in the building. For example, transients can be produced by welders, cranes, motors, induction heaters, elevators, copy machines, and other office equipment. The best way to prevent problems caused by power disturbances is to have transient-producing equipment on a separate power service than the one that supplies power to your information technology equipment.

Ground or earth

When used in reference to electrical power systems, Ground is a conducting connection between an electrical circuit and the earth or some conducting body that serves in place of the earth. The term ground is the most common name used, however it is also referred to as earth or terra in several international geographies. In this topic, these terms and other local language equivalents are interchangeable.

Ground is a critical component of an electrical power distribution system. A properly installed ground system allows for safe operation of equipment that is connected to the electrical power source under normal and electrical or equipment fault conditions. The life safety function of ground and grounding methods is addressed by the appropriate local and national electrical wiring codes. In the United States, this code is known as the National Electric Code or publication 70 of the National Fire Protection Association. Many countries have adopted the National Electric Code or have developed an equivalent code.

The National Electric Code and its equivalents have a primary objective to provide safe operation of electrical power distribution systems and electrical equipment installations. Compliance with these codes does not guarantee efficient operation of equipment connected to the power distribution systems. When sensitive electronic equipment is connected, there are often times when additional ground connections may be required. Typically, additional ground connections are recommended when there is a concern for high frequency or radio frequency (RF) interference, which may impact electronic circuits. These additional ground requirements will be found with the installation documentation for specific equipment. Additional ground requirements may also be recommendations from engineering or data center evaluations, reviews or surveys. Local or national codes allow for these additional grounds to be installed.


IBM equipment, unless double insulated, has power cords containing an insulated grounding conductor (color-coded green or green with yellow stripe) that connects the frame of the equipment to the ground terminal at the power receptacle. The power receptacles for IBM equipment are identified in the equipment documentation and should match the equipment power plug. In some cases, there may be options for different manufacturer equivalent receptacles. IBM equipment plugs should not be changed or altered to match existing connectors or receptacles. To do so may create a safety hazard and void product warranty. The connectors or receptacles for IBM equipment should be installed to a branch circuit with an equipment grounding conductor, connected to the grounding bus bar in the branch-circuit distribution panel. The grounding bus bar in the panel should then be connected back to the service entrance or suitable building ground by an equipment grounding conductor.

Information technology equipment must be properly grounded. It is recommended that an insulated green wire ground, the same size as the phase wire, be installed between the branch circuit panel and the receptacle.

For personnel safety, the ground must have sufficiently low impedance to limit the voltage to ground and to facilitate the operation of protective devices in the circuit. For example, the ground path shall not exceed 1 ohm for 120-volt, 20-ampere branch circuit devices.

The ground path impedance limit is 0.5 ohms for 120 volt branch circuits protected by 30 ampere circuit breakers. The limit is 0.1 ohms for 120 volt 60 to 100 ampere circuits.

All grounds entering the room should be interconnected somewhere within the building to provide a common ground potential. This includes any separate power sources, lighting and convenience outlets, and other grounded objects, such as building steel, plumbing, and duct work.

The equipment grounding conductor must be electrically attached to both the enclosure of the computer power center and the connector grounding terminal. Conduit must not be used as the only grounding means, and it must be connected in parallel with any grounding conductors it contains.

Figure 1. Transient grounding plate
The picture shows building structural steel, ground bar, conduit, neutral, ground, service entrance ground on one side. On the other side, it shows remote power service disconnect, main circuit breaker, neutral bus, circuit breakers, and conduit. In the bottom, it shows 3-phase and single-phase.

Transient grounding

To minimize the effects of high-frequency electrical noise, the branch circuit power panel servicing the equipment should be mounted in contact with bare building steel or connected to it by a short length of cable. If this is not possible, a metal area of at least 1 m2 (10 ft2) in contact with masonry can be used. The plate should be connected to the green-conductor common.

Figure 2. Transient grounding plate
Transient grounding using the raised floor support structure

The preferred connection is with a braided strap. If a braided strap is not available, the connection should consist of no. 12 AWG (3.3 mm or 0.0051 in.) or larger conductor and should not be more than 1.5 m (5 ft) long. To minimize this length, the preferred connection of this braided strap or conductor is to the nearest portion of the enclosure on the panel, if the enclosure is electrically continuous from the green-conductor common point to this point of connection.

The raised-floor-supporting substructure can be used as a substitute for the transient plate if the structure has a consistently low-impedance path. If the raised floor has stringers or other subframing that makes electrical connection between the pedestals, the floor itself can be used for the signal reference plane. Some raised floors are stringerless and the floor tiles lock into isolated pedestals by gravity alone. If there is no reliable electrical connection between the pedestals, a signal reference grid can be constructed by connecting the pedestals together with conductors. A minimal grid would interconnect every other pedestal in the immediate area of the power panel and extend at least 3 m (10 ft) in all directions.

Figure 3. Transient grounding using the raised floor support structure
Transient grounding using the raised floor support structure
Figure 4. Signal reference grid
Signal Reference Grid figure

Stranded bare or insulated conductor of at least no. 8 AWG (8 mm or 0.0124 in.) copper is required. This conductor provides a low-impedance path and is strong enough to make physical damage unlikely. Any connection method is acceptable as long as it provides a reliable electrical and mechanical connection.

A customer's self-contained, separately derived power system (computer power centers, transformers, motor generators), installed on a raised floor, has the same requirements.