Electrical and Environmental Guidelines

Electrical Factors

Proper design and installation of a power distribution system for a server requires specialized skills. Those responsible for this task must have a thorough knowledge and understanding of appropriate electrical codes and the limitations of the power systems for computer and data processing equipment.

In general, a well-designed power distribution system exceeds the requirements of most electrical codes. A good design, when coupled with proper installation practices, produces the most trouble-free operation.

A detailed discussion of power distribution system design and installation is beyond the scope of this document. However, electrical factors relating to power distribution system design and installation must be considered during the site preparation process.

The electrical factors discussed in this section are:

Computer room safety
Electrical load requirements (circuit breaker sizing)
Power quality
Distribution hardware
System installation guidelines

Computer Room Safety

Inside the computer room, fire protection and adequate lighting (for equipment servicing) are important safety considerations. Federal and local safety codes govern computer installations.

Fire Protection

The National Fire Protection Association’s Standard for the Protection of Electronic Computer Data Processing Equipment, NFPA 75, contains information on safety monitoring equipment for computer rooms.

Most computer room installations are equipped with the following fire protection devices:

Smoke detectors
Fire and temperature alarms
Fire extinguishing system

Additional safety devices are:

Circuit breakers
An emergency power cutoff switch
Devices specific to the geographic location, i.e., earthquake protection

Lighting Requirements for Equipment Servicing

Adequate lighting and utility outlets in a computer room reduce the possibility of accidents during equipment servicing. Safer servicing is also more efficient and, therefore, less costly.

For example, it is difficult to see cable connection points on the hardware if there is not enough light. Adequate lighting reduces the chances of connector damage when cables are installed or removed.

The minimum recommended illumination level is 70 foot-candles (756 lumens per square meter) when the light level is measured at 30 inches (76.2 cm) above the floor.

Power Quality

This equipment is designed to operate over a wide range of voltages and frequencies. It has been tested and shown to comply with EMC Specification EN50082. However, damage can occur if these ranges are exceeded. Severe electrical disturbances can exceed the design specifications of the equipment.

Sources of Electrical Disturbances

Electrical disturbances, glitches, affect the quality of electrical power. Common sources of these disturbances are:

Fluctuations occurring within the facility’s distribution system
Utility service low-voltage conditions (such as sags or brownouts)
Wide and rapid variations in input voltage levels
Wide and rapid variations in input power frequency
Electrical storms
Large inductive sources (such as motors and welders)
Faults in the distribution system wiring (such as loose connections)
Microwave, radar, radio, or cell phone transmissions

Power System Protection

Computer systems can be protected from the sources of many of these electrical disturbances by using:

A Protective Earth (PE) connection with a wire diameter of at least equal to the current carrying conductors. The neutral conductor must not be used for the PE connection. (The PE wire is GREEN with a YELLOW stripe.)
A dedicated power distribution system
Power conditioning equipment
Over- and under-voltage detection and protection circuits
Screening to cancel out the effects of undesirable transmissions
Lightning arresters on power cables to protect equipment against electrical storms

Every precaution has been taken during power distribution system design to provide immunity to power outages of less than one cycle. However, testing cannot conclusively rule out loss of service. Therefore, adherence to the following guidelines provides the best possible performance of power distribution systems for server equipment:

Dedicated power source—Isolates server power distribution system from other circuits in the facility.
Missing-phase and low-voltage detectors—Shuts equipment down automatically when a severe power disruption occurs. For peripheral equipment, these devices are recommended but optional.
Online uninterruptable power supply (UPS)—Keeps input voltage to devices constant and should be considered if outages of one-half cycle or more are common. Refer to qualified contractors or consultants for each situation.

Distribution Hardware

This section describes wire selection and the types of raceways (electrical conduits) used in the distribution system. Wire size is dictated by circuit breaker sizing and local safety codes.

Wire Selection

Use copper conductors instead of aluminum, as aluminum’s coefficient of expansion differs significantly from that of other metals used in power hardware. Because of this difference, aluminum conductors can cause connector hardware to work loose, overheat, and fail.

Raceway Systems (Electrical Conduits)

Raceways (electrical conduits) form part of the protective ground path for personnel and equipment. Raceways protect the wiring from accidental damage and also provide a heat sink for the wires.

Any of the following types may be used:

Electrical metallic tubing (EMT) thin-wall tubing
Rigid (metal) conduit
Liquidtight with RFI strain relief (most commonly used with raised floors)
Plenum-grade cables

Building Distribution

All building feeders and branch circuitry should be in rigid metallic conduit with proper connectors (to provide ground continuity). Conduit that is exposed and subject to damage should be constructed of rigid galvanized steel.

The IOX and Superdome or hp 9000 Superdome are safety grounded through the green (ground) wire in each AC power cord. In the IOX, this ground passes through the AC power cord entry into the XPC and connects internally to the XPC chassis. The XUC chassis and each ICE chassis are grounded through their respective DC power cords from the XPC. Additional safety grounding must be provided for networking equipment.

Power Routing

Power drops and interface cables from the equipment are routed down from the power panel, through a grommet-protected opening (beneath the floor level), and under the floor panels.

Grounding Systems

Superdome servers require two methods of grounding:

Power distribution safety grounding
High frequency intercabinet grounding

Power Distribution Safety Grounding

The power distribution safety grounding system consists of connecting various points in the power distribution system to earth ground using green (green/yellow) wire ground conductors. Having these ground connections tied to metal chassis parts that may be touched protects computer room personnel against shock hazard from current leakage and fault conditions.

Power distribution systems consist of several parts. Hewlett-Packard recommends that these parts be solidly interconnected to provide an equipotential ground to all points.

Main Building Electrical Ground

The main electrical service entrance equipment should have an earth ground connection, as required by applicable codes. Connections such as a grounding rod, building steel, or a conductive type cold water service pipe provide an earth ground.

Electrical Conduit Ground

To provide a continuous grounding system, all electrical conduits should be made of rigid metallic conduit that is securely connected together or bonded to panels and electrical boxes.

Power Panel Ground

Each power panel should be grounded to the electrical service entrance with green (green/yellow) wire ground conductors. The green (green/yellow) wire ground conductors should be sized per applicable codes (based on circuit over current device ratings).




	NOTE: The green wire ground conductor mentioned above may be a black wire marked with green tape.

Computer Safety Ground

Ground all computer equipment with the green (green/yellow) wire included in the branch circuitry. The green (green/yellow) wire ground conductors should be connected to the appropriate power panel and should be sized per applicable codes (based on circuit over current device ratings).

Superdome was approved by regulatory agencies around the world, and therefore requires a ground/protective earth. there are no exclusions to this regulatory approval.

High-frequency grounding between IOX and Superdome is provided by the cabinet-to-cabinet signal cabling. Whenever an IOX is connected to a Superdome cabinet, low-frequency grounding between these two cabinets is provided by a ground strap. This ground strap is shipped with each IOX. Refer to the I/O Expansion Cabinet Guide for more detail.

Newtwork-connected Equipment Ground

The installation must provide a ground connection for the network equipment. This statement is translated into the following two languages as required:




	WARNING! Sweden: Apparaten skall anslutas till jordat uttag, när den ansluts till ett nätverk.Denmark: Før tilslutning af de øvrige ledere, se medfølgende installationsvejledning.

Raised Flooring, Signal Reference Grids, and High Frequency Grounding

If a raised floor system is used, install a complete signal reference grid (SRG) for maintaining equal potential over a broad range of frequencies. The grid should be connected to power source X0 and cabinet grounds as well as to other electrical service grounds. Flat braid offers superior frequency controls to round wire. Figure 5-1 “Raised Floor Metal Strip Ground System” 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. For more information regarding raised computer floors, see NEC section 645-15.

HP recommends the following approaches:

Excellent—Add a grounding grid to the subfloor. The grounding grid should be made of flat braid copper strips in a 2 ft. by 2 ft. manner 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 or #4 AWG 2 ft. by 2 ft. copper wire grid mechanically clamped to floor pedestals and properly bonded to the building or 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 computer equipment). The use of conductive floor tiles with this style of grid further enhances ground performance.




	NOTE: The structure needs to meet all applicable safety codes and be mechanically bonded to known good grounding points.

Best Practices

Use a 2ft by 2ft grid.
Flat braid is preferred over round wire.
Exothermic welds are preferred over mechanical connections.
All transformers mounted on the floor should have an X0 bond to floor.
All air conditioning cabinets mounted on the floor with a bonding connection to the floor.
A vertical building steel grounding point is preferred over long wire runs to electrical service entrance.
Very short straps to the floor are preferred over excess lengths.

HP recommendations concerning the proper grounding of a raised floor or signal reference grid are closely aligned with other agencies such as the NEC and IEEE.

Figure 5-1 Raised Floor Metal Strip Ground System

System Installation Guidelines

This section contains information about installation practices. Some common pitfalls are highlighted. Both power cable and data communications cable installations are discussed.

Wiring Connections

Expansion and contraction rates vary among different metals. Therefore, the integrity of an electrical connection depends on the restraining force applied. Connections that are too tight compress or deform the hardware and causes it to weaken. This usually leads to high impedance causing circuit breakers to trip.




	CAUTION: Connections that are too loose have a high resistance that cause serious problems, such as erratic equipment operation. A high resistance connection overheats and sometimes causes fire or high temperatures that can destroy hard-to-replace components such as distribution panels or system bus bars.

Wiring connections must be properly torqued. Many equipment manufacturers specify the proper connection torque values for their hardware.

Ground connections must only be made on a conductive, nonpainted surface. Lockwashers must be used on all connections to prevent connection hardware from working loose.

Data Communications Cables

Power transformers and heavy foot traffic create high energy fields. Route data communications cables away from these areas. Use shielded data communications cables that meet approved industry standards to reduce the effects of external fields. Data cables that are run externally to a metal fire enclosure must have a minimum fire rating of VW-1 or VW-4 or better.

Environmental Elements

The following environmental elements can affect a Superdome server installation:

Computer room preparation
Cooling requirements
Humidity level
Air conditioning ducts
Dust and pollution control
Electrostatic discharge (ESD) prevention
Acoustics (noise reduction)
Zinc whisker control

Computer Room Preparation

The following guidelines are recommended when preparing a computer room for a Superdome server system:

Locate the computer room away from the exterior walls of the building to avoid the heat gain from windows and exterior wall surfaces.
When exterior windows are unavoidable, use windows that are double- or triple-glazed and shaded to prevent direct sunlight from entering the computer room.
Maintain the computer room at a positive pressure relative to surrounding spaces.
Use a vapor barrier installed around the entire computer room envelope to restrain moisture migration.
Caulk and vapor seal all pipes and cables that penetrate the envelope.
Use at least a 12-inch raised floor system for the most favorable room air distribution system (underfloor distribution).
Ensure a minimum ceiling height of 12 inches between the top of the server and the ceiling. Ensure all ceiling clips are in place.

Cooling Requirements

Air conditioning equipment requirements and recommendations are described in the following sections.

Basic Air Conditioning Equipment Requirements

The cooling capacity of the installed air conditioning equipment for the computer room should be sufficient to offset the computer equipment dissipation loads, as well as any space envelope heat gain. This equipment should include:

Air filtration
Cooling or dehumidification
Humidification
Reheating
Air distribution
System controls adequate to maintain the computer room within the operating ranges listed in Table 5-1 “Controlled Computer Room Environment Specifications”.

Table 5-1 Controlled Computer Room Environment Specifications

Temperature (dry bulb ^οC)^[1]		Relative Humidity %; Noncondensing		Dew Point^[2]	Rate of Chg (˚C/hr, max)
Allowable^[3]^,^[4]	Recommended^[5]	Allowable^[4]	Recommended^[5]
15 - 32 (59^ο to 90^ο F)	20 - 25 (68^ο to 77^ο F)	20 - 80	40 - 55	17	5
^[1]Dry bulb temperature is the regular ambient temperature. Derate maximum dry bulb temperature 1˚C/300 m above 900 m. ^[2]Must be noncondensing environment. ^[3]With installed media, the minimum temperature is 10˚C and maximum relative humidity is limited to 80%. Specific media requirements may vary. ^[4]Allowable: equipment design extremes as measured at the equipment inlet. ^[5]Recommended: target facility design and operational range.

Lighting and personnel must also be included. For example, a person dissipates about 450 BTUs per hour while performing a typical computer room task.

At altitudes above 10,000 feet (3048 m), the lower air density reduces the cooling capability of air conditioning systems. If your facility is located above this altitude, the recommended temperature ranges may need to be modified.

Air Conditioning System Guidelines

The following guidelines are recommended when designing an air conditioning system and selecting the necessary equipment:

The air conditioning system serveing the computer room should be capable of operating 24 hours a day, 365 days a year. It should also be independent of other systems in the building.
Consider the long-term value of computer system availability, redundant air conditioning equipment, or capacity.
The system should be capable of handling any future computer system expansion.
Air conditioning equipment air filters should have a minimum rating of 45% (based on “AShRA Standard 52-76, Dust Spot Efficiency Test”).
Introduce only enough outside air into the system to meet building code requirements (for human occupancy) and to maintain a positive air pressure in the computer room.

Air Conditioning System Types

The following three air conditioning system types are listed in order of preference:

Complete self-contained package unit(s) with remote condenser(s). These systems are available with u, or down discharge and are usually located in the computer room.
Chilled water package unit with remote chilled water plant. These systems are available with up or down discharge and are usually located in the computer room.
Central station air handling units with remote refrigeration equipment. These systems are usually located outside the computer room.

Basic Air Distribution Systems

A basic air distribution system includes supply air and return air.

An air distribution system should be zoned to deliver an adequate amount of supply air to the cooling air intake vents of the computer system equipment cabinets. Supply air temperature should be maintained within the following parameters:

Ceiling supply system—From 55° F (12.8° C) to 60° F (15.6° C)
Floor supply system—At least 60° F (15.6° C)

If a ceiling plenum return air system or a ducted ceiling return air system is used, the return air grille(s) in the ceiling should be located directly above the computer equipment cabinets.

The following three types of air distribution system are listed in order of recommendation:

Underfloor air distribution system—Downflow air conditioning equipment located on the raised floor of the computer room uses the cavity beneath the raised floor as plenum for the supply air.
Return air from an underfloor air distribution system can be ducted return air (DRA) above the ceiling, as shown in Figure 5-3 “Underfloor Air Distribution System”.
Perforated floor panels (available from the raised floor manufacturer) should be located around the perimeter of the system cabinets. Supply air emitted though the perforated floor panels is then available near the cooling air intake vents of the computer system cabinets.
Ceiling plenum air distribution system—Supply air is ducted into the ceiling plenum from upflow air conditioning equipment located in the computer room or from an air handling unit (remote).
The ceiling construction should resist air leakage. Place perforated ceiling panels (with down discharge air flow characteristics) around the perimeter of the system cabinets. The supply air emitted downward from the perforated ceiling panels is then available near the cooling air intake vents of the computer system cabinets.
Return air should be ducted back to the air conditioning equipment though the return air duct above the ceiling.
Above ceiling ducted air distribution system—Supply air is ducted into a ceiling diffuser system from upflow air conditioning equipment located in the computer room or from an air handling unit (remote).
Return air from an above ceiling ducted air distribution system may be ducted return air (DRA) above the ceiling, as shown in Figure 5-5 “Above Ceiling Ducted Air”, or ceiling plenum return air (CPRA), as shown in Figure 5-4 “Ceiling Plenum Air Distribution System”.
Adjust the supply air diffuser system grilles to direct the cooling air downward around the perimeter of the computer system cabinets. The supply air is then available near the cooling air intake vents of the computer system cabinets.

Air Conditioning System Installation

All air conditioning equipment, materials, and installation must comply with any applicable construction codes. Installation of the various components of the air conditioning system must also conform to the air conditioning equipment manufacturer’s recommendations.

Figure 5-3 “Underfloor Air Distribution System” illustrates a typical computer room underfloor air distribution system (DRA).

Figure 5-4 “Ceiling Plenum Air Distribution System” illustrates a typical computer room ceiling plenum air distribution system (CPRA).

Figure 5-5 “Above Ceiling Ducted Air” illustrates a typical computer room above ceiling ducted air distribution system (DRA).

Figure 5-2 Typical Computer Room Raised Floor Layout

Figure 5-3 Underfloor Air Distribution System

Figure 5-4 Ceiling Plenum Air Distribution System

Figure 5-5 Above Ceiling Ducted Air

Humidity Level

Maintain proper humidity levels. High humidity causes galvanic actions to occur between some dissimilar metals. This eventually causes a high resistance between connections, leading to equipment failures. High humidity can also have an adverse affect on some magnetic tapes and paper media.




	CAUTION: Low humidity contributes to undesirably high levels of electrostatic charges. This increases the electrostatic discharge (ESD) voltage potential. ESD can cause component damage during servicing operations. Paper feed problems on high-speed printers are usually encountered in low-humidity environments.

Low humidity levels are often the result of the facility heating system and occur during the cold season. Most heating systems cause air to have a low humidity level, unless the system has a built-in humidifier.

Air Conditioning Ducts

Use separate computer room air conditioning duct work. If it is not separate from the rest of the building, it might be difficult to control cooling and air pressure levels. Duct work seals are important for maintaining a balanced air conditioning system and high static air pressure. Adequate cooling capacity means little if the direction and rate of air flow cannot be controlled because of poor duct sealing. Also, the ducts should not be exposed to warm air, or humidity levels may increase.

Dust and Pollution Control

Computer equipment can be adversely affected by dust and microscopic particles in the site environment.

Specifically, disk drives, tape drives, and some other mechanical devices can have bearing failures resulting from airborne abrasive particles. Dust may also blanket electronic components like printed circuit boards causing premature failure due to excess heat and/or humidity build up on the boards. Other failures to power supplies and other electronic components can be caused by metallically conductive particles. These metallic particles are conductive and can short circuit electronic components. Use every effort to ensure that the environment is as dust and particulant free as possible.

Smaller particles can pass though some filters and, over a period of time, possibly cause problems in mechanical parts. Small dust particles can be prevented from entering the computer room by maintaining its air conditioning system at a high static air pressure level.

Other sources of dust, metallic, conductive, abrasive, and/or microscopic particles can be present. Some sources of these particulants are:

Subfloor shedding
Raised floor shedding
Ceiling tile shedding

These pollutants are not always visible to the naked eye. A good check to determine their possible presence is to check the underside of the tiles. The tile should be shiny, galvanized, and free from rust.

The computer room should be kept clean. The following guidelines are recommended:

Smoking—Establish a no-smoking policy. Cigarette smoke particles are eight times larger than the clearance between disk drive read/write heads and the disk surface.
Printer—Locate printers and paper products in a separate room to eliminate paper particulate problems.
Eating or drinking—Establish a no-eating or drinking policy. Spilled liquids can cause short circuits in equipment such as keyboards.
Tile floors—Use a dust-absorbent cloth mop rather than a dry mop to clean tile floors.

Special precautions are necessary if the computer room is near a source of air pollution. Some air pollutants, especially hydrogen sulfide (H2S), are not only unpleasant but corrosive as well. Hydrogen sulfide damages wiring and delicate sound equipment. The use of activated charcoal filters reduces this form of air pollution.

Metallic Particulate Contamination

Metallic particulates can be especially harmful around electronic equipment. This type of contamination may enter the data center environment from a variety of sources, including but not limited to raised floor tiles, worn air conditioning parts, heating ducts, rotor brushes in vacuum cleaners or printer component wear. Because metallic particulates conduct electricity, they have an increased potential for creating short circuits in electronic equipment. This problem is exaggerated by the increasingly dense circuitry of electronic equipment.

Over time, very fine whiskers of pure metal can form on electroplated zinc, cadmium, or tin surfaces. If these whiskers are disturbed, they may break off and become airborne, possibly causing failures or operational interruptions. For over 50 years, the electronics industry has been aware of the relatively rare but possible threat posed by metallic particulate contamination. During recent years, a growing concern has developed in computer rooms where these conductive contaminants are formed on the bottom of some raised floor tiles.

Although this problem is relatively rare, it may be an issue within your computer room. Since metallic contamination can cause permanent or intermittent failures on your electronic equipment, Hewlett-Packard strongly recommends that your site be evaluated for metallic particulate contamination before installation of electronic equipment.

Electrostatic Discharge (ESD) Prevention

Static charges (voltage levels) occur when objects are separated or rubbed together. The voltage level of a static charge is determined by the following factors:

Types of materials
Relative humidity
Rate of change or separation

Table 5-2 “Effect of Humidity on ESD Charge Levels” lists charge levels based on personnel activities and humidity levels.

Table 5-2 Effect of Humidity on ESD Charge Levels

Personnel Activity^[1]	Humidity^[2] and Charge Levels (volts)^[3]
	26%	32%	40%	50%
Walking across a linoleum floor	6,150	5,750	4,625	3,700
Walking across a carpeted floor	18,450	17,250	13,875	11,100
Getting up from a plastic chair	24,600	23,000	18,500	14,800
^[1]Source: B.A.Unger, Electrostatic Discharge Failures of Semiconductor Devices (Bell Laboratories,1981) ^[2]For the same relative humidity level, a high rate of airflow produces higher static charges than a low airflow rate. ^[3]Some data in this table has been extrapolated.

Static Protection Measures

Follow these precautions to minimize possible ESD-induced failures in the computer room:

Install conductive flooring (conductive adhesive must be used when laying tiles).
Use conductive wax (if waxed floors are necessary).
Ensure that all equipment and flooring are properly grounded and are at the same ground potential.
Use conductive tables and chairs.
Use a grounded wrist strap (or other grounding method) when handling circuit boards.
Store spare electronic modules in antistatic containers.
Maintain recommended humidity level and airflow rates in the computer room.

Acoustics

Computer equipment and air conditioning blowers cause computer rooms to be noisy. Ambient noise level in a computer room can be reduced as follows:

Dropped ceiling—Cover with a commercial grade of fire-resistant, acoustic rated, fiberglass ceiling tile.
Sound deadening—Cover the walls with curtains or other sound deadening material.
Removable partitions—Use foam rubber models for most effectiveness.