Input Power Quality

The plans for the data center should include a well-designed power and grounding system to maintain appropriate conditions and avoid unplanned power outages. Numerous factors can disrupt , degrade, or destroy electronic systems. High-frequency, high amplitude noise, high ground currents, low power, surges and sags in voltage, harmonic distortion, and other factors will affect the proper functioning of equipment. It is essential that power conditioning technology be used to protect the data center equipment.

The following table shows a chart that was published by the U.S. Government as a Federal Information Processing Standard or FIPS. The source is FIPS PUB 94, "Guideline On Electrical Power for ADP Installations." The U. S. Government withdrew this standard July 29, 1997 because these tolerances or tighter tolerances had been adopted as industry standards. It is presented here only as a reference.

Table 7-1. FIPS PUB 94 Tolerances Chart

		Typical Acceptable Limits for Computers and Power Sources
Environmental Attribute	Typical Environment	Normal	Critical	Units Affected and Comments
Line frequency	0.1% - 3%	1%	0.3%	Disk packs , tape, regulators
Rate of frequency change	0.5-20 Hz/s	1.5 Hz/s	0.3 Hz/s	Disk packs
Over- and under-voltage	5-6%, -13.3%	+5%, -10%	3%	Unregulated power supplies
Phase imbalance	2%-10%	5% max	3% max	Polyphase rectifiers, motors
Power source: tolerance to low power factor	0.85-0.6 lagging	0.8 lagging	<0.6 lagging or 0.9 leading	Indirectly limits power source or requires greater capacity unit with reduced overall efficiency
Tolerance to high steady state peak current	1.3-1.6 peak/rms	1.0-2.5 peak/rms	>2.5 peak/rms	1.414 normal; departures cause wave shape distortion
Harmonics (voltage)	0-20% total rms	10-20% total; 5-10% largest	5% max total 3% largest	Voltage regulators, signal circuits
DC load current capability of power source	Negligible to 5% or more	<0.1% w/exceptions	As low as 0.5%	Half wave rectifier load can saturate some power source, trip circuits
Voltage deviation from sine wave	5-50%	5-10%	3-5%	Affects regulators, signal circuits
Voltage modulation	Negligible to 10%	3% max	1% max	Voltage regulators, servo motors
Transient surges/sags	+10%, -15%	+20%, -35%	+5%, -5%	Regulated power, motor torques
Transient impulses	2-3 times nominal peak value (0-130% V-s)	Varies: 1,000-1,500V typical	Varies: 200-500V typical	Memory, disks, tapes having data transfer rates, low level data signals
RFI/EMI and "tone bursts," normal and common modes	10V up to 20 Khz; less at high freq.	Varies widely- 3V typical	Varies widely- 0.3V typical	Same as above
Ground currents	0-10 A rms + impulse noise current	0.001-0.5 A or more	0.0035 A or less	Can trip GFI devices, violate code, introduce noise in signal circuits

Power Conditioning Technology

When the power source does not meet the equipment requirements, additional hardware might be required for power conditioning. These power conditioning systems can be separate or can be integrated into UPS equipment. The use of power conditioning systems is much like the use of UPS systems. A "power sag" or "brownout" is an event that can bring the delivery of power to under 80 percent of nominal power for a brief duration, usually two seconds or less, sometimes even in the milliseconds range. You can think of a power conditioning system as a three-to-five second UPS that will maintain the power flow through a brownout.

Harmonic Content

Harmonics problems can be caused by the interaction of data center equipment with the power loads or by switching power supplies. Harmonic distortion, load imbalance, high neutral current, and low power factor can result in decreases in equipment efficiency and reliability. Eliminating harmonics problems is difficult, because the computer hardware contributes to them, and any changes in the room load or configuration to fix the problem can create new problems.

Sun Microsystems equipment has been designed to address the problems of harmonic distortion and is generally compatible with similar modern equipment. Equipment that does not have the advantages of modern harmonic-correction features should be isolated on separate circuits.

Voltage Spikes

Voltage spikes are rises in the voltage caused most often within the power distribution circuits by components turning on and off, such as the cycling of compressor motors. Large spikes can interfere with energy transfer, or the associated electrical noise can cause signal corruption.

A UPS and/or filtering system will usually stop most spikes originating upstream of the UPS. If a UPS will not be installed, some other form of regulation or surge suppression should be designed into the system.

Lightning Protection

The potentially damaging effects of lightning on computer systems can be direct or indirect. It might be on the utility power feed, directly on the equipment, or through high-frequency electromagnetic interference or surge currents. Lightning surges cannot be stopped , but they can be diverted. The plans for the data center should be reviewed to identify any paths for surge entry, and surge arrestors that provide a path to ground should be included to provide protection against lightning damage. Protection should be placed on both the primary and secondary sides of the service transformer.

Lightning protection systems should be designed, installed, and maintained in accordance with NFPA 780 (1997 edition), Standard for the Installation of Lightning Protection Systems , or any superseding local or national code.

Emergency Power Control

NFPA 70 and NFPA 75 require a single point of disconnect for all electronic systems in the data center, at each point of entry. Multiple disconnects are also acceptable, but in either case, the switches must be unobstructed and clearly marked , as shown in the following figure.

Figure 7-4. Emergency Power Disconnect and Manual Fire Alarm Pull Station

Protective covers can be placed over the buttons to avoid accidental contact, but access cannot be locked out. The switch, or switches, should disconnect power to all computer systems, HVAC, UPS, and batteries. If the UPS is located within the data center, the disconnect should stop power to the unit. If the UPS is located remotely, the disconnect should stop the supply from the unit into the room.

Though not required by code, it is recommended that all power sources in the room be controlled by the disconnect to provide the highest degree of safety to personnel and equipment.