Uncommon power - Devices to protect computer systems from unexpected power surges, losses - Includes related articles on network UPS, UPS power tips - Special focus: network management - Technology Information - Cover Story

Although on-line viruses get more attention, power-related problems are the most common and menacing threat to computer systems and networks. As computing capabilities continue to increase, so does vulnerability to inconsistent power quality.

Uncommon power - Devices to protect computer systems from unexpected power surges, losses - Includes related articles on network UPS, UPS power tips - Special focus: network management - Technology Information - Cover StoryBrownouts and blackouts account for only about 4.7% of disturbances; sags, surges, and spikes make up the other 95.3%. These disturbances are no less threatening, as they can cause electrical noise resulting in flawed data transmission, data loss, and damaged hardware.

Safeguarding systems and developing a sound power protection strategy requires knowledge of the electrical circuit, potential power-quality threats, and protection technologies.


We generally think of power problems in extremes -- blackout or brownouts -- caused by external agents like wind, lightning, or utility failures. However, many power problems originate from within.

Proximity of equipment to a building's electrical service greatly impacts power quality. The closer computers are to power services, the lower the resistance and inductance of the wires, decreasing the ability to reduce the noise's shape and magnitude. The further from the service, the higher the probability of voltage drops.

Other internal threats include inrush currents and step load changes, either of which can cause voltage sags, noise, and spikes that can interfere with the performance and current demands of computer systems.

Noise is the most common internal threat. Normal-mode noise generally occurs between the phase (hot) wire and the neutral wire. Common-mode noise, or ground noise, appears between the neutral and ground wires. Caused by lightning, electrostatic discharge (ESD), or some other element, it can cause soft processing and memory errors, pollute program results, or damage hardware.


Proper grounding is your best defense, particularly in regard to workstations and networks which use switch-mode power supplies. Such devices are generally unaffected by normal-mode noise, though highly susceptible to ground noise if not well grounded.

Most companies tie all electrically operated equipment to their facility's singlepoint ground to equalize potential differences between neutral and ground. This may not be enough to protect networks from lead inductance and/or stray capacitance. Equipment chassis can be tied together with a short and wide ground strap. Power strips can be used to provide single-point grounding close to workstations.

Power strips are effective to guard against ground loops. As frequency increases, inductive reactance of straight wire increases. A workstation plugged into a power source 50 to 200 feet away can look like a high-value resistor to a noise signal. An isolated ground can be built into the power strip to supplement the benefits of proper grounding.

One caution: if the power source is too far from the network or worstation plugged into the isolated ground, the isolated ground will actually act as an antenna for noise.


Surge suppressors vary in price and quality. The Underwriters Laboratory (UL) 1449 rating provides an effective starting point when selecting surge suppressors. Or you can simply refer to the number of joules a surge suppressor can handle. The higher the joules the greater the protection. The number of joules varies directly with the number of metal oxide varistors (MOVs) used in surge suppressor designs.

Placement of (MOVs) is equally important. MOVs are connected to the computer in parallel, allowing them to act as a low impedance path. Unwanted current is clipped off by the MOV to protect sensitive circuits. For true protection a surge suppressor should feature at least three MOVs, connected between line and neutral, line and ground, and neutral to ground.

MOVs give their lives to protect sensitive circuits, so it is important to have a warning when an MOV is no longer operational. A light or audible alarm should place a fuse in series with the MOV to serve this purpose. However, many status lights do not offer this feature.


Basically, a power conditioner is an isolation transformer with two coils, including the primary and secondary transformer windings, physically separated from each other by air and insulating materials.

It eliminates stray current paths by holding ground and anything referenced to it at the lowest possible impedance. It significantly reduces noise magnitude and strips away high-frequency components, leaving only rounder, easier-to-tolerate noise.


Installing a UPS on the network server is a given. Determining protection beyond the network server is a cost-driven consideration, in which operating departments scale the value of the corporate computing resource against the sometimes considerable cost of protecting it.

While it's a truism that power protection strategies are as unique as the systems they protect, risk management suggests several approaches.

For total network protection, we recommend a line-interactive UPS on the server and a standby power supply for each individual workstation, stand-alone PC and/or modem. The minimum configuration for a network includes a line-interactive UPS at the server and an SPS at each critical workstation.

Evaluate potential losses beyond the cost of damaged equipment, to include the cost of lost data and productivity. After all, if critical shared data and productivity were not an issue, networks and workstations would not exist to protect.

Power Tips

UPSs are designed to prevent equipment interruption in the event of power loss. They also prevent drops below a preset voltage level. The three basic types of UPSs are Stand-By Power, On-Line and Line-Interactive.

* Standby Power Systems

Current is directed through the SPS with some current diverted through the charger to keep the battery fully charged. If power goes out, or drops below a preset voltage level, the inverter turns on.

A realistic transfer time from electricity to battery power is 4 to 10 milliseconds. Since this is fast enough to switch without a workstation experiencing an outage, standby power systems are not on at all times. Additionally, the SPS inverter produces a sine wave with little distortion.

* On-Line Power Systems

UPSs feature an inverter that is on all the time, operating as part of the circuit. Incoming AC power is converted to DC by the charger, providing charging current to batteries, which float on the DC bus. The inverter converts the DC back to AC to power the load.

One drawback of an on-line UPS is that it can create neutral to ground and common-mode noise, along with harmonic distortion that can interfere with proper system operations.

* Line-Interactive Power Systems

The UPS circuit from the rectifier charger to the battery and through the inverter, is on all the time. The sensing circuit controls the current amount needed from the inverter. When a line voltage sags, the inverter provides more power to the load. When power goes out completely, the inverter supplies 100% of the power. The main advantage of a line-interactive is there is no switching time.

RELATED ARTICLE: Power: the critical link by Scott Olsen, Product Manager, and Christopher Leidigh, Team Leader, American Power Conversion

Without a UPS, your network nodes can disappear off the Network Management Station (NMS), making it nearly impossible to determine the cause of a problem. UPSes that are enabled for Simple Network Management Protocol (SNMP) can notify users of power problems and cycle power to devices that have ceased to function.

Today's uninterruptible power supplies can be used strategically to 1) increase network reliability with power protection and 2) remotely manage server and communications equipment.

The key reason for having a UPS on your internetworking equipment is to improve your network's reliability and maximize network uptime.

Incoming power is the most significant cause of downtime: the mean time before failure (MTBF) can be as little as 584 hours (far less in "problem" power areas) due to power outages. In other words, your hubs, bridges, switches and routers can reboot 1.25 times a month due to power disturbances.

Redundant supplies do not help reduce downtime that results from power sags or outages. A typical bridge with a redundant power supply has a MTBF well in excess of 100,000 hours. When power is used without going through the UPS, the resulting MTBF is only 580 hours.

Adding a UPS dramatically increases your uptime because most UpSes exhibit MTBFs far greater than those associated with internet-working equipment If you size your UPS to provide your internetworking equipment with at least five minutes of run-time, it will protect you from more than 90% of all power problems.