Causes of out-of-tolerance power

* Lightning: One of the most infrequent and devastating events affecting computer system reliability is obviously lightning. Depending upon the proximity of the strike, from 1-mile to direct building contact, up to 6,000V can enter the computer\'s ac power cord. Either immediate failure or significant electronic degradation can occur.

Working closely with many field service organizations over the years, we have found a direct correlation to increased service calls and PC board failures on systems where lightning was observed within a one mile radius of the installation. These failures did not present themselves until three to four weeks after the storm was recorded. It has been an interesting exercise for service managers to track territorial thunderstorm activity as a function of unprotected system locations as an aid to forecast CPU, memory or disk controller board spares usage.

* Power utility grid switching: The power utility company routinely switches power grids to bring on additional generation capacity during peak load periods and then to shed this capacity during light load demand. Grid switching can occur at 5 a.m., noon, 5 p.m., or at midnight each day and can be more frequent during the summer months with increased air conditioning loading. What your customers\' systems see are a fast power outage with normal and common mode noise transients accompanying this outage. This outage is so short in duration that the ride thru storage capacity within the computer\'s power supply provides uninterrupted dc power to the electronics. What degrades and disrupts the system is the transient noise present. I am sure most of you have experienced NTF service calls in which the system runs fine during the day, but mysteriously locks-up or resets sometime during non-working hours.
* Copying machines and other noise generators: Every time a copy is made, ac noise transients are reflected through the office power wiring to some degree. This high-speed noise energy can range from 20V to 100V transients. Heating, ventilating and air-conditioning motor loads, including elevators, can send reflected transients throughout an office building. Industrial manufacturing sites have greater noise exposure for computer-based systems because of large SCR controllers and power factor correction capacitors adding to the problem. Coffee pots, floor polishers, vacuums and space heaters all contribute to system logic confusion and gradual electronic degradation within the office automation environment. During one memorable site power survey, we discovered the cause of CPU re-boot on a $500,000 automated IC test system. The switching of the hotplate on a near-by 10-cup coffee maker injected 20V transients into the power safety ground common to this test system.
* Dedicated power wiring: Dedicated or isolated power wiring is widely required or recommended by computer system vendors. It can originate from a separate breaker in the nearest sub-panel on the floor in which the computer load is located or from the building\'s main service entrance. In many cases, installation is left up to the interpretation of the electrician. On the plus side, dedicated wiring ensures circuit protection geared to the current requirements of the load and isolation from other n oisy building loads in the area of the computer installation.

Unfortunately, dedicated wiring can be a two-edged sword and does not ensure a noise-free ac interface for the critical load. In most cases, long dedicated power runs result in high neutral-to-ground ac voltages creating ground loops on net-worked systems. Because load current flowing through the neutral conductor yields a voltage depending n the total resistance of this wire, we have measured between 1.5VAC to 5VAC resulting in RS232 interface board failures and random system malfunction.

In addition, if the sub-panel, which is wired to the dedicated line, is noisy because of external or internal noise generators or events, then a direct path is provided for this noise to reflect back to the computer. To the dismay of the customer after he has spent non-trivial amounts for these measures in numerous occasions we have measured higher noise levels from dedicated circuits than from nearby standard receptacles.

* Secondary grounds and mechanical wiring problems: Equipment ground connections, in addition to the green wire safety ground in the power cord, have been used as standard installation procedures for many years. Ground rods, connected to building steel or a cold water pipe for the purpose of improving system reliability, have been found t create as many problems as they solve. Because these additional grounding measures can provide another path for high-frequency noise and damaging transients (from large building loads and nearby lightning) to reach the system, many companies are eliminating these techniques.

Proper receptacle wiring can be simply and quickly tested with several inexpensive tools now available. High-impedance ground and neutral conductors, open grounds and line-to-neutral reversals can cause unending intermittent system problems as well as operator safety hazards.

About Author:
\"AC Power Specialists & Consultants\"
Wesley E. Pettinger
HiTech Marketing Associates, Inc.
http://www.theupspeople.com