| UNDERSTANDING
AC POWER PROBLEMS
By
Wesley E. Pettinger
Semiconductor
manufacturers of the microprocessors and memory devices,
of which our computer systems are based, are approaching
the smallest electronic cell element permitted by
the laws of physics. The electrical over stress required
to destroy state-of-the-art electronics, in most cases,
is 100 times less than devices available 10 years
ago. The driving force to shrink device geometries
to increase speed and storage capacity will continue
to make computers even more sensitive to ac power.
The
major cause of system malfunction and failures is
ac power noise. Semiconductor devices should last
for around 1,700 years before they run out of electrons
and fail. To provide proper protection to maximize
system reliability, it is important to consider IC
component tolerances, power supply operation and the
ac power source as an interactive system. Condensing
years of research and field test results on this issue,
the following parameters are given for ac power affecting
computer-based system performance. We all know about
transients, brownouts and spikes. But let's review
some of the facts behind the terms:
- Voltage
transients or noise greater than 10V peak-to-peak
riding on the ac sine wave (between line and neutral)
can cause internal component degradation and eventual
failure. This normal mode noise exists at high frequencies
and, depending n the amplitude, can result in immediate
system failure or a slow erosion over weeks or months.
During this degradation period, system lock-ups,
resets and data transfer errors increase.
- Voltage
transients or high-frequency noise greater than
0.5V peak-to-peak measured between neutral and safety
ground can result in system logic confusion. This
common mode noise is especially important to eliminate
because all logic components are referenced directly
or indirectly to the ac power ground. Also, computer
peripherals rely on the green wire ground as a zero-voltage
reference to ensure proper data transfer through
interface ports.
-
Neutral-to-ground voltage (measured with a standard
ac voltmeter) higher than 0.5 VAC can cause similar
logic confusion and system disruption problems.
This low-frequency voltage, commonly referred to
as ground loops, also can cause interface board
failures on networked systems.
- High-impedance
ac power source: Switch mode power supplies predominately
used in currend-day systems require a stiff power
source to draw the peak ac currents for proper operation.
In the presence of a high-impedance power feed,
ac sine wave flattopping occurs with an increase
in reflected ac nose and an increase in system sensitivity
to this noise.
- Loss
of ac voltage: Another attribute of switch mode
power supplies is the ability to ride thru momentary
power losses. The internal high-voltage capacitor
typically has sufficient energy storage to maintain
system operation for one to two ac line cycles.
Ac power loss greater than 33 msec generally results
in a halt of system operation. It is important to
understand that because of the nature of power distribution
systems, extremely high-amplitue and damaging noise
impulses always accompany loss of ac voltage.
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.
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