Charles F. Kerchner, Jr.
All rights reserved
Last revised 7 May 1992
When failure strikes a product for which we are responsible, we
will spend considerable time and money to find and resolve the
problem. Many such failures occur in electronic products because
the product design engineer failed to give adequate heed to the
two major nemeses of electronic products- overheating and power
line disturbances. The subject of this paper is the latter of
these, but both are deadly to solid-state devices. Also, power
line problems can be the cause of overheating.
There has been a quantum leap in the last few years in digital
technology. Microprocessors are everywhere. Soon they will
control virtually everything. All electrical and electronic
products are being made "smart". They are no longer limited to
computers, which everyone expects to be finicky gadgets, but are
also found in washing machines, microwave ovens, office copiers,
radios, and VCR's to just name a few. Soon every home and
business will have a dozen, if not several dozen microprocessors
operating within its confines. And most of this equipment will
be plugged into the power line. The power line is not only the
life blood of these valued products but it is also the demon
which will destroy them without adequate safeguards.
In order to discuss this we will need to make some definitions.
The following is a list of terms and definitions:
1. Steady-State Voltage - Steady-state rms voltage values
are those that stay constant for 10 seconds or longer.
2. Nominal or Normal Voltage - The normal planned voltage
for a system. For most equipment this is 110-125VAC.
3. Power Failure - A power failure is a zero voltage
condition lasting for more than one cycle (1/60 second) on any
one of the three phases being used.
4. Blackout - A total power failure lasting several
seconds to hours or more.
5. Brownout - A planned and generally announced city or
regional-wide reduction by the utility in the steady-state
voltage possible due to heavy electrical consumption.
6. Undervoltage - A continuous reduction in the
steady-state voltage below the normal voltage. Similar to
brownout but this term is generally used to refer to unplanned
reductions or localized problems within the building or
7. Overvoltage - A continuous exceeding of the normal
8. Sag - Sags are cycle-to-cycle decreases in the power
line voltage on any one of the three phases below the normal
value. The lasting time of a sag is its duration in the number
of cycles of the 60HZ line frequency that the disturbance is
below, and returns to the normal level.
9. Dip - Dips are very short (but visible in incandescent
light bulbs) decreases in the normal power line voltage. These
are similar to sags but are noticeably faster. A dip is a fast
10. Surge - Surges are cycle-to-cycle increases in the
power line rms voltage on any of the three phases above the
normal voltage. Lasting time for surges is measured the same way
as are sags.
11. Swell - Swells are longer term multi-cycle slow rising
surges lasting from a few seconds to several minutes.
12. Impulse - An impulse is a very short term disturbance
up or down superimposed on the AC sine wave that typically lasts
between 0.5 and 100 microseconds. In-phase going impulses which
increase the instantaneous voltage are called spikes.
Out-of-phase impulses which decrease the instantaneous voltage
are called notches.
13. Spike - An over voltage impulse ranging from 400V to
5600V or more which is superimposed on top of the AC sine wave.
These, when over 600V, are potentially very damaging
14. Notch - An under voltage impulse similar to a spike but
of reverse polarity to the instantaneous value of the AC sine
wave so as to take a momentary notch out of the sine wave.
Notches are typically too fast to see. These typically last as
long as spikes but can be up to several milliseconds. In
addition, spikes and notches usually come in pairs or in an
oscillating series. For every notch there is usually an
immediately following spike due to power line inductances and
15. Transient - A transient is any short-term non-normal
event on the power line. All power line disturbances are
transient by definition.
16. EMI - Broad spectrum electromagnetic noise interference
either conducted on the power line directly from the source or
radiated to the power line then conducted to the susceptible
17. RFI - Electromagnetic noise interference in the radio
18. TVI - Electromagnetic noise interference in the
19. EMP - A very large and very fast rising electromagnetic
pulse caused by catastrophic events such as lightning or nuclear
20. Harmonic - Harmonics are sinusoidal currents and
voltages with frequencies that are integral multiples of the
fundamental power line frequency. Harmonics distort the normal
21. Normal Or Differential Mode - Events which occur across
the normal current carrying wires of the power line-hot wire to
neutral wire. Also called the transverse or metallic mode.
22. Common Mode - Events which occur from the current
carrying wires, hot and neutral, relative to the safety
23. MOV - Metal Oxide Varistor. A voltage dependent
resistor. A low cost but very effective protection device. They
can handle large surges and switch in 1 to 5 nanoseconds. It
works by absorbing voltage surges and spike impulses. They are
most effective when used in groups of three to provide both
differential mode and common mode protection.
24. Silicon Avalanche Zener Diodes - A solid-state junction
device. These devices are very fast acting but have low energy
handling capability. Switching speed is in picoseconds. They
work by shunting the surge or spike impulse around the protected
25. Gas Discharge Tube - A calibrated spark gap in a gas
filled chamber. These devices are relatively slow, activating
in microseconds but can handle very large surges. They work by
shunting the surge or spike impulse around the protected
26. EMI/RFI Filter - A circuit or device containing series
inductive (load-bearing) and parallel capacitive
(nonload-bearing) components which provides a low impedance path
around the protected circuit for high frequency noise. Filters
also attenuate impulses since a Fourier's Analysis of a spike
will reveal it is composed of high- frequency waveforms. Filters
and surge suppressors when used together thus act
27. UPS - An Uninterruptible Power System which provides a
steady source of electric energy to a piece of equipment.
28. Continuous UPS - A UPS system for which the load is
continually drawing power through the batteries and inverter and
never directly from the normal AC power line.
29. Standby UPS - A UPS system which normally connects
your equipment to the normal AC power line with the batteries and
inverter in standby mode. When the power line is weak or fails
it transfers the load to the batteries and inverter without any
load malfunction and without any user action. When the power
line returns to normal the load is automatically retransferred
back to the AC power line.
30. Voltage Surge Suppressor - A fast acting circuit or
device containing MOVs, Silicon Avalanche Diodes, Gas Discharge
Tubes, or other components which suppresses voltage surges and
spikes to a safe level. The energy in the surge or spike is
either dissipated as heat in the protection components or is
diverted to earth ground by the circuitry.
31. Voltage Regulating Transformer - A continuous acting
transformer, usually ferro-resonant, which instantaneously
regulates the output voltage to normal levels despite wide swings
in the input voltage.
32. Voltage Stabilizer - A switching device which selects
appropriate windings of a transformer to maintain normal output
voltage levels despite wide swings in the input voltage.
Switching occurs after the power is non-normal for a few seconds.
This device is commonly confused with a voltage regulating
transformer but they differ greatly in response time.
Stabilizers are generally too slow to protect solid-state
33. Power Line Conditioner - A PLC is a combination of a
voltage regulating transformer with a super isolation transformer
which provides smooth, regulated, noise free, AC voltage with no
ohmic connection between input and output. A PLC will solve most
problems other than complete power failure.
How major a problem are power line disturbances? Because of the
numerous failures of otherwise well designed electronic equipment
there has been quite a bit of research done on this subject.
Much of it is not available to the general public or even to many
engineers. Not until, that is, a problem develops and nobody can
find out the cause. Then finally someone thinks of -POWER LINE
A two year study by IEEE members Martzloff and Hahn completed in
1970 shows that surges and impulse voltage spikes can occur as
frequently as twice per hour in a typical residence with peak
values of 1500V to 2500V. Spikes in industrial environments are
even more frequent and severe. Spikes as high as 5600V were
recorded during lightning storms.
A study by IBM in 29 different locations throughout the U.S.
showed that an average of 50.7 voltage spikes occur per month
which in their study was 39.5% of the total disturbances.
A two year study by Bell Laboratories showed that most locations
will experience approximately 25 power line disturbances per year
87% of which they found to be sags below 96 volts. Voltages
below 96V can cause the power supply in most equipment to
malfunction and thus lead to equipment failure.
Some independent service organizations have reported that power
line disturbances are the cause of 70% of the failures of some
equipment. I myself have frequently seen equipment improperly
operating and damaged by power line disturbances.
So indeed, while studies disagree as to which type of disturbance
is the most frequent, they all agree that power line disturbances
are a major problem and it is widespread. Once the nemesis of
esoteric products such as computers, it now is affecting us all
because of the widespread use of microprocessors and other
digital devices in everyday products. Power line disturbances
have ruined the marketability of products which will not hold up
in the field and have likewise destroyed the credibility of the
engineers and companies who designed them.
What are the solutions? Well, we begin by becoming aware of the
existence of the problem. The physical design solution boils
down to those we can put into the product and those which we use
externally to keep the disturbances outside the product or
minimize their impact.
Internal solutions involve allocating adequate time and money to
the low- tech, but very important part of the product--the power
supply--in order to make the equipment less susceptible to
powerline disturbances. Engineers must select power supplies
which have adequate operating margins to perform as required in
undervoltage and overvoltage conditions. Great care must be
given to the impulse response of the power supply so that fast
transients and high frequency noise don't sneak through and
disrupt or damage the equipment. Surge suppression devices such
as MOVs, Silicon Avalanche Zener Diodes, and Gas Discharge Tubes
and noise suppression devices such as EMI/RFI Filters should be
used to suppress any disturbances that come up the power line
before they get to the DC power supply of the equipment. Lastly,
the design engineer must test, test, test, and test some more.
All to often people use field applications as their testing
ground. It's too late then if there is a serious deficiency in
the power supply. Murphy's law will get them sure enough if they
don't do adequate testing. Accelerated testing must be done for
the anticipated life of the equipment for all of the above listed
types of power line disturbances. If an engineer ignores the
power line disturbance problem it will come back to haunt him
10,000 times over after the product is in the field.
External solutions are ideal solutions for after-the-fact design
problems and for very large disturbance problems which, as was
pointed out above, do occur. Sometimes this is the only
practical economic solution. For simple brownouts and
undervoltage the voltage stabilizer is fine. Protection from
sags, dips, notches, brownouts, and undervoltage can be provided
with voltage regulating transformers. Total protection from
blackouts, brownouts, sags, dips, notches, and total power
failures can be achieved with a fast acting emergency power
system such as a standby Uninterruptible Power System (UPS).
Protection from spikes, surges, and noise can be provided by
plug-in combination surge and noise suppressors. These devices,
when externally applied, usually perform better than similar
devices used internal to the equipment. For no matter how well
the design engineer designs his protection within the equipment
the disturbance is already in the equipment before the protection
can work. Power line disturbances propagate along the wires at
some fraction of the speed of light. Take the example of
particularly damaging disturbances such as spikes. All
protection devices have a finite turn on time. For MOVs, the
most commonly used protection device for spikes, this is
typically five nanoseconds. The disturbance will propagate
approximately four to five feet in this time. Therefore, if
there is a sneak path through the power supply, and there usually
is, protection devices within the unit will not be fast enough
without the use of super fast and very expensive protection
schemes. Also, very large disturbances can punch through the
internal defenses and then the next stop is the delicate and
expensive high-tech components. Poof! There goes a five hundred
dollar main logic board. Thus, by making your first line of
protection further upstream on the power line at the outlet or in
the service panel you get the advantage of using the intervening
wire and the typical 6' cordset on the equipment as a propagation
delay line giving the suppressors in the external protector time
to work fully before the disturbance reaches the equipment. Thus
adequate protection can be provided externally with low cost
parts. External protection, even with the best internal
protection, should always be recommended for use with high-tech
equipment as low-cost insurance if for no other reason.
In conclusion, it should be apparent that power line problems are
a real world phenomena which we have to deal with if we are to
continue to expand our use of high-tech digital equipment.
However, solutions as outlined above are available.