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March 30, 2003

                             ET NEWS
Issue No. 92            3-30-2003

- News
- ET Journal
- NICET Test Dates
- AFAA Class Schedule
- Comments & Acknowledgements


Recommendation Req. Dropped for Levels I and II
In its March 2003 meeting, the NICET Board of Governors decided
to eliminate the Personal Recommendation requirement for
certification at Levels I and II in all job task competency
programs. The Personal Recommendation --Technician Application
Part V--remains as one of the requirements for certification at
Levels III and IV.

The Board's ruling is retroactive for all candidates currently
in Pending Status. In the upcoming weeks, NICET will review
files of all applicants who have met a Level I or II exam
requirement and subsequently received Conditional Decision
Letters. Those applicants who have met the work history and work
element verification requirements will be certified. If you now
meet all remaining requirements for certification, and you have
not heard from NICET by May 30, 2003 about your certification,
please contact our Certification Staff at 888-476-4238 option 3.



Thanks to all of you who took the time to stop by the AFAA at
ISC-West and say hi! Thank you also for all of the kind words.

For an introvert like me Las Vegas doesn't have much appeal.
People seem to be in a hurry to go somewhere and do something,
which is the antithesis of Hawaii where people seem to have
nothing to do and nowhere to go. Guess where I'll be on vacation!


I'm home this week.

Have fun!



NICET Fire Alarm Systems Level III

35012 is a Level III General Non-Core Work element.

General Work Elements are categorized as either Core or Non-Core
Work Elements. All Level III General Core Work Elements
constitute a mandatory requirement for achieving certification
at Levels III and IV.

Understand the possible effects that transients and RFI/EMI may
have on fire alarm systems, the possible sources of such
interference, and measures that can be taken during design and
installation to minimize the probability of such interference.
(NFPA 70, Fire Alarm Signaling Systems, UL Standards, basic
electronic textbooks. Transient Voltage Suppression)


Fire Alarm Signaling Systems 
1994 version:
2003 version:

UL Standards

Basic electronic textbooks
My pick:

Transient Voltage Suppression

"Understand the possible effects that transients and RFI/EMI may
have on fire alarm systems, the possible sources of such
interference, and measures that can be taken during design and
installation to minimize the probability of such interference."

Glossary of terms

EMI/RFI. (Electromagnetic Interference/Radio Frequency
Interference) A form of "noise" on data transmission mediums
that can reduce data integrity & increase transmission errors.

MOS. Metal Oxide Semiconductor. One of two major categories of
chip design [the other is bipolar]. It derives its name from its
use of metal, oxide, and semiconductor layers. There are several
varieties of MOS technologies including PMOS, NMOS, CMOS.

MOV. Metal Oxide Varistor. A varistor is a very rugged voltage
clamping device capable of absorbing very large currents without
damage. By itself, a MOV is capable of holding typical power line
surge voltages down to a level of approximately 330V peak
(on a 120V circuit). The MOV is often compared with another
clamping device, known as a SILICON AVALANCHE DIODE. The MOV is
commonly thought to have a slower reaction time than the
avalanche diode, but this isn't true.

Thyristor. Also known as a Silicon Controlled Rectifier (SCR). A
SCR is basically a diode with an extra junction tied to a third
leg, known as the gate between the cathode and anode. SCRs
prevent current flow in either direction until the gate receives
a voltage signal. After receiving this trigger signal, the SCR
then becomes a diode. It remains on, regardless of what happens
at the gate, until the zero crossing, at which point current
ceases to flow.

Voltage Clamping. The peak voltage that can be measured after a
Surge Protective Device has limited or "clamped" a transient
voltage surge.

Zener Diode. A diode that maintains a relatively constant voltage
when the reverse voltage across it is increased passed a specific
point, called the zener voltage.

Voltage transients and interference are usually of short duration
and/or have low energy content. High-energy transients are
generally caused by direct lightning strikes.

Protective signaling systems are susceptible to interference
from: lightning, uneven power line conditions, and transients.
Transients are generated by switching various system components;
by capacitive, inductive, or electromagnetic coupling from
motors, neon signs, RF transmitters, or direct-coupled transients
on system wiring.

Transient protection can be provided by transient suppressors.
Transient suppressers need to conduct more current than the
corresponding voltage rise indicates. Devices such as thyrite and
metal oxide varistor (MOV) have current rises that increase
exponentially. Devices such as zener diodes and spark gaps are
break-over devices where the current rises dramatically when a
certain voltage is reached. Spark gap devices are difficult to
apply correctly, because they continue to conduct until the
applied voltage is reduced below 25 volts. Usually, protecting
each conductor will provide a more reliable system. Also, any
circuit that is not sheathed in grounded conduit is likely to
become an antenna.

NFPA 70-1999 800-30 A listed protector is to be provided on each
circuit run partly or entirely in aerial wire not confined within
a block. (The word “block” means a square or portion of a city,
town, etc.) Also, a listed protector is to be provided on each
circuit, aerial or underground, so located within the block
containing the building served as to be exposed to accidental
contact with electric light or power conductors operating at over
300 volts to ground.

The following is Copyright © Harris Corporation 1998

Silicon Zener Diodes
Zener diodes are special silicon diodes which have a relatively
low, defined breakdown voltage, called the zener voltage. At low
reverse voltages a zener diode behaves in a similar manner to an
ordinary silicon diode, that is, it passes only a very small
leakage current. If, however, the reverse bias is increased until
it reaches the breakdown region, then a small reverse voltage
increase causes a considerable increase in leakage current; the
reverse current is then called the zener current. The
characteristics of a zener diode operating under reverse
breakdown conditions are similar to those of a struck glow
discharge tube. Because of this, zener diodes can be used in a
similar way, i.e. as stabilizers, limiters, ripple reduction
elements, reference voltage sources, and also as DC coupling
elements with a constant voltage drop.

A special kind of zener diode is the bi-directional zener diode
with breakdown characteristics in both directions. The main
features are:
 o Energy absorption in both directions
 o Very fast response
 o Low zener voltage variation from standby to peak pulse power
 o After power pulse load the bi-directional zener diode
 automatically recovers to ready state

The bi-directional Zener diodes are designed to protect voltage
sensitive components, integrated circuits, MOS devices, hybrids
and complete electronic systems.

Transient Suppression Devices
There are two major categories of transient suppressors:
a) Transient suppressors that attenuate transients, thus
preventing their propagation into the sensitive circuit; and
b) Transient suppressors that divert transients away from
sensitive loads and so limit the residual voltages.

Attenuating a transient, that is, keeping it from propagating
away from its source or keeping it from impinging on a sensitive
load is accomplished with filters inserted in series within a
circuit. The filter, generally of the low-pass type, attenuates
the transient (high frequency) and allows the signal or power
flow (low-frequency) to continue undisturbed. Diverting a
transient can be accomplished with a voltage-clamping type device
or with a "crowbar" type device. The designs of these two types,
as well as their operation and application, are different enough
to warrant a brief discussion of each in general terms. A more
detailed description will follow later in this section.

A voltage-clamping device is a component having a variable
impedance depending on the current flowing through the device or
on the voltage across its terminal. These devices exhibit a
nonlinear impedance characteristic that is, ohm's law is
applicable but the equation has a variable R. The variation of
the impedance is monotonic; in other words, it does not contain
discontinuities in contrast to the crowbar device, which exhibits
a turn-on action. The volt-ampere characteristic of these
clamping devices is somewhat time-dependent, but they do not
involve a time delay as do the sparkover of a gap or the
triggering of a thyristor. With a voltage-clamping device, the
circuit is essentially unaffected by the presence of the device
before and after the transient for any steady-state voltage below
the clamping level. The voltage clamping action results from the
increased current drawn through the device as the voltage tends
to rise.

Crowbar-type devices involve a switching action, either the
breakdown of a gas between electrodes or the turn-on of a
thyristor, for example. After switching on, they offer a very low
impedance path, which diverts the transient away from the
parallel-connected load.

These types of crowbar devices can have two limitations. One is
delay time, which could leave the load unprotected during the
initial transient rise. The second is that a power current from
the voltage source will follow the surge discharge
(called "follow-current" or "power-follow"). In AC circuits,
this power-follow current may not be cleared at a natural current
zero unless the device is designed to do so; in DC circuits the
clearing is even more uncertain. In some cases, additional means
must be provided to “open” the crowbar.

The frequency components of a transient are several orders of
magnitude above the power frequency of an AC circuit and, of
course, a DC circuit. Therefore, an obvious solution is to
install a low-pass filter between the source of transients and
the sensitive load. The simplest form of filter is a capacitor
placed across the line. The impedance of the capacitor forms a
voltage divider with the source impedance, resulting in
attenuation of the transient at high frequencies. This simple
approach may have undesirable side effects, such as:
a) Unwanted resonances with inductive components located
elsewhere in the circuit leading to high peak voltages;
b) High inrush currents during switching, or,
c) Excessive reactive load on the power system voltage.

These undesirable effects can be reduced by adding a series
resistor hence, the very popular use of RC snubbers and
suppression networks. However, the price of the added resistance
is less effective clamping.

Beyond the simple RC network, conventional filters comprised of
inductors and capacitors are widely used for interference
protection. As a bonus, they also offer an effective transient
protection, provided that the filter's front-end components can
withstand the high voltage associated with the transient. There
is a fundamental limitation in the use of capacitors and filters
for transient protection when the source of transients is
unknown. The capacitor response is indeed nonlinear with
frequency, but it is still a linear function of current. To
design a protection scheme against random transients, it is often
necessary to make an assumption about the characteristics of the
impinging transient. If an error in the source impedance or in
the open-circuit voltage is made in that assumption, the
consequences for a linear suppressor and a nonlinear suppressor
are dramatically different as demonstrated by the following

Crowbar Devices
This category of suppressors, primarily gas tubes or carbon-block
protector, is widely used in the communication field where
power-follow current is less of a problem than in power circuits.
Another form of these suppressors is the hybrid circuit which
uses solid-state or MOV devices. In effect, a crowbar device
short-circuits a high voltage to ground. This short-circuit will
continue until the current is brought to a low level. Because the
voltage (arc or forward-drop) during the discharge is held very
low, substantial currents can be carried by the suppressor
without dissipating a considerable amount of energy within it.
This capability is a major advantage.

Volt-Time Response. When the voltage rises across a spark gap,
no significant conduction can take place until transition to the
arc mode has occurred by avalanche breakdown of the gas between
the electrodes.

Power-Follow. The second characteristic is that a power current
from the steady-state voltage source will follow the surge
discharge (called "follow-current" or "power-follow").

Voltage-Clamping Devices
To perform the voltage limiting function, voltage-clamping
devices depend on their non-linear impedance in conjunction with
the transient source impedance.

Three types of devices have been used: reverse selenium
rectifiers, avalanche zener diodes and varistors made of
different materials, i.e., silicon carbide, zinc oxide, etc.

Selenium Cells
Selenium transient suppressors apply the technology of selenium
rectifiers in conjunction with a special process allowing reverse
breakdown current at high-energy levels without damage to the
polycrystalline structure. These cells are built by developing
the rectifier elements on the surface of a metal plate substrate,
which gives them good thermal mass and energy dissipation
performance. Some of these have self-healing characteristics,
which allows the device to survive energy discharges in excess
of the rated values for a limited number of operations
characteristics that are useful, if not “legal” in the unsure
world of voltage transients. The selenium cells, however, do not
have the clamping ability of the more modern metal-oxide
varistors (MOV) or avalanche diodes. Consequently, their field
of application has been considerably diminished.

Zener Diodes
Silicon rectifier technology, designed for transient suppression,
has improved the performance of regulator-type zener diodes. The
major advantage of these diodes is their very effective clamping,
which comes closest to an ideal constant voltage clamp.

Since the diode maintains the avalanche voltage across a thin
junction area during surge discharge, substantial heat is
generated in a small volume. The major limitation of this type
of device is its energy dissipation capability.

Silicon Carbide Varistors
Until the introduction of metal-oxide varistors, the most common
type of varistor was made from specially processed silicon
carbide. This material was very successfully applied in
high-power, high-voltage surge arresters. However, the relatively
low a values of this material produce one of two results. Either
the protective level is too high for a device capable of
withstanding line volt-age or, for a device producing an
acceptable protective level, excessive standby current would be
drawn at normal voltage if directly connected across the line.
Therefore, a series gap is required to block the normal voltage.
In lower voltage electronic circuits, silicon carbide varistors
have not been widely used because of the need for using a series
gap, which increases the total cost and reproduces some of the
characteristics of gaps described earlier. However, this varistor
has been used as a current-limiting resistor to assist some gaps
in clearing power-follow current.

Metal-Oxide Varistors
A varistor functions as a nonlinear variable impedance. The
relationship between the current in the device I, and the voltage
across the terminals V, is typically described by a power law:
I = kVa

The term a (alpha) in the equation represents the degree of
nonlinearity of the conduction. A linear resistance has an a = 1.
The higher the value of a, the better the clamp, which explains
why a is sometimes used as a figure of merit. Quite naturally,
varistor manufacturers are constantly striving for higher alphas.
This family of transient voltage suppressors are made of sintered
metal oxides, primarily zinc oxide with suitable additives.

These varistors have a values considerably greater than those of
silicon carbide varistors, typically in the range of an effective
value of 15 to 30 measured over several decades of surge current.
The high exponent values (a) of the metal-oxide varistors have
opened completely new fields of applications by providing a
sufficiently low protective level and a low standby current. The
opportunities for applications extend from low-power electronics
to the largest utility-type surge arresters.

Standby Power
The power consumed by the suppressor unit at normal line voltage
is an important selection criterion. Peak standby current is one
factor that determines the standby power of a suppressor. The
standby power dissipation depends also on the alpha
characteristic of the device. As an example, a selenium
suppressor can have a 12mA peak standby current and an alpha of
8. Therefore, it has a standby power dissipation of about 0.5W
on a 120V RMS line (170V peak). A zener-diode suppressor has
standby power dissipation of less than a milliwatt. And a
silicon-carbide varistor, in a 0.75” diameter disc, has standby
power in the 200mW range.

High standby power in the lower alpha devices is necessary to
achieve a reasonable clamping voltage at higher currents. The
amount of standby power that a circuit can tolerate may be the
deciding factor in the choice of a suppressor. Though high-alpha
devices have low standby power at the nominal design voltage, a
small line-voltage rise would cause a dramatic increase in the
standby power. For a zener-diode suppressor, a 10% increase above
rated voltage increases the standby power dissipation above its
rating by a factor of 30. But for a low-alpha device, such as
silicon carbide, the standby power increases by only 1.5 times. 

The gas-discharge suppressor turns on when the transient pulse
exceeds the impulse sparkover voltage. For representative surge
rates; 1kV/ms and 20kV/ms when a surge voltage is applied, the
device turns on at some point within the indicated limits. At
20kV/ms, the discharge unit will sparkover between 600V and
2500V. At 1kV/ms, it will sparkover between 390V and 1500V.

The gas discharge device may experience follow-current. As the
AC voltage passes through zero at the end of every half cycle
the arc will extinguish, but if the electrodes are hot and the
gas is ionized, it may reignite on the next cycle. Depending on
the power source, this current may be sufficient to cause damage
to the electrodes. The follow current can be reduced by placing
a limiting resistor in series with the device, or, selecting a
GDT specifically designed for this application with a high
follow-current threshold. The gas discharge device is useful for
high current surges and it is often advantageous to provide
another suppression device in a combination that allows the added
suppressor to protect against the high initial impulse. Several
hybrid combinations with a varistor or avalanche diode are

Peak Pulse Power
Transient suppressors have to be optimized to absorb large
amounts of power or energy in a short time duration:
nano-seconds, microseconds, or milliseconds in some instances.
Electrical energy is transformed into heat and has to be
distributed instantaneously throughout the device. Transient
thermal impedance is much more important than steady state
thermal impedance, as it keeps peak junction temperature to a
minimum. In other words, heat should be instantly and evenly
distributed throughout the device. The varistor meets these
requirements: an extremely reliable device with large overload
capability. Zener diodes dissipate electrical energy into heat
in the depletion region of the die, resulting in high peak

Clamping Voltage
Clamping voltage is an important feature of a transient
suppressor. Zener diode type devices have lower clamping
voltages than varistors. Because these protective devices are
connected in parallel with the device or system to be protected,
a lower clamping voltage can be advantageous in certain

Speed of Response
Response times of less than 1ps are sometimes claimed for zener
diodes, but these claims are not supported by data in practical
applications. For the varistor, measurements were made down to
500ps with a voltage rise time (dv/dt) of 1 million volts per
microsecond. Another consideration is the lead effect. In
summary, both devices are fast enough to respond to real world
transient events.

It has been stated that a varistor's V-I characteristic changes
every time high surge current or energy is subjected to it. That
is not the case. The V-I characteristic initially changes on
some of the devices, but returns to within a few percent of its
original value after applying a second or third pulse. To be
conservative, peak pulse limits have been established on data
sheets. In many cases, these limits have been exceeded many fold
without harm to the device. This does not mean that established
limits should be exceeded, but rather, viewed in perspective of
the definition of a failed device. A "failed" varistor device
shows a ±10% change of the V-I characteristic at the 1mA point. 

Failure Mode
Varistors subjected to energy levels beyond specified ratings may
be damaged. Varistors fail in the short circuit mode. Subjected
to high enough energy, however, they may physically rupture or
explode, resulting in an open circuit condition. These types of
failures are quite rare for properly selected devices because of
the large peak pulse capabilities inherent in varistors. Zeners
can fail either short or open. If the die is connected by a wire,
it can act as a fuse, disconnecting the device and resulting in
an Open circuit. Designers must analyze which failure mode, open
or short, is preferred for their circuits.

When a device fails during a transient, a short is preferred, as
it will provide a current path bypassing and will continue to
protect the sensitive components. On the other hand, if a device
fails open during a transient, the remaining energy ends up in
the sensitive components that were supposed to be protected.
Another consideration is a hybrid approach, making use of the
best features of both types of transient suppressors.

Depending on the application, transient suppressor capacitance
can be a very desirable or undesirable feature. Varistors in
comparison to zener diodes have a higher capacitance. In DC
circuits capacitance is desirable, the larger the better.
Decoupling capacitors are used on IC supply voltage pins and can
in many cases be replaced by varistors, providing both the
decoupling and transient voltage clamping functions. The same is
true for filter connectors where the varistor can perform the
dual functions of providing both filtering and transient
suppression. There are circuits however, where capacitance is
less desirable, such as high frequency digital or some analog
circuits. As a rule the source impedance of the signal and the
frequency as well as the capacitance of the transient suppressor
should be considered. The current through C P is a function of
dv/dt and the distortion is a function of the signal's source
impedance. Each case must be evaluated individually to determine
the maximum allowable capacitance. The structural characteristics
of metal-oxide varistors unavoidably result in an appreciable
capacitance between the device terminals, depending on area,
thickness and material processing. For the majority of power
applications, this capacitance can be of benefit. In
high-frequency applications, however, the effect must be taken
into consideration in the overall system design.

The ABCs of MOVs
The material in this guide has been arranged in 3 parts for easy
reference; Section A, Section B and Section C.

"A" is for Applications
This section provides general guidelines on what types of MOV
products are best suited for particular environments.

"B" is for Basics
This section explains what Metal Oxide Varistors are, and the
basic function they perform.

"C" is for Common Questions
This section helps clarify important information about MOVs for
the design engineer, and answers questions that are asked most

To properly match the right MOV with a particular application,
it is desirable to know:
 o The maximum system RMS or DC voltage.
 o The MOV continuous voltage at 10 - 25% above maximum system
 o The worst-case transient energy that will need to be absorbed
 by the MOV.


What is an MOV?
An MOV is a Metal Oxide Varistor. Varistors are voltage
dependent, nonlinear devices, which have an electrical behavior
similar to back-to-back Zener diodes. The varistor's symmetrical,
sharp breakdown characteristics enable it to provide excellent
transient suppression performance. When exposed to high voltage
transients, the varistor impedance changes many orders of
magnitude – from a near open circuit to a highly conductive
level – and clamps the transient voltage to a safe level. The
potentially destructive energy of the incoming transient pulse
is absorbed by the varistor, thereby protecting vulnerable
circuit components and pre-venting potentially costly system

What is an MOV Made Of?
The MOV is composed primarily of zinc oxide with small additions
of bismuth, cobalt, manganese and other metal oxides. The
structure of the body consists of a matrix of conductive zinc
oxide grains separated by grain boundaries, which provide P-N
junction semiconductor characteristics. 

Standard varistors are available with AC operating voltages from
2.5V to 3200V. Higher voltages are limited only by packaging
ability. Peak current handling exceeds 70,000 amps, and energy
capability extends beyond 10,000 joules for the larger units.
Package styles include the tiny tubular device used in
connectors, and progress in size up to the rugged industrial

Common Questions

Connecting MOVs for Added Protection
Q. Can MOVs be connected in parallel?
A. Yes. The paralleling of MOVs provides increased peak current
and energy-handling capabilities for a given application. The
determination of which MOVs to use is a critical one in order to
ensure that uniform current sharing occurs at high transient

Q. Can MOVs be connected in series for special voltage
A. Yes. MOVs can be connected in series to provide voltage
ratings higher than those normally available, or to provide
ratings between the standard offerings.

Q. How are MOVs connected for single-phase protection?
A. FOR SINGLE-PHASE AC: The optimum protection is to connect
evenly rated MOVs from hot-to-neutral, hot-to-ground and
neutral-to-ground. If this configuration is not possible,
connection between hot-to-neutral and hot-to-ground is best.

Current Steering or Directing
Q. Does an MOV simply steer current?
A. No. It is incorrect to believe that an MOV device merely
re-directs energy. In fact, the MOV dissipates heat energy
within the device by actually absorbing this energy. The degree
or level to which this absorption can take place is dependent on
the energy rating of the device.

Failure of Device and Fuse Selection
Q. How does an MOV fail?
A. When subjected to stresses above its ratings, an MOV can fail
as a short circuit. If applied conditions significantly exceed
the energy rating of the device, and current is not limited, the
MOV may be completely destroyed. For this reason, the use of
current-limiting fuses is suggested.

Q. How do you select a fuse to prevent failure of an MOV?
A. Fuses should be chosen to limit current below the level where
damage to the MOV package could occur. Generally, the fuse should
be placed in series with either the varistor or the source ahead
of the varistor.

Lead Inductance/Lead Forms/Lead Coating
Q. Does lead inductance/capacitance affect MOV performance?
A. Yes. Transient wave forms with steep fronts (1us) and in
excess of several amps produce an increase in voltage across the
varistor. This is a characteristic of all leaded devices
including Zeners, known as overshoot. Unlike Zeners, some MOVs
are leadless and do not exhibit overshoot.

Q. Are MOVs sensitive to polarity?
A. No. Since MOVs provide bidirectional clamping, they are not a
polarized device.

Q. Are MOVs sensitive to electrostatic discharge?
A. No. In fact, MOVs are specifically designed to protect
sensitive integrated circuits from ESD transients.

Q. Generally speaking, are MOVs sensitive to chemical/ pressure
when potted?
A. No.

Speed of Response, Compared to Zeners
Q. Are Zeners significantly faster 
than MOVs?
A. No, 

not to the extent of the claims made. The intrinsic
response time of MOV material is 500 picoseconds. As the vast
majority of transients have a slower rise time than 
this, it is
of little or no significance to compare speeds of response. The
response time of a leaded MOV or Zener is affected by circuit
configuration and lead inductance.

Voltage Regulation, Voltage Limits
Q. Can an MOV be used as a voltage regulator?
A. No. MOVs function as nonlinear impedance devices. They are
exceptional at dissipating transient voltage spikes, but they
cannot dissipate continuous low-level power. 


The increasing usage of sensitive solid state devices in modem
electrical systems, particularly computers, communications
systems and military equipment, has given rise to concerns about
system reliability. These concerns stem from the fact that the
solid state devices are very susceptible to stray electrical
transients which may be present in the distribution system. The
initial use of semiconductor devices resulted in a number of
unexplained failures. Investigation into these failures revealed
that they were caused by transients, which were present in many
different forms in the system. Transients in an electrical

result from tile sudden release of previously stored
energy. The severity of, and hence the damage caused by
transients depends on their frequency of occurrence, the peak
transient currents and voltages present and their waveshapes.

In order to adequately protect sensitive electrical 
thereby assuring reliable operation, transient voltage
suppression must be part of the initial design process and not
simply included as an afterthought. To ensure effective transient
suppression, the device chosen must have the capability to
dissipate the impulse energy of the transient at a sufficiently
low voltage so that the capabilities of the circuit being
protected are not affected. The most successful type of
suppression device used is the metal oxide varistor. Other
devices, which are also used, are the zener diode and the
gas-tube arrestor.

The Transient Environment
The occurrence rate of surges varies over wide limits, depending
on the particular power system. These transients are difficult
to deal with, due to their random occurrences and the problems
in defining their amplitude, duration and energy content. Data
collected from many independent sources predict with certainty
only a relative frequency of occurrence, while the absolute
number of occurrences can be described only in terms of low,
medium or high exposure. This data was taken from unprotected
circuits with no surge suppression devices. Low exposure data
was collected in geographical areas known for low lightning
activity, with little load switching activity. Medium exposure
data came from geographical areas known for high lightning
activity, with frequent and severe switching transients. High
exposure areas are rare, but real systems, supplied by long
overhead lines and subject to reflections at line ends, where
the characteristics of the installation produce high sparkover
levels of the clearances. Investigations into the two most common
exposure levels, low and medium, have shown that the majority of
surges occurring here can be represented by typical waveform
shapes (per ANSI/IEEE C62.41-1980). The majority of surges,
which occur in indoor low voltage power systems can be modeled
to an oscillatory waveform. A surge impinging on the system
excites the natural resonant frequencies of the conductor system.
As a result, not only are the surges oscillatory but also surges
may have different amplitudes and waveshapes at different
locations in the system. These oscillatory frequencies range
from 5kHz to 500kHz with 100kHz being a realistic choice.

In outdoor situations the surge waveforms recorded have been
categorized by virtue of the energy content associated with them.
These waveshapes involve greater energy than those associated
with the indoor environment. These waveforms were found to be
unidirectional in nature.

Transient Energy and Source Impedance
Some transients may be intentionally created in the circuit due
to inductive load switching, commutation voltage spikes, etc.
These transients are easy to suppress since their energy content
is known. It is the transients, which are generated external to
the circuit and coupled into it, which cause problems. These can
be caused by the discharge of electromagnetic energy, e.g.,
lightning or electrostatic discharge. These transients are more
difficult to identify, measure and suppress. Regardless of their
source, transients have one thing in common - they are
destructive. The destruction potential of transient voltage is
defined by their peak voltage.

It should be noted that considering the very small possibilities
of a direct lightning hit it may be deemed economically
unfeasible to protect against such transients. However, to
protect against transients generated by line switching, ESD, EMP
and other such causes is essential, and if ignored will lead to
expensive component and/or system losses. The energy contained in
a transient will be divided between the transient suppressor and
the line upon which it is traveling in a way, which is determined
by their two impedances. It is essential to make a realistic
assumption of the transient's source impedance in order to ensure
that the device selected for protection has adequate surge
handling capability. In a gas-tube arrestor, the low impedance of
the arc after sparkover forces most of the energy to be
dissipated elsewhere - for instance in a power-follow
current-limiting resistor that has to be added in series with the
gap. This is one of tile disadvantages of the gas-tube arrestor.
A voltage clamping suppressor (e.g., a metal oxide varistor) must
be capable of absorbing a large amount of transient surge energy.
Its clamping action does not involve the power-follow energy 

resulting from the short-circuit action of the gap. The degree to
which source impedance is important depends largely on the type
of suppressor used. The surge suppressor must be able to handle
the current passed through them by the surge source. An
assumption of too high an impedance (when testing the suppressor)
may not subject it to sufficient stresses, while the assumption
of too low an impedance may subject it to unrealistically large
stress; there is a trade off between the size/cost of the
suppressor and the amount of protection required.

In a building, the source impedance and the load impedance
increases from the outside to locations well within the inside of
the building, i.e., as one gets further from the service
entrance, the impedance increases. Since the wire in a structure
does not provide much attention, the open circuit voltages show
little variation.

Power system wiring can be divided into three categories:
Category A covers outlets and long branch circuits over 30 feet
from category B and those over 60 feet from category C.

Category B is for major feeders and short branch circuits from
the electrical entrance. Examples at this location are bus and
feeder systems in industrial plants, distribution panel devices,
and lightning systems in commercial buildings.

Category C applies to outdoor locations and the electrical
service entrance. It covers the service drop from pole to
building entrance, the run between meter and the distribution
panel, the overhead line to detached buildings and under-ground
lines to well pumps.

Transient Suppression
The best type of transient suppressor to use depends on the
intended application, bearing in mind that in some cases both
primary and secondary protection may be required. It is the
function of tile transient suppressor to, in one way or another,
limit the maximum instantaneous voltage that can develop across
the protected load. The choice depends on several factors, but
the decision is ultimately a trade-off between the cost of the
suppressor and the amount of protection needed. The time required
for a transient suppressor to begin functioning is extremely
important when it is used to protect sensitive components. If the
suppressor is slow acting and a fast-rise transient spike appears
on the system the voltage across the protected load can rise to
damaging levels before suppression begins. On AC power lines the
best type of suppression to use is a metal oxide varistor. Other
devices occasionally used are the zener diode and the gas-tube

Gas-Tube Arresters
This is a suppression device which finds most of its applications
in telecommunication systems. It is made of two metallic
conductors usually separated by 10mils to 15mils encapsulated in
a glass envelope. This glass envelope is pressurized and contains
a number of different gases. Types specifically designed for AC
line operation are available and offer high surge current ratings.

Zener Diodes
One type of clamp-action device used in transient suppression is
the zener diode. When a voltage of sufficient amplitude is
applied in the reverse direction, the zener diode is said to
break down, and will conduct current in this direction. This
phenomenon is called avalanche. The voltage appearing across the
diode is therefore called the reverse avalanche or zener voltage.

When a transient propagates along the line with a voltage
exceeding the reverse-based voltage rating of the diode, the
diode will conduct and the transient will be clamped at the zener
voltage. This clamping voltage is lower than that of an
equivalent varistor. Some manufacturers have claimed that the
response time of a zener diode is 1ps to 2ps. In practice, the
speed of response is greatly determined by the parasitic
inductance of the package and the manner in which the device is
connected via its leads. Although zener diodes can provide
transient protection, they cannot survive significant
instantaneous power surges. Larger diodes can be used to increase
the power rating, but this is only at the expense of increased
costs. Also, the maximum tolerable surge current for a zener
diode in reverse breakdown is small when compared to tolerable
surge currents for varistors. Due to the fact that there is only
the p-n junction in a zener diode, it will need to have some
additional heat sinking in order to facilitate the rapid build-up
of heat which occurs in the junction after it has encountered a

Metal Oxide Varistor
As the name suggests, metal oxide varistors (MOV) are variable
resistors. Unlike a potentiometer, which is manually adjusted,
the resistance of a varistor varies automatically in response to
changes in voltages appearing across it. Varistors are a
monolithic device consisting of many grains of zinc oxide, mixed
with other materials, and compressed into a single form. The
boundaries between individual grains can be equated to p-n
junctions with the entire mass represented as a series-parallel
diode network. When a MOV is biased, some grains are forward
biased and some are reverse biased. As the voltage is increased,
a growing number of the reversed biased grains exhibit reverse
avalanche and begin to conduct. Through careful control in
manufacturing, most of the nonconducting p-n junctions can be
made to avalanche at the same voltage. MOVs respond to changes in
voltages almost instantaneously. The actual reaction time of a
given MOV depends on physical characteristics of the MOV and the
wave shape of the current pulse driven through it by the voltage
spike. Experimental work has shown the response time to be in the
500 picosecond range. One misconception about varistors is that
they are slow to respond to rapid rise transients. This “slow”
response is due to parasitic inductance in the package and leads
when the varistor is not connected with minimal lead length. If
due consideration is given to these effects in its installation,
the MOV will be more than capable of suppressing any voltage
transients found in the low voltage AC power system. The MOV has
many advantages over the zener diode, the greatest of which is
its ability to handle transients of much larger energy content.
Because it consists of many p-n junctions, power is dissipated
throughout its entire bulk, and unlike the zener, no single hot
spot will develop. Another advantage of the MOV is its ability to
survive much higher instantaneous power.

When designing circuits of the complex nature seen in today’s
electrical environment, the initial design must incorporate some
form of transient voltage surge suppression. The expense of
incorporating a surge protection device in a system is very low
when compared with the cost of equipment downtime, maintenance
and lost productivity, which may result as a consequence of not
having protection. When selecting surge suppressors for retrofit
to an existing design, one important point to remember is that
the location of the load to be protected relative to the service
entrance is as important as the transient entrance, which may be
present in an overvoltage situation.

Transient Voltage Suppressors (TVS’s) are devices used to protect
vulnerable circuits from electrical overstress such as that
caused by electrostatic discharge, inductive load switching and
induced lightning. Within the TVS, damaging voltage spikes are
limited by clamping or avalanche action of a rugged silicon pn
junction which reduces the amplitude of the transient to a
nondestructive level. In a circuit, the TVS should be "invisible"
until a transient appears. Electrical parameters such as
breakdown voltage (V(BR)), standby (leakage) current (ID), and
capacitance should have no effect on normal circuit performance.
The TVS breakdown voltage is usually 10% above the reverse
standoff voltage (VR), which approximates the circuit operating
voltage to limit standby current and to allow for variations in
V(BR) caused by the temperature coefficient of the TVS. When a
transient occurs, the TVS clamps instantly to limit the spike
voltage to a safe level, called the clamping voltage (VC), while
conducting potentially damaging current away from the protected



OR1 PCC Sylvania, Portland;
Test 4/26/03. Postmark deadline 3/8/03.
Test 7/26/03. Postmark deadline 6/7/03.

OR2 Clackamas Community College, Oregon City;
Test 6/21/03. Postmark deadline 5/3/03.
Test 9/27/03. Postmark deadline 8/9/03.

WA1 Bates Technical College, Tacoma;
Test 5/17/03. Postmark deadline 3/29/03.
Test 7/26/03. Postmark deadline 6/7/03.

WA2 Walla Walla Community College;
Test 5/17/03. Postmark deadline 3/29/03.
Test 9/27/03. Postmark deadline 8/9/03.

WA3 Spokane Community College;
Test 5/17/03. Postmark deadline 3/29/03.
Test 8/23/03. Postmark deadline 7/7/03.

These dates are from the NICET web site. For a complete list of
all test centers and test dates, visit:


April 1-3, 2003 Phoenix, AZ - Sponsored by AZ AFAA
Intermediate Fire Alarm Seminar.

April 7-9, 2003 New Orleans, LA - Sponsored by LA AFAA
Advanced Fire Alarm Seminar.

April 22-24, 2003 Richmond, VA - Sponsored by VA AFAA
Advanced Fire Alarm Seminar.

April 22-24, 2003 Boise, ID
Intermediate Fire Alarm Seminar.

April 29-30 & May 1, 2003 Seattle, WA
Intermediate Fire Alarm Seminar.

May 5-8, 2003 Anaheim, CA - Sponsored by CAFAA
Fire Alarm System Testing and Inspection Seminar.
Intermediate Fire Alarm Seminar.

May 6-8, 2003 Chicago, IL
Advanced Fire Alarm Seminar.
More information will be available soon.

May 13-15, 2003 Billings, MT
Intermediate Fire Alarm Seminar.
More information will be available soon.

June 9-12, 2003 Sacramento, CA - Sponsored by CAFAA
Fire Alarm System Testing and Inspections Seminar
Intermediate Fire Alarm Seminar
More information will be available soon.

June 10-12, 2003 Alexandria, LA - Sponsored by LA AFAA
Automatic Fire Detection and Fire Alarm Systems Seminar (Fund.)
Fire Alarm System Testing and Inspections Seminar
More information will be available soon.

June 17-19, 2003 Las Vegas, NV
Intermediate Fire Alarm Seminar.

July 14-17, 2003 Anaheim, CA - Sponsored by CAFAA
Plan Review Seminar
Intermediate Fire Alarm Seminar
More information will be available soon.

September 15-18, 2003 Oakland, CA - Sponsored by CAFAA
Plan Review Seminar
Intermediate Fire Alarm Seminar
More information will be available soon.

September 16-18, 2003 Boston, MA - Sponsored by New England AFAA
Fire Alarm System Testing and Inspections Seminar
Automatic Fire Detection and Fire Alarm Systems Seminar (Fund.)
More information will be available soon.

September 22-25, 2003 Lafayette, LA - Sponsored by LA AFAA
Fire Alarm System Testing and Inspections Seminar
Intermediate Fire Alarm Seminar
More information will be available soon.

October 15-17, 2003 Boston, MA - Sponsored by New England AFAA
Intermediate Fire Alarm Seminar
More information will be available soon.

November 3-6, 2003 Anaheim, CA - Sponsored by CAFAA
Plan Review Seminar
Advanced Fire Alarm Seminar
More information will be available soon.

November 4-7, 2003 Boston, MA - Sponsored by New England AFAA
Advanced Fire Alarm Seminar
More information will be available soon.


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and official position of the NFPA on the referenced subject,
which is represented only by the standard in its entirety.
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Some information may be found within this web site that is reprinted with permission from one or more of the following: NFPA 70 National Electrical Code®,NFPA 72® National Fire Alarm Code®, & NFPA 101® Life Safety Code®, Copyright© NFPA, Quincy, MA 02269.

This reprinted material is not the complete and official position of the NFPA on the referenced subject, which is represented only by the standard in its entirety.