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Low-Voltage Power System Surge
Overvoltage Protection
Dev Paul, Senior Member, IEEE
Abstract—The unpredictable threat of transient overvoltages is
ever increasing in today’s low-voltage power supplies for every aspect of industry. To calculate the magnitude, duration, and energy
of such transient overvoltages is not an easy task. Some loads are
becoming very sensitive to such overvoltages, thereby creating a
challenge for the application engineer to design a reliable power
supply system. To apply surge overvoltage mitigation devices requires technical knowledge to understand their application limitations and configuration within a power system. Technical overview
of fundamentals of transients and associated noise is presented.
The importance of understanding applicable UL, IEEE, and IEC
Standards, and thorough review of manufacturers’ data on transient voltage surge suppressors (TVSSs) is included. TVSS testing
requirements per the second edition of UL 1449 is presented. An
overview of how to design a low-voltage power supply system to
suppress transient overvoltages is included.
Index Terms—Electromagnetic compatibility, electromagnetic
interference, metal-oxide varistor, noise, spikes, surges, transient
voltage surge suppressor.
switching frequencies and switched-mode power supplies, used
for their greater efficiencies contribute to noise. The ever increasing application of silicon-controlled-rectifier (SCR)-based
power circuits and capacitors do contribute to the noise problem
for their own control systems.
Transient overvoltages are harmful to sensitive control
circuits and should be suppressed by application of transient
voltage surge suppressors (TVSSs). However, the problem is
to quantify the magnitude, duration, and energy parameters of
the surge for proper evaluation to select TVSS devices. This
paper provides a fundamental overview of transient surge and
associated noise in low-voltage power distribution systems.
The TVSS application approach is included to help design
engineers in the selection of proper transient surge-suppression
OLTAGE transients are brief and unpredictable. These
two characteristics make it difficult to detect and measure
them. Considerable work is in progress and much data is now
available from the power quality group to better understand
these transients [1]. Low-voltage power supply systems are
getting more disturbed in terms of power quality issues.
The switch-mode power supplies used for equipment such
as fax machines, printers, variable-frequency drives (VFDs)
for energy-efficient heating, ventilation, and air-conditioning
(HVAC) systems, elevators, escalators, electronic ballasts, etc.,
and a great percentage of other loads are becoming nonlinear.
Such loads generate current harmonics, leading to distorted
voltage. In addition to this distorted voltage, the switching and
lightning surges propagating through the distribution system
lead to transient overvoltages.
Both analog and digital circuits use complex solid-state components in today’s control systems and are inherently susceptible to damage or malfunction from the electrical surges. The
current trend toward more performance in smaller size has itself contributed to the noise problem. Digital circuits with high
Paper ICPSD 00–03, presented at the 2000 IEEE/IAS Industrial and
Commercial Power Systems Technical Conference, Clearwater Beach, FL,
May 7–11, and approved for publication in the IEEE TRANSACTIONS ON
INDUSTRY APPLICATIONS by the Power Systems Protection Committee of the
IEEE Industry Applications Society. Manuscript submitted for review May 12,
2000 and released for publication July 26, 2000.
The author is with EARTH TECH, Oakland, CA 94612-3060 USA (e-mail:
Publisher Item Identifier S 0093-9994(01)00283-3.
Electrical noise generated within the facility by transient
surges corrupts low-voltage power supplies. In discussing the
characteristics of transient surges and associated noise, it is
helpful to understand other related terms.
A. Definitions—Noise and Noise-Related Terms
1) Noise can be defined as any form of electromagnetic energy other than the desired signal and its harmonic components.
2) Transients are the momentary amplitude changes in
voltage or current or both;
3) Surge is a term for either high-voltage noise or long-duration transients.
4) Electromagnetic interference (EMI) is the impairment of
a desired electromagnetic signal by an electromagnetic
disturbance such as noise. Since noise-related EMI often
occurs in the radio frequency range of 10 kHz to 30 MHZ,
the term radio frequency interference (RFI) is often used
instead of the general term EMI;
5) Electromagnetic compatibility (EMC) is a device’s capability to perform its intended function within a given electromagnetic environment without adversely affecting or
being adversely affected by other devices sharing that environment;
6) Electromagnetic environment is a set of conditions characterized by: 1) the presence of one or more kinds of disturbance, energy, rise time, frequency, duration, and amplitude of disturbance and 2) the effect that these disturbances have on equipment within the environment.
0093–9994/01$10.00 © 2001 IEEE
Fig. 1.
Fig. 2.
Fig. 4.
Voltage spikes.
Fig. 5.
Oscillatory decaying disturbances.
Common-node (CM) noise.
Normal-mode (NM) noise.
Fig. 3. Normal-mode (NM) noise on primary winding transformed into
common-mode (CM) through distribution transformer.
B. Modes of Noise
Various modes of noise and their characteristics are defined
as follows.
1) Common-Mode Noise: The term “common” indicates
that the noise signals on each of the current-carrying
conductors are in phase and equal in magnitude. Thus,
a voltage signal is not generated between these conductors. The common mode is also known as the longitudinal mode (see Fig. 1).
2) Normal-Mode Noise: This is defined as the noise
appearing between the current-carrying conductors. It
is also known as transverse-mode, differential-mode,
metallic-mode, or symmetrical RFI (see Fig. 2).
3) Normal-to-Common-Mode
Common-mode noise is more troublesome than
normal-mode noise. Noise is always transmitted
through a distribution transformer as common-mode
noise, regardless of the mode in which it was generated
(see Fig. 3).
4) Low-Voltage Noise: Noise with peak voltage less than
2000 V is considered low voltage.
5) Voltage or Current Spikes (Impulses): They are called
fast transients due to their fast rise times in the order of 1
ns–10 s. They occupy a broad frequency spectrum from
4 kHz to 5 MHz, occasionally reaching 30 MHz. A typical duration is within 100 ns–150 s. Voltage reaches
150% or more of the peak nominal line voltage. They
may occur in bursts lasting for as long as one cycle of
line voltage (see Fig. 4).
6) Oscillatory Decaying Disturbances: These disturbances
have a frequency range of 400 Hz–5 kHz or more (see
Fig. 5).
7) High-Voltage Surges: These range upwards from 2000
V. A typical amplitude of surges at a 120-V receptacle
is between 100–500 V. In very rare circumstances, the
magnitude reaches 4000–6000 V. The upper limit results
because sparkover (arcing between the conductors) will
occur at about 6000 V, at the distance between the terminal blocks of the indoor power system supply rated at
240 V.
8) Broad-Band Noise: The spectral characteristics of
a noise waveform dictate the frequency attenuation
requirements of a noise protection device. The shorter
the rise time of a noise pulse, the broader is the band of
energies that must be attenuated. Voltage spikes contain
a power spectrum of broad frequency range and are
considered broad-band noise.
9) Narrow-Band Noise: Such noise has much lower frequency content than broad-band noise, in the less than
1-MHz frequency range.
10) Attenuation of Common-Mode Noise: Power transformers do not transform high-frequency signals in
the same way as they do 60-Hz power. To high-frequency transients, the transformer is nothing more
than a network of capacitance and iron-core reactance.
The iron core of the transformer cannot respond to
the high frequency and, thus, becomes a negligible
factor. In some applications, the use of an isolation
transformer or a shielded transformer is effective to
attenuate the common-mode noise, which otherwise
could disturb the performance of sensitive equipment.
In the case of an isolation transformer, the degree
Fig. 6.
Attenuation of common-mode noise through isolation transformer.
Fig. 7.
Attenuation of common-mode noise through shielded transformer.
of attenuation depends upon the relative magnitudes
of transformer inter-winding capacitance
(see Fig. 6).
winding-to— ground capacitance
In the case of a shielded transformer, the introduction
of a grounded shield between the windings results in
much better attenuation to common-mode transients.
The electrostatic charge around the primary winding is
conducted to ground by the shield before it can induce
voltage into the secondary winding. The small amount
of coupling of the electrostatic field around the shield
and is
is generally called “effective capacitance”
much smaller than the inter-winding capacitance
of an unshielded transformer (see Fig. 7).
11) One or more of the following parameters can characterize noise distortion:
• single impulses or oscillatory signals;
• rise and/or fall times;
• duration;
• rate of repetition;
• amplitude;
• frequency.
The attenuation is generally described in decibel (dB) units.
The logarithmic relationship of dB to attenuation in volts or amperes is shown by
or dB
Thus, 60 dB could be stated as
resulting in:
*In some locations, sparkover of clearances may limit
the overvoltages.
Fig. 8. Rate of surge occurrences versus voltage level at unprotected locations
for different exposures.
The various causes of surge overvoltage and associated noise
are well documented.
1) Lightning: Lightning can produce various modes of noise
and transient disturbances as follows:
• lightning and common-mode noise;
• lightning-induced electrostatic coupling;
• lightning-induced magnetic coupling;
• lightning-induced conductive coupling;
• lightning and normal-mode noise.
2) Power-Factor-Correction Capacitors: Oscillation frequencies are in the range of 1–20 kHz.
3) Power System Switching: Included are fault-clearing devices and load switching and electronic switching devices.
4) Lighting Exposure and Surge Intensity: The probability of
surge voltage exceeding specific peak values is related to
the levels of lightning exposure, as shown in Fig. 8 [12].
Working groups of IEEE and IEC Standards have developed
different standard surge waves for testing TVSS devices meant
for outdoor and indoor application locations to the low-voltage
power distribution system [16].
1) Outdoor: Combo Wave—1.2/50 s voltage wave and
8/20 s current wave are predominant at the service
entrance outdoor location. However, lightning discharges
Fig. 9.
Test waveforms as described by ANSI/IEEE C62.41-1991 [16].
induce oscillations, reflections that ultimately appear as
decaying oscillations in a low-voltage power system.
2) Indoor: Ring Wave—A surge impinging on the system
excites the natural resonant frequencies of the conductor
system. As a result, not only are the surges typically
oscillatory, but also may have different amplitudes and
wave shapes at different places in the low-voltage power
system. These oscillatory frequencies of surges range
from 5 to more than 500 kHz. Based upon such conclusions, a Ring Wave, 0.5 s with 100 kHz which rises in
0.5 s, then decays while oscillating at 100 kHz, each
peak being 60% of the preceding peak. Such waves are
depicted in Fig. 9.
There is a lack of definite data on the duration, waveform, and
source impedance of transient overvoltages in ac power circuits.
These three parameters are important for estimating the energy
that a transient can deliver to a suppressor. The impedance presented by a source of energy to the input terminals of a device
or network is defined as the source impedance. Because of the
wide range of possible source impedance and the difficulty of
selecting a specific value, three broad categories of building
locations as defined in Table I [17] have been considered for
the application of surge suppressors in the low-voltage power
system. The degree to which the source impedance is important depends largely on the type of surge suppressors that are
used. The surge suppressors must be able to withstand the current passed through them by the surge source.
Source impedance should not be confused with the surge
impedance. Surge impedance is the concept relating to the
parameters of a line to the propagation of the traveling wave,
for the low voltage wiring system it may be in the range of
150–300 .
Clamping surge protective devices work only if there is a finite source impedance [15].
Demarcation between location Categories B and C is arbitrary, taken to be at the main meter or at the main disconnect
or at the secondary of the service transformer if the service is
provided to the user at a higher voltage.
Fig. 10.
Power quality pyramid-relative cost.
The majority of the power quality problems in the
low-voltage distribution system are being solved by the
application of TVSSs and their combinations with other
noise-filtering devices. TVSSs go by a variety of names
provided by individual manufacturers, creating a challenge for
the power system designer for proper selection and application.
In the low-voltage power distribution system, series-connected
TVSSs are available, however, such devices carry normal
current continuously and need to have short-circuit withstand
capability without creating a dangerous situation. Increased
application demand of TVSSs to solve power quality problems
in general is dedicated by the relative cost of such devices as
compared to other devices shown in the power quality pyramid
(see Fig. 10).
Broad categories of TVSS devices are defined as follows.
1) Clamps: A metal-oxide varistor (MOV) has high-energy
absorption capability and response time of nanoseconds
2) Crowbar: Gas tubes do not response quickly; transient
may occur faster than the device can respond.
3) Hybrid: These devices may be considered as a combination of an MOV and a Crowbar.
4) Sine-Wave Tracking: New technology tracks the ac sine
wave; this enables response to minor spikes and transients
that may pass through other categories of protection devices.
5) Others: These include data/signal/telephone TVSSs and
noise-filtering devices [15].
The second edition of UL Std. 1449 [9] has important changes
affecting the test requirements for safety and performance of
permanently connected TVSS products. This edition includes
the "end of life" mode testing, which could result in damage to
products from a large surge event, a sustained overvoltage such
as a loss of neutral, or installation error such as improper bonding
or misoperation. MOV element of TVSS can cause a short circuit
condition when its goes into a thermal runaway condition. If not
properly contained, equipment damage may result.
The TVSS must pass the following tests without the evidence
of risk of fire or electric shock [9].
1) Overvoltage: The TVSS must withstand 110% of rated
voltage for seven hours.
2) Abnormal Overvoltage, Full Phase Voltage—High Current: The TVSS must withstand 25 kA at full phase
voltage of 208 V for 120 V and 480 V for 277 V. The
overvoltage is applied for seven hours.
3) Abnormal Overvoltage, Full Phase Voltage—Limited Current: The TVSS must withstand5 A (also 2.5, 0.5, and0.125
A) at full phase voltage of208 V for 120 V and 480 Vfor277
V. The overvoltage is applied for seven hours.
4) Voltage-Limiting Test and Duty Cycle (Pulse Life) Test:
The test is performed for L-L, L-N, L-G, and N-G connections of TVSS subjecting an impulse surge of 6 kV
and 0.5 kA, limiting voltage is measured and recorded.
The device is then subjected to ten consecutive 6-kV and
3-kA positive impulses at 60-s intervals, and ten consecutive 6-kV and 3-kA negative impulses at 60-s intervals.
Following the duty cycle test, the TVSS is subjected to another 6 kV and 0.5-kA impulse and the limiting voltage
is measured and recorded. The average of the limiting
voltage test is to fall within the minimum suppressed
voltage ratings and is not to exceed the suppressed voltage
rating by greater than 10%.
5) Surge Current Test: This test is designed to qualify each
device to withstand a standard Category C3 transient. The
device must withstand two consecutive (one positive and
one negative) 6-kV and 10-kA surge impulses (C3). After
testing, the device must stay connected to the service at
rated voltage for seven hours or until thermal equilibrium.
It is important that the application engineer has the basic understanding of the published literature on transient surge overvoltage environment and the surge wave shapes used in testing
the TVSS [17], [18].
To avoid guesswork and misapplication of the TVSS, the application engineer, perhaps, should develop his/her own list of
evaluation factors. These evaluation factors, in general, will es-
tablish the roadmap for analysis and solution to the problem of
voltage transients and associated high-frequency noise.
1) Develop a complete one-line diagram of the low-voltage
power distribution system showing main transformer,
main switchboard, subpanels, feeders distribution transformers, type of loads, and metering devices at each
panel. Show control, data communication, and other sensitive loads at appropriate locations throughout the facility in one— line representation (see Fig. 11).
2) Review the utility company incoming primary power
source grounding method and configuration in order to
select proper primary surge arrester to be located at the
primary side of the main power transformer.
3) Establish an equipment layout sketch indicating
raceway and wiring types, configuration, and approximate lengths.
4) Review grounding, bonding, and shielding provisions of
the distribution system as needed for equipment protection, personnel safety, and high-frequency shielding of
data and control systems [6], [7].
5) Establish maximum continuous operating voltage at
each main power distribution panel based upon the
utility voltage regulation and the effect of the transformer taps, if any.
6) Perform three-phase and single-phase short-circuit analysis to establish maximum expected short-circuit current at all critical locations within the power distribution
system where TVSSs are to be applied.
7) Review facility site in association with configuration of
power distribution system and loads to determine the degree of outside transient surge exposure as well as the
internal switching surge propagation [3], [12].
8) Analyze power distribution system design for possible
ferroresonance condition which could lead to failure of
surge arresters [13].
9) For each location, establish the TVSS category type, protection modes, high-frequency noise-filtering requirements, and the means of connection to the equipment
that needs surge protection.
10) Establish TVSS local or remote monitoring requirements.
11) At Category A locations, make analysis of the choice
between the series-connected or the parallel-connected TVSS devices. This may require reviewing the
problem with the equipment vendor to assure proper
12) Review design of grounding, bonding, and shielding, a
key element to the success of TVSS application [1].
13) Carefully review the TVSS vendor literature for each
category for specific requirements and safety test requirements [9]. If possible, contact the vendor to address
questions and concerns to obtain clarification of TVSS
characteristics and performance specifications. TVSS
performance criteria and specifications shall include at
least the following:
a) surge current capability: performance, safety;
b) fail-safe design;
Fig. 11.
Category classification defined by IEEE Std. C62.41 and IEC Std 664 for TVSS application.
UL listing, testing, and suppression voltage;
modes of protection;
EMI/RFI filtering capability;
life-cycle testing;
monitoring features;
short-circuit withstand capability and fusing
i) response time.
To avoid guesswork and misapplication of TVSSs, an
engineering analysis of transient surge overvoltages and
high-frequency noise is required. Application engineers must
learn to evaluate the transient environment and should have
technical knowledge to fully understand the limitations of
commercially available TVSSs for each specific location in
the power system. Some manufacturers promote their TVSSs
merely by their surge-chopping and energy-absorption capability without proper application guidance and criteria. A new
UL standard [9] requires their test withstand capability and
not all TVSSs could meet such requirements. With a thorough
understanding of the applicable standards, and knowledge of
surge propagation through the power distribution system, the
design engineer should be able to make a proper selection of
TVSS for the application.
Proper application of TVSS devices in a low-voltage power
distribution system may require all or a combination of the following.
1) Apply both primary and secondary surge-suppressor devices at the main transformer [13].
2) Implement integrated surge-suppressor device at the
main panel [11].
3) Apply “cascaded network” approach [1], [13].
4) Apply power quality pyramid for design of distribution
system [11].
5) Use integrated surge protection devices approach, especially for a new facility, as it provides better margin of
protection. In retrofit projects, use as short as possible
the specially designed low-impedance coaxial cable to
install a surge-suppression device outside the panel.
6) Use twisted shielded pair wires and ground shield at either both ends or at one end, depending upon the overall
configuration of equipment ground and shield ground.
7) Provide reference ground grid for sensitive electronic
equipment [7], [8].
8) Use of ferro-resonant line conditioners can provide
better attenuation than isolating power transformers for
fast line-to-line transients [5].
9) Fuses applied to the TVSS shall have surge current withstand capability and shall isolate the device in case of its
failure by surge current higher than its let through capability. However, in the case of thermal runaway of the
TVSS device, the current is still too low to operate the
fuse; only a thermal cut-out in close proximity to the
surge device can clear this fault. Development of such
proper disconnectors is a challenge to the manufacturers
of surge devices.
10) Install series-hybrid-type filters as close to the critical
loads as possible, such as in front of fire alarm systems,
control devices, programmable logic controllers, cash
registers, etc.
11) Install surge protection equipment on all data and communication lines, especially noise-suppression devices.
12) At noncritical loads which generate disturbance such as
harmonics and noise, installation of less expensive surge
strips may be adequate.
13) If possible, the grounding system of the surge arrester
shall have minimum resistance to ground in order to minimize ground potential rise above remote earth which
acts as reference ground to other sensitive equipment in
the facility.
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[2] The New IEEE Standard Dictionary of Electrical and Electronics Terms,
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Dev Paul (M’73–SM’90) received the B.Sc. degree
with honors in mathematics and the B.E. (Honors)
and M.S.E.E. degrees in electrical engineering from
Punjab University, Chandigarh, India, in 1965, 1969,
and 1971, respectively. He completed further studies
in power systems at the University of Santa Clara,
Santa Clara, CA, in 1975.
In 1972, he joined EARTH TECH (formerly
Kaiser Engineers), Oakland, CA, as a Design
Engineer and has worked on a variety of heavy
industrial, cogeneration, commercial, DOD and
DOE facilities, and rapid transit rail projects. In his present position as a
Principal Electrical Engineer, he is responsible for the overall design, analysis,
studies, specification, installation, project engineering, startup work, and
system integration. He has authored papers published in IEEE Industry
Applications Society (IAS) and APTA conference proceedings. His main fields
of interests are power system analysis, protection, grounding, and harmonics.
Mr. Paul is an active member of several IAS Committees. He received the
Award of Distinction for his M.S.E.E. thesis work on power system stability.
He is a Registered Professional Engineer in the States of California, Nevada,
and Oregon.