Welcome to the World of Transients
The Nature of Transients
An electrical transient (surge, impulse, or spike) is a temporary excess in voltage and/or
current on an electrical circuit that has been disturbed. These electrical transients are high-energy,
fast rise time, short duration phenomena. Typically, the rise times are in the nanosecond to microsecond
range. The voltage and current magnitudes at a facility service entrance can be in the thousands of volts
and thousands of amperes range. The durations of these disturbances range from nanoseconds to
milliseconds. These transients, impulses, and spikes are shorter in duration than the cycle of the
power-frequency sine wave (1.67 milliseconds at 60 Hertz).
Transient Generation
Externally Generated Transients
Externally generated transients are transients that originate outside of a given facility. They
account for about twenty percent of the total transient activity at a facility. Lightning strikes
and utility company grid switching are examples of external transient sources. Cloud-to-cloud
lightning discharges are capable of producing electric field intensities in the hundreds to
thousands of volts per meter range. If this type of transient appears across an unprotected
power, telephone, data, or coaxial line the result can be system destruction. The electrical
utilities have little or no control over the surges induced on their power lines. Thus, it is the
responsibility of the utility customers to protect their equipment from these transients. Noisy
electrical neighbors sharing your electrical distribution system, such as welding shops, can also
be a major source of transients. Table 1 below, provides examples of external transient impulse
sources.
Table 1. Characteristics of Steep Front Short Duration (SFSD) Impulses.
Field
Strength:
Impulse Shape:
Rise time / Time to half
Value
Peak Current
/ Peak Voltage:
Lightning:
< 40 kV/m
< 160 A/M
200-500 ns / 5 -20 µs
< 200 kA / ---
Utility System
Generated:
Nuclear:
< 10 kV/m
< 300 A/m
< 50 kV/m
< 1000 A/m
< 10 ns / 1-5 µs
--- / 2-3 X System
Voltage
< 10 kA / 2000 kV
Characteristics:→
Source: ↓
< 10 ns / 10-200 ns
Source: IEEE Power Engineering Review, October 1987, Pg. 54
Lightning Generated Transients
Lightning surges may be caused by a direct strike to the power system, telephone lines or
other communications conductors. They may also be caused by a strike to the structure of a
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facility or to nearby structures. Direct lightning strikes to the soil also produce lightning generated
transients. Distance cloud-to-cloud or cloud-to-ground lightning flashes also produce transients in
the electrical distribution system and other signal carrying conductors of an installation.
Direct damage is the obvious physical damage resulting from a lightning strike. The
magnitudes of voltage and current associated with a lightning strike are enormous. Lightning
strikes have voltages on the order of 100,000,000 volts and currents up to 340,000 amperes.
Structural lightning protection systems (lightning rods) mitigate damage to a facility and protect
its inhabitants. Properly designed and applied surge protection systems protect the critical and
valuable electrical and electronic systems within the facility.
Magnetic fields accompany a cloud-to-cloud or a cloud-to-ground lightning strike. The
magnetic fields are produced by charge in motion as the lightning discharge occurs. Magnetic
field intensities on the order of hundreds of amperes per meter from a nearby strike are capable
of coupling large surges into power and data conductors. For example, a magnetic field
strength of 200 amperes per meter is capable of inducing a 2,000-ampere transient into a
conductor whose length is 10 meters.
Electric fields are created by electrical charge and are capable of coupling transient energy
into power and signal conductors. The coupling mechanism is through the stray distributed
capacitance that exists whenever two conductors are separated by an insulating material. The
insulating material may be air or the electrical insulation of wires. A lightning discharge six
tenths of a mile away may produce an electric field intensity of thirty volts per meter. A
conductor 10 meters in length could potentially be exposed to a 300 volt transient.
Electromagnetic radiation is caused by lightning’s electrical charge in motion and the
recurring on/off action of a lightning strike. Charge in motion causes energy to be radiated.
The radiated energy is like the energy radiated by a TV or radio station. The radiated
electromagnetic field can induce electrical currents in conductors and wires at a distance that is
measured in miles from the strike.
Ground Surges are caused by the difference in ground potential between two points. As a
negatively charged cloud passes over a facility, a positive charge is induced in the earth and
grounded objects below the cloud. The induced charge shifts the ground potential and allows
current flow between equipment that is not physically located, grounded and bonded at the same
point. This situation may lead to mysterious “clear day” equipment disruptions, damage and
destruction. Should lightning strike near a facility, the charge on the grounded objects is
suddenly redistributed which causes a current surge to flow through grounding impedance of
grounded objects. The current surge through the grounding impedance produces large voltage
differences, which may equalize through electrical and electronic equipment. The problem is
accentuated when physically different points are used to ground equipment. Using a “dedicated
ground rod” or building steel used to locally ground electrical equipment, such as telephone
equipment, computer systems or machine tools creates ground loops. These ground loops are
also transient and noise loops. They allow potential equalization through ground and connected
equipment, data cables and shields that are grounded at a different points.
Generally, installing surge-protective devices on all incoming ac power lines, panels, phone
lines, and data cables provides adequate surge protection. If the fields associated with a
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lightning strike are strong enough, destructive transients may be induced on conductors
downstream from their surge protection. In lightning prone areas, data and communication
lines exceeding thirty feet in length should be protected on each end to eliminate this source of
equipment damage. The facility grounding and bonding must meet current codes and standards
to provide for personnel safety and proper equipment operation.
Internally Generated Transients
Internally generated transients are transients which are created within a given facility.
These transients account for approximately eighty percent of the total transient activity at an
installation. Abruptly changing the current flowing through an electrical circuit produces these
transients. Switching a light, motor, copy machine, or other electrical device abruptly changes
the current in the circuit and creates transients. Internally generated transients are ether random
or repetitive with respect to their timing and the power-line voltage sine wave. The switching
of a lighting circuit or the operation of a copier tends to be random in nature. The operation of
inverters, variable frequency drive, and SCR light dimmers produce repetitive transients.
A rapid current change in an electrical circuit causes a rapid change in the magnetic flux
linking the same circuit. This change in the magnetic field causes a voltage to be induced in the
circuit. The induced voltage “E” is proportional to the rate of change of the current “di” divided
by the time increment of the current change “dt” multiplied by the inductance “L” of the circuit.
A negative sign is placed before “L” to indicate that the induced voltage opposes the current
change which produced it. If the induced voltage aided the current change, we would open the
door to the possibility of perpetual motion and infinite voltages being created. This relationship
may be expressed by the formula:
Equation 1. E (volts) = -L di/dt
For a constant circuit inductance “L” and constant change in current “di”, as the time
increment “dt” decreases, the value of “E” increases. The induced voltage or transient voltage
increases directly with the circuit inductance “L”, directly with the current change “di” and
inversely with the time increment “dt”. Small time increments “dt” are characteristic of the fast
turn-on and turn-off times caused by switching electrical loads on or off and lead to large
induced voltages. Occasionally, people hear a “pop” and see a small flash under the wall switch
plate when the dining room lights (or other electrical circuit) are switched off. This is a
switching transient. The energy stored in the magnetic field of a circuit that can become
transient energy is given by:
Equation 2. W (joules) = ½ L I2
Where “W” is the energy stored in joules, “L” is the inductance, and “I” is the instantaneous
circuit current. The energy stored in a circuit increases directly with an increase in inductance
“L” and with the square of the current “I”. Thus, if the current in a circuit is doubled (all other
factors being held constant) the stored energy will quadruple. Thus, for larger ampacity
electrical services and larger loads, stronger surge suppression devices should be considered.
Refer to Equations 1 and 2, above and observe that circuit inductance “L” directly affects
the magnitude of induced transient voltage “E” and the transient energy “W” magnitude. The
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transient voltage “V” increases directly with the current change in the circuit “di”. Thus, the
transient energy quadruples when the instantaneous current “I” is doubled. Note that shorter
time increments “dt” result in lager transient voltages. Short time increments “dt” are produced
when electrical loads are switched on or off. The operation of circuit interrupt devices (i.e.,
fuses or breakers) also produces short switching time increments “dt”.
Typically, switching larger load currents produces larger transients and larger transient
energies. Examples of larger load currents include electric welders, electric motors and HVAC
compressors. Switching inductive loads, such as motors, transformers, contactors, relays,
solenoids, and HVAC compressors produce higher amplitude and more energetic transients.
Other common transient sources include variable frequency drives, copiers, printers, and
inverters. The normal operation of most electrical equipment creates electrical transients.
These voltage surges and spikes vary in magnitude, duration and energy.
As previously mentioned some of transients can produce immediate equipment damage and
disrupt an organization’s operations. This damage may be noticeable due to the presence of
smoke and the odor of destroyed electronic components. Other transients produce disruption
and cumulative damage. Cumulative damage takes place little by little over a period of time
and results in equipment failure. The effect is similar to tire wear. Each short trip does not
cause much tire wear. But, after a total of 50,000 miles the tire may need to be replaced.
The constant advances in microelectronics have resulted in smaller, higher density, faster, and
more transient sensitive integrated circuits. These components are operating at higher speeds and
at lower voltage and current levels. This makes them more susceptible to upset, data loss,
cumulative transient damage and destruction.
Symptoms of Electrical Transients, Equipment Susceptibility
Trends and the Impact of Transients
Transient disruption or upset should be suspected when a system exhibits erratic operation,
show signs of unexplained memory loss and invalid data, is plagued by mysterious program
failures which vanish when the system is reset, or frequently needs to be rebooted and/or
reprogrammed.
Transient damage may be confused with the “normal failure” of electrical and electronic
systems. Over time, personnel may become accustomed to the transient induced failures and
ultimately budget for them. Examples of transient damage include frequent circuit board
replacement and a high level of system maintenance, the need to replace lamps and electronic
ballasts too early and too often, damaged variable frequency drives, the need for frequent motor
rewinds due to burned windings, and plant or process shutdowns.
Equipment destruction may also be confused with “normal failure” when the destruction is
the result of cumulative damage caused by numerous low level transient events. The damage
also may be caused by large and readily identifiable transient events such as lightning strikes or
major power disruptions.
Trends in Electrical and Electronic Equipment Transient Sensitivity and Transient
Susceptibility
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Over time, the characteristics of electrical and electronic equipment that is used to
distribute, control and convert electrical energy from the point of generation to the point of use
has changed. And, this change continues to take place.
When the distribution of electrical energy began, the electrical loads consisted largely of
electrical incandescent lightning, resistive heating devices and motor driven devices. These
devices and their controls were relatively robust. The invention of the vacuum tube diode (a
two element device), the “Fleming Diode” in 1904 and the “Audion” or vacuum tube triode (a
three element device) by Lee de Forest in 1907 made possible practical wireless-telegraph and
set the stage for the development of long range land based and wireless telephony, radio,
television, motion picture sound, rf power, radar, computers, medical devices and the other
electronic devices and systems that we take for granted.
The figure below depicts the transient sensitivity and susceptibility situation as technology
progressed from robust electrical loads, to vacuum tube technology, to transistor based
technology, to integrated circuits technology, to the increasingly smaller, denser, faster, lower
voltage operating voltage, sensitive integrated circuit technology. The progression of electronic
technology is expected to accelerate leading to faster, smaller geometry discrete components at
the “chip” level. The side effect of these advances is increasingly more transient susceptible
and sensitive systems. Today, more than ever before, professional grade transient protection is
a necessity; it is no longer an option or a luxury.
Electrical System Transient Sensitivity and Susceptibility
Transient
Sensitivity
and
Susceptibility
Time
The Economic Impact of Transients on Organizations
Transient activity impacts organizations through excessive repair and replacement costs,
increased labor and overtime costs, lost sales and revenue, and customer dissatisfaction.
Transient Voltage Surge Suppressor or Surge-Protective Device
Overview
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A Transient Voltage Surge Suppressor (TVSS) or Surge-Protective Device (SPD) is
essentially a voltage controlled, high-speed, high-power switch. When the transient voltage
exceeds the TVSS switching level, it “switches” the transient away from downstream
equipment and protects the equipment from the disruption, damage and destruction that
transients cause.
How Surge Suppression Works
The SPD, when activated by a transient, creates a temporary equipotential bond between
all of the disturbed conductors. When the transient has passed, the SPD resets to its normal
ready state. The on and off switching speed is accomplished in billionths of a second by
modern high-powered solid-state SPD designs. The temporary bond limits the potential
difference between the disturbed conductors. By limiting the potential difference between
disturbed conductors, the tendency for damaging transient flow to enter the protected equipment
or systems is mitigated. This accomplishes two basic functions. First the SPD limits the
transient voltage to “safe” levels. Second the SPD diverts transient current away from the
protected equipment. A portion of the transient energy is converted directly into thermal energy
in the SPD. A portion of the energy is diverted away from the protected load. And, a portion
of the transient energy is reflected harmlessly back toward the transient source.
The Need for Surge Protective Devices
Everyone who uses electricity (grid supplied or locally generated), telephone lines (voice
only or fax/modem/data), data lines (data collection, data transmission, or computer
communication) needs protection against external and internal transient activity. In short,
anyone who uses wire to transmit electrical energy or data needs effective and reliable surge
suppression.
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