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P62.42.4 October 2013
Draft Guide for the Application of Surge-Protective Components in Surge Protective Devices and Equipment Ports - Part 4
Thermally Activated Current Limiters
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P62.42.2™/D2
Draft Guide for the Application of
Surge-Protective Components in Surge
Protective Devices and Equipment
Ports - Part 4 Thermally Activated
Current Limiters
Sponsor
Surge Protective Devices Committee
of the
IEEE Power and Energy Society
P62.42.4 October 2013
Draft Guide for the Application of Surge-Protective Components in Surge Protective Devices and Equipment Ports - Part 4
Thermally Activated Current Limiters
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Abstract: <Select this text and type or paste Abstract—contents of the Scope may be used>
Keywords: <Select this text and type or paste keywords>
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
P62.42.4 October 2013
Draft Guide for the Application of Surge-Protective Components in Surge Protective Devices and Equipment Ports - Part 4
Thermally Activated Current Limiters
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Introduction
A PTC device is one which undergoes a change in resistance of typically several decades over a very
narrow temperature range. These devices consist working material arranged between two metal electrodes.
The working material is a special type of either ceramic (see clause 5.1), or polymer (see clause 4.1). PTCs
are widely used as coordinating elements in telecom applications (see clause 6.1), where they switch to a
high resistance when subjected to a fault, thus mitigating damage to equipment, but in normal operation
have a low resistance which minimizes the amount of power dissipated.
This standard describes PTC construction, their voltage-current characteristics, characteristic properties,
ratings and circuit examples
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Thermally Activated Current Limiters
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Contents
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1. Overview .................................................................................................................................................... 4
1.1 Scope ................................................................................................................................................... 4
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2. Normative references.................................................................................................................................. 5
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3. Definitions .................................................................................................................................................. 5
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4. Polymeric PTC ........................................................................................................................................... 5
4.1 Construction ........................................................................................................................................ 5
4.2 Operation ............................................................................................................................................. 6
4.3 Production............................................................................................................................................ 7
4.4 Ratings ................................................................................................................................................. 7
4.5 Characteristics ..................................................................................................................................... 8
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5. Ceramic PTC .............................................................................................................................................10
5.1 Construction .......................................................................................................................................10
5.2 Operation ............................................................................................................................................12
5.3 Ceramic PTC Production ....................................................................................................................15
5.4 Ratings ................................................................................................................................................16
5.5 Characteristics ....................................................................................................................................16
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6. Applications...............................................................................................................................................18
6.1 Telecom Coordinating Element ..........................................................................................................18
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1. Overview
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1.1 Scope
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The C62.42 guide series covers surge protective components (SPCs) used in power and telecom surge
protective devices (SPDs) and equipment ports. This part on Positive Temperature Coefficient Devices
(PTC) technology SPCs covers:
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- Construction
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- Operation
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- Production
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- Ratings
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- Characteristics
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- Application examples
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2. Normative references
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The following referenced documents are indispensable for the application of this document (i.e., they must
be understood and used, so each referenced document is cited in text and its relationship to this document is
explained). For dated references, only the edition cited applies. For undated references, the latest edition of
the referenced document (including any amendments or corrigenda) applies.
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none
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3. Definitions
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For the purposes of this document, the following terms and definitions apply. The IEEE Standards
Dictionary: Glossary of Terms and Definitions1 should be consulted for terms not defined in this clause.
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4. Polymeric PTC
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4.1 Construction
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The active material in a polymeric PTC is made by loading a conductive medium, typically carbon black,
into a molten polymer. This polymer must have an adequate amount of crystallinity when cooled, for
reasons that will become clear from the following discussion. The amount of conductive medium added
depends on the nature of the medium, and on the cold resistance to be achieved.
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As the mixture is cooled, it starts to crystallize. As it does, the conductive medium is expelled from the
crystalline region into the amorphous region. The result is an increasing concentration of conductive
medium in the amorphous region. As the crystalline region of the polymer grows during the cooling
process, the amorphous region shrinks, until the density of conductive medium is sufficient to allow the
formation of conductive chains. Further shrinkage of the amorphous region causes the chains to grow.
When the chains are long enough to form a bridge between the electrodes, the resistance of the device
drops - slowly at first, and then precipitously. Further cooling results in a further decrease in resistance, but
at a lower rate. This process is characterized by an R-T curve, such as that shown in Figure 1.
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IEEE Standards Dictionary: Glossary of Terms and Definitions is available at http://shop.ieee.org.
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1.0E+05
Relative resistance
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0.0
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40.0
60.0
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100.0
120.0
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180.0
Temperature, °C
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Figure 1 — An example of an RT curve for a polymeric PTC
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4.2 Operation
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Operation of the device occurs by traversing the R-T curve in the opposite direction from that just
described. The temperature of the device on the R-T curve is a combination of the ambient temperature
and an incremental temperature due to I2R heating. Because of this combination, the current required to
switch a device is a function of the ambient temperature, and is characterized by a derating curve such as
that shown in Figure 2.
1.8
Multiply 23 o C hold or trip
current value by
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-40
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Temperature, C
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Figure 2 — Example of a derating curve for either hold or trip current
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4.3 Production
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The production process begins by extruding the mixture of conductive medium and polymer into relatively
thin sheets. Foil electrodes are then attached to both sides of the sheet, forming what is often called plaque.
Devices are made by punching chips out of the plaque, and attaching leads to the chips. The resulting
device may, or may not be encapsulated, depending on the application.
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4.4 Ratings
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The ratings listed on a data sheet typically include a hold current, a trip current, and a resistance range.
Among these, the cold resistance range is the key parameter, because it determines the hold current, trip
current, and time-to-trip ratings.
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4.4.1 Cold resistance
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The cold resistance of the device is the resistance measured at a standard ambient temperature, usually
20 oC or 25 oC. As described in clause 4.1, the cold resistance of the device depends on its material
composition. Because the density of carbon chains is not precisely uniform over the plaque, the resistance
of any given chip punched from the plaque varies over a range of values. The minimum and maximum
values of resistance [Rmin and Rmax, respectively] for devices made from these chips are specified on the
data sheet for the device.
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4.4.2 Hold and trip currents
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For a typical device, a plot of hold/trip current [they are the same for a given device] for a possible range of
production lots is shown in Figure 3. The upper and lower lines shown in the plot are the 99.8%
confidence lines for the distribution. If the confidence level is reduced, the confidence lines will move
closer together; and the hold and trip currents will also move closer together. However reducing the
confidence level will increase the probability that the customer will receive a part that is out of spec; unless
a selection step is added to the production process to eliminate out-of-spec parts.
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In Figure 3, the trip current is determined as the value of current corresponding to Rmin, and the hold
current is determined as the value of current corresponding to R1max (the resistance one hour after a trip
event). Moving the resistance window will change the hold and trip currents, as will expanding or
contracting the resistance window. Data is typically taken over a wider resistance range than will occur in
production, to better establish the regression line, and to better show options for placement of the resistance
window. Plots similar to Figure 3 can be made for the process variables of any device.
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Regression
99.8% Confidence
Rmin
R1max
Current
Trip Current
Rmin
R1max
Hold Current
Resistance
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Figure 3 — An illustration of a dispersion plot of hold/trip current versus resistance
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4.5 Characteristics
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The Characteristics listed on a data sheet typically include a maximum operating voltage, a maximum
abnormal current, and a time-to-trip value at a specified current. Resistance recovery time [a characteristic]
may also be measured, but is generally not specified.
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4.5.1 Time to Trip
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As was the case for hold and trip currents, the time for a device to trip at a specified current is also a
function of resistance. Figure 4 shows a typical time-to-trip versus current plot, for a single device at room
temperature. Devices of lower resistance will have a time-to-trip curve that lies above the curve shown in
Figure 4. Devices of higher resistance will have a time-to-trip curve that lies below the curve shown in
Figure 4. At higher temperatures the line moves down (faster trip times for a given current). At lower
temperatures the line moves up (longer trip times for a given current). The curve moves up or down with
temperature, according to the derating curve for the device (see Figure 2).
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Fault Current
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Time-to-Trip
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Figure 4 — A typical time-to-trip versus current for a single device at room temperature
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4.5.2 Resistance Recovery Time
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A device which is tripped by a fault current greater than the rated trip current for the device will stay in the
tripped state until the fault power is removed. Once the power is removed the device will cool, and the
resistance will drop toward the cold resistance. The rate at which it returns has a significant dependence on
a number of things, including the amount of the fault current, the rate of heat transfer out of the device, and
the size of the device. An example of one of many possible resistance recovery curves is shown in
Figure 5. Typically an hour is allowed for the resistance to stabilize near its cold value; and on the data
sheet this value of resistance is listed as R1max. Often a value of resistance near R1max is achieved in a
much shorter time than one hour. The time scale of the figure depends significantly on a number of things,
including the amount of the fault current, the rate of heat transfer out of the device, and the size of the
device.
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Relative resistance
1000000
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Relaxation time, seconds
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Figure 5 — The general shape of a resistance recovery curve.
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5. Ceramic PTC
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5.1 Construction
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PTC thermistors are made of doped polycrystalline ceramic on the basis of barium titanate. Generally,
ceramic is known as a good insulating material with a high resistance. Semiconduction and thus a low
resistance are achieved by doping the ceramic with materials of a higher valency than that of the crystal
lattice. Part of the barium and titanate ions in the crystal lattice is replaced with ions of higher valencies to
obtain a specified number of free electrons which make the ceramic conductive.
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The material structure is composed of many individual crystallites (figure 6). At the edge of these
monocrystallites, the socalled grain boundaries, potential barriers are formed. They prevent free electrons
from diffusing into adjacent areas. The result is high resistance of the grain boundaries. However, this
effect is neutralized at low temperatures. High dielectric constants and sudden polarization at the grain
boundaries prevent the formation of potential barriers at low temperatures enabling a smooth flow of free
electrons. The PTC resistance RPTC is composed of individual crystal and grain boundary resistances. The
grain boundary resistance is strongly temperature dependent.
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RPTC = Rgrain + Rboundary
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Rboundary = f (T)
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Figure 6 — A representation of the polycrystalline structure of a ceramic PTC thermistor
Above the ferroelectric Curie temperature, dielectric constant and polarization decline so far that there is
strong growth of the potential barriers and thus of resistance. In a certain range of temperature above the
Curie temperature TC, the resistance of the PTC thermistor rises exponentially. Beyond the range of the
positive temperature coefficient the number of free charge carriers is increased by thermal activation. The
resistance then decreases and exhibits a negative temperature characteristic (NTC) typical of
semiconductors (see figure 7). In Figure 7, TC = ferroelectric Curie temperature and α = temperature
coefficient. With rising temperature, the resistance of the PTC thermistor initially decreases and rises then
steeply. Beyond the range of the positive temperature coefficient the resistance again decreases, which can
lead to thermal run-away if the thermistor is operated in this region.
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Figure 7 — An example of an RT curve for a ceramic PTC
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5.2 Operation
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A current flowing through a thermistor may cause sufficient heating to raise the thermistor's temperature
above the ambient. As the effects of self-heating are not always negligible, a distinction has to be made
between the characteristics of an electrically loaded thermistor and those of an unloaded thermistor. The
properties of an unloaded thermistor are also termed "zero-power characteristics".
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5.2.1 Unloaded thermistors
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The zero-power resistance value R (T) is the resistance value measured at a given temperature T with the
electrical load kept so small that there is no noticeable change in the resistance value if the load is further
reduced. For test voltages, please refer to the individual types (mostly ≤ 1.5 V).
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Figure 8 shows the typical dependence of the zero-power resistance on temperature. Because of the abrupt
rise in resistance (the resistance value increases by several powers of ten), the resistance value is plotted on
a logarithmic scale (ordinate) against a linear temperature scale (abscissa).
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Figure 8 — Typical resistance/temperature characteristic
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In Figure 8:
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RPTC = f (TPTC)
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RR
Rated PTC resistance (resistance value at 25 °C)
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Rmin
Minimum resistance
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Temperature at Rmin
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Reference resistance: Rref = 2 • Rmin
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Reference temperature (resistance value reaches Rref = 2 • Rmin)
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5.2.2 Electrically loaded thermistors
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When a current flows through the thermistor, the device will heat up more or less by power dissipation.
This self-heating effect depends not only on the load applied, but also on the thermal dissipation factor G th
of the thermistor itself.
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Self-heating of a PTC thermistor resulting from an electrical load can be calculated as follows:
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P V I 
dH
dT
 Gth  (T  T A )  C th 
dt
dt
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P Power applied to PTC
T Instantaneous temperature of PTC
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V Instantaneous value of PTC voltage
TA Ambient temperature
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I Instantaneous value of PTC current
Cth Heat capacity of PTC
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dH/dt Change of stored heating energyover time
dT/dt Change of temperature over time
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Gth Thermal dissipation factor of PTC
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The properties of electrically loaded PTC thermistors (in self-heated mode) are better described by the I/V
characteristic than by the R/T curve (see figure 9). It illustrates the relationship between voltage and current
in a thermally steady state in still air at 25 °C.
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Figure 9 — I/V characteristic of a PTC thermistor
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In Figure 9:
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Ir
Residual current at applied voltage Vmax (current is balanced)
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Vmax
Maximum operating voltage
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VR
Rated voltage (VR < Vmax)
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VBD
Breakdown voltage (VBD > Vmax)
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Figure 10 illustrates the two operating states of a PTC fuse. In rated operation of the load the PTC
resistance remains low (operating point A1). Upon overloading or shorting the load, however, the power
consumption in the PTC thermistor increases so much that it heats up and reduces the current flow to the
load to an admissible low level (operating point A2). Most of the voltage then lies across the PTC
thermistor. The remaining current is sufficient to keep the PTC in high-resistance mode ensuring protection
until the cause of the overcurrent has been eliminated.
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Figure 10 — Operating states of a PTC thermistor for overcurrent protection
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In Figure 10:
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a) Rated operation
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b) Overload operation
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RL Load resistance
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IS Switching current
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IR Rated current
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RPTC PTC resistance
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5.3 Ceramic PTC Production
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Mixtures of barium carbonate, titanium oxide and other materials whose composition produces the desired
electrical and thermal characteristics are ground, mixed and compressed into disks, washers, rods, slabs or
tubular shapes depending on the application. These blank parts are then sintered, preferably at
temperatures below 1400 °C. Afterwards, they are carefully contacted, provided with connection elements
depending on the version and finally coated or encased.
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The ratings listed on a data sheet typically include rated current, switching current, and a rated resistance.
Among these, the cold resistance (rated resistance) is the key parameter, because it determines the hold
current, trip current, and time-to-trip ratings in combination with the physical dimensions.
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5.4.1 Rated Resistance RR
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The rated resistance RR is the resistance value at temperature T R. PTC thermistors are classified according
to this resistance value. The temperature TR is 25 °C, unless otherwise specified.
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As described in Clause 5.2.1 the resistance is determined by the grain boundaries of the ceramic material.
Because of impurities during the production process the potential barriers and therefore the rated resistance
varies over a range of values. The resistance tolerance is specified on the data sheet for the device.
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As opposed to most PTC thermistors made of polymeric materials, ceramic PTC thermistors always return
to their initial resistance value, even after frequent heating/cooling cycles. This is an issue only if the PTC
resistance is a significant fraction of the total circuit resistance
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5.4.2 Rated current IR and Switching current IS
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It is important to know at which current the PTC thermistor will not trip and at which currents the
thermistor will reliably go into high-resistance mode. For this reason we specifiy the rated current I R and
the switching current IS.
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Rated current IR: At currents ≤ IR the PTC thermistor reliably remains in low-resistance mode.
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Switching current IS: At currents ≥ IS the PTC thermistor reliably goes into high-resistance mode.
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The currents specified in the data sheets refer to TA = 25 °C, unless otherwise stated.
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5.5 Characteristics
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The characteristics listed on a ceramic PTC data sheet typically include a time-to-trip value at a specified
current, a maximum operating voltage, and a maximum switching current. The voltage dependence of
resistance may also be shown.
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5.5.1 Switching time ts
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The switching time tS. is the time it takes at applied voltage for the current passing through the PTC to be
reduced to half of its initial value. The tS values apply to TA = 25 °C.
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The dynamic heating behavior of the PTC thermistor is determined by the specific heat capacity of the
titanate material, which is approx. 2.7 Ws/Kcm3. At short switching times (less than 5 s with commonly
used overcurrent protection devices) heat dissipation through the surface and lead wires is virtually
negligible. Almost the entire electrical dissipation is consumed to heat up the ceramic material, to increase
the temperature above the reference temperature and thus to produce a stable operating point on the R/T
characteristic. When dissipation increases with rising difference between device temperature and ambient,
only a small amount of excess energy remains for heating the component and the result is the switching
time curves as a function of switching current shown in Figure 11.
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versus switching current IS (measured at 25 °C in still air)
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5.5.2 Maximum Switching current ISmax
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In electrically loaded PTC thermistors electrical power is converted into heat. The high loads generated for
a short period of time during the heating phase (the PTC thermistor is in low-resistance mode when the
operating voltage is applied) are limited by the specification of the maximum permissible switchting
current ISmax and the maximum operating voltage V max in the specification.
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The number of heating processes is also an important criterion. The permissible number of switching cycles
not affecting function or service life is given in the specification as well.
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A ceramic PTC thermistor which is tripped by a fault current greater than the rated switching current for
the device will stay in the tripped state until the fault power is removed. After a few minutes only ceramic
PTC thermistors return to their initial resistance value thus enabling very fast resumption of operation of
the protected equipment.
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5.5.3 Voltage dependence of resistance
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The resistance of the ceramic PTC thermistor is composed of the grain resistance and the grain boundary
transition resistance. Particularly in the hot state, the strong potential barriers are determining resistance.
Higher voltages applied to the PTC thermistor therefore drop primarily at the grain boundaries with the
result that the high field strengths dominating here produce a break-up of the potential barriers and thus a
lower resistance. The stronger the potential barriers are, the greater is the influence of this "varistor effect"
on resistance. Below the reference temperature, where the junctions are not so marked, most of the applied
voltage is absorbed by the grain resistance. Thus the field strength at the grain boundaries decreases and
the varistor effect is quite weak.
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Figure 12 shows the typical dependence of resistance on field strength. It can be seen that the difference in
resistance is largest between R(E1), R(E2) and R(E3) at temperature Tmax and thus in the region of
maximum resistance. Due to this dependence on the positive temperature coefficient of the field strength,
operation on high supply voltages is only possible with PTC thermistors that have been designed for this
purpose by means of appropriate technological (grain size) and constructional (device thickness) measures.
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This behavior is characteristic of ceramic PTCs, but not of polymeric PTCs which do not exhibit a voltage
dependence of resistance.
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Figure 12 — Influence of field strength E on the R/T characteristic (varistor effect), where
αR1 > αR2 > αR3
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6. Applications
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6.1 Telecom Coordinating Element
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For telecommunications circuits, a major application is as a coordinating element between primary and
secondary protectors. Figure 13 shows a generic circuit for a protector. The primary protector is a device
which limits high amplitude lightning surges, but which can have a significant surge let-through in the
process. Typically this device is a GDT. The secondary protector is a device which can’t handle high
amplitude surges, but which can limit the remnant surge to the lowest value consistent with the operating
voltage of the system. Typically this device is a thyristor which has a lower operating threshold than the
GDT.
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Equipment
Primary Protection
Secondary Protection
Coordination Element
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Figure 13 — Reference circuit for a surge protector
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In Figure 13 the coordinating element, generally a resistor, is placed between the primary and secondary
protection, and is put there so that an incident surge current will develop enough voltage to fire the primary
protector. In the absence of a coordinating element the secondary protection (thyristor) would operate
before the primary protection (GDT), due to its lower operating threshold. When this happens the
secondary protector is subjected to the full surge, which generally destroys it.
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A PTC is ideal for this resistor for three reasons:
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
Because the PTC switches to a high resistance state when subjected to a fault, the amount of
power it has to dissipate is minimized. So if V is the fault voltage, a fixed resistor of value Rlow
would have to dissipate V2/Rlow, whereas a PTC only has to dissipate V2/Rhigh, where Rhigh >> Rlow
As a result the PTC can be relatively small, and doesn’t reach a high temperature. In contrast, a
fixed resistor needs to be large enough to handle the highest current reached in a power contact
condition, and generally gets quite hot while doing so.
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
A PTC Thermistor’s resistance is depending on the temperature of the component which itself
depends on the power consumption of the PTC as well as the ambient temperature situation. These
two parameters are exactly those limiting the maximum energy of any following component to be
protected. Unlike other technologies (e.g. semiconductor solutions) a PTC thermistor does not use
single parameters like voltage or current to detect whether to fuse or not. Instead it is only reacting
on the energy consumption in total. Therefore energy peaks that do not harm any of the following
components do not cause an interruption of a telecom line.
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Advantages of a ceramic PTC:

For many telecom applications the balance between the tip and ring (often referred to as
longitudinal balance) is essential. As opposed to other technologies (not self resettable fuses,
polymeric PTC Thermistors), ceramic PTC thermistors always return to their initial resistance
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value, even after frequent heating/cooling cycles. The balance of the line is maintained under all
circumstances.

A ceramic PTC thermistor which is tripped by a fault current greater than the rated switching
current for the device will stay in the tripped state until the fault power is removed. After a few
minutes only ceramic PTC thermistors return to their initial resistance value thus enabling very
fast resumption of operation of the protected equipment.
Advantages of a polymeric PTC:
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
Unlike a ceramic PTC the resistance of the active material in a polymeric PTC is independent of
frequency (reactive elements may be introduced by the packaging), minimizing the impact on the
performance of systems with high frequency signals
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
The resistance of a polymeric PTC is independent of voltage; unlike ceramic PTCs where the
series impedance drops as voltage is increased. Because of this effect, the DC resistance of a
ceramic PTC has to be higher than a polymeric PTC for a given coordinating impedance, resulting
in a higher insertion loss.
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