Background of the Invention
This invention relates generally to devices for general circuit protection
including electrical distribution and motor control applications. In particular,
the invention relates to a current limiting device comprising an inhomogeneous
resistance structure and an electrically conducting material, where a substantial
fraction of the current is carried by a conductor with a low pyrolysis or thermal
ablation temperature. The conductor may be an organic conductor.
There are numerous devices that are capable of limiting the current
in a circuit when a short-circuit occurs. One known limiting device includes a
filled polymer material which exhibits what is commonly referred to as a PTCR
(positive-temperature coefficient of resistance) or PTC effect. The unique attribute
of the PTCR or PTC effect is that at a certain switch temperature the PTCR material
undergoes a transformation from a more conducting material to a more resistive
material. In some of these prior current limiting devices, the PTCR material (typically
polyethylene loaded with carbon black) is placed between pressure contact electrodes.
Current limiting devices are used in many applications to protect
sensitive components in an electrical circuit from high fault currents. Applications
for current limiting devices include applications to protect sensitive components
in an electrical circuit from high fault currents. Applications range from low
voltage/current electrical circuits to high voltage/current electrical distribution
systems. A important retirement for many applications is a fast current limiting
response in order to minimize the peak fault current that develops.
In operation, current limiting devices are placed in a circuit to
be protected. Under normal circuit conditions, the current limiting device is in
a highly conducting state. When a short-circuit occurs, the PTCR material heats
up through resistive heating until the temperature is above the switch temperature.
At this point, the PTCR material resistance changes to a high resistance state
and the short-circuit current is limited. When the short-circuit is cleared, the
current limiting device cools down over a time period that may be long to below
the switch temperature and returns to the highly conducting state. In the highly
conducting state, the current limiting device is again capable of switching to
the high resistance state in response to future short-circuit events.
U. S. Patent 5,382,938 describes a PTC element comprising a body
of an electrically conductive polymer composition having a resistivity with a
positive temperature coefficient, the body defining two parallel end surfaces and
two electrodes arranged in contact with the end surfaces for carrying current
through the body. The polymer composition of the body includes a polymer material
and an electrically conductive powdered material distributed in the polymer material.
The term PTC element is the accepted term for an element which exhibits a positive
temperature coefficient of resistance with a switch temperature as shown in Fig.
1 of U. S. Patent 5,382,938. At least one of the parallel surfaces on the body
is in free contact with an electrode or with a parallel surface on another body
of electrically conductive polymer composition. A pressure device inserts a pressure
directed perpendicularly to the parallel surfaces on the body, or the bodies, on
the electrodes. The pressure device is preferably provided with a pressure-exerting
device with the ability to be resilient. After changing from a low resistance
to a high resistance state, the PTC element returns to the initial resistance and
is reusable after having been subjected to short-circuit currents. The parallel
surfaces on the body, or the bodies, of polymer composition may be concentric.
PTC elements are used in electric circuits as overcurrent protection.
U. S. Patent 5,313,184 describes an electric resistor having a resistor
body arranged between two contact terminals. The resistor core includes an element
with PTC behavior, which, below a material-specific temperature, forms an electrically
conducting path running between the two contact terminals. The resistor can be
simple and inexpensive, but still having high rate current-carrying capacity protected
against local and overall overvoltages. This is achieved by the resistor core
additionally containing a material having varistor behavior. The varistor material
is connected in parallel with at least one subsection of the electrically conducting
path, forming at least one varistor, and is brought into intimate electrical contact
with the part of the PTC material forming the at least one subsection. The parallel
connection of the element with PTC behavior and the varistor can be realized both
by a microscopic construction and by a macroscopic arrangement.
European Patent 0,640,995 A1 describes an electrical resistance element
containing a resistive material that has PTC characteristics and is arranged between
two plane-parallel electrodes that are subjected to pressure, whereby the resistive
material consists of a polymer matrix and two filler components that consist of
electrically conducting particles, wherein the two filler components are embedded
in the polymer matrix. In the event of a short-circuit current, the resistivity
of the resistive material changes, in a step-like manner above a limiting temperature
value, in a surface layer that lies on the electrodes and that contain at least
the first of the two filler components. The second of the two filler components
is selected in such a way that a composite material that contains at least a polymer
matrix and the second filler component exhibits PTC characteristics with a step
characteristic that is higher by at least one order of magnitude, relative to
the surface layer. At the same time, this composite material has a resistivity
that is lower, by at least one order of magnitude, than a composite material that
is formed from the polymer matrix and the first filler component.
European Patent Appln. 0,762,439 discloses a current limiting device.
This current limiting device relied on a composite material and an inhomogeneous
distribution of resistance structure.
Known current limiting devices utilize a composite material comprising
a low pyrolysis or vaporization temperature binder and an electrically conducting
filler combined with an inhomogeneous distribution of resistance structure. The
switching action of these devices occurs when joule heating of the electrically
conducting filler in the relatively higher resistance part of the composite material
causes sufficient heating to cause pyrolysis or vaporization of the binder material.
However, the switching time is limited by time required for heat to diffuse from
the conducting filler to the surrounding binder material. Thus, rapid switching
is not readily available with known current limiting devices.
Despite the efforts described above to provide simpler, more durable
current limiting devices, a need still exists for a simpler, quicker, more durable,
reusable potentially low cost current limiting device for general circuit protection
in electrical distribution and motor control applications that can be tailored
to a plurality of applications. Moreover, the switching action should be relatively
quick to avoid disadvantageous effects to the circuitry from the time delay.
Current limiting devices are used in many applications to protect
sensitive components in an electrical circuit from high fault currents. Applications
range from low voltage/current electrical circuits to high voltage/current electrical
distribution systems. The present invention provides a relatively quick, simple,
reusable, potentially low cost current limiting device that can be tailored to
a plurality of applications.
Accordingly, it is an object of the invention to provide a quick,
simple, reusable, potentially low cost current limiting device, where the current
limiting device overcomes the above noted and other disadvantages of the related
It is a further object of the invention to provide a current limiting
device, where a conducting filler is chosen so that a substantial fraction of the
electrical current is carried by a low pyrolysis temperature material, such as
an organic conductor. Therefore, switching during short circuit conditions occurs
faster than in known current limiter devices because the switching is instigated
by pyrolysis of the organic conductor. Thus, a time delay to heat diffusion from
the conductor to the surrounding low pyrolysis temperature binding material can
Embodiments of the invention will now be described, by way of example,
with reference to the accompanying drawings, in which:
- Figure 1 is a schematic representation of a current limiting device in accordance
with the invention; and
- Fig. 2 is a schematic representation of a current limiting device with separate
conductive composite materials, in accordance with the invention.
In accordance with one example of the invention, a current limiting
device is constructed using an electrically conductive composite material so that
there is an inhomogeneous distribution of resistance throughout the device, where
a conducting filler is chosen so that a substantial fraction of the electrical
current is carried by a low pyrolysis temperature material such as an organic conductor.
To be a reusable current limiting device, the inhomogeneous resistance
distribution is arranged so at least one thin layer of the current limiting device
is positioned perpendicular to the direction of current flow and has a much higher
resistance than the average resistance for an average layer of the same size and
orientation in the device. In addition, the current limiting device is under compressive
pressure in a direction perpendicular to the selected thin high resistance layer.
The compressive pressure may be inherent in the current limiting device or exerted
by a resilient structure, assembly or device, such as but not limited to a spring.
One example current limiting device, in accordance with the invention,
comprises a highly conducting composite material with low pyrolysis temperature
binder and conducting filler that is pressure contacted to electrodes so that there
is a significant contact resistance between the material and one or both electrodes,
where the conducting filler(s) is (are) chosen so that a substantial fraction
of the electrical current is carried by a low pyrolysis temperature material such
as an organic conductor.
In operation, the device is placed in the electrical circuit to be
protected. During normal operation, the resistance of the limiting device is low,
i.e., in this example the resistance of the current limiting device would be equal
to the resistance of the highly conducting composite material plus the resistance
of the electrodes plus the contact resistance. When a short-circuit occurs, a high
current density starts to flow through the device. In initial stages of the short-circuit,
the resistive heating of the device is believed to be adiabatic. Thus, it is believed
that the selected thin, more resistive layer of the current limiting device heats
up much faster than the rest of the current limiting device. With a properly designed
thin layer, it is believed that the thin layer heats up so quickly that thermal
expansion of and/or gas evolution from the thin layer cause a separation within
the current limiting device at the thin layer.
The invention, in accordance with the invention comprises a fast-acting
current limiting device 1. As illustrated in Fig. 1, the current limiting device
1 comprises electrodes 3 and a composite material 5, which comprises a low pyrolysis
or vaporization temperature binder and an electrically conducting filler combined
with inhomogeneous distributions 7 of resistance structure under compressive pressure
P. However, the scope of the invention includes any suitable construction where
a higher resistance is anywhere between the electrodes. For example, the higher
resistance may be between two Composite materials 55, as illustrated in Fig. 2.
However, this is merely exemplary and is not meant to limit the invention in any
The binder should be chosen such that significant gas evolution occurs
at a low (about approximately <800°C) temperature. The inhomogeneous distribution
structure is typically chosen so that at least one selected thin layer of the current
limiting device has much higher resistance than the rest of the current limiting
The conducting filler is selected so that a substantial fraction
of the electrical current is carried by a low pyrolysis temperature material, such
as for example an organic conductor. With such a construction, switching during
short circuit conditions occurs faster than previous current limiting devices
because the switching can be instigated by pyrolysis of the organic conductor
itself. Thus, any time delay due to heat diffusion from the conductor to the surrounding
low pyrolysis temperature binding material is eliminated.
The inhomogeneous distribution of resistance is arranged so that
at least one thin layer positioned perpendicular to the direction of current flow
has a predetermined resistance, which is at least about ten percent (10%) greater
than an average resistance for an average layer of the same size and orientation.
Further, it is positioned proximate to at least one electrode electrically conductive
composite material interface.
It is believed that the advantageous results of the invention are
obtained because, during a short-circuit, adiabatic resistive heating of the thin
layer followed by rapid thermal expansion and gas evolution from the binding material,
which leads to a martial or complete physical separation of the current limiting
device at the selected thin layer, and produces a higher over-all device resistance
to electric current flow. Therefore, the current limiting device limits the flow
of current through the short-circuited current path. When the short-circuit is
cleared externally, it is believed that the current limiting device regains its
low resistance state due to the compressive pressure built into the current limiting
device allowing thereby electrical current to flow normally. The current limiting
in accordance with the invention is reusable for many such short circuit conditions,
depending upon such factors, among others, as the severity and duration of each
Examples of low pyrolysis temperature conducting filler materials,
in accordance with the invention, comprise conductive polymers, such as but not
limited to, polythiophene, polypyrrole, polyaniline, and also organic conductive
materials, such as but not limited to, tetrathiafulvalene-tetracyanoquinodimethane.
These compositions can be used as a sole electrically conducting element in the
composite material. Alternatively, they can be used together with metal or ceramic
In a current limiting device in accordance with the invention, it
is believed that the vaporization and/or ablation of the composite material causes
a partial or complete physical separation at the area of high resistance, for example
the electrode/material interface. In this separated state, it is believed that
ablation of the composite material occurs and arcing between the separated layers
of the current limiting device can occur. However, the overall resistance in the
separated state is much higher than in the nonseparated state. This high arc resistance
is believed due to the high pressure generated at the interface by the gas evolution
from the composite binder combined with the deionizing properties of the gas. In
any event, the current limiting device of the present invention is effective in
limiting the short-circuited current so that the other components of the circuit
are not harmed by the short circuit.
After the short-circuited current is interrupted, it is believed
that the current limiting device, of the present invention, when properly designed,
returns or reforms into its nonseparated state due to compressive pressure which
acts to push the separated layers together. It is believed that once the layers
of the current limiting device have returned to the nonseparated state or the low
resistance state, the current limiting device is fully operational for future
current-limiting operations in response to other short-circuit conductors.
Alternate embodiments of the current limiting device of the present
invention can be made by employing a parallel current path containing a resistor,
varistor, or other linear or nonlinear elements to achieve goals such as controlling
the maximum voltage that may appear across the current limiting device in a particular
circuit or to provide an alternative path for some of the circuit energy in order
to increase the usable lifetime of the current limiting device.
Further, in accordance with the invention, third phase fillers can
be added to the current limiting device. This third phase filler is usable to
improve specific properties of the composite, such as the mechanical properties;
dielectric properties; or to provide arc-quenching properties or flame-retardant
properties. Materials which could be used as a third phase filler in the composite
material include: a filler selected from reinforcing fillers, such as, fumed silica,
or extending fillers, such as, precipitated silica and mixtures thereof. Other
fillers can include titanium dioxide, lithopone, zinc oxide, diatomaceous silicate,
silica aerogel, iron oxide, diatomaceous earth, calcium carbonate, silazane treated
silicas, silicone treated silicas, glass fibers, magnesium oxide, chromic oxide,
zirconium oxide, alpha-quartz, calcined clay, carbon, graphite, cork, cotton sodium
bicarbonate, boric acid, alumina-hydrate, etc.. Further, other additives may include:
impact modifiers for preventing damage to the current limiter such as cracking
upon sudden impact; flame retardant for preventing flame formation and/or inhibiting
flame formation in the current limiter; dyes and colorants for providing specific
color components in response to customer requirements; UV screens for preventing
reduction in component physical properties due to exposure to sunlight or other
forms of UV radiation.
The invention in accordance with the invention further comprises
binder material having a low pyrolysis or vaporization temperature (<800°C)
such as: a thermoplastic (for example, polytetrafluoroethylene, poly(ethyleneglycol),
polyethylene, polycarbonate, polyimide, polyamide, polymethylmethacrylate, polyester
etc.); a thermoset plastic (for example, epoxy, polyester, polyurethane, phenolic,
alkyd); an elastomer (for example, silicone (polyorganosiloxane), (poly)urethane,
isoprene rubber, neoprene, etc.); an organic or inorganic crystal; combined with
an electrically conducting filler, such as a conducting filler where a substantial
fraction of the electrical current is carried by a low pyrolysis temperature material,
such as an organic conductor.
In accordance with another preferred embodiment of the invention,
the binder material in the current limiting device can be eliminated altogether.
A pure organic conductor can be utilized in place of the composite material, as
long as an inhomogeneous resistance structure is maintained within the current