WO1991003822A1

WO1991003822A1 - Conductive polymer device

Info

Publication number: WO1991003822A1
Application number: PCT/US1990/005102
Authority: WO
Inventors: Arthur F. Emmett
Original assignee: Raychem Corporation
Priority date: 1989-09-08
Filing date: 1990-09-10
Publication date: 1991-03-21
Also published as: EP0490989A1; ATE187013T1; CA2066254A1; DE69033364D1; JPH05500884A; DE69033364T2; CA2066254C; KR920704316A; EP0490989B1

Abstract

A melt-extrudable conductive polymer composition which contains a polymer, a particulate conductive filler and a particulate non conductive filler. When a standard strip heater is made from the composition and tested in a UL VW-1 test, it has comparable performance to a heater made from a second composition which is the same as the composition but which does not contain the non conductive filler. When tested in a standard arcing fault test, the standard heater will trip a fuse in less time than is required by the second heater, i.e. in less than 30 seconds. A preferred nonconductive filler is Sb2O3.

Description

ONDUCTIVE POLYMER DEVICE

BACKGROUND £E IHE. INVENTION

Field Q£_ the Invention

This invention relates to conductive polymer compositions and strip heaters comprising them, in particular self-regulating strip heaters which comprise a pair of elongate metal electrodes embedded in an elongate core of a conductive polymer composition which exhibits PTC behavior.

Introduction £_&. iJae. Invention

Conductive polymer compositions and electrical devices comprising such compositions are well known. A conductive polymer composition comprises a polymeric component and, dispersed or otherwise distributed therein, a particulate conductive filler. Strip heaters, particularly self-regulating strip heaters comprising conductive polymers, are also well- known. In this application, the term strip heater is used to mean a conductive polymer resistive element into which elongate electrodes are embedded. In operation, such strip heaters provide a varying level of heat in response to changes in the thermal environment. Under normal operating conditions, this self-regulating feature serves to limit the maximum temperature which the heater achieves, thus providing reliability and safety. However, under certain circumstances where the busbars are exposed by external damage or by faulty installation, and when the heater is electrically powered and exposed to an electrolyte, an arc can occur between the electrodes. Unless the arc is interrupted, the conductive polymer may burn and could possibly result. One way to minimize this danger is to develop appropriate conductive polymer compositions in which the polymer itself is flame-retardant or which contain conventional flame retardant additives to work in conjunc¬ tion with the strip heaters. Another method to minimize risks from arcing faults is to use fuses or other circuit protection devices, e.g. arc fault interruptors or ground fault detectors, as part of the strip heater circuit in order to remove power from the circuit if an arc should occur.

SUMMARY OF THE INVENTION

I have now found that the presence of a nonconductive filler in the conductive polymer composition in a strip heater can reduce the trip time of a fuse which forms part of a strip heater circuit, and thus reduce the danger that the heater will burn and cause damage. In a first aspect, this invention discloses a strip heater assembly which comprises

(A) a strip heater which comprises

(1) a resistive element which is composed of a first conductive polymer composition which comprises

(a) a polymer,

(b) a particulate conductive filler, and

(c) a particulate nonconductive filler, and

(2) two electrodes which can be connected to a source of electrical power to cause current to flow through the resistive element, and (B) a fuse ,

the particulate nonconductive filler being such that when the first composition is made into a standard strip heater as hereinafter defined and the standard heater is tested in a standard arcing fault test as hereinafter defined, it trips the fuse in less than 30 seconds.

In a second aspect the invention discloses a strip heater assembly which comprises

(A) a strip heater which comprises

(1) a resistive element which is composed of a conductive polymer composition which comprises

(a) a polymeric component which comprises polyethylene,

(b) a particulate conductive filler which comprises carbon black, and

(c) a particulate nonconductive filler which comprises Sb2θ3, and

(2) two elongate wire electrodes which are embedded in the resistive element and which can be connected to a source of electrical power to cause current to flow through the resistive element, and

(B) a fuse which is a very fast acting fuse and which has a rating of 10A, 125/250V,

the particulate nonconductive filler being such that when the composition is made into a standard strip heater and the standard heater is tested in a standard arcing fault test it trips the fuse in less than 30 seconds.

In a third aspect the invention discloses a strip heater circuit which comprises

(A) a strip heater which comprises

(a) a polymer,

(b) a particulate conductive filler, and

(c) a particulate nonconductive filler, and

(2) two electrodes which can be connected to a source of electrical power to cause current to flow through the resistive element,

(B) a fuse, and

(C) a power supply,

The invention further includes, in a fourth aspect, strip heaters which are useful in the assemblies and circuit defined above and are novel in their own right, namely, a strip heater which comprises a first conductive polymer composition comprising ( 1 ) a polymer ,

(2) a particulate conductive filler, and

(3) a particulate nonconductive filler

and which

(a) when tested following the procedure of UL VW-1, has a performance which is similar to that of a second heater made from a second conductive polymer composition which is the same as the first composition except that it does not comprise the particulate nonconductive filler and

(b) when tested in a standard arcing fault test

(i) trips the fuse in less time than is required by the second heater, and

(ii) trips the fuse in less than 30 seconds.

In a fifth aspect the invention discloses a strip heater which comprises a first conductive polymer composition comprising

(1) a polymer,

(2) a particulate conductive filler, and

(3) a particulate nonconductive filler

and which

(a) when tested following the procedure of UL VW-1, does not pass the test; and (b) when tested in a standard arcing fault test

(i) trips the fuse in less time than is required by a second heater made from a second conduc¬ tive polymer composition which is the same as the first composition except that it does not comprise the particulate nonconductive filler, and

(ii) trips the fuse in less than 30 seconds.

The invention further includes, in a sixth aspect, con¬ ductive polymers which are useful in the assemblies, circuit and strip heaters defined above and are useful in their own right, namely a melt-extrudable first conductive polymer composition which comprises

(1) a polymer,

(2) a particulate conductive filler, and

(3) a particulate nonconductive filler

said first composition being such that when the first composition is made into a standard strip heater

(i) when the standard heater is tested following the procedure of UL test VW-1 its performance is similar to a second heater which is made from a second conductive polymer composition which is the same as the first composition except that it does not comprise the particulate nonconductive filler, and

(ii) when the standard heater is tested in a standard arcing fault test (a) the time it requires to trip a fuse is less than the time required to trip a fuse for the second heater, and (b) it trips the fuse in less than 30 seconds.

In a seventh aspect the invention describes a melt- extrudable first conductive polymer composition which comprises

(1) a polymer,

(2) a particulate conductive filler, and

(3) a particulate nonconductive filler

(i) when the standard heater is tested following the procedure of UL test VW-l it does not pass the test, and

(ii) when the standard heater is tested in a standard arcing fault test (a) the time it requires to trip a fuse is less than the time required to trip a fuse for a second heater which is made from a second conductive polymer composition which is the same as the first composition except that it does not comprise the particulate nonconductive filler, and (b) it trips the fuse in less than 30 seconds.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is a cross-sectional view of a standard strip heater of the invention;

Figure 2 is a top view of a strip heater of the invention; and Figure 3 is a cross-sectional view of a strip heater along line 3-3 of Figure 2.

DETAILED DESCRIPTION ΩZ TEE. INVENTION

The first conductive polymer composition of this invention comprises a polymer which may be an organic polymer (such term being used to include siloxanes) , preferably a crystalline organic polymer, an amorphous thermoplastic polymer (such as polycarbonate or polystyrene) , an elastomer (such as polybutadiene or ethylene/propylene/diene (EPDM) polymer) , or a blend comprising at least one of these. Suitable crystalline polymers include polymers of one or more olefins, particularly polyethylene; copolymers of at least one olefin and at least one monomer copolymerisable therewith such as ethylene/acrylic acid, ethylene/ethyl acrylate, and ethylene/vinyl acetate copolymers; melt-shapeable fluoropolymers such as polyvinylidene fluoride and ethylene tetrafluoroethylene; and blends of two or more such crystalline polymers. Suitable polymers and compositions comprising them may be found in U.S. Patent Nos. 4,188,276, 4,237,441, 4,388,607, 4,470,898, 4,514,^'β20, 4,534,889, 4,560,498, 4,591,700, 4,624,990, 4,658,121, 4,774,024 and 4,775,778; and European Patent Publication Nos. 38,713, 38,718, 74,281, 197,759 and 231,068.

Crystalline polymers are particularly preferred, although not required, when it is desired that the composition exhibit PTC (positive temperature coefficient) behavior. The term "PTC behavior" is used in this specifi- cation to denote a composition or an electrical device which has an R14 value of at least 2.5 or an Rτ.oo value of at least 10, and preferably both, and particularly one which has an R30 value of at least 6, where R14 is the ratio of the resistivities at the end and the beginning of a 14°C range, RT_QO ^{is tne} ratio of the resistivities at the end and the beginning of a 100°C range, and R30 is the ratio of the resistivities at the end and the beginning of a 30°C range.

The composition also comprises a particulate conductive filler which is dispersed or otherwise distributed in the polymer. The particulate conductive filler may be, for example, carbon black, graphite, metal, metal oxide, or a combination of these. Alternatively, the conductive filler may itself comprise a conductive polymer in which a par¬ ticulate conductive filler is distributed in a polymer matrix and the matrix is then ground into particles before being dispersed in another polymeric matrix. The par¬ ticulate conductive filler is present in the composition in an amount suitable for achieving the desired resistivity, normally 5 to 50% by weight of the composition, preferably 10 to 40% by weight, particularly 15 to 30% by weight.

The particulate nonconductive filler comprises a material which is electrically insulating, i.e. has a resistivity of greater than 1 x 10^ ohm-cm. Preferably the nonconductive filler has a melting temperature of less than 1000°C. Suitable materials include metal oxides which are easily reduced, e.g. Sb2θ3, Pbθ2, B2O3, M0O3, and Bi2θ3. In this application, easily reduced means that the material has a reduction potential of less than +0.5 volts, preferably less than +0.4 volts, particularly less than +0.375 volts. Particularly preferred is Sb2θ3. For ease of dispersion in the polymer matrix, the filler is preferably in the form of particles which have a particle size of 0.01 to 50 μm, particularly 0.05 to 50 μm, especially 0.10 to 10 μm. The nonconductive filler may be single material or it may comprise a blend of metal oxides or a metal oxide and another particulate filler. Although a blend of Sb₂θ₃ and decabromodiphenyloxide (also known as decabromodiphenylether) , DBDPO, is commonly used as a flame retardant package in polymers, the presence of the DBDPO or any other halogenated material is not necessary for satisfactory performance in the compositions of the invention.

The conductive polymer composition may also comprise inert fillers, antioxidants, prorads, stabilizers, dispersing agents, or other components. Mixing is preferably effected by melt- processing, e.g. melt-extrusion. Subsequent processing steps may include extrusion, molding, or another procedure in order to form and shape the composition. The composition may be crosslinked by irradiation or chemical means .

The conductive polymer composition may be used in any current carrying electrical device, e.g. a circuit protection device, a sensor, or, most commonly, a heater. The heater may be in the form of either a strip or a laminar sheet in which the resistive element comprises the composition of the invention. Strip heaters may be of any cross-section, e.g. rectangular, elliptical, or dumbell ("dogbone") . Appropriate electrodes, suitable for connection to a source of electrical power, are selected depending on the shape of the electrical device. Electrodes may comprise metal wires or braid, e.g. for attachment to or embedment into the conductive polymer, or they may comprise metal sheet, metal mesh, conductive (e.g. metal- or carbon- filled) paint, or other suitable materials.

In order to provide environmental protection and electrical insulation, it is common for the resistive element to be covered by a dielectric layer, e.g. a polymeric jacket (for strip heaters) or an epoxy layer (for circuit protection devices). The dielectric layer may comprise flame retardants or other fillers. For some strip heater applications, a metallic grounding braid is present over the dielectric layer in order to provide physical reinforcement and a means of electrically grounding the strip heater.

The compositions of this invention are particularly useful when, in the form of strip heaters, they are used in conjunction with a fuse and act to "trip" the fuse faster than strip heaters comprising conventional materials. A fuse "trips" when the current in the circuit comprising the fuse exceeds the rated value of the fuse. Fuses are categorized based on their overload fusing characteristics, i.e. the relationship between the value of current through the fuse and the time for the fuse to open as described in Bulletin SFB, "Buss Small Dimension Fuses", May 1985, the disclosure of which is incorporated herein by reference. Of the major categories (slow blowing, non-delay, and very fast acting), it is very fast acting fuses which are most useful in this invention. These fuses have little, if any, inten¬ tional delay in the overload region. Although the selection of a specific fuse is dependent on the normal operating con¬ ditions of the strip heater and the anticipated fault con¬ ditions, fuses which are particularly preferred are very fast-acting ceramic ferrule fuses with a current rating of 10 amperes and a voltage rating of 125/250 volts. Such fuses are available, for example, from the Bussman Division of Cooper Industries under the name Buss GBB"-10. The fuse may be an independent component in the circuit or it may be in a fused plug assembly, i.e. an assembly in which the fuse is part of the plug which connects the strip heater to the power source, e.g. an outlet or a power supply.

Strip heaters of the invention are commonly used in a strip heater assembly which comprises the strip heater and a fuse.. Alternatively, the strip heater is a component of a strip heater circuit which comprises the strip heater, a power supply, and a fuse. The power supply can be any suitable source of power including portable power supplies and mains power sources. Other components, such as resistors, thermostats, and indicating lights, may also be present in the circuit.

In this specification, a "standard strip heater" is defined for testing purposes. A "standard strip heater" is one in which a conductive polymer composition is melt- extruded around two 22 AWG stranded nickel/copper wires to produce a strip heater of flat, elliptical shape as shown in Figure 1. The heater has an electrode spacing of 0.10 inch (0.25 cm) from, the center of one electrode to the center of the second electrode. The thickness of the heater at a point centered between the electrodes is 0.08 inch (0.20 cm). The heater is jacketed with a composition which contains- 31.9% by weight of a standard flame retardant package as described in Example 1. The jacket is 0.030 inch (0.076 cm) thick.

The standard strip heater is tested by means of a "standard arcing fault test". In this test (which is more fully described in Example 1), a standard strip heater is connected in a circuit to a power supply and a 10A, 125/250V fuse. An arc is initiated between two exposed electrodes of the heater and the time to interrupt the current and extinguish the arc by means of tripping the fuse is recorded. I have found that a standard strip heater which comprises the composition of the invention (i.e. a first conductive polymer composition) trips the fuse faster than a second strip heater which comprises a second conductive polymer composition, i.e. a composition which is the same as the first composition except that it does not comprise the nonconductive particulate filler. The time to trip a fuse for the standard heater generally will be at least two times as fast, preferably at least three times as fast, par¬ ticularly at least five times as fast, e.g. five to eight times as fast as the second heater. Thus the standard heater will trip the fuse in at most half the time required to trip the fuse in a circuit which comprises a second heater. When tested in the standard arcing fault test, a standard strip heater of the invention normally will trip the fuse in less than 30 seconds, preferably in less than 25 seconds, particularly in less than 20 seconds, e.g. in 5 to 10 seconds. An additional aspect of the invention is that the addition of the nonconductive particulate filler results in an increase in the number of current spikes observed during the arcing fault test. Even if the amplitude of the spikes is similar for both types of heaters, there generally will be at least 2 times, preferably at least 3 times, par¬ ticularly at least 4 times as many current spikes in a given period, e.g. 30 seconds, for the heater comprising the first composition.

A second test which is conducted on heaters comprising the first composition of the invention is the UL VW-1 vertical-wire flame test (Reference Standard for Electrical Wires, Cables, and Flexible Cords, UL 1581, No. 1080, August 15, 1983. In this test, a heater sample is held in a vertical position while a flame is applied. In order to pass the test, the sample cannot "flame" longer than 60 seconds following any of five 15-second applications of the test flame. The period between sequential applications of the test flame is either 15 seconds (if the sample ceases flaming within 15 seconds) or the duration of the sample flaming time if the flaming lasts longer than 15 seconds. In addition, combustible materials in the vicinity of the sample cannot be ignited by the sample during the test. In this specification, when the performance in this test of the heater of the invention is said to be "similar" to that of a second heater which comprises a second conductive polymer composition, it means that if ten different samples of the standard heater are tested, eight of them (i.e. 80%) must have the same result (i.e. pass or fail) as ten samples of the second composition.

The invention is illustrated by the drawing in which Figure 1 shows a cross-section of a standard strip heater 1. Electrodes 5, 7 are embedded in the first conductive polymer composition 3 (the resistive element) . A polymeric jacket 9 surrounds the heater core. Figure 2 shows a top view of strip heater 1 which has been prepared for the arcing fault test described below. A V-shaped notch 11 is cut through the polymeric jacket 9 and the conductive polymer composition 3 on one surface of the heater in order to expose electrodes 5 and 7. The cross-sectional view of the heater along line 3-3 is shown in Figure 3. Electrodes 5, 7 remain partially embedded in the conductive polymer 3. The invention is illustrated by the following examples.

Example 1 (Comparative Example)

The ingredients listed in Table I were preblended and then mixed in a co-rotating twin-screw extruder to form pellets. The pelletized composition was extruded through a 1.5 inch (3.8 cm) extruder around two 22 AWG stranded nickel/copper wires to produce a strip heater. The heater had an electrode spacing of 0.106 inch (0.269 cm) from center-to-center and a thickness of 0.083 inch (0.211 cm) at a center point between the wires. The heater was jacketed with a 0.030 inch (0.076 cm) layer of a composition containing 10% by weight ethylene/vinyl acetate copolymer (EVA), 36.8% medium density polyethylene, 10.3% ethylene/ propylene rubber, 23.4% decabromodiphenyloxide, 8.5% antimony oxide, 9.4% talc, 1.0% magnesium oxide, and 0.7% antioxidant.

The heater was tested using the standard arcing fault test described below. The results are shown in Table II. In a related test, the amplitude and frequency of the current spikes produced when a heater was tested following the procedure of the arcing fault test but without the use of a fuse were recorded. In this modified arcing fault test, the samples were allowed to burn for three minutes after a flame was initiated. The results are shown in Table III.

The heater was tested following the procedures of the UL VW-1 vertical-wire flame test (Reference Standard for Electrical Wires, Cables, and Flexible Cords, UL 1581, No. 1080, August 15, 1983). Of the 10 samples tested, five passed the test. These results are shown in Table IV. S an-ard Arcing Faul Test

A standard, jacketed 25 inch- (64 cm-) long strip heater was prepared by stripping one inch (2.5 cm) of jacket and core material from a first end to expose the two electrodes. A transverse v-shaped notch was cut half-way through the thickness of the heater 2 inches (5.1 cm) from the second end and the jacket and core polymer were removed from the top half in order to expose part of each of the two electrodes. The electrodes at the first end were connected in a circuit in series with a 120V/1OOA power supply, a contactor relay, a 0.1 ohm/100 watt shunt resistor, and a 10A, 125/250V very fast acting fuse (Buss GBB™-10, available from the Bussman Division of Cooper Industries) . A chart recorder was connected across the shunt resistor in order to measure the voltage drop. When the relay was closed, the sample was powered. A sufficient quantity of 10 to 20% saline solution was applied to the exposed v-notch until an arcing fault was initiated. The chart recorder was monitored until the current was interrupted and the arc was extinguished (i.e. until the fuse tripped) . Both the time duration of the arc, as determined from the current spikes on the chart, and the distance of arc fault propagation on the strip heater were measured. In some instances, the number of current spikes present during the arcing fault was also determined.

Examples ? to 6

For each example, pellets of the composition of Example 1 were preblended with the inorganic materials in the proportions shown in Table I. After mixing in a co-rotating twin screw extruder and pelletizing, the cόmpositions were extruded to form strip heaters with the same geometry as that of Example 1 and were jacketed as in Example 1. The results of the arcing fault test and the vertical flame test are shown in Tables II and IV. It is apparent that those compositions which contain Sb2θ3 have significantly faster trip times in the arc fault test than comparable materials which do not contain the filler.

A strip heater formed from the composition of Example 2 was also tested following the modified arcing fault test described in Example 1. As shown in the results in Table III, the amplitude of the current spikes and the burn rate were comparable for both the conventional composition (Example 1) and the composition of the invention (Example 2). The major difference occurred in the frequency of the current spikes; the spikes were much more prevalent for the composition of the invention than for the conventional material.

Example 7

Following the procedure of Example 1, the ingredients listed in Table I were preblended, mixed in a co-rotating twin screw extruder, and pelletized. The pellets were extruded over two 16 AWG 19-strand nickel-coated copper wire electrodes to produce a strip heater with a wire spacing of 0.285 inch (0.724 cm) center-to-center and a thickness of 0.057 inch (0.145 cm) at a position intermediate to the electrodes. The heater was jacketed with the same material as in Example 1. The results of testing are shown in Tables II and IV. Example 8

Pellets of the composition of Example 7 were blended with 11.7% by weight decabromodiphenyloxide and 4.3% Sb2θ3 before extrusion into pellets. The pellets were extruded to form a strip heater as in Example 7. The results of testing are shown in Tables II and IV.

Tahl e I

CONDUCTIVE POLYMER FORMULATIONS (Components in Percent by Weight)

Component __L

EEA 51.7 43.4 49.6 47.5 43.4 35.2 29.3 24.6

CB 30.3 25.5 29.1 27.9 25.5 20.6 17.2 14.5

MDPE 17.2 14.4 16.5 15.8 14.4 11.7

HDPE 32.4 27.2

Antioxidant 0.8 0.7 0.8 0.8 0.7 0.5 0.5 0.4

Sb₂0₃ 4.3 4.0 8.0 4.3

ZnO 20.0 16.8

Al₂0₃ 3H₂0 16.0 32.0

DBDPO 11.7 11.7

Process Aid 0.6 0.5

Notes to Table I:

EEA is ethylene/ethyl acrylate copolymer.

CB is carbon black with a particle size of 28 nm.

MDPE is medium density polyethylene.

HDPE is high density polyethylene.

Antioxidant is an oligomer of 4,4-thio bis (3-methyl 1-6-t-butyl phenol) with an average degree of polymerisation of 3 to 4, as described in U.S.

Patent No. 3,986,981.

Sb₂0₃ is antimony trioxide with a particle size of 1.0 to 1.8 μ .

ZnO is zinc oxide with a particle size of 0.15 μ .

Al₂0₃ 3H₂0 is alumina trihydrate with a particle size of 0.15 μ .

DBDPO is decabromodiphenyl oxide (also known as decabromodiphenylether) , Table II

ARCING FAULT TEST RESULTS

*The test was discontinued after 5 minutes, even though the fuse did not trip.

Table III

MODIFIED ARCING FAULT TEST RESULTS

Table IV

VERTICAL WIRE FLAME TEST (UL VW-1)

Example % Pass

1 50% 2 100 3 100 4 100 7 100 8 100

Claims

1. A melt-extrudable first conductive polymer composition which comprises

(A) a polymer,

(B) a particulate conductive filler, and

(C) a particulate nonconductive filler,

said first composition being such that when the first composition is made into a standard strip heater and the standard strip heater is compared to a second heater which is made from a second conductive polymer composition which is the same as the first composition except that it does not comprise the particulate nonconductive filler, one of the following conditions occurs,

(1) when the standard heater is tested

(a) following the procedure of UL test VW-1 its performance is similar to the second heater and

(b) in a standard arcing fault test (i) the time it requires to trip a fuse is less than the time required to trip a fuse for the second heater, and (ii) it trips the fuse in less than 30 seconds, and

(2) when the standard heater is tested

(a) following the procedure of UL test VW-1 it does not pass the test and

(b) in a standard arcing fault test (i) the time it requires to trip a fuse is less than the time required to trip a fuse for the second heater and (ii) it trips the fuse in less than 30 seconds .

2. A composition according to claim 1 wherein the nonconductive filler comprises an inorganic oxide.

3. A composition according to claim 2 wherein the oxide is Sb₂0₃.

4. A composition according to claim 2 or 3 wherein the nonconductive filler further comprises decabromodiphenyloxide .

5. A composition according to any one of the preceding claims wherein the composition exhibits PTC behavior.

6. A strip heater which comprises a first conductive polymer composition according to any one of the preceding claims and which

(1) when tested following the procedure of UL VW-1 either

(a) does not pass the test or (b) has a performance which is similar to that of a second heater which is made from a second conductive polymer composition which is the same as the first composition except that it does not comprise the particulate nonconductive filler, and

(2) when tested in a standard arcing fault test (a) trips the fuse in less than time than is required by the second heater, and (b) trips the fuse in less than 30 seconds .

7. A strip heater assembly which comprises

(A) a strip heater which comprises (1) a resistive element which is composed of a conductive polymer composition according to claim 1, 2, or 3 and

(2) two electrodes which can be connected to a source of electrical power to cause current to flow through the resistive element, and

(B) a fuse,

the particulate nonconductive filler being such that when the first composition is made into a standard strip heater and the standard heater is tested in a standard arcing fault test it trips the fuse in less than 30 seconds.

8. A strip heater assembly according to claim 7 wherein the fuse is part of a fused plug assembly.

9. A strip heater assembly according to claim 7 or 8 wherein the fuse is a very fast acting fuse.

10. A strip heater circuit which comprises

(A) a strip heater which comprises

(1) a resistive element which is composed of a conductive polymer composition according to claim 1, 2, or 3 and

(B) a fuse, and

(C) a power supply, the particulate nonconductive filler being such that when the composition is made into a standard strip heater and the standard heater is tested in a standard arcing fault test it trips the fuse in less than 30 seconds.