CA1331399C

CA1331399C - Assemblies of ptc circuit protection devices

Info

Publication number: CA1331399C
Application number: CA000605672A
Authority: CA
Inventors: David A. Horsma; Hundi P. Kamath; Shou-Mean Fang
Original assignee: Raychem Corp
Current assignee: Raychem Corp
Priority date: 1988-07-15
Filing date: 1989-07-14
Publication date: 1994-08-09
Anticipated expiration: 2011-08-09
Also published as: EP0429489A1; EP0429489B1; US4967176A; KR900702614A; HK1006899A1; ATE122827T1; KR0178953B1; JPH04500138A; WO1990000825A1; AU4035689A; DE68922733D1; DE68922733T2

Abstract

ASSEMBLIES OF PTC CIRCUIT PROTECTION DEVICES
ABSTRACT
A device assembly in which a plurality of PTC circuit protection devices are connected in series. Assemblies of this type are useful in providing protection under voltage conditions which would be unsafe for an individual protection device. In a preferred system the device assembly is connected in series with a circuit breaker.

Description

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ASSEMBLIES OF PTC CIRCUIT PROTECTION DEVICES

BACKGROUND OF THE INVENTION
Field of the Invention This invention relates to electrical devices comprising PTC materials.
Backqround of the Invention There are a number of known materials whose resistivity increases sharply with temp~rature over a relatively small temperature range. Such materials are said to be "PTC
materials" or to "exhibit PTC behaviorn, PTC being an abbre-viation of "positive temperature coefficientn. For many purposes, it is preferred that a PTC material should exhibit an R14 value of at least 2.5 and/or an Rloo value of at least 10, and particularly preferred that it should have an R30 value of at least 6, where R14 is the ratio of the resistivities at the end and the beginning of a 14C range, Rloo is the ratio of the resistivities at the end and the beginning of a 100C range, and R30 is the ratio of the resistivities at the end and the beginning of a 30C range.
Many PTC materials show increases in resistivity which are very much greater than these minimum values. A plot of the log of the resistance of a PTC element ~i.e. an element composed of a PTC composition) against temperature will often show a sharp change in slope over a part of the temperature range in which the composition has an Rloo value of at least 10. The term "switching temperature~ (usually abbreviated Ts) is usedihereih to denote the temperature at the intersection point of extensions of the substantially straight portions of such a plot which lie either side of .
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the portion showing the sharp change in slope. The term "peak resistivity" is used herein to denote the maximum resistivity which the composition exhibits above Ts, and the term "peak temperature" is used to denote the temperature at which the composition has its peak resistivity.
PTC elements have proved particularly useful as com-ponents of self-regulating heaters and of circuit protection devices. The PTC materials which have been used or proposed for use in such electrical devices are certain ceramics and certain conductive polymers, the term "conductive polymer~
being used herein to denote a composition which comprises an organic polymer ~this term being used to include poly-siloxanes) and, dispersed or otherwise distributed in the organic polymer, a particulate conductive filler. Suitable ceramic materials include doped barium titanates, and suitable conductive polymers include crystalline polymers having carbon black dispersed therein. PTC ceramics generally exhibit a sharp change in resistivity at the Curie point of the material, and PTC conductive polymers generally exhibit a sharp change in resistivity over a temperature range just below the crystalline melting point of the poly-meric matrix. The PTC ceramics which are used in commercial practice generally show-a sharper rate of increase in resistivity than do the PTC conductive polymers. PTC
ceramics generally have a resistivity of at least 30 ohm-cm at 23C, whereas PTC conductive polymers can have a lower resistivity at 23C, e.g. down to about 1 ohm-cm or lower.
PTC ceramics tend to crack and thus to fail suddenly if exposed to excessive electrical stress, whereas PTC
conductive polymers tend to degrade relatively slowly.
Documents which disclose circuit protection devices comprising PTC conductive polymers include U.S. Patent Nos.

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~3~ 1331399 4,237,441, 4,239,812, 4,255,698, 4,315,237, 4,317,027, 4,329,726, 4,352,083, 4,475,138, 4,481,498, 4,639,818, 4,647,894, 4,645,896, 4,685,025, 4,689,475, 4,724,417, and 4,774,024; European Publication No. 38,713 (published September 2, 1987); International Publication No. WO89/03162 (published April 6, 1989); and the trade pamphlets published by Raychem Corporation in January 1987 and entitled "A
General Approach to Circuit Design with PolySwitch Devices", "Protection of Subscriber Line Interface Circuits with PolySwitch Devices", "Protection of PBX and Key Telephone Systems with PolySwitch Devices", ~Protection of Telecommunications Networks with PolySwitch Devices", "Protection of Loudspeakers with PolySwitch Devices", and "Protection of Batteries with PolySwitch Devicesn.
("PolySwitch" is a registered trademark of Raychem Corporation.) The term "hold current" (or "pass current") is used to denote the maximum steady current which can be passed through a PTC circuit protection device without causing it to trip (i.e. be converted into a high temperature, high resistance state such that the circuit current is reduced to a very low level). The hold current of a device depends upon the rate at which heat is lost from the device; for example, the higher the ambient temperature, the higher the hold current. It is known to connect a plurality of substantially identical devices in parallel to provide a PTC
protection asse~bly having a hold current which is sub-stantially equal to the sum of the hold currents of the individual devices. The performance characteristics of a PTC circuit protection device depend importantly on the voltage which is dropped across it in the tripped state; the higher the voltage, the greater the danger that the device ,` : .

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wlll be damaged and wlll thus fall to provlde the deslred protec-tlon and/or wlll fail ln a hazardous way, e.g. wlll explode or burn. As ls apparent from the patents and appllcatlons referred to above, much effort has been devoted to lncreaslng the voltage which can safely be dropped over PTC conductlve polymer clrcult protectlon devlces. In general, the greater the dlstanc between the electrodes, and the greater the extent of the crossllnklng of the conductlve polymer, the hlgher the voltage whlch can be em-ployed. Whlle there are avallable protectlon devlces whlch can safely handle a voltage of about 600 volts RMS, protectlon agalnst ¦ hlgher voltages remalns a problem. Another unsolved problem ls the provlslon of devlces whlch wlll protect agalnst voltages that .
can be handled by exlstlng devlces, but whlch are easler to manu-facture than exlstlng devlces (e.g. requlre less or no crossllnk-lng) and/or whlch have a more convenlent shape (the shape often belng largely determlned by the conflguratlon and separatlon of the electrodes), elther for lnstallatlon or ln use (e.g. on a ~-~- prlnted clrcult board or ln other sltuatlons where space is at a ~```'; . .
premlum) and/or for thermal balance conslderatlons.
SUMMARY OF THE INVENTION
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As noted above, lt ls known to connect a plurallty of ¦ substantlally ldentlcal PTC protectlon devlces ln parallel ln order to provlde a protectlon assembly havlng a hold current substantlally equal to the sum of the hold currents of the lndivl-~;~ dual devlces. It ls not known, however, to connect a plurallty of ' PTC protectlon devlces ln serles ln order to provlde an assembly j whlch can safely handle a voltage hlgher than can be handled by ~ any of the devlces 133~ 39~

individually. The reason for this is as follows.
Theoretical considerations make it clear that this desirable result should be achieved by a plurality of series-connected devices which are precisely identical and which are in precisely identical thermal environments. However, those skilled in the art have held the belief that this desirable result would never in fact be achieved because it is not in practice possible to make devices which are precisely identical or to place them in precisely identical thermal environments, and because even the smallest difference, under fault conditions, would cause a single one of the devices to increase in resistance much more rapidly than the others and thus to shoulder the whole of the voltage burden.
Consequently, those skilled in the art have believed that the ability of a number of devices, connected in series, to control excessive current is no greater than the ability of the single device which trips.
We have discovered that there are many circumstances in `~
which this belief is not justified. In particular, we have discovered that if due account is taken of the dynamic variables during the tripping process ~e.g. the rate of change of the current, the rate of change of resistivity with temperature, and the rate at which heat is removed from the devices, including in some cases transfer of heat between devices), the electrical stress can be shared between the devices. In some cases, the sharing of the electrical stress will last only for a limited time, and maintenance of the fault condition which caused tripping will result in substantially all of the electrical stress being concentrated on a singie device. In some such cases, this is a satisfactory outcome because the single device, ~`;"
` having been converted into the tripped condition over a ..

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-6- 133~39.9 substantialiy longer time period because of the temporary sharing of the electrical stress, can safely handle the voltage which is being dropped over it in the steady state condition. In other such cases, this is not true, but the time during which electrical stress is shared is neverthe-less highly significant because it is long enough to allow another desired change (e.g. the making or breaking of a contact) to take place under conditions which are substan-tially less severe than they would otherwise be, for example if only one or a lesser number of protection devices had been present. Providing such other change (a) taXes place before there is an excessive electrical stress on the device which is bearing the greatest share of the stress and (b) interrupts the circuit (or otherwise prevents the exertion of excessive stress on that device~, then the series of devices will safely handle a substantially higher voltage than any one of the devices, alone. In other cases, the electrical stress is also shared in the steady state con-dition, with more than one of the protection devices being in a tripped condition; in some cases one or more of the tripped devices is also in a latched condition (i.e. the device remains in the high resistance state even if the fault condition is removed, unless power is removed from the circuit).
one very valuable use of this discovery is in circuit protection applications wherein a plurality of PTC protec-tion devices are connected in series to form a device ''assembly. The device assembly can be used on its own, if it will withstand the electrical, stress exerted on it when a~
fault condition occurs. Alternatively, the assembly can be used in conjunction with a circuit breaker. In the latter embodiment, the dev~ce as=erlbly ho1ds and controls the ' :

.. ,, . . ..... , ... .... , ;.. . . ~ . ,. , .. . .. ; . . ... . .. ... ; ., ... .. .. .. . , ~7~ ~3~3 excessive current for a period of time which is short but which nevertheless substantially reduces the electrical stress on the circuit breaker, whose cost and complexity can, therefore, be substantially reduced.
Another very valuable use of this discovery is in switching apparatus which incorporates a device assembly comprising a plurality of PTC protection devices connected in series. The device assembly is preferably connected in series between the terminals (which are being separated or engaged) during the switching operation but is disconnected when the switch is fully open and is disconnected or con-nected in parallel when the switch is fully closed. In this way, the device assembly controls the current during the critical period while the terminals are being separated or engaged, and reduces the danger that an arc will be struck between the terminals.
BRIEF DESCRIPTION OF THE DRAWING
The invention is illustrated in the accompanying drawing, in which Figure 1 is a circuit diagram of the use of the invention for circuit protection, Flgure3 2A, 2B and 2C are diagrammatic representations of successive stages of operation of a switch making use of the invention as the switch is opened, Figures 3 and 4 are cross-sections of composite device assembl~es of the invention, Figure 5 i9 a plan view of another composite device assembly of the invention, and i ;:
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Figure 6 is a cross-section on line 6-6 of Figure 5.
DETAILED DESCRIPTION OF THE INVENTION
The number of protection devices which are connected in series is generally at least three, preferably at least five, and can be many more, e.g. up to 100. The devices will often all be devices which have been made by the same manufacturing process. However, this is not necessary. In general, when using devices which have been rated for use up to a particular voltage (A volts), and the voltage across the assembly in the fault condition is B volts, the number of device~ connected in series will be B/A. However, since the rating is generally a conservative one, a number of devices which is less than B/A can be used, particularly when a large number of devices are employed. It is of course important to ensure also that the hold current of the device assembly is sufficiently high, and for this purpose a plurality of sets of devices in series can be placed in parallel with each other. For example a device assembly for protecting a 6KV 600 amp circuit might comprise 600 sets, connected in parallel, each set being made up of ten 600 volt 1 amp protection devices.
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Ii The device assembly can be operated under adiabatic -conditions, or can be such that heat is transferred between ~' the devices during the tripping operation. For example, the ~ devices can be separated from each other, e.g. by an inert .$~ insulating liquid, or (particularly when laminar devices are ~i~ employed) can be stacked one on top of the other or secured ~, to a thermally conductive substrate.
d`` The invention is illùstrated in the drawing in which ~ Figure 1 is a circuit diagram in which a device assembly 1 .;i ,1 r 133~9 is connected in series with a circuit breaker 12, a switch 13, a source of power 14 and a load RL.
Figure 2 shows the sequential opening of a switch which comprises a stationary portion 21 and a slidable portion 22. Electrical connection from the device assembly 1 is made through stationary terminal 23. When the switch is closed (Figure 2A), terminal 23 is in physical contact with slidable portion 22. When a specific event (e.g. a voltage surge) occurs, the contact is broken between the portions of the switch, and slidable portion 22 moves away from stationary portion 21 (Figure 2B). When the switch is completely open, the terminal 23 is physically separated from the slidable portion 22 of the switch (Figure 2C).
Figures 3 and 4 show cros~-sectional views of composite device assemblies 30 of the invention. Each assembly shown comprises three devices 31 which are adjacent to, and electrically in series with, one anotner. The devices comprise a PTC element 32 and two electrodes 33, although in some embodiments in which the devices are in physical and thermal contact, some or all of the devices need have only a singIe electrode. Electrical leads 34 are attached to the opposite faces of the assembly stack in order to make electrical connection to a power supply or circuit. The assembly of Figure 3 comprises devices of the same size, although, as shown in Figure 4, devices of different sizes and/or comprising compositions of different resistivities may be used.
Figure 5 is a plan view of a composite device assembly 50. A substrate 51 which comprises a PTC composition is laminated, printed, or otherwise supplied with metal . ,. :

,. . ~ . . , --lo- 13313~9 electrode strips 52. Slots 53 may be machined or etched through the thickness of the PTC substrate and lead wire 54 may be attached to individual devices 55 in order to produce the desired series/parallel configuration.
Figure 6 shows a crbss-sectional view on line 6-6 of Figure 5 in which the PTC substrate 51 comprises a conductive polymer.
I . EXAMPLES
; The invention is illustrated by the following examples.
Exam~e 1 A conductive polymer composition was prepared by mixing the following ingredients (by volume) in a Banbury mixer:
56.7% high density polyethylene (Marlex~ 6003, available from. Phillips Petroleum), 25.1% carbon black (Sterling~ SO, : available from Cabot), 16.5% silane-coated alumina ~ri-hydrate (SoIem~ 916SP, available from J. M. Huber), and 1.7% antioxidant (an oligomer of 4,4-thio bis(3-methyl ~;
1-6-t-butyl phenol) as described in U.S. Patent No.
~-~ 3,986,981). Using a Brabender crosshead extruder fitted with a dogbone-shaped die, pellets of the composition were melt-extruded around two 20 AWG 19/32 nickel-coated copper wires which had been coated with a graphite/silicate com-position IElectrodag~ 181, available from Acheson Colloids).
~: The extrudate was cut into pieces, and the conductive ~t``'' polymer was removed from part of the device to expose the `: electrodes. The devices were heat-treated at 150C in ~:: nitrogen for~one hour, irradiated with a 1.5 MeV electron 't beam to a dose of 20 Mrad, heat-treated a second time, ~ irradiated to a dose of 150 Mrad, and heat-treated a third :, `
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~' 13~13~9 time. After processing, the devices had a resistance of 16.5 to 18.5 ohms and had maximum voltage and current ratings of 600 volts and 1 amp, respectively. `
Ten devices were electrically connected in series and were then inserted into a beaker which was filled with a thermally dissipating liquid (Fluorinert~ FC-75, available from DuPont). The beaker was placed in a water bath heated to 100C and the devices were allowed to equilibrate to the temperature. The devices were connected to a series ballast resistance of 500 ohms and were then powered at 6000 volts/2 amps rms for a period of 0.4 seconds. The voltage and current were monitored with an oscilloscope during the test, and the resistance of each device was measured at the start and conclusion of the test. The oscilloscope traces indi-cated that the devices which tripped did so within three AC cycles.
The resistances for three different experimental groups of ten devices are listed in Table I. Those devices which did not trip during the test are indicated by an asterisk *). When the ratio of resistance after the test (Rf) to initial resistance (Ri) was greater than 1.2, the device was deemed to have tripped; those between 1.10 and 1.19 did not completely trip. During each test, 50 to 70% of the devices tripped.
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TABLE I
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Device No. 1 2 3 4 5 6 7 8 9 10 Ri (ohms) 17.617.2 17.318.5 17.616.917.8 17.3 17.2 17.6 Rf (ohms) 17.924.1 23.822.7 23.417.224.3 17.7 17.3 18.0 Rf/Ri 1.02*1.401.38 1.331.02*1.371.02*1.02*1.01*1.02*

GrouP 2:
Device No. 1 2 3 4 5 6 7 8 9 10 Ri ( obms ) 17.0 16.5 17.2 17.5 17.6 16.7 17.4 16.7 17.0 17.5 Rf ~ohms)18.7 18.3 18.0 22.7 24.2 25.5 25.1 25.6 18.1 18.1 -Rf/Ri 1.10* 1.11* 1.05* 1.30 1.38 1.53 1.44 1.53 1.06* 1.03*

Group 3:
., i `: ., ~ Device No. 1 2 3 4 5 6 7 a 9 lo Ri (cbDns)16.8 17.0 16.6 17.7 18.4 18.1 17.4 17.2 18.3 18.5 - -Rf (obms)24.0 23.1 18.8 25.3 24.8 27.0 25.9 18.1 24.0 18.8 Rf/Ri 1.43 1.3S 1.13* 1.43 1.35 1.44 1.49 l.OS* 1.31 1.02*

A conductive polymer composition with a resistivity of about 4 ohm-cm was prepared by mixing 56.1 vol% high density polyethylene ~Marlex~ HXM 50100, available from Phillips Petroleumj, 26`.7 vol~ carbdn black ~Statex~ G, available from Columbian Chemicals), 15.5 vol~ magnesium hydroxide (Kisuma~ :~
::; 5A, available from Kisuma), and 1.7 vol% antioxidant ~as :~

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described in Example 1) in a Banbury mixer. Pellets of the - composition were extruded to produce a sheet with a thickness of 0.040 inch (0.10 cm). After laminating the sheet on each side with electrodeposited nickel fo,il electrodes (available from Fukuda), the sheet was irradiated to a dose of 10 Mrad. Devices with a diameter of 0.360 inch (0.914 cm) and a resistance of 0.66 ohms were cut from the ; plaque. Each device was nominally capable of withstanding 60 volt/40 amp electrical powering. Three devices were stacked together and two metal leads were soldered to the top and bottom surfaces of the stack. During the lead ; attachment process, solder stuck to the exposed edges of the electrodes of each of the three devices, fusing the devices together to give a composite device assembIy with a resistance of 2.56 ohms. When tested under 600 volt/l amp -~
impulse conditions, the composite device assembly survived 12 to 18 seconds before failing. When tested at 300 volts/l amp, the composite device assembly tripped in less than 17 seconds and survived 132 test cycles. Under conventional ~; testing, the individual devices comprising the composite device assembly would not survive voltage impulses of 300 to 600 volts. -~
Example 3 .`` Five devices as described in Example 2 were stacked to `` produce a composite device assembly with a resistance of ;~` 3.94 ohms. When tested at 600 volts/l amp, the composite device assembly qurvived 17 seconds before tripping. When ` tested at 300 volts/l amp, the composite device assembly survlved 145 cycles.
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-14- 13313~9 Example 4 A conductive polymer composition with a resistivity of about 1 ohm-cm was prepared by mixing 65.8 vol~ high density polyethylene tMarlex~ 6003, available from Phillips Petroleum) with 34.2 vol% carbon black (Raven~ 600, available from Columbian Chemicals) in a Banbury mixer. The composition was extruded, laminated with metal foil, and irradiated as described in Example 2. Two devices with a diameter of 0.360 inch ~0.91 cm) and a resistance of 0.148 ohm were cut from the laminated sheet. Using the procedure of Example 2, these devices were positioned on either side of a device as described in Example 2 to produce a composite device assembly with a resistance of 1.185 ohms. When tested at 600 volts/l amp, the composite device assembly survived 45 seconds before tripping. At 300 volts/l amp (power applied for 40 seconds), the composite device assemblies survived 145 cycles.
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Example 5 A device as described in Example 2 (diameter 0.360 inch) was sandwiched between two devices with a diameter of '~ 0.250 inch ~0.64 cm) cut from the sheet described in Example `~ 4 to produce a composite device assembly with a resistance ~-~ of 2.1 ohms. When powered at 600 volts/l amp, the composite device assembly tripped in 11 seconds. At 300 volts/
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~; 1 amp/40 sec, the composite device assemblies survived 20 to j~` 120 cycles.

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Claims

1. An electrical circuit breaking system which comprises (1) a circuit breaker, and (2) a device assembly which is connected in series with the circuit breaker and which comprises a plurality of PTC circuit protection devices connected in series.

2. A circuit breaking system according to claim 1 wherein the device assembly comprises a least three devices connected in series.

3. A circuit breaking system according to claim 2 wherein the device assembly comprises at least three laminar PTC
elements, each of said PTC elements being sandwiched between two laminar metal electrodes, and the PTC elements being stacked on top of each other and separated by laminar metal electrodes.

4. A circuit breaking system according to claim 1 wherein the devices have all been manufactured by the same manufacturing process.

5. A circuit breaking system according to claim 1 wherein the device assembly comprises a plurality of sets of devices connected in series, the sets being connected in parallel with each other.

6. A circuit breaking system according to claim 1 wherein each of the devices comprises a PTC element composed of a conductive polymer.

7. A circuit breaking system according to claim 1 wherein each of the devices comprises a PTC element composed of a ceramic.

8. A circuit breaking system according to claim 1 wherein the devices are immersed in an insulating liquid.

9. An electrical switching system which has a closed configuration, an intermediate configuration, and an open configuration, and which comprises (1) a first metal contact surface, (2) a second metal contact surface, and (3) a device assembly which comprises a plurality of PTC circuit protection devices connected in series, the first and second surfaces being in direct physical contact when the system is in the closed configuration; the first and second surfaces being physically separated from each other but electrically connected to each other through the device assembly when the system is in the intermediate configuration; and the first and second surfaces being physically and electrically separated from each other when the system is in the open configuration.

10. An electrical switching system according to claim 9 wherein each of the PTC elements has been made by the same manufacturing process.

11. An electrical switching system according to claim 9 wherein each of the PTC elements is composed of a conductive polymer.