Abstract:
Use of PTC circuit protection devices in motor vehicle wiring systems. The devices can replace some or all of the fuses conventionally used. Preferred devices contain PTC conductive polymers. Because the PTC devices can be reset without physical access to them, the wiring system can be arranged more efficiently and more economically. For example, the size of at least some of the wires of the harness can in many case be reduced.

Description:
This application is a continuation of application Ser. No. 08/566,588 filed Nov. 30, 1995 now U.S. Pat. No. 5,645,746, the disclosure of which is incorporated by reference. Application Ser. No. 08/566,588 is a file wrapper continuation under 37 CFR 1.62 of application Ser. No. 08/110,593, filed Aug. 23, 1993 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to positive temperature coefficient (PTC) circuit protection devices. 
     2. Introduction to the Invention 
     PTC circuit protection devices are well known. Under normal operating conditions of the circuit, a PTC circuit protection device is in a low temperature, low resistance state. However, if a fault occurs (eg. if the current through the PTC device increases excessively, and/or the ambient temperature around the device increases excessively, and/or the normal operating current is maintained for more than the normal operating time), then the PTC device will be “tripped”, i.e. converted into a high temperature, high resistance state such that the current in the circuit is reduced to a safe level. Generally, the PTC device will remain in the tripped state, even if the fault is removed, until the device has been disconnected from the power source and allowed to cool. In a batch of PTC devices made by the same manufacturing process, uncontrollable variations in the process can cause substantial variation in the conditions which will trip any individual device. The largest steady state current which will not cause any of the devices in the batch to trip is referred to herein as the “pass current” (I PASS ) or “hold current”, and the smallest steady state current which will cause all of the devices to trip is referred to as the “trip current” (I TRIP ). In general, the difference between I PASS  and I TRIP  decreases slowly as the ambient temperature increases. Depending on the particular type of device, I TRIP  may for example be 1.5 to 2.5 times I PASS  at 20° C. For any individual device, the pass current and the trip current are the same. However, in this specification, reference is made to a PTC device having an I PASS  and a different I TRIP , because as a practical matter, the manufacturer of a wiring system must make use of PTC devices taken from a batch of such devices. Generally, the higher the ambient temperature, the lower the pass current and the trip current. This phenomenon is referred to as “thermal derating”, and the term “derating curve” is used to denote a graph of temperature against pass current. 
     Generally, the way in which pass current changes with ambient temperature i.e. the shape of the derating curve, depends mainly on the PTC material, while the pass current at any particular temperature depends also on the other factors which determine the resistance and the thermal transfer characteristics of the device. Thus, using a given PTC material, it is possible to make devices which have different pass currents but derating curves of the same shape. But to make a PTC device having a derating curve of a different shape, a different PTC material must be employed. 
     PTC circuit protection devices contain PTC elements which may be composed of a PTC conductive polymer or a PTC ceramic, eg. a doped barium titanate. PTC ceramics have been used commercially for several decades, but they are brittle and their resistivities are higher than is desirable. Over the last decade, PTC conductive polymers which do not suffer from these disadvantages have been developed, and the use of circuit protection devices based on them has increased rapidly. Reference may be made for example to U.S. Pat. Nos. 4,237,441, 4,255,698, 4,238,812, 4,315,237, 4,426,633, 4,780,598, 4,800,253, 4,907,340, and 5,089,801, and copending commonly assigned U.S. application Ser. Nos. 07/893,626, now abandoned, 07/894,119 now U.S. Pat. No. 5,378,407, and 07/910,950 now abandoned. The disclosure of each of those patents and applications is incorporated herein by reference for all purposes. 
     In automobiles and other road vehicles, there is always a risk that in one way or another, e.g. through an insulation failure, one or more of the insulated wires or other components of the electrical wiring system, e.g. connectors. splices, connection blocks, switches and operative devices, will be accidentally grounded (i.e. connected, e.g. via the chassis or bodywork of the vehicle, to the ground pole of the battery), thus creating a short circuit current which, if maintained for too long, can overheat the wire (or another component in the short circuit), and thus cause damage, e.g. as a result of melting the insulation on the wire. It is conventional, in order to prevent such damage, to protect each of the wires by a fuse which, if the wire is accidentally grounded, will blow before the wire can overheat. The fuses are generally placed in fuse boxes which are readily accessible, so that blown fuses can be easily replaced. The system may also contain one or more circuit breakers for substantially the same reason; however, circuit breakers generally cost more than fuses and tend to lose their calibration (i.e. the current required to trip them changes with time and usage). 
     The number of operative electrical devices in automobiles and other road vehicles has risen sharply over recent years, and is still increasing. For example, many automobiles now contain electric motors to control windows, seats and door locks, and a wide range of sensors and other diagnostic apparatus, as well as interior and exterior lights. With the large number of electrical devices now present in automobiles., it is impractical to have a separate fuse to protect the wiring for each device. Many of the fuses, therefore, must protect the wiring for a number of different devices. Such shared fuses must have a current-carrying capacity substantially greater than the sum of the maximum current requirement of each of those devices, taking into account also any transient inrush currents which any electric motors, lamps or other devices may draw when first switched on. The relatively cheap fuses conventionally used in automobiles have very rapid response times, and when using such fuses, transient inrush currents often determine the capacity of the fuse, which may need to be larger than that required to handle steady state currents. Consequently, it is necessary to use wires having a still greater current-carrying capacity, even in branches which serve only devices having much smaller current requirements, and even though the fuse capacity may be determined by inrush currents which would not damage the wire. 
     Even with such shared fuses, however, and even with two or more fuse boxes at different locations in the vehicle, it is a significant design problem to find appropriate, accessible location(s) for the fuse box(es) without taking up space which is needed for other purposes. In addition, the use of shared fuses results in disconnection of all the devices protected by that fuse, even if only the wiring to one of them is shorted out. Present wiring systems are, therefore, expensive; they are difficult to install and maintain; they add significantly to the weight of the vehicle and, therefore, have an adverse effect on fuel consumption; and they result in disconnection of groups of devices, whereas separate wiring for each of the devices would result in disconnection of only one of the devices. 
     It has been proposed to replace fuses in a conventional automobile wiring harnesses by PTC devices, in order to take advantage of the fact that a PTC device can be reset without physical access to it, thus making it possible to place the PTC devices in locations which are inaccessible (and which would not, therefore be satisfactory for conventional fuses); see published German Patent Application No. P 40 15 816. 
     SUMMARY OF THE INVENTION 
     I have now discovered, in accordance with the present invention, that when at least some of the electrical wiring in the wiring system of a vehicle is protected by PTC circuit protection devices, substantial advantages can be obtained by matching the time/current combinations which will trip the PTC devices to the time/current combinations which will damage the wiring or other components which they protect. For any particular type of wiring, therefore, it is usually possible to design a PTC device which will protect the wiring but, unlike a fuse, will not be tripped by transient inrush currents which will not damage the wiring. In particular, I have found that it is possible 
     to reduce the required current-carrying capacity of some of the wiring, and/or some of the switches, connectors, splices, connection blocks or other components of the system, with a consequent reduction in material costs and/or weight. 
     Similar advantages can be obtained in similar electrical systems, for example those installed in airplanes and ships. 
     The present invention differs from the known use of PTC circuit protection devices in automobiles in order to protect operative electrical devices, i.e. devices which are switched on by the vehicle operator in order to produce a predetermined effect. e.g. window lift motors. In that known use, the PTC device protects the operative device (eg. from overheating) and is usually very close to, or integrated into, the protected device; it does not provide protection against a grounding fault to the wiring which leads to the protected device, such protection being provided by a conventional fuse or circuit breaker (whose function, in accordance with the present invention, can be directly or indirectly replaced by one or more PTC devices). For the avoidance of confusion, a PTC device which is used, in accordance with the present invention, in order to protect wiring, a switch, or other component which is not an operative device, will be referred to herein as a branch PTC device or BPTC device; and a PTC device which is used to protect an operative device in the known way will be referred to herein as an appointed PTC device or APTC device. 
     An important part of this invention is my recognition of the benefits that can be obtained by appropriate matching of the properties of the BPTC device and of the wiring or other components that it protects. The current-carrying capacity of insulated wires (and other electrical components) depends upon the size and duration of the current and the ambient temperature. It also depends upon the extent of the change in the insulation which the user regards as tolerable. For example, softening of the insulation might be regarded as intolerable, i.e. as “damage”, for an insulated wire which was wrapped around a corner, but tolerable for the same wire passing through a straight duct. Users of insulated wires, e.g. manufacturers of automobile wiring harnesses, may for example stipulate that the temperature of the wire must not exceed a particular temperature, and this stipulation can be used to determine the maximum steady state current which the wire is permitted to carry at any particular ambient temperature, without damage to the insulation. In this specification, the term “damage current” is used to denote the smallest steady-state current which will cause “damage” to the insulated wire (or other electrical component), with damage being defined by criteria set by the manufacturer of the electrical system containing the wire. For example, in one case the damage current might be that which resulted in substantial softening of the insulation; in another case it might be that which caused fuming of the insulation; in another case it might be the maximum temperature which the wire is permitted to reach. The term “damage curve” is used herein to denote a graph of ambient temperature against damage current. For an insulated wire, the way in which the damage current changes with ambient temperature, i.e. the shape of the damage curve, depends mainly on the insulation material, while the damage current at any particular temperature depends also on the size of the wire and the thickness of the insulation. Thus, using a given insulation material, it is possible to make insulated wires which have different damage currents (usually determined in large measure by the size of the wire) but damage curves of the same shape. But to make an insulated wire having a damage curve of a different shape, a different insulating material must be employed. Generally speaking, the smaller the damage current of an insulated wire or other component, the cheaper and lighter it will be. 
     Appropriate correlation of the BPTC device and the insulated wiring (and other electrical components) requires that, throughout the temperature range of interest, e.g. −40° to +85° C. in most parts of automobiles, the pass current of the BPTC device is greater than the maximum possible steady state load current, and the trip current is less than the damage current of the wiring (and other electrical components). However, so far as possible, bearing in mind manufacturing procedures and tolerances, safety margins, and the need to accommodate transient currents higher than the maximum steady state load current, it is preferred that the margin between the maximum load current and the pass current should be small, and that the margin between the trip current and the damage current should also be small. In this way, the system can make use of lighter and/or less costly components. 
     The maximum load current in any part of a wiring system may decrease to some extent with increasing ambient temperature, but the extent of any such decrease is generally much less than the decrease with ambient temperature of the pass current of a BPTC device. The pass current of the BPTC device is, therefore, determined by the load current at the top end of the applicable ambient temperature range. With the PTC conductive polymers which are at present commercially available, the decrease in trip current with ambient temperature is substantially greater than the decrease in damage current with ambient temperature of insulated wires and other electrical components. Under these circumstances, the necessary characteristics of the insulated wire (or other component) are determined by the trip current of the BPTC device at the bottom end of the applicable ambient temperature range. Using improved PTC devices, with improved derating curves, smaller and/or lighter and/or less expensive components can be used. Thus the aim is (for any particular insulated wire) to use a BPTC device whose trip current derating curve is a little below the wire damage curve throughout the ambient temperature range of interest. 
     In a first aspect, this invention provides an electrical harness suitable for installation in a road vehicle, the harness comprising a plurality of branch circuits connected in parallel with each other, at least one of the branch circuits comprising 
     (a) an insulated wire having a gauge size of less than 18 AWG and, 
     (b) a PTC device which protects the insulated wire from damage by excessive currents. 
     In a second aspect, this invention provides an electrical harness suitable for installation in a road vehicle, the harness comprising a plurality of branch circuits connected in parallel with each other, at least one of the branch circuits comprising 
     (a) an insulated wire, 
     (b) a switch, and 
     (c) a PTC device which protects the insulated wire and the switch from damage by excessive currents; 
     said components (a), (b) and (c) being connected in series with each other, and the switch having a current-carrying capacity which is less than the current-carrying capacity of an 18 AWG insulated wire. 
     In a third aspect, this invention provides an electrical circuit which has a maximum steady state current under normal operating conditions I LOAD  and which comprises 
     (1) a source of electrical power; 
     (2) a PTC protection device having a pass current I PASS  and a trip current I TRIP  and containing a PTC element which is composed of a conductive polymer exhibiting PTC behavior; 
     (3) an operative electrical device which is spaced apart from the PTC protection device; and 
     (4) an insulated electrical wire which has a damage current I DAMAGE  and which connects the PTC device to the operative electrical device 
     and in which 
     (a) at each temperature within a range T 1  to T 2 , where T 1  is 0° C. and T 2  is 80° C., 
     (i) I PASS  is at least 1.1 times I LOAD  and 
     (ii) I DAMAGE  is at least 1.1 times I TRIP ; 
     (b) at at least one temperature within the range T 1  to T 2 , I PASS  is at most 1.5 times I LOAD ; and 
     (c) at at least one temperature within the range T 1  to T 2 , I DAMAGE  is at most 1.5 times I TRIP . 
     In a fourth aspect, this invention provides, in the electrical system of a road vehicle which includes 
     (1) a source of electrical power, and 
     (2) a plurality of branch circuits connected in parallel with each other, at least one of the branch circuits comprising 
     (a) an insulated wire, 
     (b) an operative device connected to the insulated wire, 
     (c) a switch, and 
     (d) a PTC device which has a trip current I TRIP  at 20° C. and which protects the insulated wire from damage by excessive currents at temperatures from −40° C. to +85° C.; 
     said components (a), (b), (c) and (d) being connected in series with each other; 
     the improvement which comprises matching the time/current combinations which will trip the PTC device to the time/current combinations which will damage the insulated wire or the switch, whereby at least one of 
     (i) the insulated wire, and 
     (ii) the switch 
     has a current-carrying capacity which is smaller than would be possible if the PTC device was replaced by a fuse having a rated current equal to I TRIP . 
     In a fifth aspect, this invention provides, in the electrical system of a road vehicle which includes 
     (1) a source of electrical power, and 
     (2) a plurality of branch circuits connected in parallel with each other, at least one of the branch circuits comprising 
     (a) an insulated wire, 
     (b) an operative device connected to the insulated wire, 
     (c) a switch, and 
     (d) a protective device which protects the insulated wire from damage by excessive; 
     said components (a), (b), (c) and (d) being connected in series with each other; 
     the improvement which comprises using, as the protective device, a PTC device which is tripped by selected time/current combinations, and matching the time/current combinations which will trip the PTC device to the time/current combinations which will damage the insulated wire or the switch, whereby at least one of 
     (i) the insulated wire, and 
     (ii) the switch 
     has a current-carrying capacity which is smaller than would be possible if the PTC device was replaced by a fuse whose capacity is determined by inrush currents which would not damage the insulated wire. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is illustrated in the accompanying drawings, in which 
     FIG. 1 is a conceptual representation of a part of an automobile electrical system of the known type; 
     FIG. 2 is a conceptual representation of a part of an automobile electrical system of the present invention corresponding to FIG. 1; 
     FIG. 3 is a conceptual representation of the physical lay-out (or “architecture”) of an automobile electrical system of the known type; 
     FIG. 4 is a conceptual representation of the architecture of an automobile electrical system of the present invention corresponding to FIG. 3; 
     FIG. 5 is a conceptual circuit diagram of an automobile electrical system of the known type; 
     FIG. 6 is a conceptual circuit diagram of an electrical system according to the invention which corresponds to the known system of FIG. 5; 
     FIG. 7 is an illustration of a module for use in a system according to the invention which contains BPTC protection devices and which can be connected to a standard ISO junction box in an automobile electrical system; 
     FIGS. 8 and 9 are illustrations of junction boxes for use in a system according to the invention which contain BPTC protection devices and which can form part of an automobile electrical system; 
     FIG. 10 illustrates BPTC devices which can be used in an automobile electrical system of the invention; and 
     FIG. 11 shows a part of an electrical system of the invention which comprises a principal node and a sub-node; 
     FIGS. 12 and 13 show a node in an electrical system of the invention in which a power input line is connected to a plurality of branch circuits through a corresponding plurality of electrical devices, namely silicon-controlled rectifiers in FIG.  12  and relays in FIG. 13; 
     FIGS. 14,  15  and  16  show parts of systems of the invention in which the voltage on a line connecting a branch PTC device and an operative device is sensed and is used to indicate if the device is (or has been) in a tripped state; 
     FIG. 17 shows the relationship between ambient temperature and the steady state currents which can be tolerated by various circuit components, and 
     FIG. 18 shows the relationship between time and the transient currents which can be tolerated by various circuit components. 
     In the drawings, the abbreviation OP DEV is used to denote an operative device, and the abbreviation JNCT BOX is used to denote a junction box. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The system of the invention contains a plurality of branch circuits, at least two of which, and preferably many or even all of which, are PTC-protected branch circuits. The other branch circuits (if any) will normally be protected by other means, e.g. a fuse or a circuit breaker. Provision must be made for disconnecting the BPTC device from the power supply. This is preferably done by means of a switch which can be operated to disconnect at least one of the PTC-protected branch circuits. The term switch is used herein in a broad sense to denote any electrical device which achieves connection and disconnection, including make-and-break switches, relays, field effect transistors (FET&#39;s), including MOSFETS, JFETS, LFETS, and PFETS, bipolar devices including Darlingtons, pnp transistors and npn transistors, and silicon-controlled rectifiers (SCR&#39;s). An advantage of the present invention is that the presence of BPTC device often reduces the maximum current to which the switch can be exposed, and thus makes it possible to use a cheaper and/or smaller switch. Especially is this so when a single fuse has been replaced by two or more BPTC devices. It is often convenient for a switch and the BPTC device(s) which the switch controls to be located within a switch module, preferably within an insulating container (e.g. a box composed of an insulating polymer) which can be secured to the bodywork of the vehicle. The switch module can of course contain a plurality of such switches and BPTC devices. 
     The system will often contain at least one power distribution node which comprises a power input line and a plurality of outgoing lines, at least one of said outgoing lines, and preferably many or all of said outgoing lines, being electrically connected to the input line through a BPTC device. The number of BPTC devices can be the same as or less than the number of outgoing lines. The connections between the input lines and the outgoing lines can be made through a printed circuit board, a flexible printed circuit, a metal frett, or otherwise. The BPTC device is preferably mounted on, e.g. soldered or welded to, the circuit board, flexible printed circuit or metal frett. The BPTC devices and the connections between them and the input line and the outgoing lines are preferably contained within a container, particularly a container which is made of insulating material and which can be reopened and reclosed. The container can comprise vents and/or thermal insulation (inside and/or outside the container) which affect the time which the PTC device(s) take to trip, and which also help to ensure that the PTC device(s) therein do not trip as a result of overheating which has nothing to do with the electrical system. The BPTC devices can be offset so as to reduce the extent to which heat is transferred between them. For example if the BPTC devices are planar, they can be mounted parallel to each other and positioned, if space permits, so that adjacent devices overlap, when viewed at right angles to the plane of the devices, by at most 75%, preferably at most 50%, of the area of the smaller device (if their areas are different), for example so that they do not overlap at all. 
     The branch PTC-protected circuits can be connected to the supply connection directly or through one of a plurality of subsidiary supply means which are electrically connected to the principal supply means in parallel with each other. These subsidiary supply means make it possible to avoid a multiplicity of wires which have to be routed over relatively long distances, especially when the operative devices are controlled by relays which respond to signals generated at a different location. Often at least one of said subsidiary supply means 
     (i) contains a subsidiary circuit protection device which is the weakest current-carrying point in the subsidiary supply means and which is converted into a high resistance state if the current in the subsidiary supply means exceeds a value I SUB  amps and thus reduces the current to a value substantially below I SUB , and 
     (ii) is connected to a plurality of PTC-protected branch circuits, in each of which I PTC  is less than I SUB . 
     Like the systems disclosed in published German Patent Application No. P 40 15 186, the systems of the present invention provide greater freedom to design the physical and electrical layout of the system (the so-called “system architecture”). Thus in a conventional system, the requirement for accessibility of fuse boxes generally sets a practical upper limit of five or less on the number of locations at which fuse boxes are located (there may of course be two or more fuse boxes adjacent to each other at a single location). In the systems of the invention, by contrast, the BPTC devices can be at a larger number of spaced-apart locations, for example at at least four spaced-apart locations, e.g. at six or more spaced-apart locations, which are typically separated from each other by a distance of at least 50 cm, with at least one, often at least two, of the locations being separated from at least one of the other locations by a distance which is at least 250 cm, often at least 300 cm. Through use of the present invention, at least some of the distances between the operative devices and the power distribution nodes which feed current to them can be much smaller than in a conventional system. Thus it is preferred that in systems of the present invention, if there are p PTC-protected branch circuits, then in at least 0.5 p, particularly at least 0.75 p, of those circuits, the length of the PTC branch connection means is less than 300 cm, more preferably less than 200 cm, particularly less than 100 cm. 
     Also as noted previously, another advantage which can result from the present invention is a reduction in the needed current-carrying capacity of some of the wiring or other components (e.g. switches, connectors, splices and connection blocks) of the system. This advantage is obtained by correlating the characteristics of the BPTC device in a PTC-protected branch circuit with the characteristics of the other components and the ambient temperature of the air surrounding the electrical system, under both steady state and transient conditions. At all temperatures at which the system may operate, the pass current of the BPTC device in a PTC-protected branch circuit must be more than the maximum steady state current drawn by the operative device(s) in that branch circuit, and the trip current of the BPTC device must be less than the steady state current which will cause substantial damage to the wire insulation (or any other component of the system) in that branch circuit. In addition (again at all temperatures at which the system may operate), the BPTC device must be converted to its tripped state in a time which is sufficiently short that the currents in the branch circuit, during the time it takes to trip the BPTC device, will not cause substantial damage to the wire insulation (or any other component of the system). Fulfillment of these conditions does not, however, necessarily make it possible to achieve the objective of reducing the cost and/or weight of the wiring or other electrical components of the system. To do this, it is necessary to select a BPTC device whose pass current (I PASS ) is not excessive having regard to the maximum expected load current (I LOAD ), and to select wiring (and/or other electrical components) whose damage current(s) (I DAMAGE ) is not excessive having regard to the trip current of the BPTC device (I TRIP ). The minimum level for I PASS  is set by the need to ensure that the system is not subject to an unacceptable level of “nuisance tripping”, i.e. tripping by transient or steady state currents which would not damage the wiring or other components of the system. Once this need has been met, however, any further increase in I PASS  increases the minimum current-carrying capacity which is required in the wiring and other components. It is preferred, therefore, that at each temperature within the applicable ambient temperature range, e.g. 0° to 80° C., or −40° to +85° C., I PASS  is at least 1.1 times, particularly at least 1.2 times, I LOAD , and that at at least one temperature in the range, I PASS  is at most 1.3 times, preferably at most 1.1 times, I LOAD . The minimum current-carrying capacity of the wiring and other components is set by the value of I TRIP , which is higher than, but corresponds to I PASS . Since a margin of safety is desirable, it is preferred that at all temperatures within the applicable ambient temperature range, I DAMAGE  is at least 1.1 times, particularly at least 1.2 times I TRIP . However, once a margin of safety exists, there is no reason, electrically speaking, to use wiring or other electrical components which are capable of carrying currents substantially higher than those which will trip the BPTC device, and in general, the less the current-carrying capacity of an electrical component, the cheaper and/or lighter it will be. In many cases, therefore, it is preferred to use wiring and/or other electrical components having, at at least one temperature within the applicable ambient temperature range, an I DAMAGE  which is at most 1.5 times, particularly at most 1.3 times, I TRIP . However, it should be noted that in some cases, insulated wire having such an I DAMAGE  is of such small size that it may require special handling techniques. 
     In existing road vehicle systems (in which the power source is a battery, usually of a nominal 12 or 24 volts, for example an actual 12-14.7 or 24-29 volts, and the return connection means may be a separate wire and/or structural elements of the vehicle), insulated wires of 18 AWG or more are often used throughout the system. The insulation employed is often polyvinyl chloride (PVC) or crosslinked polyethylene, but more expensive insulation materials, e.g. polyesters and fluoropolymers, are sometimes used, particularly on wires which are likely to become very hot or to be exposed to gasoline or transmission fluids or other fluids which might damage conventional insulation. Through use of the present invention it is possible to replace at least some of the wiring by wires of a smaller gauge, e.g. 22 gauge or even less, and/or by wires having cheaper or thinner insulation. 
     Preferred systems of the present invention comprise at least one PTC-protected branch circuit in which at all temperatures between 0° and 80° C., preferably at all temperatures between −40° and +85° C., I DAMAGE  is equal to or lower than I 22 , where I 22  is the I DAMAGE  of a conventional PVC insulated 22 AWG copper wire (cross-sectional area 0.324 mm 2 ). Preferably the system contains a plurality of such PTC-protected branch circuits. Thus in one embodiment, the system contains a total of p PTC-protected branch circuits, and at least 0.1 p, particularly at least 0.2 p, especially at least 0.5 p, of those circuits have PTC-protected branch connection means having an I DAMAGE  which is equal to or less than I 22 , for example PTC-protected branch connections means which consist of or contain 22 AWG insulated wires. In another embodiment, the total length of the PTC-protected branch connections means in which I DAMAGE  is greater than I 22  is not more than 10 times, preferably not more than 5 times, particularly not more than 2 times, the total length of the PTC protected branch connection means in which I DAMAGE  is equal to or greater than I 22 . 
     An operative device in a PTC-protected branch circuit can be protected against damage by an APTC device which is connected in series with the operative device and is adjacent to (or forms a part of) that operative device. The APTC device can be tripped by excessive temperature or excessive current (including a current which lasts for an excessive time) or both. 
     The systems of the invention can also include diagnostic means which will indicate when a BPTC device is in a tripped condition and/or if it has been in a tripped condition but has been restored to a normal operating condition. Such diagnostic means preferably comprises a voltage sensor which senses the voltage across the BPTC device or at a point on the PTC-protected branch connection means. When the PTC-protected branch circuit is connected to the power supply, this voltage will be high (generally equal to the voltage of the power supply) under normal conditions but will drop sharply if the BFTC device is tripped. The sensor can for example generate a visible light signal, e.g. in a light-emitting diode (LED), or an electronic signal which can be fed to a computer. When there are a plurality of voltage sensors, the system can include means for sequentially scanning the sensors. 
     The BPTC devices used in this invention preferably comprise a PTC element which is composed of a conductive polymer (i.e. a composition which comprises a polymeric component and, dispersed in the polymeric component, a particulate conductive filler, preferably carbon black). PTC circuit protection devices based on PTC conductive polymer elements have become increasingly popular over the last ten or twelve years, as continuing developments have made it possible to reduce the size and resistance of such devices, while increasing their reliability and versatility to meet a wide range of physical and electrical requirements. The BPTC devices used in this invention generally have a resistance of less than 3 ohm, preferably less than 1 ohm, especially less than 0.25 ohm, at 23° C. Preferably they comprise two laminar electrodes, generally metal foil electrodes, with a laminar PTC element sandwiched between the electrodes. For further details of suitable BPTC devices, reference should be made to U.S. Pat. Nos. 4,237,441, 4,238,812, 4,315,237, 4,329,726, 4,426,633, 4,685,025, 4,689,475, 4,774,024, 4,800,253, 4,935,156, 5,089,801 and 5,178,797, the disclosures of which are incorporated herein by reference. 
     Referring now to the drawings, FIG. 1 is a conceptual representation of a part of an automobile electrical system of known type. The wiring which serves operative devices  101 ,  102  and  103  is protected by fuse  12 . Similarly, the wiring for devices  104 - 106 ,  107 - 109 , and  110 - 112 , is protected by fuses  13 ,  14 , and  15  respectively. All of the fuses are within a single, easily accessible fuse box, as indicated by the dashed lines, even though operative devices  107 - 109  and  110 - 112  are located a considerable distance from the fuse box. 
     FIG. 2 shows a part of a system of the invention which is similar to that shown in FIG. 1 but in which fuse  12  has been replaced by three branch PTC devices (“BPTC devices”)  201 ,  202 , and  203 . BPTC device  201  protects only the wiring which serves operative device  101 ; BPTC device  202  protects only the wiring which serves operative device  102 ; and BPTC device  203  protects only the wiring which serves operative device  103 . Fuses  14  and  15  have been replaced by BPTC devices  204 - 206  and  207 - 209  respectively, each BPTC device protecting the wiring for one of operative devices  107 - 112 . Furthermore, the BPTC devices  204 - 209  are placed in a junction box (indicated by the dotted and dashed lines) which need not be easily accessible (since the BPTC devices are remotely resettable after correction of a fault, as opposed to conventional fuses, which must be manually replaced) and which can therefore be strategically placed for economy and convenience in manufacturing. 
     FIG. 3 is a conceptual representation of the physical lay-out (“architecture”) of an automobile electrical system of the known type. Battery  1  is connected by an unbranched supply connection means  7  to a fuse box  9  and by return connection means  8  to the bodywork of the automobile. The fuse box is easily accessible. A plurality of wires run from the fuse box to locations all around the automobile, each wire serving a single operative device or a relatively small number of operative devices. 
     FIG. 4 shows a system of the invention which is similar to that shown in FIG. 3, but in which the supply connection means  7  is replaced by a plurality of branches  71 ,  72 ,  73 ,  74 ,  75 , etc., which are severally connected to junction boxes  91 ,  92 ,  93 ,  94 ,  95 , and  96 . Wires run from each junction box to operative devices located nearby. Within each junction box, the wires are connected through BPTC devices to a branch of the supply connection means. Because the BPTC devices operate remotely and do not need to be replaced, the junction boxes do not need to be easily accessible, and are placed at strategic locations, i.e. locations selected for ease of installation of the boxes and the wiring which runs to and from them, and for reduction of the amount of wire employed, having regard to the location of the operative devices served through a particular junction box. 
     FIG. 5 is a conceptual circuit diagram of an automobile electrical system of known type. A battery  1  is placed at the front of the car and is connected by return connection means  8  to the bodywork of the car, and by supply connection means  7  to the remainder of the circuit. Fuse link  2  and ignition switch  3  form part of the connection means  7 . The supply connection means  7  is connected to the rest of the circuit through fuses  11 ,  12 ,  13 ,  14 ,  15 ,  16  and  17 , all of which are placed within a readily accessible fuse box indicated by the dashed lines in the Figure. Fuse  11  protects the wiring which leads to operative devices  101 ,  102  which are always on when the ignition switch  3  is on (e.g. sensors); it also protects the wiring which lead to operative devices  114  and  115  (e.g. electric motors) which are controlled by switch  160  and which are protected from by excessive currents by appointed PTC devices (“APTC devices”)  173  and  174 . Fuse  12  protects wiring which leads to operative devices  106  and  107  at the front of the automobile and  117  and  118  at the rear of the automobile (e.g. parking lights) which are controlled by switch  150 . Fuse  12  also protects wiring which leads to operative device  103  (e.g. a horn) which is controlled by switch  151  and protected by APTC device  170 . Fuse  13  protects wiring which leads to operative devices  104  and  105  (e.g. headlights) which are controlled by switch  152 . Fuse  14  protects wiring which leads to operative devices  108  and  109  at the front of the automobile and operative devices  119  and  120  at the rear of the car (e.g. flashing emergency lights), all of which are controlled by switch  153 . Fuse  14  also protects wiring which leads to operative device  121  (e.g. a rear window heater) which is controlled by switch  154  and protected by APTC device  175 . Fuse  15  protects the wiring which leads to operative device  122  (e.g. a rear window wiper) at the rear of the automobile. Fuse  16  protects the wiring which leads to operative devices  123 ,  124  and  125  in the rear of the automobile, device  123  (e.g. a trunk light) being controlled by switch  162  and devices  124  and  125  (e.g. rear courtesy lights) being controlled by switch  163 . Fuse  16  also protects the wiring which leads to operative  110  and  111  (e.g. seat motors) which are controlled by switches  156  and  157  respectively. Fuse  16  also protects the wiring which leads to operative device  116  (e.g. front courtesy lights) which is controlled by switch  161 . Fuse  17  protects the wiring which leads to operative devices  112  and  113  (e.g. window lift motors) which are controlled by switches  158  and  159  respectively and protected by APTC devices  171  and  172  respectively. 
     The size of the wires in FIG. 5 is indicated by the number of parallel lines used to depict the wires. Thus the supply connection means has the largest size; the wiring to the operative device  122  has the smallest size; and all the other wiring has an intermediate size. The wiring to the operative device  122  can be of the smallest convenient size for the current requirements of the operative device  122 , because it is the only wiring protected by the fuse  15 , which has a current-carrying capacity corresponding to that device alone. The wiring and fuses leading to the other operative devices must be of larger size because they are determined by the sum of the requirements of the operative devices which they serve. 
     FIG. 6 is a conceptual circuit diagram of an electrical system according to the invention which corresponds to the known system of FIG. 5, but in which many of the fuses have been replaced by branch PTC devices (“BPTC devices”), and in many cases the wiring has been replaced by lower gauge wire. Fuse  11  has been replaced by three BPTC devices, BPTC device  209  protecting the wiring to operative devices  114  and  115 , BPTC device  210  protecting the wiring to operative device  102 , and BPTC device  211  protecting the wiring to operative device  101 . Fuse  12  is unchanged. Fuse  13  has been replaced by BPTC device  203  having a pass current lower than the current which would blow fuse  13 , because the BPTC device responds slowly to the transient currents generated when the headlights  104 ,  105  are first switched on. Fuse  14  has been replaced by two BPTC devices, BPTC device  212  protecting the wiring to operative devices  119  and  120  and BPTC device  213  protecting the wiring to operative device  121 . Fuse  15  has been replaced by a single BPTC device  201 . Fuse  16  has been replaced by four BPTC devices, BPTC device  204  protecting the wiring to operative devices  116 ,  123 ,  124  and  125 . BPTC device  205  protecting the wiring to operative device  110 , BPTC device  206  protecting the wiring to operative device  111 , and BPTC device  207  protecting the wiring to operative device  112 . Fuse  17  has been replace by BPTC device  208  protecting the wiring to operative device  113 . 
     As in FIG. 5, the size of the wires is indicated by the number of parallel lines used to depict the wire. Where a fuse has been replaced by a number of BPTC devices, the size of the wiring protected by the BPTC devices can be reduced to a smaller size, because the number of operative devices served by each branch of the wiring has been reduced. 
     FIG. 7 is an exploded schematic view of a module containing three BPTC devices  201 ,  202  and  203 . The module contains a base  1  and a vented cap  2  which is a snap fit on the base. The BPTC devices are mounted on a printed circuit board  3  having traces thereon which connect the BFTC devices to prongs  4  which extend outwards from the base and which fit the sockets of a standard ISO junction box. 
     FIG. 8 is a cross-sectional schematic view of a junction box containing a power distribution node in which a power input line  7  feeds outgoing lines  71 ,  72  and  73  through BPTC devices  201 ,  202  and  203  which are mounted inside a container  2 . 
     FIG. 9 is a cross-sectional schematic exploded view of a junction box containing a power distribution node. The box comprises a first cap section  81  in which power input line  7  is split into five terminals and a second cap section  82  in which five outgoing lines  71 ,  72 ,  73 ,  74  and  75  are formed with terminals. The two caps fit onto opposite ends of a center section  10  which contains five BPTC devices  204 ,  205 ,  206 ,  207 , and  208 , thus connecting the power input line to the outgoing lines through the BPTC devices. 
     FIG. 10 shows part of the base  5  of a conventional fuse box which comprises sockets  51 ,  52  for fuses. Fitted into the sockets are BPTC devices  201 ,  202  and  203 , which comprise laminar metal foil electrodes  81 ,  82  with a laminar conductive polymer PTC element  83  sandwiched between them. Leads  84  and  85  are soldered to foils  81  and  82  respectively at one end of the BPTC device and fit into the sockets  51 ,  52 . At the opposite end of the device, insulating cap  86  fits over the PTC element and the metal foils so that the device can be easily handled. 
     FIGS. 11,  12  and  13  are circuit diagrams for power distribution nodes between a power input line  7  and outgoing lines  71 ,  72 , and  73  which are individually protected by BPTC devices  201 ,  202  and  203 . In FIG. 11, outgoing line  73  extends to a sub-node between it and two outgoing lines  731  and  732  which are protected by BPTC devices  2031  and  2032 , and lines  71  and  72  are controlled by make-and-break switches  150  and  151  respectively. In FIG. 12, lines  71  and  72  are controlled by bipolar transistors  165  and  166 . In FIG. 13, lines  71  and  72  are controlled by relay switches  167  and  168  respectively. 
     FIGS. 14,  15  and  16  are circuit diagrams for parts of systems in which a voltage sensor is used to sense the voltage on a line connecting a BPTC device to an operative device. An outgoing line  71 ,  72 ,  73  or  74  is connected through a BPTC device  201 ,  202 ,  203  or  204  to a power input line  7 , and extends outwards to an operative device  117 ,  118 ,  119  or  120 . Between the BPTC device and the operative device, a voltage sensor line is connected to the outgoing line. In FIG. 14, the voltage sensor line is connected to a logic circuit  710  which sends a logic signal to a computer  711  which processes the signal and indicates when the voltage on the line  71  drops below a certain level because the BPTC device  201  has tripped. In FIG. 15, the voltage sensor line is connected through a light emitting diode (LED) to the power input line  7 ; the LED lights when the voltage on the line  71  drops below a certain level. FIG. 16 is similar to FIG. 14 but shows a plurality of outgoing lines  71 ,  72 ,  73  and  74  and a corresponding number of logic circuits  710 ,  720 ,  730  and  740  which feed signals to a clock-controlled shift register  715 . The shift register produces a serial date stream indicating the state of the BPTC devices. 
     FIG. 17 shows the relationship between ambient temperature and (a) the steady state load current, I LOAD , in a typical branch circuit of about 4 amps (b) the pass current, I PASS , and the trip current, I TRIP , of a typical BPTC device in which the PTC element is a conductive polymer containing polyethylene, and (c) the damage current for typical PVC-insulated wires of 18 AWG, 20 AWG and 22 AWG. It will be seen that at all ambient temperatures between −40° and about 75° C. the BPTC device will not be tripped by I LOAD . Above about 75° C. the device will be tripped by I LOAD , even though the insulated wires will not be damaged by I LOAD  unless the ambient temperature is over 90° C. (so-called “nuisance tripping”, because a normal load current trips the device although it is not necessary to do so to protect the circuit). It will also be seen that at all ambient temperatures between −40° and about 90° C., the BPTC device will protect the 18 AWG wire (since it will be tripped by fault currents which would damage the 18 AWG wire). However, it protects the 20 AWG wire only between about −20° and about 90° C., and the 22 AWG wire only between about 50° and 85° C. 
     As is made clear by FIG. 17, if the thermal derating curve had a lower slope, or if the spread between I PASS  and I TRIP  was lower, this would reduce the required wire size still further. PTC conductive polymers based on polyvinylidene fluoride typically have better thermal derating characteristics than those based on polyethylene. 
     Also shown on FIG. 17 is the current required to blow a fuse (I FUSE ) of the kind typically used in the prior art to protect a circuit having a steady state load current of about 4 amps but which is subject to much higher transient inrush currents. I FUSE  is for example about 12 amps because the fuse must accept those inrush currents. This in turn means that it is necessary to use 18 AWG wire if the ambient temperature may be greater than about 80° C. 
     FIG. 18 shows diagrammatically the way in which the relationship between time. (on a log scale) and (a) the load current in a circuit containing a motor or lamp or other device drawing an inrush current, (b) the current which will blow a fuse of the kind used in the prior art in such a circuit (I FUSE(1) ), (c) the current which will blow a fuse having, under steady state conditions, a current-carrying capacity similar to a PTC device (I FUSE(2) ), (d) the transient currents need to damage insulated 18, 20 and 22 AWG wires, and (e) the transient currents which will always be passed by (I PASS ) and which will always cause tripping of (I TRIP ), the devices in a typical batch of a particular type of PTC device. It will be seen that the very rapid response of the fuses to transient currents means that a fuse which will pass normal transient inrush currents will also pass a relatively high steady state current (well above the steady state load current, as in FIG.  17 ), and that this in turn means that the wiring in the circuit must be of 18 AWG size, or larger diameter. The PTC device, by contrast, reacts much more slowly than a fuse, and therefore permits the use of a wire of 20 AWG or even 22 AWG. A fuse having, under steady state conditions, a current-carrying capacity similar to the PTC device, will be blown by transient inrush currents which do not trip the PTC device.