PTC element

A PTC element has at least one body and two electrodes. The body or bodies has/have and is/are two parallel surfaces, of an electrically conductive polymer composition with a positive temperature coefficient. At least one of the parallel surfaces on the body, or on one of the bodies, of polymer composition is in free contact with an electrode or with a parallel surface on another body of electrically conductive polymer composition. A pressure device exerts a pressure directed perpendicularly to the parallel surfaces on the body, or the bodies, on the electrodes. The pressure device is preferably provided with pressure-exerting devices with the ability to be resilient. After changing from a low resistance to a high resistance state, the PTC element returns to the initial resistance and is reusable after having been subjected to short-circuit currents. The parallel surfaces on the body, or the bodies, of polymer composition may be concentric.

BACKGROUND AND SUMMARY OF THE INVENTION 
A PTC element often comprises a body of an electrically conductive polymer 
composition a resistivity with a positive temperature coefficient, the 
body defining two parallel end surfaces, and two electrodes arranged in 
contact with the end surfaces for carrying current through the body. The 
polymer composition of the body includes a polymer material and an 
electrically conductive powdered material distributed in the polymer 
material. The expression PTC element is the accepted term for an element 
whose resistivity has a positive temperature coefficient. PTC elements are 
used in electric circuits as overcurrent protection. 
The resistance of a PTC element of the above-described kind is low, for 
example a few m.OMEGA., in the normal operating range of the element, 
which may extend to, for example, 80.degree. C. and increases slightly 
with the temperature. If the temperature of the element exceeds this 
value, for example because of an overcurrent, the resistance increases 
more rapidly, and when exceeding a certain temperature, the element 
suddenly changes from a low resistance to a high resistance state in which 
the resistance may amount to one or a few tens of k.OMEGA.. It is 
well-known that the resistance of the PTC element, after changing from low 
resistance to high resistance state, does not return to the initial 
resistance. In more serious cases when it is subjected to very great 
electrical stresses, such as short-circuit currents, blistering and crack 
formation occur in central or other parts of the polymer composition of 
the PTC element, so that the the PTC element can no longer function, that 
is, the element becomes destroyed. 
In the known PTC elements the aim has been to secure the electrodes as 
efficiently as possible to the body of polymer composition to achieve the 
best possible electrical contact and hence minimize the contact 
resistance. To achieve the good securing of the electrodes, they are 
normally formed with an uneven surface structure on the side facing the 
body of polymer composition, so that the polymer composition during the 
manufacturing of the PTC element is able to melt and penetrate into 
cavities in the electrode surface. Usually, the electrodes consist of 
metal foils and are applied by being pressed onto the body with heating. 
According to the present invention it has proved to be possible to 
counteract or completely eliminate the abovedescribed unfavourable 
processes during operation of the PTC elements and to produce a PTC 
element which, after transition from a low resistance to a high resistance 
state, returns to the initial resistance, and which is reusable also after 
having been subjected to short-circuit currents. Further, the PTC element 
changes its resistance in case of a lower energy development, that is, its 
current limiting properties are improved. 
According to the invention, the above result is obtained by producing at 
least one electrical contact between an electrode and a body of 
electrically conductive polymer composition or between two bodies of 
electrically conductive polymer composition in the PTC element by the 
electrode making free contact with the mentioned body or that the 
mentioned bodies make free contact with each other while maintaining a 
pressure directed perpendicular to the contact surface in question by 
means of a pressure device. 
More particularly, the present invention relates to a PTC element 
comprising a body, provided with two parallel surfaces, of an electrically 
conductive polymer composition with a positive temperature coefficient, in 
which the parallel surfaces on the body of polymer composition or one of 
the parallel surfaces on the body of polymer composition and one of the 
parallel surfaces on another body included in the PTC element or on one of 
several other bodies included in the PTC element and arranged 
geometrically parallel to the first-mentioned body, which latter body or 
bodies, respectively, is/are of the same or a different electrically 
conductive polymer composition with a positive temperature coefficient, 
are arranged in electrical contact with electrodes, included in the PTC 
element, for carrying current through the body, or bodies, of polymer 
composition, characterized in that the body of polymer composition, if 
only one such body is included in the PTC element, with at least one of 
its parallel surfaces is adapted to make free contact with one of the 
electrodes, or at least one of the bodies of polymer composition, if more 
than one such body is included in the PTC element, with at least one of 
its parallel surfaces is adapted to make free contact with one of the 
electrodes or with the other body, or one of the other bodies, of polymer 
composition, and that a pressure device is adapted to exert on the 
electrodes a pressure directed perpendicular to the parallel surfaces on 
the body, or bodies, of polymer composition. The expression "to make free 
contact" here and in the following means that the respective elements 
making free contact are not fixed to each other but make contact with each 
other only by abutment. 
A feasible explanation of the result obtained according to the invention 
may be the following. During normal passage of current, a low contact 
resistance is maintained between the elements which make free contact with 
each other due to the pressure exerted on the contact surface. In case of 
short-circuit currents, electrodynamic repulsion forces occur between the 
elements making free contact with each other, which leads to a separation 
of the elements and hence a reduction of the number of contact points 
between electrode and conducting particles in the body of the polymer 
composition making free contact therewith or between conducting particles 
in bodies of the polymer composition making free contact with each other. 
This leads to a current concentration at the remaining contact points, 
which causes molten phases to occur in the polymer composition at the 
contact surface and the PTC element to trip at the contact surface without 
the rest of the polymer composition being subjected to any stress with 
ensuing unfavourable effects. Since the pressure against the contact 
surface remains, when the surface is still hot after the short circuit, 
the original contact and the original contact resistance may be 
reestablished between the elements making free contact with one another. 
The polymer composition may be of a known kind and its composition or 
constituents constitute no part of the present invention. Thus, the 
polymer material may consist of thermoplastic resins, elastomers, 
thermosetting resins or mixtures thereof used in prior art polymer 
compositions with PTC behaviour. As examples of suitable polymer materials 
may be particularly mentioned polyolefins such as polyethylene, 
crosslinked polyethylene, polypropylene, polybutene and copolymers or 
ethylene and propylene. The polymer material preferably has a 
crystallinity of at least 5%. The conducting powdered material preferably 
consists of conducting carbon black or conducting soot. However, it is 
possible, per se, to use, together with or instead of the mentioned 
powdered material, conducting particles of another kind, such as particles 
of metallic materials, for example nickel, cobalt, copper and silver. A 
suitable particle size of the powdered material is 0.01-10 .mu.m and a 
suitable content of the powdered filler is 10-60 percent of the total 
volume of the powdered filler and the polymer material. The resistivity of 
the polymer composition is preferably within the range of 10 
m.OMEGA.cm-100 .OMEGA.cm and has the ability, after a transition, to 
exhibit a resistivity of 1 .OMEGA.cm-1 k.OMEGA.cm. If more than one body 
of electrically conductive polymer composition is included in the PTC 
element, the bodies may be of the same or different polymer composition 
and then with the same or different resistivity. 
In those embodiments of the present invention in which at least one of the 
electrodes is firmly secured to a body of conducting polymer composition, 
such an electrodes or such electrodes may be of a conventional kind. They 
may consist of metal foils or of thin metal plates or metal nettings which 
are rolled on the polymer composition in heated state or fixed in some 
other way thereto, such as by spraying or vapour deposition. Usually, the 
surface facing the polymer composition has an irregular structure. The 
electrodes may also consist of a combination of two or more elements, for 
example a thin foil of nickel and a thicker plate of copper fixed thereto. 
According to one embodiment of the present invention, such fixed 
electrodes may consist of a tight plate or foil or metal, which on the 
side facing the polymer composition is coated with a layer of metal with 
an irregular surface structure, applied by thermal spraying such as plasma 
spraying, flame spraying or arc spraying, which metal layer comprises 
protruding portions with a height of 1-50 .mu.m and a width of 1-50 .mu.m. 
In those embodiments of the present invention where at least one of the 
electrodes makes free contact with a body of conducting polymer 
composition, such an electrode may in itself, that is, apart from the fact 
that it is not fixed to the body of conducting polymer composition, be of 
the same kind as indicated in the preceding paragraph. It may also consist 
of a more or less thick plate with a surface with an even structure facing 
the body of conducting polymer composition. It is also possible, per se, 
to use other high conductivity materials than metallic materials in the 
electrodes, such as polymer material containing very high contents of 
electrically conductive powdered fillers, such as conducting carbon black, 
conducting soot, copper or nickel. 
The pressure which is maintained on the electrodes perpendicularly to the 
parallel surfaces on the body, or bodies, of polymer composition 
preferably amounts to at least 0.1 MPa (14.7 psi). Especially preferred is 
a pressure of 0.1 MPa-10 MPa (14.7 to 1470 psi). 
The pressure on the electrodes of the PTC element may be achieved purely 
mechanically or by utilizing forces generated by electric currents. When 
utilizing purely mechanical forces, the pressure device may, for example, 
consist of two plates arranged in parallel with the parallel surfaces of 
the body, or bodies, of electrically conductive polymer composition and 
arranged outside the electrodes, and of drawing devices or clamping 
devices arranged in or adjacent the plates. When utilizing forces 
generated by electric currents, the pressure device may comprise plates of 
high conductivity material making contact with the electrodes, in which 
the current paths are adapted to be substantially parallel to the parallel 
surfaces of the body, or bodies, and directed in substantially the same 
direction as the plates, or yokes of a ferromagnetic material arranged 
outside the plates of high conductivity material. 
According to a particularly preferred embodiment of the invention, the 
pressure device is provided with pressure-exerting devices with the 
ability to be resilient. Such a design of the pressure device 
significantly facilitates a separation of the electrode and the body of 
polymer composition making free contact with each other, or of bodies of 
polymer composition making free contact with each other, at short-circuit 
currents. In a preferred embodiment the pressure device comprises two 
pressure-exerting parts, making contact with the electrodes of the PTC 
element by abutment, and a layer of rubber or another elastic material, 
for example an elastic plastic, is arranged between one of the electrodes 
and one of the pressure-exerting parts. 
According to another preferred embodiment the pressure device comprises 
parts exerting pressure against the electrodes of the PTC element, and at 
least one resilient element arranged between the pressure-exerting parts 
outside en electrode. 
According to an additional embodiment of the invention, a PTC element 
according to the invention is stacked on top of another PTC element 
according to the invention in the same pressure device, which is thereby 
adapted to exert a pressure directed perpendicularly to the parallel 
surfaces of the body, or the bodies, of polymer composition in each one of 
the PTC elements, and the outwardly-facing electrodes are electrically 
parallel-connected as are the inwardly-facing ones, which may possibly be 
designed to form one single electrode. Preferably, the device is formed 
with separate inwardly-facing electrodes and with a layer of rubber or 
other elastic material, such as elastic plastics, arranged between the 
electrodes. These embodiments with parallel-connected outwardly-facing and 
inwardly-facing electrodes have very good current-limiting properties also 
in case of a total area of the PTC elements which is relatively large. 
According to yet another embodiment of the invention, the body of polymer 
composition, if only one such body is included in the PTC element, or the 
bodies of polymer composition, if more than one such body is included in 
the PTC element, at surfaces exposed between the electrodes is/are 
surrounded by a casing of insulating material, which may advantageously be 
of the same kind and thus have the same physical properties as the polymer 
material which is included in the polymer composition. The casing of 
insulating material increases the dielectric strength of the PTC element. 
The reason is that the casing prevents flash-over between particles of the 
conducting powdered material, which may be exposed at surfaces of the 
polymer composition, which are exposed between the electrodes, when the 
PTC element is subjected to short-circuit currents. 
According to a particularly preferred embodiment of the invention, the 
pressure device comprises a pressure-absorbing frame arranged around the 
electrodes and the body, or bodies, of electrically conducting polymer 
composition, with wedge-shaped elements insertable into the frame. The 
frame may have the shape of an apparatus housing provided with a lid and 
at least one of the wedge-shaped elements may be arranged in the apparatus 
housing itself and at least one of the wedge-shaped elements may be 
secured to the lid. 
Another embodiment of the invention is characterized in that the parallel 
surfaces on the body, or bodies, of electrically conductive polymer 
composition consist of concentric surfaces. The electrodes are thereby 
concentric with the mentioned concentric surfaces of the body, or bodies, 
of polymer composition. This embodiment requires an extremely small space. 
A PTC element of the kind mentioned in the preceding paragraph may be 
designed such that, inside an electrode which is arranged at an 
inwardly-facing concentric surface on the body, or on that body of polymer 
composition which is arranged furthest towards the interior, there is 
arranged a body which is expansible in a direction perpendicular to the 
concentric surface and that, outside an electrode which is arranged at an 
outwardly-facing concentric surface of the body, or on that body of 
polymer composition which is located furthest towards the exterior, there 
is arranged a counter-support or this electrode is itself designed to 
constitute a counter-support.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows the resistivity R as a function of the temperature T for the 
electrically conductive polymer composition which is included in the PTC 
element according to the present invention. T.sub.t is the temperature, 
the transition temperature, at which the polymer composition changes from 
a low resistance to a high resistance state. 
The PTC element according to FIG. 2 comprises a centrally arranged body 10 
of an electrically conductive polymer composition with a positive 
temperature coefficient, for example consisting of 67 percent by volume 
polyethylene (e.g. LUPOLEN 6031M from BASF, Germany) and 33 percent by 
volume carbon black (e.g. N 550 from Degussa AG, Germany), in the form of 
a rectangular 1 mm thick plate as well as two electrodes 11 and 12 with 
associated terminals 13 and 14, respectively, arranged at the parallel end 
surfaces 10' and 10" of the body (the flat sides of the plate). The 
electrode 11 consists of an 0.5 mm thick plate of nickel with an even 
surface structure on both sides. On the outside, the plate is coated with 
a thin layer of copper. It makes free contact with the body 10, that is, 
makes contact only by abutting the body, and thus not being fixed to the 
body. The electrode 12 consists of an 0.3 mm thick copper foil which on 
the side facing the body 10 is coated with an 0.1 mm thick layer of copper 
with uneven surface structure, applied by plasma spraying. The electrode 
12 is fixed to the polymer by being pressed against the body 10 when the 
body has been heated, so that the polymer material in liquid state may 
penetrate into irregularities in the inwardly-facing side of the 
electrode. 
The device according to FIG. 3 includes two bodies, 10a and 10b, of the 
same electrically conductive polymer composition as that in the body 10 in 
FIG. 1, in the form of 1 mm thick rectangular plates with the parallel 
surfaces 10a', 10a" and 10b', 10b", respectively These plates make 
electrical contact only by abutment without being fused or otherwise fixed 
to each other, that is, make free contact with each other. The electrodes 
11 and 12 may be of the same kind as the electrodes 11 and 12 in FIG. 1. 
However, the electrode 11 may also be of the same kind as the electrode 12 
and, like this electrode, be fixed to the body 10a. In the device 
according to FIG. 3, the bodies 10a and 10b may be of different 
electrically conductive polymer compositions and have different 
resistivities to modify the properties of the PTC element. The device may 
also be modified so as to comprise more than two bodies (10a and 10b) of 
conductive polymer composition with the same or a different resistivity. 
If the electrodes are fixed to adjacent bodies of electrically conductive 
polymer composition, at least one of the bodies must make free contact 
with another one of the bodies. If one of or both of the electrodes make 
free contact with an adjacent body of electrically conductive polymer 
composition, all the polymer bodies may be fixed to each other, for 
example by fusion. 
FIG. 4 illustrates a device according to the invention in which a PTC 
element according to FIG. 2 is arranged in a pressure device comprising 
two plates 15, 16, which are parallel to the parallel end surfaces of the 
body 10 (the flat sides of the plate 10) and to the electrodes 11 and 12. 
The terminals 13 and 14 are not shown in the figure. The plates are of 
electrically insulating material, for example resin reinforced with glass 
fibre. The pressure against the electrodes and against the end surfaces of 
the body 10 is brought about by tightening a number of bolts 17. Between 
the bolt heads 17a and one of the plates 16 of the pressure device, stiff 
springs 17b are arranged which are not completely compressed when the PTC 
element is pressurized and prepared for normal operation. In case of a 
short circuit, the springs may therefore be further compressed, which 
permits the electrode 11 to be separated from the body 10. 
The device according to FIG. 5 differs from the device according to FIG. 4 
in that the springs 17b are not included and in that a 1 mm thick mat 30 
of rubber is arranged between the electrode 11 and the plate 15. Such a 
mat of rubber may possibly be arranged between the electrode 12 and the 
plate 16 as well. 
Instead of the PTC element according to FIG. 2, a PTC element according to 
FIG. 3 may be arranged, in an analogous manner, in the devices according 
to FIGS. 4 and 5. 
In the device illustrated in FIGS. 6 and 7, a PTC element according to FIG. 
2 or 3 is provided with plates 32, 33, arranged outside electrodes 11, 12, 
of copper or other high conductivity material between two opposite yokes 
18 and 19, respectively, of iron or other ferromagnetic material. The iron 
yokes are U-shaped with a base 18a and 19a, respectively, with a flat 
shape and with short legs 18b and 19b, respectively. Alternatively, only 
plates corresponding to the base 18a and 19a, respectively, may be used. 
By connecting the PTC element to the network in such a way that the 
current paths 20 and 21, respectively, become parallel to the end surfaces 
of the body 10 and directed in the same direction, as shown by dashed 
lines and arrows in FIG. 6, an attractive force is achieved between the 
yokes, which is illustrated by dashed arrows in FIG. 7. In this way, the 
body 10 is exposed to pressure between the end surfaces when current is 
conducted through the PTC element. 
In the device according to FIG. 8, two PTC elements of the kind shown in 
FIG. 2 are arranged one above the other with an intermediate mat 30a of 
rubber. The two electrodes 11 are parallel-connected as are the two 
electrodes 12. The PTC elements can be subjected to pressure in a manner 
analogous to that of the PTC elements according to FIG. 2 in the devices 
according to FIGS. 4 and 5 with the two electrodes 12 making contact with 
the plates 15 and 16. When arranging a rubber mat 30a between the two 
electrodes 11 in accordance with FIG. 8, no advantage is gained by 
utilizing springs 17b according to FIG. 4 or a rubber mat 30 according to 
FIG. 5, If the PTC element according to FIG. 8 is formed without a rubber 
mat 30a, that is, with the two electrodes 11 making contact with each 
other or replaced by one single electrode which on both sides makes 
contact with bodies 10 of electrically conductive polymer composition and 
this PTC element is arranged in the pressure devices according to FIGS. 4 
and 5, it is an advantage to retain the springs 17b and the rubber mat 30. 
Instead of the PTC element according to FIG. 2, a PTC element according to 
FIG. 3 may be used in the cases described with reference to FIG. 8. As 
explained before, a PTC element according to FIG. 8 has particularly good 
current limiting properties. 
The PTC element according to FIG. 9 is of the same kind as that shown in 
FIG. 2 but is provided with a casing 31 of insulating material, preferably 
of polyethylene, which is the polymer material included in the body 10 of 
electrically conductive polymer composition. The casing 31 is adapted to 
surround the surfaces 10"' on the body 10 which are exposed between the 
electrodes 11 and 12, which, as explained above, increases the dielectric 
strength of the PTC element. The PTC element according to FIG. 9 may be 
arranged in the same way as the PTC element according to FIG. 2 in the 
pressure devices according to FIGS. 4 and 5. Instead of a PTC element 
according to FIG. 2, a PTC element according to FIG. 3 may be used, in an 
analogous manner, in a PTC element according to FIG. 9, the casing 
surrounding the surfaces 10a"' and 10b"' on the bodies 10a and 10b. The 
casing 31 of insulating material may be applied around the body 10 or the 
bodies 10a and 10b by applying a ring of insulating material around the 
body 10 or the bodies 10a and 10b in a tool suitable therefor and be 
brought to fuse together, by heating, with the edges on the body 10 or the 
bodies 10a and 10b. 
The device according to FIGS. 10a, b, c and 11 comprises three identical 
PTC elements, one of which is shown with the parts separated in FIG. 10b. 
The body 10 of electrically conductive polymer composition, for example of 
the same kind as in FIG. 2, is arranged in a plastic frame 40 which 
extends around the body. The two electrodes 11 and 12 consist of 1 mm 
thick silver-plated plates of copper, which make free contact with the 
body 10, that is, only by abutting it, and thus not being fixed to it. 
Each electrode is provided with alternative conductor terminals 13a and 
14a for cable connection and 13b (not shown) and 14b for connection to a 
bar, respectively. Outside the electrodes 11 there is arranged a 
wedge-shaped, loose plate 41 and outside the electrode 12 there is 
arranged a loose spring plate 42 of thin sheet metal. The spring plate is 
provided with a plurality of tongues 42a cut out and bent out from the 
plate, which give the plate its resilient properties. A package comprising 
the plates 41 and 42 and electrodes 11 and 12 arranged therebetween and 
with an intermediate body 10 of conducting polymer composition is arranged 
in each one of three compartments 43a, 43b and 43c in an apparatus housing 
43 with two partitions 43d and 43e forming the compartments. The 
partitions are parallel to two opposite, parallel walls 43f and 43g in the 
apparatus housing. The apparatus housing is provided with hollows 44a and 
44b for alternative connection of a conductor to the attachment 14a or 14b 
in the compartment 43a, with hollows 45a and 45b for alternative 
connection of a conductor to the attachment 14a or 14b in the compartment 
43b, and with hollows 46a and 46b for alternative connection of a 
conductor to the attachments 14a or 14b on an electrode 12 in the 
compartment 43c. On the opposite, not visible wall of the apparatus 
housing, there are corresponding recesses for connection of conductors to 
the attachments 13a and 13b on each electrode 11. The pressure between the 
electrode 11 and 12 and the body 10 in each one of the packages in the 
compartments 43a, 43b and 43c is brought about by applying a lid 47 with 
wedge-shaped plates 48, 49 and 50, fixed to the lid, on the apparatus 
housing, a wedge-shaped plate being inserted into each one of the 
compartments 43a, 43b and 43c adjacent a plate 41 located there. The lid 
is provided with mechanical attachments 47a, 47b, 47c and 47d for 
mounting. As will be clear from FIG. 11, the apparatus housing is provided 
with plane-parallel reinforcing walls 51 and 52 on the pressure-absorbing 
sides. The device according to FIGS. 10a, b, c and 11 is intended to be 
connected into a three-phase cable with a PTC element connected into each 
of the three phase conductors. The apparatus housing 43 with outer walls 
43f and 43g, reinforcing walls 51 and 52, lid 47 and wedge-shaped plates 
41, 48, 49 and 50 are all manufactured from an electrical insulating 
material, for example a polyamide, to which a filler, for example in the 
form of short glass-fibres, has been added. The PTC element according to 
FIG. 12 comprises a tubular body 10 of an electrically conductive polymer 
composition with a positive temperature coefficient, for example of the 
same composition as that used in the body 10 in FIG. 2. The interior 
cylindrical envelope surface is designated 10' and the exterior also 
cylindrical envelope surface is designated 10". In a cross section 
perpendicular to the centre line of the tubular body, each one of the 
envelope surfaces forms a circle, The wall thickness of the tubular body 
amounts to at least 1 mm. The PTC element also has two electrodes 11 and 
12 which are arranged in contact with the envelope surfaces 10' and 10" 
and concentric therewith. The electrodes are provided with terminals 13 
and 14, respectively. The electrode 11 consists of an 0.5 mm thick tube of 
nickel with an even surface structure on both sides. On the outside the 
tube is coated with a thin layer of copper. The tube is slitted in the 
longitudinal direction so that one edge at the slit is able to slide over 
the other. The electrode 11 makes free contact with the body 10, that is, 
makes contact only by abutting the body along the envelope surface 10' 
under pressure, and thus not being fixed to the body. The electrode 12 
consists of an 0.3 mm thick copper foil which on the side facing the body 
10 is coated with an 0.1 mm thick layer of copper with an uneven surface 
structure, applied by plasma spraying. The electrode 12 is fixed to the 
polymer by being pressed against the body 10, after the body has been 
heated so that the polymer material in liquid state may penetrate into 
irregularities in the inwardly-facing side of the electrode. Inside the 
electrode 11 there is arranged a body 31, which is expansible in a 
direction perpendicular to the envelope surface 10' and the electrode 11 
and outside the electrode 12 there is a counter-support 32, for example in 
the form of a tube of resin reinforced with fibre-glass, which is 
manufactured in situ, for example by applying the glass-fibre material and 
the resin in uncured state onto the electrode 12, and thereafter curing 
the resin. The expansible body 31 consists in the exemplified case of a 
tubular body 31a of rubber or other elastic material, which by means of a 
bolt 31b, arranged centrally in the body, with a bolt head 31c and a nut 
31d and with washers 31e and 31f arranged at the end surfaces, inside the 
bolt head and the nut, may be brought to expand while achieving a desired 
pressure against the electrode 11. Instead of providing the PTC element 
with a separate counter-support, the electrode 12 may be formed as a tube 
with sufficient thickness to serve itself as a counter-support, such as in 
the form of a tube of copper which has been drawn down, on the spot, to 
the intended dimension to make contact directly with the envelope surface 
10" on the body 10. In this case both electrodes of the PTC element are 
adapted to make free contact, under pressure, with the envelope surfaces 
10' and 10" on the body 10. Such a copper tube may alternatively be 
adapted to serve only as a counter-support and then to make contact with 
the electrode 12. Also, when using a separate counter-support, the 
electrode 12 may, instead of being fixed to the body 10, be adapted to 
make free contact with the body 10. In a manner analogous to that of a 
copper tube, a sleeve of memory metal may be used as a separate 
counter-support or both as a counter-support and an electrode, the sleeve 
in expanded state being arranged on the spot around the PTC element and 
thereafter brought to resume its smaller dimension before the expansion. 
If the expansible body 31 consists of a body of metallic material, it may 
be suitable or necessary to arrange a layer of rubber or other 
electrically insulating material between the expansible body and the 
electrode 11. 
The device according to FIG. 13 includes two bodies 10a and 10b, of the 
same electrically conductive polymer composition as that in the body 10 in 
FIG. 12, in the form of concentric tubes with a wall thickness of 1 mm. 
The concentric envelope surfaces are designated 10a', 10a" and 10b', 10b", 
respectively. The two bodies 10a and 10b make free contact with each 
other, that is, make contact only by abutment under pressure, and thus not 
by being fused or otherwise fixed to each other. The electrodes 11 and 12 
may be of the same kind as the electrodes 11 and 12 in FIG. 12. However, 
the electrode 11 may also be of the same kind as the electrode 12 in the 
first exemplified case, when this is fixed to the body of polymer 
composition, and thus be fixed to the body 10a. In the device according to 
FIG. 13, the bodies 10a and 10b may be of different electrically 
conductive polymer composition and have different resistivity to modify 
the properties of the PTC element. The device can also be modified so as 
to contain more than two concentric bodies (10a and 10b) of conductive 
polymer composition with the same or with different resistivity. If the 
electrodes are fixed to adjacent bodies of electrically conductive polymer 
composition, at least one of the bodies must make free contact with 
another one of the bodies. If one of or both of the electrodes make free 
contact with an adjacent body of electrically conductive polymer 
composition, all polymer bodies may be fixed to one another, for example 
by fusion. The device 31 with its parts for maintaining the electrodes 11 
and 12 and the bodies 10a and 10b pressed against each other in the PTC 
element according to FIG. 13 is in the exemplified case the same as that 
in the PTC element according to FIG. 12. 
The PTC element according to FIG. 14 differs from the PTC element according 
to FIG. 12 in that a 1 mm thick layer 33 of rubber or other elastic 
material is arranged between the electrode 12 and the counter-support 32 
to increase the capacity of the pressure-exerting parts to be elastic. As 
in the case according to FIG. 12, instead of being fixed to the body 10, 
the electrode 12 may make free contact with the body, that is, only by 
abutting the body along the envelope surface 10". 
In the device according to FIG. 15, the electrode consists of a solid wire 
of copper or nickel with a thin coating of nickel. The electrode 11 makes 
free contact with the body 10, that is, only by abutting the body along 
the envelope surface 10'. The body 10 and the electrode 12 are of the same 
kind as the corresponding elements in the device according to FIG. 12. The 
electrodes 11 and 12 and the body 10 are maintained pressed against each 
other by a drawing or clamping device 34, which, for example, may consist 
of a plate 34a arranged around the electrode 12 with an intermediate 
elastic insulating layer (not shown), the plate being provided with 
overlapping edges in the axial direction of the body 10, so that the 
diameter of the tubular body formed by the plate can be reduced by 
tightening a screw device 34b, in a manner analogous to that of a hose 
clamp, or by compressing two jaws with an internally semicylindrical shape 
in a direction perpendicular to the envelope surfaces 10' and 10" of the 
body 10. In the illustrated case, the electrode 11 itself serves as a 
counter-support. It is, of course, possible, instead of using an electrode 
11 in the form of a solid wire, in accordance with FIG. 15, to use an 
electrode 11 in the form of a layer of the kind illustrated in FIGS. 12-14 
and a counter-support arranged inside such an electrode in the form of a 
rigid body or an elastic device, for example of the kind designated 31 in 
FIGS. 12-14. 
The PTC element according to FIG. 16 differs from the PTC element according 
to FIG. 15 in that a 1 mm thick layer 33 of rubber or other elastic 
material is arranged between the electrode 12 and the plate 34a to 
increase the capacity of the pressure-exerting parts to be elastic. 
Instead of the drawing of clamping device 34, it is possible to use a 
sleeve of memory metal, the sleeve of memory metal in expanded state being 
arranged in place around the PTC element and thereafter being brought to 
assume a smaller diameter. Such a sleeve may at the same time serve as an 
electrode in the case illustrated in FIG. 15. 
FIG. 17 exemplifies the use of the PTC element according to the invention 
as overcurrent and short-circuit protective device in an electric circuit 
26 comprising a motor 25. The PTC element 22, for example of the kind 
shown in FIG. 4, 5, 12 or 13, is series-connected to a contact device 23. 
In parallel with the contact device there is an excitation coil 24, which 
is included in a fast magnetic tripping device for the contact device. The 
tripping device is adapted to influence the contact device for opening 
this at an over-current.