Temperature Sensitive Trip Device

The present invention relates to a temperature-sensitive trip device.

The present invention is particularly useful with electrical surge arrestors employed to protect signal transmission lines against induced voltage surges. An electrical surge arrestor in its simplest form comprises two electrodes hermetically seated to an insulating envelope by which the electrodes are held spaced apart to form an arcing gap. One of the electrodes is connected to a transmission line and the other to earth so that when a surge is experienced on the transmission line, an arc is struck across the gap and the excess voltage represented by the surge thereby taken to earth. The most usual form of arrestor now employed is the dual device providing two line electrodes for connection to respective ones of a transmission line pair, and commoned earth electrodes along with two envelopes, to form a unified structure in which two gaps are provided viz: between each line and earth.

Some known arrestors are "fail-safe", that is to say, a line electrode and an earth electrode and designed to fuse together or one of them to soften and collapse into the other, when the energy dissipation in the line electrode exceeds a certain level so that a direct connection is provided to earth.

Experience has now shown that surge arrestors as above described, whether fail-safe or not, can, at energy dissipation levels considerably below those for which fail safe operation is provided, heat up to temperatures which give rise to a fire risk. To counteract this, arrestors have been provided with temperature-sensitive earth trips which act to bridge electrode terminal pieces of an arrestor with a spring urged, rigid contact member at a pre-determined temperature so as to provide a direct short between the arrestor terminals to reduce the energy dissipation in the electrodes and thus the temperature generated by the energy dissipation.

The existing trip devices, however, are unsatisfactory. They consist of several separate parts some at least of which are held in contact with one another only by the resilient bias of the spring and can, therefore, be dislodged by mechanical shock. In some of the devices, the parts are not visible in the completed assembly with the result that a part could be omitted during assembly and the omission not subsequently noticed. This danger would also arise if the arrestor were dismantled for any reason following manufacture e.g. during installation. Again, all the existing devices use a discrete fusible pellet or slug which may not melt symmetrically and consequently may cause the contact member to jam or make poor contact with an electrode terminal piece. Further, all the existing devices necessarily require some kind of base or mounting device on which the trip device can be assembled and this increases the expense and physical size of the device.

It is an object of the present invention to provide a temperature-sensitive trip device to overcome the abovementioned disadvantage.

It is a further object of the present invention to provide a temperature-sensitive trip device which is also of a more general utility.

Briefly, the invention uses a contact member which itself is resilient and is mounted on the arrestor; and a rigid, flexible material is solidified in contiguous relationship with the contact member normally to hold it resiliently biassed away from a position in which it forms a connection between a line electrode and an earth electrode, so that when the fusible material melts (its melting point being chosen at the temperature above which a fire hazard would arise) the contact member is released and moves under the force of its resilient bias to assume said position amd thus short circuit the two electrodes.

The contact member may be a coil spring carried round the insulating envelope of the arrestor or a respective one thereof; or it may be a spring arm secured at one end to one electrode of a line and earth electrode pair, of a length to span the two electrodes and providing a contact surface at its other end to engage the other electrode. In the case of the coil spring, the spring may be held compressed by the fusible material so as to expand on melting of the fusible material to make contact at its ends respectively, with the line electrode and earth electrode concerned, if the latter are suitably formed, or with suitable terminals attached to the electrodes. The terminals may also serve to prevent removal of the coil spring from the arrester, and thus prevent inadvertent loss of the spring, although the spring could be made captive on the arrestor by other means, for instance, by a deposition of wax or other suitable fusible material of a melting point not higher than the material used to hold the spring in the resiliently biassed condition. In the case of the spring arm, the fusible material may be solidified against it adjacent its point of attachment to the electrode concerned and at its side facing the arrestor, to act as, a block between the arrestor and the arm supporting the arm so as to hold it tensed away from the arrestor whereby on melting of the fusible material, the arm is released to bring its contact surface into engagement with the other electrode under the force of the resilient bias of the arm.

For a dual surge arrestor, two contact members are used. Thus, two coil springs would be used, one mounted round each envelope, or two spring arms would be used which could take the form a single spring strip secured at substantially its mid point to the commoned earth electrodes.

The spring arm, or each of them, may be made of any suitable metal, e.g. copper, and may be soldered or brazed to the electrode. The coil spring, or each of them, may consist of steel (suitably coated for tinning) or of beryllium or copper.

The arrestor of the present invention, may be packaged in a plastics envelope, preferably a clear plastics such as that sold under the trade marks "DEROTON" or "VALOX", which is a glass filled, fire retardant polyester of a melting point of 222.degree. to 230.degree. C.; and it may be provided with flying leads from the electrodes or electrode terminals so that arrestor can be directly connected to the line and earth terminals of the transmission system.

The fusible material may be a solder or other fusible, eutetic alloy or a wax. For most applications, a standard solder will be found suitable, but it will be understood that the fusible material chosen will depend on the particular application in which the arrestor is to be used since it is this that will determine what temperatures may be tolerated and what melting characteristics the fusible material should have. For instance, it might be that higher temperatures could, or might need to be tolerated outdoors than indoors, and consideration would need to be given to the materials surrounding the arrestor in its use.

Generally, however, the melting point of the fusible material will not be less than 60.degree. and not more than 400.degree. C.

Standard solders will give melting points in the upper part of the above range; and suitable eutetic alloys including a lead free solder, of melting points below those of standard solders, are shown in Table I hereinbelow. If melting points above 400.degree. C. are required, brazing and silver solders may be used.

It may also be feasible to use thermoplastics.

The invention will now be described, by way of example only, with reference to the accompanying drawings in which like reference numerals are used to indicate like parts and in which:

FIG. 1 is a plan view of an encapsulated and electrical surge arrestor employing a trip device according to the present invention, shown before operation of the trip device;

FIG. 2 is a plan view of the arrestor of FIG. I showing the trip device after operation; the scale being enlarged in the interests of clarity;

FIG. 3 is an end elevation of the arrestor as shown in FIG. I;

FIG. 4 is a side elevation of a dual electrical surge arrestor employing a further trip device according to the invention, showing the device before operation; and

FIG. 5 is a side elevation of the arrestor of FIG. 3, after operation of the trip device.

Referring now to FIGS. 1 to 3, the arrestor comprises a first line electrode 1, a second line electrode 2, respective earth electrodes (not shown) joined to a common terminal 3, line terminals 4 and 5 welded respectively to the line electrodes 1 and 2, flying leads 6, 7 welded respectively to the line electrode terminals 4, 5, and flying common earth lead 8 welded to the common terminal 3, and insulating envelopes 9,10 to which respectively are hermetically sealed, line electrode 1 and the corresponding earth electrode, and line electrode 2 and its corresponding earth electrode. The common terminal 3 and the earth electrodes are formed to provide a communicating passageway (not shown) between the interiors of the respective insulating envelopes.

The trip device comprises, in respect of each pair of electrodes, a coil spring 11 or 12 (shown in section in FIG. 1) mounted around the respective insulating envelope and held in a compressed condition by the solidification thereon of a rigid fusible material 13, in this case, a lead-tin solder. The deposition of the solder may be carried out by mounting a number of the coil springs on a rod coated with release agent, with each two adjacent springs separated by a washer coated with release agent, axially compressing the springs on the rod, dipping the springs in molten solder, withdrawing the rod and removing the springs therefrom when the solder has solidified thereon so that the solder itself then maintains the springs in a compressed condition. A spring is then mounted around each insulating envelope before the electrode terminals are welded to the electrodes. To prevent the springs from shifting about on the insulating envelopes, they may be located by a deposit of wax conveniently in the form a single strand 13' joining the springs at their adjacent ends.

When in use of the arrestor, the solder melts, one or both spring uncoils under the force of the resilient bias of its compression to cause its ends respectively to make contact with a line electrode terminal and the opposing face of the common earth terminal as shown in FIG. 2, thus short circuiting the respective line electrodes to earth.

The arrestor of FIGS. 1 and 2 is encapsulated in a plastics box 14 having slots 15 in a lateral wall to permit entry of the flying leads; and the box is closed with a press-on lid 16 as shown in FIG. 3.

Since the surge arrestor per se of FIGS. 4 and 5 is identical with that of FIGS. 1 to 3, it will not be further described; and, in the interests of clarity, it is shown without the encapsulation illustrated in FIGS. 1 and 3.

The trip device employed with the surge arrestor of FIGS. 4 and 5, comprises a single strip of spring metal 17 welded at substantially its mid point to the common terminal 3 of the arrestor. The strip forms two spring arms 18, 19 respectively spanning the axial distance between the common terminal 3 and line electrode 1, and the axial distance between common terminal 3 and line electrode 2. Each spring arm, at its end adjacent a line electrode, provides a contact surface in the form of a pointed stud 20, 20'. Between each spring arm, adjacent its point of attachment to the common terminal 3, and a sleeve 3', 3" integral with common terminal 3, a fusible material 21, again, in this instance, a solder is solidified so that the material forms a block lying between the arm and the respective insulating envelope to hold the spring arm tensed and resiliently biassed so as to urge its contact surface towards the respective line electrode. As a result, when the solder melts, the end of the arm bearing the contact surface is brought firmly into contact with the surface of the respective line electrode to short the electrode to earth.

The present invention is not limited to use with an electrical surge arrestor. Thus, the contact element could be used in many electrical arrangements to provide a temperature-sensitive trip action; or its mechanical action on release by the fusible material, on melting of the latter, could be used to operate, say, a valve or a movable electrical contact.