Thermo-activating breaker

Abstract

 A normally-closed bimetal switch 1 comprises a pair of electrical switch contacts (6,8), a bimetallic strip (7), which carries one of the contacts (6), and a transparent glass envelope (2), within which the contacts (6,8) and the bimetallic strip (7) are hermetically sealed. The envelope (2) contains a gas, such a neon, at low pressure. The composition and pressure of the gas are so chosen that on opening of the contacts (6,8) a gas discharge is set up which is visible through the envelope (2), and thus acts as an alarm, and which produces sufficient heat to prevent or substantially delay the re-closing of the contacts (6,8).

Description

[0001] The invention relates to a bimetal switch and in particular to a bimetal switch in a sealed transparent envelope which provides a gas discharge when two contacts are separated which is sufficient to prevent or substantially delay re-closing of the contacts and which acts as a visible alarm.

[0002] In conventional known bimetal switches electrical resistance losses are used to provide the heat which distorts the bimetallic strip of the switches. The heat is provided by passing a current through the bimetallic strip itself, or through a wire which is wound around the strip, or through some other suitably positioned component. The switch contacts are generally part of the circuit through which the current is passed to heat the bimetallic strip. Thus, as soon as the bimetallic strip has deformed sufficiently to open the switch contacts, the circuit is broken and the bimetallic strip will no longer be heated and will begin to cool and move back to its original position. The circuit will therefore only be broken for a relatively short period, which may be too short for some purposes. For example, if such a conventional switch is in a situation where the conditions for the opening of the contacts frequently occur, or occur for long periods then the switch may have a shorter life than would be the case if the switch contacts were separated for a longer period.

[0003] Conventional switches generally also give no positive indication of when their contacts have been opened. The contacts may be examined but it may be difficult determine whether they are touching or are separated.

[0004] GB555892 discloses an example of a known bimetal switch which is used for starting and operating an electric discharge lamp of the type having electrodes which require preheating. The switch includes a pair of contacts which are normally closed. One of the contacts is mounted on the bimetallic strip of the switch, and the strip and the contacts are sealed within a transparent envelope together with a heating element which is provided adjacent to the strip. The heating element and the strip are so designed that at the end of the time period required for preheating the electrodes to their operating temperature, the bimetallic strip has distorted sufficiently to open the contacts. A further bimetallic strip may be provided which may mount the second contact.

[0005] According to the invention there is provided a normally-closed bimetal switch having a plurality of switch contacts and a bimetallic strip sealed within a transparent envelope, characterised in that the envelope contains gas at low pressure, the composition and pressure of gas being so chosen that on opening of the contacts a gas discharge is set up which is visible through the envelope and which produces sufficient heat to prevent or substantially delay the re-closing of the contacts.

[0006] Thus, the period for which the contacts are open is lengthened so that a switch according to the invention can be used in circumstances where the relatively short recovery time of a conventional switch makes its use impractical. Furthermore, there is a positive indication, in the form of the gas discharge, of when the contacts have been opened so that a person can monitor the switch more easily.

[0007] The said gas may be of any suitable composition and may be a single type of gas or a combination of types of gases. Preferably said gas is neon.

[0008] Where the heat for the bimetallic strip is provided by passing a current through a separate component, the position of the component in relation to the bimetallic strip will affect the amount of distortion of the bimetallic strip which is caused by the heat from the component. Switches of this type must therefore be accurately manufactured to ensure that the switches have consistent characteristics and that may increase the expense of manufacture. Preferably, therefore, the bimetallic strip is heated by passing an electrical current through it.

[0009] However, if the bimetallic strip is heated solely by passing a current through it, then the distortion of the strip in relation to the current passing through it, for a strip of given materials and of a given size, will not be controllable except by adjusting the stress which is initially exerted by the contacts on each other and which the strip must overcome in order to distort to open the contacts. The bimetallic strip may thus be heated by a separate heat source and preferably is heated both by a separate heat source and by passing a current through it.

[0010] The separate heat source may be an electrical resistor and preferably is an electrical resistor which is connected in series with the bimetallic strip.

[0011] A component having externally adjustable resistance may be connected in parallel with the said electrical resistor. The current passing through the electrical resistor, and hence the heat produced by it, can thus be controlled.

[0012] The bimetallic strip may have contacts on both sides.

[0013] The envelope may be made of insulating materials and preferably is made of glass.

[0014] The connecting lines to the switch contacts within the envelope may extend through the region which the envelope was sealed.

[0015] A further bimetallic strip may be provided and at least one switch contact may be mounted on the said further bimetallic strip.

[0016] Two embodiments of the invention will now be described of way of example and with reference to the accompanying drawings, in which:

Fig. 1 is an elevational view in cross-section of one embodiment;

Fig. 2 is the view of Fig. 1, but with the switch contacts shown as open; and

Fig.3 is an elevational view in cross-section of a further embodiment.

[0017] Fig. 1 shows a switch (1) which includes a hermetically sealed transparent envelope (2) containing gas at low pressure. A contact mounting leg (3) of conducting material, mounts a first contact (6) and a strip mounting leg (4) also of conducting material, mounts a bimetallic strip (7) which in turn mounts a second contact (8). The two legs (3,4) each extend through the wall of the envelope (2) at a thickened part (5) of the envelope (2) where the envelope was sealed. The contact leg (3) is connected to one terminal of an electrical supply (9) and the strip mounting leg (4) is connected to the other terminal of the supply (9) via a load (10) so that a circuit (11) is formed.

[0018] In use, if the current load in the circuit (11) is increased, the heat generated in the bimetallic strip (7) due to resistance heating will increase. The arrangement is such that it the circuit is overloaded, then the increase in temperature of the bimetallic strip will cause the bimetallic strip (7) to bend. The right hand side of the strip (7), as it is shown in the Figures, is made of a metal or metal alloy which has a greater coefficient of thermal expansion than the metal or metal alloy on the left hand side of the strip (7), and the strip (7) thus bends to the left when heated as shown in Fig. 2 and opens the contacts (6,8) to break the circuit. The pressure and composition of the gas are so chosen that on opening of the contacts (6,8) a gas discharge is set up which produces sufficient heat to prevent or substantially delay the recovery of the bimetallic strip, and hence the re-closing of the contacts (6,8), and which is also visible through the envelope (2). One suitable gas for producing both visible light and sufficient heat under the conditions described is neon.

[0019] The embodiment shown in Fig. 3 shows all the features of the embodiment of Figs. 1 and 2 except for the electrical connections from the legs (3,4). It has in addition however, a further resistor mounting leg (12) of conducting material, which extends through the wall of the envelope (2) at the thickened part (5), and mounts a resistor (13), which is connected inside the envelope between the resistor mounting leg (12) and the strip mounting leg (4). The resistor (13) acts as a further heat source for the bimetallic strip (7). A component having externally adjustable resistance, in this case a variable resistor (14), is connected outside the envelope between the resistor mounting leg (12) and the strip mounting leg (4), i.e. in parallel with the resistor (13). The current passing through the resistor (13) and the strip (7), and hence the temperature of the bimetallic strip (7), can thus be adjusted and the switch can therefore be adapted for use in circuits with different overload currents.

[0020] The envelope (2) may be made of glass.

Claims

1. A normally-closed bimetal switch having a plurality of switch contacts and a bimetallic strip sealed within a transparent envelope, characterised in that the envelope (2) contains a gas at low pressure, the composition and pressure of gas being so chosen that on opening of the contacts (6,8) a gas discharge is set up which is visible through the envelope (2) and which produces sufficient heat to prevent or substantially delay the re-closing of the contacts (6,8).

2. A switch as claimed in Claim 1, characterised in that said gas is neon.

3. A switch as claimed in Claim 1 or Claim 2, characterised in that the bimetallic strip (7) is heated by passing an electrical current through it.

4. A switch as claimed in Claim 1, 2 or 3, characterised in that the bimetallic strip (7) is heated by a separate heat source (13).

5. A switch as claimed in Claim 4, characterised in that the separate heat source (13) is an electrical resistor (13).

6. A switch as claimed in Claim 5, characterised in that, in the case in which the bimetallic strip is heated by passing an electrical current through it, the said electrical resistor (13) is connected in series with the bimetallic strip (7).

7. A switch as claimed in claim 6, characterised in that a component having externally adjustable resistance (14) is connected in parallel with the said electrical resistor (13).

8. A switch as claimed in any preceding claim, characterised in that the bimetallic strip (7) has contacts (6,8) on both sides.

9. A switch as claimed in any preceding claim, characterised in that the envelope (2) is made of insulating materials.

10. A switch as claimed in Claim 9, characterised in that the envelope (2) is made of glass.

Drawing