GB2136646A - Line circuit protection - Google Patents

Abstract

To protect equipment such as a telephone subscriber's instrument or other terminal, which may include electronics it is necessary to deal with the relatively low overvoltage which is inadequate to blow fuses but which if it persists will damage that equipment. To do this there is a module which includes a spring-loaded junction (J) connected in series with one wire of the line and a zener diode (VZ) connected across the line but in heat conductive relation therewith. Then an overvoltage ouch as referred to will, if it persists, melt the junction, which is then pulled apart by its spring loading. <IMAGE>

Description

SPECIFICATION Line circuit protection This invention relates to the protection of electrical or electronic equipment against damage due to overvoltage.

An overvoltage on a line produces an electrical current whose value may exceed the safety level for equipment connected to that line. A difficulty which may arise with such protection equipment is that a voltage whose value is at, near, or even below the normal fuse-blowing voltage may, if it persists, cause damage. This invention seeks to provide means to overcome this difficulty.

According to the invention there is provided an electrical protection module, for the protection of an electrical or electronic equipment connected to a two-wire balanced line, which module includes a spring-ioaded fusible element connected in series with one wire of the line, and a voltage clamping device formed by a zener diode or the like connected between the two wires of the line on the equipment side of the fusible element, wherein the voltage clamping device and the fusible element are so located that there is substantial heat conduction from the device to the fusible element, whereby if an overvoltage of relatively long duration occurs on the line the electrical conduction in the clamping element due to that overvoltage causes the generation of heat which by its influence on the fusible element, which rupture is also assisted by the spring loading of that fusible element.

It will be seen that this is similar to the arrangement of our Application No. 8231652 (A.J. Tricker et al 2-1-4), which describes a number of arrangements for protecting a telephone line circuit and its equipment. In the present case the main interest resides in the protection of telephone or other terminal equipment at the subscriber's end of the line.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which Figure lisa circuit diagram of, and Figure 2 is a mechanical representation of, a protection module embodying the invention.

The module to be described, Figure 1, is connected to a two-wire balanced line, to protect electrical or electronic equipment to be protected against overvoltages of the sort referred to above. This equipment in the present case is a telephone or other terminal equipment, which often includes electronics. The module is a four-terminal device, having input terminals 1 and 3 connected respectively to the A and B wires of the line, and output terminals 2 and 4 connected respectivley to the A and B wires of the equipment to be protected.

The module includes a spring-loaded low melting point junction, e.g. a suitable soldered joint J. Such a joint has a relatively low electrical resistance so that it is not likely to melt when influenced by the normal line current. However, an excessive voltage will generate enough current to melt it, so that it is in effect a spring-loaded fuse.

Connected across the line on the protected equipment side of the junction J there is a voltage clamping device Vz mounted to be in good heat conductive relation with the fusible junction J. Under normal conditions the device Vz, which may be a zener diode, either does not conduct, or it conducts at a low level, insufficient to generate much heat. If the line is subjected to an overvoltage as a result of a relatively slowly increasing amount in the line due to some external influence, such current if allowed to persist for a long period may be enough to damage the equipment. Such a current, however, increases the current bled from the line via the device Vz, which therefore generates more heat. If the offending condition persists for long enough, the heat melts the junction J, and the spring then assists in its rupture.

It is assumed that the impedance of the line between the exchange and the protected equipment is sufficient to avoid the need to include series resistors in the line. Such resistors would ensure that the primary protection arrangements for the exchange - and also the protection arrangements for the subscriber's terminal - are not misoperated in normal operating conditions. If the impedance is in fact not adequate for this, then series resistors can be included.

Figure 2 shows the construction of one module for use as shown in Figure 1. This has a printed circuit board base PCB with four terminals 1,2,3 and 4, as shown connected between terminals 2 and 4 we see a device Vz such as a standard zener diode. This can be a Semitron zener diode, with plastics encapsulation, or without such encapsulation but in a glass passivated sealed unit. Alternatively it could be a Mullard glass encapsulated zener diode. Thetermin- als 3 and 4 are interconnected by a printed circuit track T and the terminals 1 and 2 are interconnected by a spring S of low electrical resistance one end of which is secured to the terminal 2 by a low melting point junction J.

If the fuse is blown, i.e. the junction J melts, the connection is readily remade when the fault has been cleared with a soldering iron.

1. An electrical protection module, for the protection of an electrical or electronic equipment connected to a two-wire balanced line, which module includes a spring-loaded fusible element connected in series with one wire of the line, and a voltage clamping device formed by a zener diode or the like connected between the two wires of the line on the equipment side of the fusible element, wherein the voltage clamping device and the fusible element are so located that there is substantial heat conduction from the device to the fusible element, whereby if an overvoltage of relatively long duration occurs on the line the electrical conduction in the clamping element due to that overvoltage causes the generation of heat which by its influence on the fusible element, which rupture is also assisted by the spring loading of that fusible element.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (2)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Line circuit protection This invention relates to the protection of electrical or electronic equipment against damage due to overvoltage. An overvoltage on a line produces an electrical current whose value may exceed the safety level for equipment connected to that line. A difficulty which may arise with such protection equipment is that a voltage whose value is at, near, or even below the normal fuse-blowing voltage may, if it persists, cause damage. This invention seeks to provide means to overcome this difficulty. According to the invention there is provided an electrical protection module, for the protection of an electrical or electronic equipment connected to a two-wire balanced line, which module includes a spring-ioaded fusible element connected in series with one wire of the line, and a voltage clamping device formed by a zener diode or the like connected between the two wires of the line on the equipment side of the fusible element, wherein the voltage clamping device and the fusible element are so located that there is substantial heat conduction from the device to the fusible element, whereby if an overvoltage of relatively long duration occurs on the line the electrical conduction in the clamping element due to that overvoltage causes the generation of heat which by its influence on the fusible element, which rupture is also assisted by the spring loading of that fusible element. It will be seen that this is similar to the arrangement of our Application No. 8231652 (A.J. Tricker et al 2-1-4), which describes a number of arrangements for protecting a telephone line circuit and its equipment. In the present case the main interest resides in the protection of telephone or other terminal equipment at the subscriber's end of the line. An embodiment of the invention will now be described with reference to the accompanying drawings, in which Figure lisa circuit diagram of, and Figure 2 is a mechanical representation of, a protection module embodying the invention. The module to be described, Figure 1, is connected to a two-wire balanced line, to protect electrical or electronic equipment to be protected against overvoltages of the sort referred to above. This equipment in the present case is a telephone or other terminal equipment, which often includes electronics. The module is a four-terminal device, having input terminals 1 and 3 connected respectively to the A and B wires of the line, and output terminals 2 and 4 connected respectivley to the A and B wires of the equipment to be protected. The module includes a spring-loaded low melting point junction, e.g. a suitable soldered joint J. Such a joint has a relatively low electrical resistance so that it is not likely to melt when influenced by the normal line current. However, an excessive voltage will generate enough current to melt it, so that it is in effect a spring-loaded fuse. Connected across the line on the protected equipment side of the junction J there is a voltage clamping device Vz mounted to be in good heat conductive relation with the fusible junction J. Under normal conditions the device Vz, which may be a zener diode, either does not conduct, or it conducts at a low level, insufficient to generate much heat. If the line is subjected to an overvoltage as a result of a relatively slowly increasing amount in the line due to some external influence, such current if allowed to persist for a long period may be enough to damage the equipment. Such a current, however, increases the current bled from the line via the device Vz, which therefore generates more heat. If the offending condition persists for long enough, the heat melts the junction J, and the spring then assists in its rupture. It is assumed that the impedance of the line between the exchange and the protected equipment is sufficient to avoid the need to include series resistors in the line. Such resistors would ensure that the primary protection arrangements for the exchange - and also the protection arrangements for the subscriber's terminal - are not misoperated in normal operating conditions. If the impedance is in fact not adequate for this, then series resistors can be included. Figure 2 shows the construction of one module for use as shown in Figure 1. This has a printed circuit board base PCB with four terminals 1,2,3 and 4, as shown connected between terminals 2 and 4 we see a device Vz such as a standard zener diode. This can be a Semitron zener diode, with plastics encapsulation, or without such encapsulation but in a glass passivated sealed unit. Alternatively it could be a Mullard glass encapsulated zener diode. Thetermin- als 3 and 4 are interconnected by a printed circuit track T and the terminals 1 and 2 are interconnected by a spring S of low electrical resistance one end of which is secured to the terminal 2 by a low melting point junction J. If the fuse is blown, i.e. the junction J melts, the connection is readily remade when the fault has been cleared with a soldering iron. CLAIMS

1. An electrical protection module, for the protection of an electrical or electronic equipment connected to a two-wire balanced line, which module includes a spring-loaded fusible element connected in series with one wire of the line, and a voltage clamping device formed by a zener diode or the like connected between the two wires of the line on the equipment side of the fusible element, wherein the voltage clamping device and the fusible element are so located that there is substantial heat conduction from the device to the fusible element, whereby if an overvoltage of relatively long duration occurs on the line the electrical conduction in the clamping element due to that overvoltage causes the generation of heat which by its influence on the fusible element, which rupture is also assisted by the spring loading of that fusible element.

2. An electrical protection module substantially as described with reference to the accompanying drawings.