There is a need for a d. c. power transient suppresser to allow telecommunications equipment to meet the requirements of surviving a fuse-blowing transient. Survival of such a transient is required by various telecommunications authorities. For example, BTR2511 Issue 3 section 3. 3"Transient Voltages and Source Impedance-Conventional Distribution System"issued by British Telecom, requires equipment to survive the type of transient generated by a fuse rupturing. The fuse rupture causes a large rate of change of current through the cable. Cable inductance then generates a back electromotive force (EMF) of the order of-300V (this can be occasionally as high as 400V). It is this transient which the equipment must survive.

The majority of telecommunication transient suppression is designed to limit high- voltage, low-energy transients. Metal oxide varistors (MOV's) and/or transient voltage suppressors (transorbs) can usually suppress these transients satisfactorily and prevent equipment damage. However, high-voltage, high energy transients can destroy transorbs causing them to go short circuit (protective failure mode) and blow the input supply fuse (s). Metal oxide varistors are not necessarily damaged by such transients but have a finite bulk impedance so that the more current they draw the higher the voltage across

them becomes. There is a large difference between the nominal value of a MOV (i. e. the voltage at which it switches on) and the clamping voltage of the MOV (i. e. the voltage which the device clamps to when drawing its rated maximum current). Repeatability is also very questionable when using MOV's : their characteristics change quite rapidly when stressed.

Gas discharge tubes (GDTs) are used to suppress such high-voltage, high-energy transients. GDTs are designed to turn on in the presence of sufficiently high voltage and do not turn off again until the voltage drops below a second, lower level. For telecommunications applications, with a very wide range of possible input supply voltages (-38.5V to-75V), it is not possible to select a GDT which turns on for voltages above the normal supply range and then switches off again under normal telecommunications supply voltages. With a GDT connected as a shunt element directly across typical telecommunications power supply lines, the GDT, once fired, may never switch off (depending on supply voltage). This would cause further supply fuses to blow.

The invention provides a voltage protection circuit comprising a gas discharge tube (GDT) connected in series with a capacitance, the potential difference across the capacitance being arranged to increase in use until the GDT turns off.

The invention also provides a voltage protection circuit comprising a gas discharge tube connected in series with an capacitance; in which the capacitance is arranged to experience substantially zero voltage in a first state with the GDT non-conducting; in

which the capacitance is arranged to experience an increasing voltage in a second state with the GDT conducting; in which the potential difference across the capacitance is arranged to increase in the second state until the GDT turns off.

The invention also provides a method for limiting voltage transients exceeding voltages in a design range across a pair of conductors in which the pair are intended to carry a voltage within the design range; the method comprising the steps of connecting across the pair of conductors a gas discharge tube (GDT) in series with a capacitance.

The voltage protection circuits of the invention are suitable for use as d. c. power transient suppressers, and are especially suitable for allowing telecommunications equipment to survive a fuse-blowing transient.

According to a preferred embodiment, the invention provides a leakage path to allow the capacitance to discharge, once the GDT has turned off.

Embodiments of the invention will now be described by way of example with reference to the drawings in which: Figures 1 and 2 show an example of the power distribution schemes used in telecommunications exchanges of the prior art; Figure 3 shows an example of the transient suppression schemes of the present invention

Figure 1 shows the power distribution system in a typical telephone exchange. A power source, e. g. 48 volt battery feed, is connected to several sets of telecommunications (telecomms) equipment. As can be seen from Figure 1, each set of equipment, e. g. a sub- rack of equipment, is protected by a"top-of-rack"fuse in series with the power supply line. Each group of such sub-racks is, in turn, protected by an end-of-suite fuse connected in series with a common power supply line that provides power via a number of the"top- of-rack"fuses to the equipment. The supply from the battery feed is very high power.

Each end-of-suite fuse forms a branch point from the main power supply and, necessarily in the event of an equipment failure, has a much higher current rating than the individual top-of-rack fuses it supplies. The separate top-of-rack fuses protect each set of equipment in order to prevent one piece of faulty telecomms equipment bringing down the rest of the sub-racks in a suite. These power feeds are typically duplicated for redundancy purposes so that each piece of equipment has a feed A and a feed B (not shown) which come from separate top of rack fuses, separate end of suite fuses and separate power supplies.

As shown in Figure 2, a DC power transient suppresser may be connected as a shunt element, one per suite, near to the point of load. The suppresser is connected in parallel with the equipment and top-of rack fuses and in series with the end-of-suite fuse for that suite.

Figure 3 shows details of a transient suppression circuit according to the present invention. The circuit consists of a gas discharge tube (GDT) in series with a pair of electrolytic capacitors (C). Preferably, a set of varistors (V) across the input prevents the GDT from tripping due to fast, low-energy, high-voltage-transients. A pair of diodes (D) are connected across the electrolytic capacitors to prevent a damaging reverse potential being developed across the capacitors, e. g. during EMC tests such as electrical fast transient tests. A pair of wire-wound resistors (R) are also connected across the electrolytic capacitors in order to discharge them after the GDT has fired and then turned off again. Typical values for equipment with an end-of-suite fuse rated at 20 or 32 amp : the capacitors are 5600 micro Farad (uF) each, the resistors are 2700 ohms each. A 90- volt GDT is selected.

In its quiescent state, with the GDT open-circuit, the transient suppresser capacitors are discharged, thus appearing as a short circuit to any transient voltage applied across them.

For any incident voltage transient, this effectively leaves the GDT connected directly across the power supply. The GDT is rated at 90 volt, i. e. it should nominally fire and begin to conduct at a trip point of 90V +/-10% and turn off at around 50V. When the supply voltage rises above this trip point the GDT breaks down, the voltage across it rising slightly and then collapsing. The series capacitors charge up due to the current being passed by the GDT until the voltage across the GDT is less than the turn-off voltage. At this point, the GDT switches off and the capacitors are discharged via the wire-wound resistors. The resistor rating ensures that the discharge time far exceeds the transient pulse duration. After approximately 40 seconds the capacitors are sufficiently

discharged for the unit to protect against another transient. The example of 40 seconds allows the capacitors to discharge rapidly but without damage to themselves or to the resistors (self resetting). In practice, this is a much shorter time than the that taken to replace an end-of-suite fuse.

The voltage across the GDT equals the supply voltage minus the voltage across the capacitors. If a fuse rupture produces more energy than typical, the capacitors charge up quickly and can invert the voltage across the GDT sufficiently for it to conduct in the opposite direction to the normal case, described above.. When this happens the capacitors are discharged rapidly via the GDT until the GDT turns off.

Electrolytic capacitors are preferred as they are commonly available with suitably large capacitance values and suitable surge current ratings, however, other types of capacitor may be used. Wire wound resistors are preferred as they are able to provide a suitably large power dissipation and allow the specified power rating to be exceeded for short periods of time without damage, however, other types of resistor may be used.

In normal operation the suppresser appears transparent, drawing no current.

The particular design described will tolerate a fuse blow of either 20 or 32 amps, but the invention can cope with a wide range of transient voltage levels, e. g. transients from larger fuse ruptures can be protected against by increasing the amount of capacitance.

Although described with reference to telecommunications applications, the invention is not limited to this kind of application but finds wide application in dealing with high- energy transients.