GB2111771A

GB2111771A - Surge protection circuits

Info

Publication number: GB2111771A
Application number: GB08138001A
Authority: GB
Inventors: Om P Ahuja
Original assignee: Porta Systems Corp
Current assignee: North Hills Signal Processing Corp
Priority date: 1981-12-08
Filing date: 1981-12-16
Publication date: 1983-07-06
Also published as: CA1187929A; DE3151539A1; FR2518838B1; GB2111771B; SE445409B; SE8107348L; JPS58107023A; FR2518838A1

Abstract

A surge protection circuit (10) for use with telephone lines or circuits having logic elements or the like incorporated therein includes the usual gap arrester (15), a heat-sensitive switch (16), and a transient suppressor (17) providing a parallel interconnection between the protected line and source of ground potential (14). The transient suppressor (17) has a breakdown voltage substantially below that of the gap arrestor (15) and has a capability to react with a much shorter time period than that required by the arrestor. The switch (16) is responsive to heat generated in the heat coil (19), to protect against surge currents, and is also responsive to heat generated by the arrestor (15) due to prolonged overvoltage. <IMAGE>

Description

SPECIFICATION Surge protection circuits This invention relates generally to current and voltage surge protection circuits for use in conjunction with telephone lines or circuits having componenets which are sensitive to fast voltage rise rates, e.g. logic elements and/or other transistorised circuitry.

In the telephone protector module art, most test specifications address the problem of excess voltage and current transients and lightning strikes as well as power line crosses by requiring the protector to pass surges having a variety of rise rates to given voltages. Typical are 1.2/50, 2/50, 4/40, 4/200, 6/150, 8/20, 10/50 and 10/1,000, at various current levels.

With a faster rise rate, the permitted breakdown voltage is correspondingly higher to allow for the typical ionization time of commonly used discharge components, such as gas tubes and air gap arrestors. Typical rise rates range from 0.1 to 10 microseconds.

However, standard TTL logic circuits and other similar circuits may respond to signals as fast as 3 ns. Thus, transients greater than 3 ns are likely to create error readings. Further, most MOS devices, as well as many discrete semiconductors, especially fast switching transistors, are very vulnerable to fast rise rate transients, even at such low energy levels as ten microjoules. Given such sensitivity, it is apparent that known protective devices are not adequate.

The problem is not solved by the mere provision at the board level of low clamping voltage transient suppressors alone. The grounding available at the board level is not adequate for such large currents which may be fed through standard protective devices on the main frame. Such currents may be up to 10 microseconds in duration, at voltages up to 1,250 volts or more and at currents of several hundred amperes.

According to the present invention there is provided a current and voltage surge protection circuit for use in conjunction with a telephone line or circuit having components which are sensitive to fast voltage rise rates, the protection circuit comprising a heat coil in series with the telephone circuit and a plural connection of the telephone circuit to ground potential including, in delta, a gap arrestor, a heat-sensitive switch and a transient suppressor, the heat-sensitive switch being responsive to heat developed in the heat coil and the arrestor, and the transient suppressor having a breakdown voltage substantially lower than that of the arrestor and a response time substantially faster than that of the arrestor.

In an embodiment of the invention described below, as a transient wave front voltage rises the transient suppressor clamps the rising voltage as soon as it reaches the breakdown voltage of the suppressor, with virtually no delay. Thus, the maximum voltage seen by central office equipment is the voltage across the suppressor. Most of the current is conducted to ground at the main frame level. As the voltage of the wave front continues to rise, the heat coil drops the remainder of the voltage. When the applied voltage reaches the breakdown voltage of the arrestor, a further clamp is applied at approximately 30 volts. If the current is sustained, the heat coil and heat-sensitive switch will function in normal manner.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawing, in which: Figure 7 is an electrical schematic view of a protection circuit constituting an embodiment of the invention; and Figure 2 is a graph of voltage against time showing a typical transient wave front.

Figure 1 shows a protection device or circuit 10 embodying the invention, the circuit being connected in series with a typical telephone input line 11 having an outside plant (OP) side 12 and a central office (CO) equipment side 13. The line 11 is connected to a source of ground potential 14 through the delta interconnection of an arrestor (G) 15, a heat-sensitive switch (SW) 16 and a transient suppressor (TS) 17. A heat coil (HC) 19 operates the heat-sensitive switch 16, and is in series with the line 11,as is well known in the art.

Figure 2 is a graph illustrating the progress of a transient wave front, in which the x axis 25 is calibrated in terms of time, showing intervals of 1 ns at 26 and 1 ms at 27. They axis 28 indicates voltage rise, including a first level 29 corresponding to the breakdown voltage of the transient suppressor 17, and a second level 30 indicating the breakdown voltage of the arrestor 15. (Although a gas tube type gap arrestor is illustrated in the drawing, the equally well known air gap type may be substituted to perform an equivalent function). The rising wave front of a typical surge is illustrated by reference character 31.

As the transient wave front voltage rises, the transient suppressor 17 clamps the same as soon as it reaches the breakdown voltage of the suppressor, with virtually no delay. From this point on, most of the current is now conducted through the transient suppressor 17 to ground. As the voltage on the wave front continues to rise, the heat coil 19, in some instances, drops all of the remaining voltage.

In the case of stronger surges, as soon as the voltage of the wave front reaches the breakdown voltage of the arrestor 15, a further clamp occurs at approximately 30 volts. If the surge is of a sustained nature, the heat developed in the arrestor 15 will operate the heat-sensitive switch 16 causing permanent grounding.

Should the surge be continuous, and be of less voltage than the normal firing voltage of the arrestor 15, but higher than the breakdown voltage of the transient suppressor 17, the drop across the heat coil 19 will continue to build up heat, eventually causing the heat-sensitive switch 16 to close. This is ensured by placing the heat coil 19 in series with the transient suppressor 17.

Finally, should the surge current be higher than the transient suppressor 17 can accommodate, it will permanently fail short, causing all of the voltage drop to be across the heat coil 19 to accelerate the operation of the heat-sensitive switch 16.

It may be seen that as a result of the abovedescribed structure, the factor of rise rate is eliminated, with resultant protection of circuitry of the associated telephone line or circuit that is sensitive to fast voltage rise rates, e.g. logic elements and/or other transistorised circuitry. The sensitivity of the transient suppressor 17 being so much faster than that of the arrestor 15, it serves to protect the protected circuitry against the ictus of the wave front surge, without disturbing the normal functions of the arestor 15 and the heat-sensitive switch 16.

Claims

1. A current and voltage surge protection circuit for use in conjunction with a telephone line or circuit having components which are sensitive to fast voltage rise rates, the protection circuit comprising a heat coil in series with the telephone circuit and a plural connection of the telephone circuit to ground potential including, in delta, a gap arrestor, a heat-sensitive switch and a transient suppressor, the heat-sensitive switch being responsive to heat developed in the heat coil and the arrestor, and the transient suppressor having a breakdown voltage substantially lower than that of the arrestor and a response time substantially faster than that of the arrestor.

2. A protection circuit according to claim 1, in which the transient suppressor is in series connection with the heat coil.

3. A current and voltage surge protection circuit substantially as herein described with reference to the accompanying drawing.