US2298468A - Contact protection - Google Patents

Description

Patented Oct. 13, 1942 UNITED STATES PATENT OFFICE Bell Telephone Laborato ries, Incorporated,

New York, N. 1., a corporation of New York Application October 4, 1938, Serial No. 233,242

4 Claims.

Two general types of transients have been identified as a result of cathode ray oscilloscope studies of the discharge as contacts make and break in the range of currents and inductive loads usually met in telephone switching practice. One of these, which we shall call the "A" type, is a gas glow discharge and is less destructive than a second type which consists of a succession of sparks which we shall call the B type. The characteristics of these two different discharges will be discussed in detail hereunder. A means is disclosed herein for controlling the type of discharge which occurs in an electrical circuit as contacts are made and broken, so that,

under circuit conditions conducive to the establishment generally of the destructive B type transient, the relatively harmless A type transient is made to predominate, greatly extending the useful life of the contacting members. This is performed by connecting a non-reactive resistance such as a carbon rod in series with the contacting member and the load and immediately adjacent to the contacts to be protected, or the non-reactive resistance may be divided into two units both connected in series with the load and one adjacent to each contacting member. Connection of the non-reactive resistance immediately adjacent to the contacts is important, as it has been found that connecting the same resistance elsewhere in the series connection, say adjacent bers making and breaking contact in an electrical circuit against deterioration.

It is a further object of this invention to supv press radiations of waves which cause interference in radio receivers by means of a device effective also to protect contacts.

It is a further object of this invention to control the type of transient discharge occurring when contacts in an electrical circuit make and break, so that a type identified as destructive to said contacts and which also causes interference in radio receivers is suppressed.

It is a further object of this invention to protect the contacting members in an electrical circuit by the use of an inexpensive non-reactive resistance connected in series with and adjacent to the contacting members- A feature of this invention is the use of a pure resistance to protect contacts.

A further feature of this invention is the connection of non-reactive resistance in series with electrical contacts to protect said contacts.

A further feature of this invention is the connection of non-reactive resistance in series with and adjacent to electrical contacts to protect said contacts and suppress radiation.

The invention may be understood from the following description when read with reference to the associated drawing in which:

Fig. 1 represents a typical electrical circuit in which the contacts of relay I02, which make and break, are protected by non-reactive resistance llll, which is located in the circuit immediately adjacent to the front contact of the relay.

Fig. 1A is a modification of the arrangement of Fig. 1 in which the non-reactive resistance is arranged in two units, "IA and "IE, each located immediately adjacent one of the making and breaking contacts.

Fig. 2 is a drawing, reproducing an actual oscillogram, of the destructive type of transient discharge accompanying a circuit break, in a circuit generally comparable electrically with Fig. 1, except that the method of contact protection of this invention is not employed. Instead of being arranged as in Fig. 1, when the wcillogram per Fig. 2 was taken, the non-reactive protective resistance MI was located adjacent relay I". The ordinates in Figs. 2 to 6, inclusive, represent voltage or current as specified and the abscissae represent time.

Fig. 3 is a drawing, reproducing an actual oscillogram, of a single current surge accompanying one spark-over of the voltage indicated in Fig. 2. The horizontal scale of this figure is approximately fourteen times larger than that of Fig. 2. The peak of the current surge reaches 9 amperes although the steady state current in the circuit before the'contact opened was only .022 ampere.

Fig. 4 is a drawing, reproducing an actual voltage time oscillogram, of the more desirable type of transient which is found to predominate, on the breaking of the circuit arranged per this invention as in Fig. 1.

Fig. 5 is a drawing, reproducing an actual oscillogram of some of the current surges accompanying the voltage transient of Fig. 4.

Fig. 6 is a drawing, reproducing an actual osciliogram, of a current surge on closing the circuit arranged as in Fig. 1, except that the protective resistance is not located according to this invention, but is located adjacent the winding of relay I03. The ordinate represents current which reaches .4 ampere.

It was not possible to obtaini an' oscillo'gram showing a current surge on closure of the circuit per Fig. 1 arranged in accordance with this invention, as the surges on closure are reduced to about one-tenth their original value and were not within the range of the oscillograph sensitivity.

Referring now to Fig. 1, a simple electrical circuit is shown. A path may be traced from ground, through battery, the winding of relay I03, conductor Mignon-reactive resistance IOI, to the front contact of relay I02. The circuit is open at the armature. If current is supplied to the winding of relay I02, its core is energized, attracting the armature-and closing the circuit heretofore traced to ground. When the relay winding is deenergized, the armature separates from its contact under the influence of spring tension in the armature spring. The conductor 104 represents a length of sixty feet of 22 gauge wire which was used when the oscillograms were made. It is one of a pair of wires, the mate to which, conductor I05, is grounded at each end at the positions 01' relays I02 and I03 as indicated. Its length was chosen as representing a common conductor length in a telephone switchreactive resistance IN is 1000 ohms. The steady state current is 22 milliamperes.

When the non-reactive resistance It is located adjacent-relay I03, it is found on opening the contacts as indicated in Fig. 2, that the energy in the circuit is dissipated in a succession of sparks, often several hundred, at increasing voltages. Thevalue of these voltages is indicated by the son-1213f fig. 2. It may be observed that they reachapproximately 650 volts. This is more than fourteen times the full voltage of the power source in Fig. 1. I The ordinate in Fig. 3 indicates that 9 amperes flow through the contacts during a single voltage spark-over per Fig. 2. This is approximately 400 times the steady state current. Fig. 3 does not represent the maximum current surge that has been observed for this condition. Current surges as great as 15 amperes have been observed.

When non-reactive resistance IOI consisting of a carbon rod is located as in Fig. 1 adjacent the contact of relay I02, in accordance with this invention, it has been found that the character of I the transient voltages at the contacts when ,itransient comparatively small as indicated by :theordinats in Fig. 5 which represent the current accompanying a series of voltage surges per Fig. 4 near the beginning. The current surges for a contact in a circuit per Fig. 1 protected per this invention would equal approximately .3 of an ampere which is about fourteen times the steady state current, as opposedto 400 to 680 times the steady state current with the same nonzleactive resistance in circuit in a'difl'erent loca- The above description provides a comparison of the transient voltages and currents developed when a circuit per Fig. 1 is broken under two different conditions, namely, (1) when a non-inductive resistance is located adjacent the relay contact and (2) when the same non-inductive resistance is located adjacent the relay winding. We will now compare the transient currents developed when the circuit is closed for the two conditions.

Fig. 6 shows a current surge when the circuit per Fig. 1 is closed with the non-reactive resistance IOI located adjacent the winding of relay I03. It reaches .4 ampere or approximately twenty times the steady state current condition. This was compared with the current condition on closure or a circuit per Fig. 1 with the noninductive resistance located immediately adjacent the contact. It was found that for this conditron the surge was too small to be measured with the apparatus available indicating that the nonreactive resistance aflords protection also for the circuit closure condition" in that it substantially ewiiii ninates surges due to the discharge of the line The effect illustrated herein, due to the connection of a resistance adjacent to a contact and separating the contact from the line wire leading to an inductive load may be explained briefly as follows: Assume first that the line is directly connected to a contact and the two contacts commence to separate. The circuit is ruptured, leaving energy stored in the inductive load which flows into the line wire (considered now as a capacity) and charges it to a voltage at which a spark takes place between the separating contacts. The energy in the line is discharged through a minute area of the contacts, violently fusing it and reopening the circuit. The voltage between the contacts at the instant of rupture may be of either sign, depending on the phase of the oscillation of the line wire at which the rupture occurs, but it is below the sparking voltage, and the circuit remains open and the hue wire is recharged by the energy still remaining stored in the inductive load until sparking voltage is reached a second time. is repeated many times until the distance between the constantly separating contacts is too great to permit the decreasing stored energy in the inductive load to charge the line wire to a spark- This process ing voltage. The repeated sparks and explosive reopenings volatilize the contact metal and produce repeated powerful high frequency oscillations in the line wire which are radiated and cause noise in adjacent circuits. This phenomone has been called for convenience the type B transient.

When, however, the capacity or the line wire shunting the contacts and the inductive load is very small, the rate of flow of energy from the inductance as the contacts separate is so rapid that the voltage is not permitted to pass through zero after the first few spark-overs occur, but is maintained in the neighborhood of 300 volts and a gas glow discharge is established. This spreads out over the contact surface (reducing the current density) while the stored energy flows through the glow discharge at a steady moderate rate until it is nearly all dissipated, the reduced concentration of current resulting in less erosion of the contacting areas. It is usually not practical to reduce the length 01 the line wire as much as is required to produce the glow discharge (A type) transient, but the eflect of the line -wire can be very much reduced by the method of this invention. By interposing a substantial non-inductive resistance between contact and line wire, the rate of discharge of the line is reduced sufllciently so that the contact voltage is maintained at or above the ionization voltage or air and the stored energy is then dissipated continuously at a relatively low rate through the diilused gas glow discharge.

As stated above, the non-reactive resistance may be divided into two units, and one of each of said units connected to each of the making and breaking contacts as indicated by resistances HI IA and IBIB of Fig. 1A. The magnitude of the resistance and reactance o! the two combined resistances INA. and IMB should not aggregate more than that of the single resistance l! 0! Fig. 1. They should be located each immediately adjacent its respective contact. The relative amounts of the resistance connected to each contact does not appear to be important. The values and form of the voltage and current curves described for Fig. 1 apply equally to Fig. 1A.

What is claimed is:

1. In a telephone or telegraph switching system, an electrical circuit comprising a ground, a battery, a relay winding, a first electrical conductor, a carbon resistance, a pair of cooperating relay contacts adapted to open and close, a second electrical conductor, an electrical conducting path extending sequentially in series from said ground, through said battery, through said wind ing, through said first conductor, through said resistance, said resistance being connected immediately adjacent one of said contacts, through said contacts, through said second conductor and thence to said ground, the magnitude of the reactance of said resistance being of the order to promote the establishmentdominantly of a gas glow transient discharge as said contacts make and break to protect said contacts against deterioration.

2. In a telephone or telegraph switching system, an electric circuit, an electromagnetic switching relay therein, a pair of cooperating metallic electrical contacts on said relay adapted to open and close to interrupt an electrical current through said contacts and a lumped resistance, of the order of reactance of carbon rod. connected immediately adjacent one of said contacts, to protect said contacts against deterioration as said contacts make and break to successively close and open the path or the current.

3. In a telephone or telegraph switching system, a circuit extending sequentially in series from ground, through a battery, through a lumped inductance, through a conducting line, through a first non-reactive type resistance located immediately adjacent one or a pair of current making and breaking cont-acts, through said contacts through a second non-reactive type resistance located immediately adjacent the other or said pair or contacts and thence to ground, the magnitudes of the resistances and reactances oi. said non-reactive type resistances being of the order to protect said contacts against deterioration.

4. In a telephone or telegraph switching circuit, a circuit including a source of electromotive force, a lumped inductance, a conducting line, a pair or non-reactive type resistances, one of said pair located immediately adjacent a first metallic contact and the other of said pair located immediately adjacent a second metallic contact, means for making and breaking said contacts to open and close said circuit, the electrical characteristics of said resistances being chosen to protect said contacts from deterioration.

AUSTEN M. CURTIS.

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