DE504544C - Circuit arrangement for telephone systems with main and sub-exchanges - Google Patents

Description

The invention relates to a circuit arrangement for telephone systems Main and sub-exchanges, in particular with relay dialer operation, where to connect a subscriber of the sub-exchange to the main exchange the same for dialing numbers for both directions of traffic Clutch relay is used.

-The invention consists in that the clutch relay, which itself forms a link in the relay chain to be excited, to establish a callback connection between two subscribers of the same sub-exchange de-energized and so the Number dialing is made available again by the original excitation circuit remains open until the callback has been dealt with.

Such a sub-office should meet the following conditions:

1. Connections from the sub-office to the main office and vice versa,

2. Connections between subscribers of the same sub-office (callback),

3. Charge the sub-office battery via the connecting cables from the battery of the Main office.

The invention now particularly relates to sub-offices in which relays are used to couple the lines together. It is also known the clutch relay can also be used for dialing numbers. Satisfying the above requirements is difficult when

z. B. for callbacks one of the coupling relays has connected the caller to the exchange, but should serve as a number relay again when called back. Also prevents in the event of malfunctions the clutch relay energized as a result of the malfunction allows a later number to be dialed. It is known that the calling party when making the callback hangs his handset, so that the low-impedance Resistance of his microphone does not short-circuit the wake-up current for the desired point. This hanging up of the listener at the right moment is, however, with fully automatic Operation not secured. The invention therefore leaves the listener at the calling station during the establishment of the callback posted.

In Figs. 1 and 2, an embodiment of the invention is shown. The figures are divided into fields to make it easier to find the reference symbols given in the description. In the description, z. B. j [b 1 the contact b ι of the relay B lying in the field 7 of Fig. I.

Fig. 1 shows the additional facilities for a connecting line 4 / a, b in the main office. This additional device consists of a number of relays and two stepping dials. One rotary selector serves as a follow-up mechanism with three brushes zjDtn i, SjDm 2 and 8 [Dm 3 and the magnet SfDm, the other rotary selector is a pick-up with two brushes ¹ J [Da 1 and 8 [Da 3, a working contact y [Da 2 and the

μν

Magnet SjDa. In the field ι of Fig. Ι a selection VW is indicated, which can be of any type. A call finder could also be put in its place. In field 5 of Fig. 1, the known three brushes of a line selector LWa, LWb and LWc are shown, which itself can be of any design and circuit. The other voters of the main office are not signed.

Fig. 2 shows a sub-office with 10 subscriber connections Ni to No. Only relays are used as switching means. The battery of the shown in field 4

t5 sub-office has a much lower one Voltage than the main office battery.

I. Connection from subscriber N2 to the main office

i. In the quiescent current, a charging current flows for the sub-office battery:

Fig. 2: 4 / earth, battery, fuse Si, resistance Wi, connecting wire α », ijp 3, S, α wire to the office,

Fig. Ι: 4 / c 2, 3 / w 3, y 3, Ü, ν r ι and 2JSp, battery, earth.

In this circle, Sp is constantly excited. S does not respond to this weak current.

2. Subscriber N 2 lifts the receiver: Fig. 2: Earth, winding I of 3 / Γ2, i / 23, N 2, 1 21, / 21, ακ-wire, i \ p 3, S, α- Vein to office,

Fig. 2: Earth, winding I of 3 / T 2, t \ 23, battery.

In this circuit is J speaking? and T. 5 ¹ excited M:

Fig. 2: Earth, % \ s 2, M, battery. The relay T first closes a hold circuit via battery, 8 / Γ 2 II, t 22, F 2 I, lojst 3 and g 3, 1 04, P ₁ m 1, earth.

P and F 2 respond, whereas F 2 picks up at the moment without any consequences, ijp 3, on the other hand, opens the connection lead, so that no further Γ relay can respond. Although the winding I of the T-relay receives a calling line power from the local battery, z. B. Earth, 4 / Γ0Ι, £ 03, No, toi, foi, resistance Wi, battery of the sub-office.

Since the battery voltage is low and the 4JWi resistance is very high, Γ ο Ι cannot respond.

Contact SJP ² energizes H: earth, $ js 2, p 2, X H, battery.

- .. τ Jh 2 switches the & -wires through to the office. 3 / ^ 21 and 3 / i23 switch the calling subscriber line to the exchange line.

3. During the subsequent number dialing, relay 5 does not drop because it is delaying is.

The state: T 2, S, M, H, P, F 2 excited, remains until it is triggered.

4. As soon as the calling line is switched through in the sub-office, the following comes Supply circuit established:

Fig. 1: Battery, 2JSp, 3 / w 11, Ü, 3/3, u 3, 4 / c 2, α-wire to the sub-office, subscriber loop, fr-wire from the sub-office, 3 / c 3, y 2, impulse relay A, earth.

A responds. g / a 2 energizes the delay relay V: battery, gjV, a 2, earth.

2 / α ι closes the loop for the preselector line relay R: battery, ijR, loop, f-contact of the Γ relay in the preselector (not shown), transformer winding Ü, αϊ, £ 3, t, earth.

The preselector (or call finder) occupies a group selector (or number memory) in a known manner. At gjv 13, V applies earth to an auxiliary wire h . The drag contact 3 / ü 11 switches off Sp and puts the battery back on the α-wire without interruption. The drop in Sp opens the test circuit for the line selector at contact 6jsp 1 and thus blocks the connection line against incoming connections.

5. The number dialing consists in a known manner in short interruptions of the supply current. The impulse relay ^ jA follows the interruptions of the supply current and establishes the connection _go via the selector in a known manner with its contact 2 / α ι.

6. The follower Dm follows the number selection: Earth, 9 / ^ 13, pilot wire h, 8jf 2, 03, rotary magnet Dm of the follower, battery.

Since the subscriber does not dial the code number 9s of his sub-office, the brush 8 / Dm 2 comes to a contact connected ^{to the relay 8 / i 7 II.} The control relay gjW was energized during the current impulse: earth, gjv 13, W, a 2 at rest, battery.

In the interval after a current pulse series W falls due to short circuit by a FII 2, and energized: Earth, 9 / ^ 13 h pilot wire, 8 / i «4, w i, Dm brush 2, F II, battery.

8 // 2 opens the circuit for the rotary magnet Dm of the follower mechanism, so that Dm is not switched further. 8 // 2 binds F II via g \ v 13.

7. If the subscriber hangs up after the call, the main office sjA n ₀ drops off continuously. 2 / α ι opens the loop to the main office voters who trigger in a known manner. 9 / a 2 shorts V so that it drops. The circuit for relay 8jF II is interrupted by contact gjv 13. The step switch mechanism returns to its rest position on earth interrupter Syun River ν, 12, b 2, since contact arm 3, Dm contact arm 3, the rotary magnetic Dm, battery.

Via contact 3/2/11 the feed relay Sp is again connected to the α-wire to the sub-office.

8. In the sub-office (Fig. 2), S drops out when the subscriber hangs up the phone. 5 / j 2 opens the hold circuit of M and H. lojm 1 brings P, F 2 I and Γ 2 II to fall, so that all relays of the sub-center have returned to the rest position. P is the last falling relay. It closes the charging circuit again at ijp 3, so that the relay 2JSp responds in the main office (Fig. 1), which in turn enables access to the connection line from the line selector (Fig. 1) through contact 6jsp ι.

II. Connection to sub ¹ ^ nehmerNo

9. The line selector (Fig. 1, field 5) is set to the tenth contact by a subscriber in the main office. Are on the line selector, both the a- and for fins of all numbers that are connected in the sub-office, switched frequently, as only one connecting line leads to the sub-office. The c-lamellas, on the other hand, lead individually to test resistors. After the single selection, the line selector checks for free: earth, test relay of the line selector ^ P LW, brush / JFc, 6 / wi o, sp 1, p 3, C, battery.
C responds. 2Jc 1 excites B and holds Sp:

Earth, 2 each 1, B, Sp, battery.

4Jc 2 and 3 / c 3 switch the connection lines to the sub-office to the wires sa and sb arriving from the line selector. 6 / c 4 energized /: earth, 6 / c 4, p 2, y 5, III 2, fr 1, /, battery.

10. The recruitment of the voters of the sub-office should take place slowly because of the tension the battery there is low (6 to 10 volts) and the magnets are therefore slow work. The singles election is therefore repeated in an arbitrarily slow amount of time. It the processes in the main office should first be described.

By moving the contact 3 / c 3, the battery is connected to the fr wire. 4 / fr wire, 3 / c 3, u 2, 4/1 3, battery.

Via 6 / c 4 / is excited, and 4 / i 3 opens the fr wire to the sub-office as the first number current surge. The gripper starts up: battery, S [Da, i 2, earth.

Contact Da 2 of the tap energizes relay III: earth, 6 / c 4, p 2, 3/5, γ j Da 2, relay III, battery.

Contact 7 / ΙΠ 2 opens the excitation circuit of /, which drops out. 8 / i 2 switches off the rotary magnet Da of the tap, and 7jDa 2 interrupts the circuit for relay III. When contact 7 / III2 drops, / is re-energized. / switches the rotary solenoid Da on again via contact 8 / i 2. /, Da and III work as self-interrupters, with the contact 4 / i sending 3 current surges to the lower office until the pickup brush 7JDa ι reaches the slat 10 occupied by the line selector. In this position relay P responds: battery, j \ P, i 1, Da 1, lamella 10, line selector brush LWc, test relay PLW, earth.

Z \ p ι connects the outgoing α-wire to the line selector. 6 / p 2 closes a ground current for C and opens the excitation circuit for /. 6 p 3 opens the test circuits of other line selectors (c-slats).

11. Now the line selector sends in the usual way via the brushes LWa lineio, zip 1, Ui, 4 / c 2, α-wire, subscriber alarm clock, earth ringing current to the point N o, as will be shown later. If the receiver is picked up at point N o, direct current flows via the path just mentioned and the relay 3 / C / I responds. In addition, the wake-up current is switched off in the line selector in a known manner, ζ / ιι 2 switches the fr wire through to the line selector, so that feed current can now flow from the line selector. As soon as the current loop has come about from the line selector via the subscriber line, the caller initiates the count.

12. If the caller hangs up the phone, go triggers the line selector and P's hold circuit is opened. Relay C drops out via contact GlP 2. 2Jc 1 opens the circuit for B and Sp. Via breaker 8 / Un and contact ν i2, for 2 the pickup Da runs home until contact brush Da 3 runs off the segment when it reaches the rest position.

13. The following processes take place in the sub-office: As soon as a line selector has checked the main office (see section 10), the fr-wire is connected to the battery. This battery excites V and U.

Fig. Ι: battery, 4 / i 3, zl ^{u 2} > ^c 3> fr-vein. Fig. 2: ι lh 3, V, sti, U, earth.

14. Number dialing.

Each time contact 4 / j 3 (Fig. 1) is opened, the fr wire is de-energized.

First power interruption: V holds the armature because it is delayed. U, on the other hand, drops no, initially T 1 is excited:

Fig. 2: Battery, 7 / Γ 1 II, p 1, 6 / w 1, g 2, ν i, h i, earth.

T ι binds via battery, yjT 1 II, t 12, relay FiI, io / w 2, 1 04, P, ν 3, w, earth.

P speaks. yfp 1 opens the excitation circuit for T i, ijp 3 blocks the connection line against outgoing assignments. St responds to: battery, 6jSt I, g 1, u 3, ν ι, h 1, earth.

15. First power supply: Fig. Ι: Battery, 4 / i 3 ,. 3 / «2, c 3, fr-vein, Fig. 2: ijh 3, V, st i, St II, G II, earth.

G speaks and holds on battery, 6] G I, gi, u 3, vx, hi, earth.

16, Second power interruption:

St drops and excites Γ 2: earth, 6 / fe ι, ν ι, g 2, st 2, 7 \ t 14, 8 / Γ 2 II, battery.

Γ 2 binds: battery, 8 / Γ 2 II, ί22, F 21, io / ji 3, ί 04, P ₁ ν 3, w, earth.

17, Second power cut.

Fig. Ι: Battery, φ 3, sju 2, c 3, & -Ader, Fig. 2: 1 / Ä2, F, j # i, C /, earth.

6 / «3 opens the holding circuit for winding G I, which drops, io / m 2 opens the holding circuit for T ι and F 1. The current state differs from that of the occupancy only in that T 2 and P are excited. At the third interruption, U drops out again and excites Γ 3: earth, 6 [hi, ν τ, g2, ui, 8 / i 24, T 3 II, battery.

The alternating play of U, St and G ao is repeated until finally gjT ο II is excited at the tenth current impulse, which binds itself via the battery, 9 / Γ 0 II, t 02, Fo I, xo / P, ν 3, w , Earth.

4 / ioi and £ 03 connect the subscriber microphone unit N ο to the connection line. In section 11 it is described that after dialing the number wake-up current is sent over the α-wire: Ringing current source in the line selector (not shown), line selector brush LWa, line Sa, 2> \ p ^τ > U I, 4 / c 2, α-wire to the sub-office,

Fig. 2: i / S, φ οι, TVo, ί03, ι / Α 2, earth.

When lifting the handset at the intercom station N ο, direct current flows so that 6 * responds. $ [s 2 excites M and H. i \ h 2 switches through the fr wire. 1 / A3 opens the surge circuit, ν drops. ι o / m 1 keeps the excitation circuit for T o and P closed.

18, tripping.

If the listener is hung up at JV o, 6 "drops, 5 / s 2 opens the circuit for M and H. io / m ι breaks the circuit for P, T o and F o. All relays of the sub-office are thus in the Returned to rest position.

III. recall

(Connection from subscriber N 2 to subscriber N ο of the same sub-office)

19. When subscriber JV 2 picks up the receiver, the processes described in sections 1 to 5 occur. The caller N 2 first dials the code of his sub-office, z. B. 223. As described in section 6, the brush Dm 2 of the follower mechanism is now placed on the contact 8 / ra 1 by the first series of impulses. The relay g \ W energized while the dial is running drops out again and energizes relay I: Battery, 7 / I, 8 / w 1, Dm 2, wi, 114, g \ v 13, earth.

Relay I binds via 7 // 3, 11, gjv 13, earth.

The second series of impulses causes the traveling mechanism to reach contact 8 / ra 2. As soon as relay g \ W has dropped out again, relay II is energized: battery, 7 / II, 12, 8 / »2, Dm 2, w i, u 4 , 9 / w 13, earth.

Relay II connects via battery, 7 / II, I 2, II i, 9Jv 13, earth.

The third series of impulses causes the follow-up mechanism to get to contact 8 / ro 3. After the dial has run out, relay gjW drops out again and now energizes via its contact 8 [w 1 relay y \ G and III: battery, 7 / ΙΠ, GI, II3 , 8 / »3, Dm 2, wi, u 4, gjv 13, earth.

Relay III connects via: battery, 7 / ΙΠ, III i, II i, gfv 13, earth.

G binds itself via its second winding: battery, gjG II, g 4, υ 13, earth.

As described earlier, the contact 2Yes ι hires the voters of the main office. However, since a recall is to be established, these voters are no longer necessary. Contact 2 / ^ 3 therefore opens the loop between the selectors that trigger.

In the follow-up movement, contact η 3 is followed by ten further contacts. The participant now selects the number zero (single row 1 to o) as the fourth series of impulses and thus places the follow-up mechanism on the tenth of these slats. After this series of impulses has elapsed , relay W drops out again and now energizes 8JF I and Y. At the same time, the heat relay Th is energized: battery, 8 [TH ₁ Y, F I, III 3, lamella o, Dm 2, wi, u4, gjv 13, earth.

F binds: battery, 8 / i ⁷ II, / 2, g [v 13, earth. too

The excitation of relay Y enables the transmission of the series of current impulses selected by subscriber IV 2 (10 current impulses corresponding to the selected number "zero") to the & wires via contacts 4 / i 3 and y 4 Among other things, by means of the magnet 8 \ Da and the relay 7 / ΠΙ, which works as a pick-up. The series of current impulses appears in the sub-office as dialing numbers, and the use of the relay T for this dialing is made possible, even if one of them, relay T 2, is already energized by the caller N 2.

20. 4/3/4 connects the & -wires via 3 ^ 2 and 4Ji 3 to the battery and also via 4Jx r and AC relay WE to a ringing current source RM. Furthermore, 3/3/3 opens the α-wire, so that the supply current is interrupted. Therefore, in the sub-office 5 "(Fig. 2) i _ao drops. Thereupon 5 / s 2 opens the circuit for M and H. M drops with a delay, H then-

against immediately. Via the normally closed contact i [Ji 3, relays V and U are connected to the & -wire, which is connected to the battery in the main office. V therefore closes the hold circuit for P, S / i ⁷ 21 and T 2 II at io \ v 3 before io [m 1 opens. The two corresponding AC relays (Fig. 1) 4JWE and (Fig. 2) ι [W move their contacts.

Fig. Ι: Battery, Rufstrommaschine 4 [RM, WE, χι, y 4, & -Ader zum Unteramt, Fig. 2: Capacitor i / C 1, W, earth. In the sub-office (Fig. 2), 10 / iF first closes the circle: Earth, 10 / ^ 8 // 22, ί22, T2IL battery and at the same time opens the holding current for 8 / F 2 I. First, F 2 drops and opens the contact 8 // ^: 22, only then does relay T 2 drop out. This order is necessary so that F does not bind.

In the main office (Fig. 1) after a short time 4Jwe energizes relay X : battery, 4 / X, we, gjv 13, earth.

X connects via 4 [x 2, 4 [x 1] opens the alternating current circuit for the & -wires, so that i \ W also drops in the sub-office (Fig. 2). Earth is again connected to contact \ o \ v 3 via the normally closed contact of 10 / «/. The relay 10 / P drops out as soon as relay 8 [T 2 II has opened its contact 122 . Now the intercom station N 2 is connected to the connection lead with the receiver lifted and via ijp j to the α-wire. But since the main office (Fig. 1) 3/3/3 has switched off the battery from the α-wire, the apparent call from JV 2 has no effect. The state in the sub-office is as follows:

N 2 picked up the receiver, but does not generate an xA.nruf. The relays V and U are energized as with a simple assignment from the line selector (see section 10). All T and ^ relays are idle. The Γ relays can therefore be used for the subsequent number selection effected by the pickup DA (single selection). 2i. Single choice.

In the meantime, the following has happened at the main office:

After the last number dialing, ('Fig. Ι) 8 / F II responded and 2Jf 1 closed the following circuit: Earth, 2 / g 1,

/ i, B, Sp, battery.

B and Sp respond. C is excited via contact 6 [sp 1: earth, 2 [g 1, f 1, brush Dm 1 of the stepping mechanism via lamella o, 6 / wi o, sp i, p 3, C ₁ battery.

4jc 2 and 3 / c 3 flip over without any effect, since the α wire has already been disconnected from the battery via contact 3/3/3. After a short time, as already described, 4 / X responds, and this closes the excitation circuit for J, since the excitation of F means that the hold circuits for relays I, II and III are already open: Earth, 6 / c 4, ρ 2, χ 3, 7 / III2, b ι, J, battery.

Through the excitation of /, the rotary magnet of the tap Da is excited via contact 8 / i 2, and 7 [Da 2 excites relay III. Contact III2 opens the excitation circuit from / again. The voter in the sub-office is controlled via 4/1 3, as already described in section 10, whereby /, Da and III work as self-interrupters. The game repeats itself until the brush 7 [Da 1 comes to lamella 0 and P responds: Battery, 7 [P, ii, Da ι on lamella ο of the tap, lamella ο of the tap, lamella ο of the follower in field 2, Dm 1, f 1, g 1, earth.

6 [p 2 closes a hold circuit for C. 6 [p 3 opens all test circuits for the line selectors. Zip ι puts over 31 1 ringing current to the α-wire.

22. In the sub-office, the single choice (10 current impulses) of the type described in section 14 and below had energized the relay T ο, and T ο had also bound itself. During this number dialing, T 2, which, as described in section 20, had dropped off after relay F, responded again, but without consequences, because no feed current could arise for N 2 , since the α-wire in the main office is dead. The ringing current flows from the main office (Fig. 1) via: _go

Fig. Ι: Battery, ringing current machine, 2, [RM, y 1, p 1, Ui, 4Jc 2, α-wire to Unteramt, Fig. 2: i / S, 4 / ioi, JV o, £ 03, 1 / / 12, earth.

A small branch current flows through the capacitor 2 / Co ₁ 3 // 21, 121, N 2, 1 23, Γ 2 I to earth as a sign for the waiting subscriber JV 2 that subscriber JV o is being called.

23. When JV 0 answers, the microphone from JV ο closes a direct current path in which $ [U I (Fig. 1) in the main office and ijS respond in the sub-office (Fig. 2). In the main office (Fig. 1) S / w 4 opens the holding circle for Y, who drops out. 3/3 '2 puts the ^ -relay, which had held itself during the singles selection via y 2 (excited), back to the & -wires, and 3/3/3 closes the feed circuit:

Fig. 1: Battery, $ \ v 11, 3/3, gi, 4 / c 2, α-wire to the lower office,

Fig. 2: i [S, N o, ijh 2, earth. 'no

ι [S responds, and 5 / j 2 excites M and H. 1 [h 2 closes the supply circuit to the ^ 4 relay in the main office, ι [h 3 opens the circuit for V and U. During the fall time of V is the following Path closed: α-wire, i / 5Y ν 2, m 2, connection wire on, 3 // 21, # 21, JV 2, t 23, T 2 I, earth.

T 2 responds and connects JV 2 to the speech wires. The two participants begin the conversation.

24. If the desired subscriber JV ο does not answer, the

Caller N 2 can be switched back on to the connection so that he can release it. The heat relay 8/77 / in fig. After about 20 to 30 seconds, TH energizes relay U: battery, 3 / {7 II, th 1, b 3, earth.

U binds itself via 3 / «5. 8 / m 4 opens the excitation circuit for Y and TH until the heat relay has cooled down so far that it opens its contacts; the α-wire is now on the battery: battery, 3 / ife 2, ti 3, gj, 4 / c 2, α-wire to the lower office.

Furthermore, 3/3 switches? 2 over ^ back earth to the & veins, so that V drops off in the sub-office. Since A ^T o had not answered, so .S "and therefore M are not excited, \ a \ v 3 opens the holding circuit for P, T o and F o, which drop. Now i //> 3 connects those in voltage

standing α-wire to the connecting end , so that now T 2 I can respond. The waiting subscriber JV 2 now draws power and keeps relay A energized in the office until he recognizes the non-response from subscriber N o due to the absence of the call sign. As soon as he hangs up his receiver, he interrupts the supply circuit, so that relay A drops out in the main office. 9 / a 2 short-circuits in the main office ijjV until it drops out and initiates the triggering of the tap in the manner described earlier. After the pick-up has returned to the rest position, the follow-up mechanism also runs home on its way: battery, SjDM, segment DM 3, Da 3 (at rest), b 2, ν 12, interrupter Un, earth.

25. Should the waiting subscriber JV 2 forego the connection, i.e. hang up the receiver before the heat relay TH in the main office made the switchover described, the triggering takes place as follows:

Nothing changes in the state until TH responds. Since JV 2 had hung up the phone, the feed current does not come about and the ^ relay in the main office drops out immediately. It then initiates the release as already described.

Claims (6)

Patent claims: i. Circuit arrangement for telephone systems with main and sub-exchanges, in particular with relay dialer operation, in which the same coupling relay, which is also used for dialing numbers, is used to connect a subscriber of the sub-exchange to the main exchange for both traffic directions, characterized in that the coupling relay (e.g. T 1 ), which itself forms a link in the relay chain to be excited by dialing the number (T ι to T o, windings II), de-energizes to produce a callback connection (between two subscribers of the same sub-exchange) and is thus made available to dialing numbers again by the original excitation circuit (connection lead on, p 3-contact, α-wire, exchange battery) remains open until the callback has been dealt with. 2. Circuit arrangement for telephone systems according to claim 1 for the production of callbacks, in which the caller does not hang up the handset during the production of the callback, characterized in that a winding (Tl) of the coupling relay over the handset of the calling subscriber station during the production of the callback (T) is applied to the Anschalteader {an) , which in turn is switched off by the exciting potential (-60 volts in the main office; at ιIp 3). 3. Circuit arrangement for telephone systems according to claim 1 and 2, characterized in that after the production of the point desired in the callback, the connection potential (60 volts on the α-wire) is temporarily applied to the connection lead (on) (via ijv 2 and m 2 ). 4. Circuit arrangement for telephone systems according to claim 1 to 3, characterized in that the clutch relay (T) has two windings, one of which (7 'I) in the connection circuit (T 2 I, t 23, N 2, t 2i, on, α-wire), the other winding (TII) is in the number range (field 8). 5. Circuit arrangement for telephone systems according to claim 1 to 4, characterized characterized in that the connection circuit for connections from one of the subscriber lines to the connection line the charging current of the battery of the connection device leads. 6. Circuit arrangement for telephone systems according to claim 1 to 5 for callbacks, characterized in that in the time of the disconnection of the excitation potential (-60 volts on the α-wire) of the connecting lead (on) the battery of the switching point only a fault current through the clutch relay sends (earth, T 2 I, JV2, 4Jwi, Si, battery, earth). For this purpose 2 sheets of drawings