GENERAL

 Each C-switch unit comprises a 5 x 10 reed-relay matrix with full availability between its inlets from the B switches and its outlets to the D switches and junctors. 

Each C-switch unit may be interconnected with others (either 5 x 10 or 5 x 5) to increase the number of outlets to D switches and junctors. When this is done, the access relays and alarm circuit are redundant in all the units except the one using the positive-battery feed. The connections of the CM, CT, H and HD leads are explained in notes on the diagram. The AM, CA, CF, CM, CT and PA leads are used only on the first unit of a 'horizontal' section, and are left disconnected on units 2, 3, etc. A C switch may comprise several 'horizontal' sections, each one serving a particular Major- group. 

FACILITY SCHEDULE Provision is made for:

 (1) Operating the access relays on receipt of signals from the A-switch pattern relays and the C-switch selector.

 (2) Connecting any of the five inlets to any of the ten outlets. 

(3) Access to the H wire of each 8-C link to determine whether the link is free or busy. (4) Marking the five crosspoint relays associated with each inlet from the B switches.

 (5) Holding the selected crosspoint relay when the mark condition is removed. 

(6) The 8-switch selector to detect short-circuit diodes in the crosspoint matrix.

 (7) Lighting an alarm lamp on the alarm indicating panel (identifying the Major), and initiating a prompt alarm, if a diode is short circuit.

 (8) The C-switch selector to initiate a prompt alarm and light the alarm lamp should the access relays remain held. 

(9) Preventing calls from being set up through a section of a C switch, common to five inlets from the B switches, if one of the units in that section is jacked-out or busied.

 ( 1 0) Manually busying the switch section from service by preventing the operation of the access relays.

 (11) Access, via test points to the P, H and MD leads. 

(12) Determining the path taken by a given connection, using switch indicators and a call-trace pulse generator. 

CIRCUIT DESCRIPTION Outline When a connection is to be established between a B switch and a D switch or junctor, the C-switch selector applies a negative potential to theCA lead of a tentatively-chosen C-switch section belonging to the same 'Major' as the 8 switch. This potential, together with an earth received on the AM lead from the A-switch pattern relays, operates the access relays in the first unit of that section. The access relays extend to the B-switch selector the CT and CM leads controlling the five B-C links, and prepare the testing circuit. The B-switch selector tests the state of the links, chooses a free one, and marks its controlling CM lead. This mark, together with the positive potential on the H-wire from the D switch or junctor, operates the crosspoint relay common to the chosen 8-C link and the C-D or C-junctor link. When successful switching between a line circuit and a junctor is detected by Call Control, the earth mark to the line circuit is replaced by negative battery, and the positive potential on the H wire from the junctor is replaced by an earth. The C-switch access relays are released. At the conclusion of the call, the H-wire earth from the junctor is disconnected, and the crosspoint relay releases. The appearance of negative potential on the CM lead, after switching is completed, indicates that the diode associated with the operated crosspoint relay is short circuit. This causes an alarm to be given, but the unit is allowed to remain in service. An alarm is initiated by the C-switch selector if the access relays are operated prematurely, but the unit is allowed to remain in service. 

Detail 

Normal Operation

 When a connection is to be established through this unit, a negative potential is present on the CA lead and an earth on the AM lead, operating the access relays TA, T8, TC.

 Relays TA, TB, TC operating:

• TA1-4, TB1-4 and TC1-2 connect the 'test' and 'mark' wires of B-C links 1-5 to the 8-switch selector. 

• TC3 connects relay ALto the CF lead for 'permanent-access' and diode-fault checks. 

• TC4 connects positive battery to the testing circuit of each link. 

The B-switch selector examines the potential on the CT leads from the five B-C links. The presence of a positive potential on a CT lead indicates that the link is free; a negative potential indicates that it is busy. 

The B-switch selector chooses a free B-C link and connects a +15V 'mark' to its controlling CM lead. 

For example, if a connection is to be established via the first B-C link and the first outlet to a D switch or junctor, then a positive potential will be present on the H wire from the junctor or D switch (depending on whether the call is originating or terminating); also, a positive 'mark' (+15V) will be present on the CM1 lead. Hence, relay 1 XA operates.

 Relay 1 XA operating: 

• 1 XA 1 extends the positive potential on the H-wire to the 'B' switch, and holds relay 1 XA. 

• 1 XA2 extends the P wire from the junctor or D switch to the 8 switch. 

• 1 XA3 and 1 XA4 extend the positive and negative wires from the junctor or D switch to the B switch. 

When Call Control detects successful switching from the junctor to the required line circuit, the earth 'mark' applied to the A switch is replaced by negative battery, and the positive potential on the H wire from the junctor is replaced by an earth. The access relays are thef! released, thus disconnecting the 'test' and 'mark' wires from the 8-switch selector. At the conclusion of the call, the earth on the H wire from the junctor is disconnected, and the crosspoint relay releases. 

Diode Fault 

The appearance of a negative potential on the CM lead, when switching is completed, indicates a short-circuit diode associated with the operated crosspoint relay, and is detected in the B-switch selector, which earths the CF lead to operate relay AL. 

Relay AL operating: 

• AL1 locks relay AL. 

• AL2 lights the Major-identifying alarm lamp on the alarm indicating panel. 

• AL3 initiates a prompt alarm. 

The access relays TA, TB, TC are then released. 

Although a prompt alarm has been initiated, the switch unit remains in service. 

Access Relays Fail to Release 

The C-switch selector comprises two 1-out-of-5 selectors, the two being connected together to form a 1-out-of-10 selector. 

When ·one section of the selector is in use, an earth is applied to the CF leads of the C switches associated with the other section of the selector (which is therefore not in use at that instant) to check that none of the C-switch access relays is operated. 

If any are operated, this constitutes a fault. Relay AL operates via contact TC3 and the Major-identifying alarm lamp glows on the alarm indicating panel. 

Subsequent circuit operation is as described for a diode fault. 

When the fault has been cleared, relay AL may be released by the insertion of a U link between: TPA 1 and TPA2, earthing the resistive-battery feed to the relay. 

MAINTENANCE

 A test jack is provided on the front of the unit for maintenance purposes. Into the test jack may also be plugged switch indicators to determine whether the switch section is busy or free, and, if busy, to determine, in conjunction with the call-trace pulse generator, the path taken by a particular connection. The 'switch free' indicator prevents the operation of relays TA, TB, TC, by linking TPA 19 to TPA20, suppressing the negative potential on the C-switch selector CA lead, and thereby busying out this 'horizontal' section of the C switch.