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 junction. 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. 

PLUG IN UNIT CONNECTOR

Backplane connections

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 B-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. 

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, TB, 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. 

Relays TA,TB, TC operate

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.