Parkend Line Circuits, Line Finders and Group Selectors
These are selectors that were originally used in PABX4 equipment
Getting Dial Tone
When a caller loops his line, relay LS in his line circuit operates. This busies the final selector multiple, marks the calling outlet on the line finder banks and extends a start earth to a free first group selector. The first selector is determined by the postion of the start distribution uniselector. If the first selector chosen happens to be busy then a chain of contacts within the first selectors extend the start earth to the next free selector.
On receiving dial tone, the caller can dial the first digit into the selector. The dial pulses cause the selector to step up to the level dialled. It then drives into the bank, seeking a free outlet. This will be denoted by a 250 ohm battery on the H wire. The selector stops on the free outlet, earths the outlet to busy it and extends the caller's loop through to the next selector on the - and + wires. This siezes the next selector and causes it to return a holding (and busying) earth on the H wire. The selector and line circuit are held during the duration of the call by this H wire earth.
Dialling Level 9
Level 9 is used for accessing exchange lines to the public network. It is necessary to be able to bar some callers from using this facility. A BD-FT strap is inserted in the line circuit of all callers who are permitted level 9 access. This strap forwards an earth on the FT wire on permitted calls to operate the BZ relay in the selector.
On a normal call, the selector K and H relays are held by the earth returning on the H wire from the next stage. At the end of the call, the earth is removed momentarily to release relays K and H. This causes the rotary magnet to continue to self drive out of the bank and return to the normal position where the N springs release. At this point the selector is made free again by connecting the ST relay through to the bank of the start distribution uniselector.
Should the selector be siezed (eg by a faulty line) and then the caller does not dial, the selector times out and releases itself. 30 second S&Z pulses are used for the timing, though at Parkend the pulses are about a minute apart. The Z pulse occurs first in the sequence and is not used by the selector. It is followed immediately by the S pulse which operates relay BZ. After the timing period the Z pulse reappears to operate the H relay which in turn disconnects the A relay so releasing the selector.
In both of these situations it is possible that the caller's loop is still there. If so, it holds the line circuit CO relay and prevents the complete release of the line circuit. This would be a "PG" situation and CO operated with LS released brings up the PG alarm. Should the caller clear, CO releases and the PG alarm ceases.