Group Selector P Wire Battery Testing

1

• Group Selector P Wire Battery Testing
• When a group selector hunts around a level for a
  free outlet, it needs to have the free outlets
  designated by a condition different from that
  when the outlet is busy. An earth condition is
  present when the outlet is busy as this condition
  can also be used to hold the connection on the
  busy outlet.
• In large exchanges it is usual for the free
  condition to be a "dis" on the P wire.
• Normally the testing relay can be pre-operated
  and can hold to the busy earths and release when
  it reaches a free "dis" P wire outlet. The
  testing relay is also the switching relay and has
  to carry a heavy spring load, making it fast to
  release and therefore suitable for testing to a
  "dis".
• A 200 outlet group selector can be made with only
  five relays using this technique, making the
  switch smaller and cheaper and enabling more
  shelves to be fitted on a rack, all important
  qualities in a larger exchange. The disadvantage
  is that banks must be kept clean and wipers and
  cords in good condition.

2

• Small exchanges are generally unmanned and dirty
  banks etc would be a major hazard to a "dis"
  testing selector, particularly as traffic may be
  light and subsequent calls would be likely to
  follow the same route through the exchange and
  come across the same fault leaving a caller
  isolated.
• Battery testing is preferable in these
  circumstances as an inadvertent "dis" on the p
  wire would simply mark the outlet as busy and the
  caller's selector would find a later selector.
• However battery testing requires a relay to
  operate when the selector wipers pass over a free
  outlet. Such a relay has to be very fast to
  operate and cannot therefore have a large
  springset load. A separate switching relay must
  be employed.

3

• In a PABX a high speed relay is used for
  testing.
• In a UAX a very lightly loaded 3000 type relay
  is used. It also has a very small winding with
  only eleven ohms resistance and little inductance
  in order to ensure its fast to operate
  characteristic.
• Such relays and testing circuits also have to be
  arranged so that two selectors attempting to
  switch simultaneously to the same free outlet
  will not succeed, in other words, two testing
  relays cannot operate in parallel to the
  resistance battery from the free outlet.

4

• The UAX13 Group Selector
• When the selector is not in use and is free, a
  150 ohm battery is presented to the associated
  linefinder on the incoming P wire.
• On seizure, the A relay is operated. Relays B
  and CD operate as described previously. B3 earths
  the incoming P wire. A CC relay is also operated
  at this time. This connects dial tone to the
  caller and the connection back through the
  linefinder and line circuit is held by the B3
  earth.

The selector then steps under the control of the
pulse train with B, CD and CC holding to reach
the desired level. At this point the selector
has to decide whether or not the caller is
allowed access to the dialled level. It does this
by using signals sent forward from the line
circuit determining whether the caller is
"ordinary" or "coinbox", and by using a vertical
marking bank to determine which level has been
dialled. eg "ord" callers may be allowed to dial
8 whilst "CCB" callers may be barred. If the
call is barred, CD releases at the end of
dialling but CC is held via the VMB. CC1 prevents
the selector from entering the bank and CC3
connects NU tone to the caller.
5

• If the level is permitted, relay CC releases
  following the release of CD. CC1 starts the drive
  into the level.
• Relay HX inspects each outlet as the wipers pass
  it. If no battery is present, HX remains released
  and the switch drives on. On finding an outlet
  with a 150 ohm battery on the P wire, HX operates
  quickly and at HX1 cuts the drive. It also
  operates relay H to the B3 earth on the P wire.

Relay H holds to the B3 earth via its H6 contact.
H7 short circuits the HX relay which releases. H4
prevents the switch from driving on. H2 and H3
extend the caller's loop to the next selector,
releasing relay A and starting the slow release
of the B relay. However the next selector A and
B relays operate to return a holding earth on the
P wire before B3 can remove its local earth. The
loop has therefore been extended forward to the
next stage and the selector is left holding to
the earth returning on the P wire ie "forward
loop, backward holding".
6

• If all ten outlets are busy the selector steps
  onto the eleventh outlet and operates its
  internal S springs. The 150 ohm battery is
  connected to the eleventh step P wire outlet on
  all ten levels and this will operate the HX
  relay. HX1 will disconnect the drive, stopping
  the selector on the eleventh step with its S
  springs operated.

S1 prevents the H relay from switching the caller
away from the A relay, which continues to hold.
This allows S2 to return busy tone to the
caller. One point to note is that the incoming P
wire does not have its battery reconnected and
therefore become free until the selector has
restored to normal and contact N2 restores. ie
the "homing" of the selector is guarded.
7

The PABX4 First Group Selector This is a
somewhat more complicated circuit as the selector
also controls the uniselector linefinder. The
testing relay FT first finds the caller's line
circuit and then has to find a free outlet on the
level dialled. In this article we will only deal
with looking for a free outlet on a level.

• When the selector is returning dial tone, relays
  A, B and K are operated. CD is not pre-operated.
  CD only operates when stepping starts and magnet
  current flows through the relay. However at the
  end of stepping CD releases as usual and connects
  the drive circuit to the rotary magnet. The
  selector now looks for a 250 ohm battery on the H
  wire, stepping over outlets with earths or
  disconnected H wires.

8

• On reaching a free outlet, the 250 ohm battery
  operates the high speed relay FT via the 7 ohm
  winding and the 56 ohm resistor. This reduces the
  voltage on the H wire to cause it to test "busy".
  
• FT1 cuts the drive and operates relay H via its
  30 ohm winding by placing it in parallel with the
  56 7 ohms of the FT circuit.
• H5 provides a hold circuit for relay H via the K
  relay to the B2 earth. H4 earths the outgoing P
  wire to short circuit FT and cause its release.

H1 prevents the rotary magnet from taking any
more steps. H2 and H3 extend the caller's loop to
the next stage and release relay A. This in turn
releases relay B slowly. The following selector
is now seized via the - and wires, its A and B
relays operate and a holding earth is returned on
the H wire. This maintains relay K which is in
series with The H relay and the LS and CO relays
in the line circuit.
9

• When the call is released, the earth on the H
  wire is removed and relays K and H restore. This
  reconnects the drive circuit for the rotary
  magnet at H1 and the selector restores to normal.
• The selector can distinguish between extensions
  that are permitted exchange line calls and those
  that are not. A strap between BD and FT wires
  (not shown) is provided on extensions permitted
  exchange line access. This operates a BZ relay in
  the selector.

If level 9 is dialled, then NP springs operate in
the selector. With BZ4 operated the selector acts
as normal as it can test the H wires on level 9.
If BZ4 is not operated the NP1 springs
disconnect the H wire and the switch steps to
outlet eleven. The S springs operate, and S1
operates FT which holds to the battery, stopping
the selector. H cannot operate as S1 has its 30
ohm winding disconnected. Relays A and B remain
operated and NU tone is returned to the calling
extension. Similarly should the selector reach
outlet eleven on any other level the S springs
operate to operate FT and cause the selector to
stop. Busy tone will be returned.