Introduction to Strowger
Dial Limits : The nominal dial characteristics are speed = 10 pps, ratio = 2/3 break, 1/3 make.
Factors Affecting Pulsing : The dial pulses, in the end, operate and release an A (pulsing) relay in selectors and relay sets. The speed is generally set by the dial but the ratio is affected by many other conditions.
Modern selectors overall have a great amount of tolerance to pulse distortion.
A transmission bridge is required in each exchange that the call passes through. The A and B relays in each exchange hold the call through that exchange via the B contact earth on the P wire holding any group selectors that will have been used to set up the call.
Local lines and junction circuits are simply two wire circuits, but each exchange connection requires the third P wire circuit inorder to test and hold the call through the exchange. Please recall from the "Transmission Bridge" paper that barretters can serve both caller and called customers when a junction is involved in the call.
On a "junction" call the originating exchange uses an outgoing auto - auto relay set to convert the three wire exchange connection to the two wire condition of the junction. Any intermediate exchanges that the call passes through will also require an auto - auto relay set. The terminating exchange uses a final selector to provide the necessary transmission bridge.
Any auto - auto relay set therefore has to provide facilities to convert the three wire exchange condition to the two wire junction condition and this requires that the relay set has a transmission bridge. Any pulsing must therefore be repeated across the transmission bridge to the junction and following exchange.
The P wire normally has a low resistance battery condition on it to denote that the junction is free. The relay set is seized by the customer's loop being extended from a group selector level on the - and + wires and relay A operates. This in turn operates relay B which earths the incoming P wire to hold the connection from the group selector.
Junction Limits : A2 also loops the junction with the 400 ohm high impedance I relay. This "initial pickup" is the condition that usually limits the resistance permitted in the junction pair. This limit can go up to 2000 ohms with the total loop resistance being presented to the distant A relay being 2400 ohms. Above this value the A relay will not operate reliably. During pulsing the I relay is short circuited and the distant A relay will receive somewhat higher line current.
Pulse Repetition : When the caller dials, the A relay releases during the pulse break periods. Contact A2 repeats the break to the junction, contact A1 operates relay C and C2 operates relay CA. These two relays hold during the pulse train. Contact C1 short circuits relay I and, on the next operation of relay A, presents a zero ohm loop to the junction. Pulsing continues with either a break or a zero ohm loop being extended to the junction.
At the end of the pulse train relays C and then CA release slowly. C1 removes the short circuit from the junction and leaves the I relay and a 400 ohm resistor in parallel across the line. Flux builds in the I relay whilst the distant A relay is held by the current flowing via the 400 ohm resistor. When CA1 releases the line current will all flow via the I relay, but as the relay has been partially fluxed, the line current will not drop below the "hold" value for the distant A relay. This arrangement is known as "Two Stage Dropback" and is an essential feature in all loop disconnect relay sets.
If the 400 ohm resistance circuit had not been provided, then the drop back from the short circuit to an unfluxed I relay would have caused the line current to have dropped momentarily to zero. This would have caused a momentary release of the distant A relay and so would have produced an extra pulse. eg if six had been dialled, seven pulses would have been received.
The A relay in the auto - auto relay set could also be adversely affected if a "one stage drop back" had been employed. During pulsing the transmission bridge capacitors build a charge from the A relay battery and earth via the short circuit of C1. When C! releases the capacitors would discharge to the junction line potentials and this would produce a current in the A relay in opposition to the line current. The A relay may therefore itself produce an extra break which it would repeat to the junction.
However when large exchanges with many junction groups are involved, it is usually more economic to provide the relay sets back in the exchange between group selector ranks. The outgoing side of the relay set is then connected to a further group selector which in turn will select which junction is used. Contact B3 provides the forward holding earth to busy and hold the next group selector. The forward B3 earth is also generally taken to the bush of the test jack of the selected junction to mark it as busy to any testing engineer.