Dial Limits : The nominal dial characteristics are speed = 10 pps, ratio = 2/3
break, 1/3 make.
In practice dials are set to maintenance limits of speed = 9-11 pps, ratio
break = 63-70% break.
Equipment is designed to accept pulsing at 7-12 pps, ratio break = 63-70%.
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.
Line resistance : This reduces current and flux values and their
rate of growth. Generally the operate lag of the A relay is
increased and the release lag is reduced. the break pulse tends
to increase.
Line inductance : This also slows growth of current and flux
and again tends to increase the break pulse.
Line leakance : This holds the flux in the A relay to a certain
minimum value and keeps the relay partially fluxed. This tends
to slow the release of the relay and therefore tends to reduce
the length of the break pulse.
Line capacitance : This provides a path for the back EMF of the
relay when the break pulse disconnects it. This tends to hold
the relay operated thus reducing the break pulse. However it
also restricts the rate of growth of current and increases the
operating lag of the relay. The break pulse therefore tends to be
displaced in time.
Dial springs spark quench : This is usually a 100 ohm resistor in
series with a capacitor of about 2 microfarads across the dial springs.
The capacitor has no real affect on the make as the capacitor will be
short circuited. On the break though the capacitor charges from the
line voltage and provides a circuit for the A relay back EMF. This slugs
the release of the relay and tends to hold it operated and so reduces
the break period.
Exchange battery voltage : Low voltage acts in a similar way to line
resistance in reducing relay flux and the rate of growth of flux, again
the operate lag of the A relay is increased and the release lag reduced,
leading to an increase in the break pulse length.
Transmission bridge capacitors : Final selector capacitors are
disconnected during pulsing and therefore have no effect. Auto to
auto relay sets have two capacitors connected in series across the A
relay during pulsing, which act rather like line capacitance and
therefore cause the pulsing to be displaced
Typical Selector Stepping
In a stepping circuit, relays B and CD
have to remain held during the reception
of a train of pulses.
In particular relay B has to remain held
during the break period of the pulse.
During this time the B relay is short
circuited by the A1 contact and the
slugging effect provides a release lag of
around 225 ms.
This is achieved by using the whole
winding space for the 1300 ohm coil and
allows the relay to hold to a 46 volt
supply, 12 pps and a break that can
reach or exceed 95%. The relay can also
have a high number of springsets with
its high mechanical load on the
Relay CD has to remain held during the make period of the pulse when the magnet
current ceases. This is achieved by having a short circuit 700 ohm winding which
provides a release lag of around 150 ms. Generally the magnet will fail to step
before the CD relay will fail.
Modern selectors overall have a great amount of tolerance to pulse distortion.
Junction calls :
When calls are made
involving more than
one exchange, the
interconnecting the
exchanges are called
"junction" circuits
between the customer
and the exchange use
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 in order 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
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 the following exchange.
The Auto - Auto Relay Set
This diagram shows the
essential features of an auto auto relay set that permit P
wire holding and pulse
repetition to a junction.
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. A1 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.
A junction of 2000 ohms however implies a transmission loss of perhaps 20 db
and this will not be acceptable in nearly all cases. To overcome this the
junction will need amplifiers to reduce the loss to between 3 and 6 db
depending upon where in the network the junction is.
In turn, amplifiers generally mean a four wire junction being used which in turn
reduces the line resistance by half.
To offset this gain though, many transformers would be introduced into the
signalling path along with capacitors across the signalling path. These factors
generally reduce the junction signalling limits to around 1200 ohms.
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
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 C1 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.
This again would be an extra pulse.
Trunking :
The diagram and write up
assumes that the auto - auto
relay set is provided on a
one per outgoing junction
basis. This is generally true,
particularly in the smaller
However when large exchanges with many junction groups are involved, it is
usually more economic to provide the relay sets further back in the selector chain,
between the group selector ranks. There are usually more junctions connected to
the levels of second selectors than there are second selectors.
The outgoing side of the relay set is then connected to a second group selector
which in turn will select which junction is used.
Contact B3 in the auto auto relay set 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.
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