INTRODUCTION
A Relay is an electrically operated switch for opening and closing
circuits. In the operation of telephone and telegraph equipment
many of the complex switching operations are performed by relays.
In a large automatic exchange there maybe as many as 120,000 relays and
the completion of a successful call can depend upon the correct
functioning of 200 or more relays.
OPERATION
When current flows through the winding a magnetic field is built up; the
magnetic flux path is shown above. The armature is attracted. to
the pole face thereby operating the Lever Spring(s). When current
flow ceases the magnetic field collapses and the tension of the springs
cause the armature to release. The residual air gap prevents a
closed. magnetic circuit and ensures prompt release when the operate
path is disconnected.
- A fixed residual stud of Phosphor - Bronze, 4., 12 or 20 mils
thick (A,. B and C respectively) or an adjustable residual for more
stringent adjustments.
- Twin contact domes are pro1ei for greater reliability.
- The standard contact material is Silver, used to disconnect
currents up to 300 mA at 50 Volts.
For currents up to 1 AMP, Platinum (pt) or more recently Palladium
(Pd) contacts are used. These two contact materials are identified
as follows:-
- Four types of contact action are available.
- Up to Four coil windings maybe provided.
- A maximum number of 9 Springs can be fitted each side of the
Buffer Block.
COIL TAG IDENTIFICATION & SPRING NUMBERING
LABELLING
A guide to the adjustments of a relay is given by the PO Code Label.
If WHITE label relay has 14 MIL springs.
If GREEN label relay has 12 MIL springs.
If RED label special adjustments in some respects refer to
adjustment card.
HIGH IMPEDANCE
Relays connected to Speech wires in circuits providing transmission
bridge facilities must offer High Impedance to speech frequencies or
they will shunt the ac signal. To achieve this the relay must
have high Inductance, this is done by fitting three Nickel-Iron
sleeves. Identified by 3N on cheek.
