The 3000 Type Relay
January 1935

Since the early days of automatic telephony the facilities demanded of an automatic telephone exchange have increased considerably, and in consequence, telephone circuit design has become more complex, and the conditions imposed upon the switching apparatus have become more severe. These two facts have necessitated the adoption of the very soundest of principles in the design of both the circuits and the apparatus, and developments in these two directions have gone hand in hand to ensure good telephone service.

The British Post Office engineers have had considerable experience of the performance of all types of circuits and designs of apparatus under service conditions, and during recent years this knowledge has resulted in the adoption of certain 'standards' which are now specified for use on all automatic exchange contracts. The majority of the switching functions in the more recent exchanges are performed by equipment of uniform design, each of which represents the best all round solution of its particular mechanical and electrical problem when all factors are considered.

The latest and most important apparatus component to be standardized is the telephone relay. One design of relay, known as the 3,000 type, has been adopted for use in all the main switching circuits and incorporates the best features of previous well known designs. In addition it has certain new features which are particular to itself.

The responsibility of choosing a design for so important a component as a telephone relay is indeed a great one, and this fact will be readily appreciated when one bears in mind that a director exchange may contain as many as 120,000 relays, and that 1,500 contact operations may be involved in completing a call.

It is not, however, on account of quantities alone that the relay must be considered important, but primarily because of the precision of performance expected of it in many cases. The circuit designer is often forced to take full advantage of the possibilities of the relay, demanding controlled operate and non-operate current figures or close hold and release; in addition, definite time sequences must be guaranteed, involving operate and release lags controlled between fixed limits. Moreover, the contact carrying capacity of the relay must not be too limited or the number of necessary ' relief' relays will be seriously increased.

These and many other considerations affect the choice of a relay design, and a nice balance must be obtained between reasonable initial cost and easy maintenance. This latter requirement is of vital importance, not only on account of low costs but also because it is so very closely allied to good maintenance.

In the design of the 3,000 type relay the maintenance aspect has received very careful consideration, and the method of adjustment and test has been laid out on a straightforward mechanical basis.

Details of Relay
Magnetic Circuit. In considering the features of the 3,000 type relay it will be noticed that the magnetic circuit is made up of three items, coil, yoke and armature, the assembly to the yoke, latter being suspended on the yoke in the familiar knife edge manner. This magnetic system is highly efficient and is further improved by increasing the core cross section at the armature end by a disc of soft iron, and by the use of nickel finish on all parts.

Such a magnetic circuit is a sound foundation upon which to build a relay design, for high magnetic efficiency offers the possibility of good contact pressures, consistent time lags, economy in current consumption and minimum flux leakage to adjacent relays.

The cheek at the armature end of the winding assembly is made of copper and serves to reduce parasitic armature oscillation. This feature contributes to a reduction of contact bounce on release of the relay. In certain particular instances, however, e.g. impulsing relays, the fitting of this metal cheek is not desirable and the usual bakelite cheek is fitted instead.

The bobbin can be fitted with 5 connecting tags, thus allowing the relay to carry several windings depending upon the number of  'common' points permissible in any particular circuit.

In order to limit the dust collecting area of the springs, the relay is designed for side mounting, consequently a means of holding the armature in position when current is not flowing has been arranged. This takes the form of a spring washer, kept in position by the armature retaining screw which passes through a clearance hole in the armature and screws into the yoke. When screwed home this arrangement applies a small but sufficient pressure on the armature to ensure a correct location on the knife edge.

Depending upon the particular circuit requirements, armatures with fixed studs or adjustable screws are used, the stud lengths having nominal values of 4 mils. 12 mils and 20 mils.

Spring Set
The contact springs are of nickel silver and are split at the free end in such a manner that two independent tips are formed, each of which carries a contact. The moving springs are controlled by brass pins which project through a hole in the spring below, the pins being set centrally between the two contacts thus ensuring equal pressure on both contact points. Silver contacts are used for light current circuits but selector magnets etc. are controlled by platinum. The springs forming a contact unit are clamped together by means of one screw, while two screws are used to fix the contact.

Contact assemblies, shown below, are fixed one on each side of the buffer block, the latter occupying a central position on the yoke.

Buffer Block
The buffer block is made of a resinous compound and is moulded to form a number of steps, which in conjunction with a lug on each fixed spring provide a means of controlling minimum contact pressures irrespective of the travel of the moving springs. The block is white in colour and in addition to acting as a background to facilitate the observation of spring movements during adjustment, it also serves to a certain extent as a protection for the relay springs when a relay-set cover is being removed.

Several sizes of block have been standardized to cover the requirements of any combination of spring units, the smallest size accommodating three springs on each side while the largest allows nine springs on each side. This maximum is limited by the requirement of mounting two relays, side by side on a standard selector mounting plate.

Method of Adjustment
As previously mentioned, the adjustment of the 3,000 type relay is on a mechanical basis, that is, the springs are adjusted to conform to definite tension limits, and tests to confirm these adjustments are made by direct measurements on the springs  themselves. Electrical tests normally have no place in confirming the existence of correct mechanical adjustment.

This method of adjustment has advantages over any method which uses 'gauging' to ensure a certain minimum travel after two springs have made contact, for in the latter case, as the control of the tension is indirect, it is impossible to control tension variations as closely as might be desired. With a buffered spring, however, spring tensions are set directly, and variations in spring thickness do not prevent a small tolerance on a nominal figure from being guaranteed.

The procedure for the adjustment of the relay is firstly to set the residual and travel within the specified limits, measurements being made by means of a feeler gauge. All the buffered springs are then tensioned against the buffer block to a certain figure and each lever spring is tensioned until its associated back spring is just moved away from the buffer block. This latter movement is not measured as it is only necessary to ensure that the contact pressure is something in excess of the pressure previously applied between the back spring and the buffer block.

All lever springs are tensioned to definite limits against the lever spring below or the armature itself, thus ensuring a uniform load on the relay and avoiding clearance between stud and pin.

The physical tolerances on the component parts of the relay and the tolerances on each nominal adjustment are such, that if the relay springs are reasonably straight the contact opening is always a safe figure, and the front springs are lifted clear of the buffer block by their associated lever springs.

A test current is finally applied to the relay as an overall operate test to confirm that the relay is up to specification as a complete unit.

By using this standard method 0f adjustment it is possible to adjust and maintain, without reference to individual relay adjustment cards, all relays which do not have difficult circuit conditions to fulfil.

All standard adjustments are given in a general specification and can be readily memorized; but residual lengths may vary between one relay code and another without making a relay 'non-standard,' so to cover this point the required residual length is shown on the label at the bottom of the relay coil cheek. Further, when a definite sequence of action exists between contacts on a relay, a longer travel is necessary, and to emphasize this the label is marked 'X' early action, or 'Y' late action, as the case may be. These sequences are arranged in the build up of the relay and are not obtained by the bending of springs.
All relays, therefore, equipped with standard 14 mil springs and requiring standard adjustments carry a white label on the coil cheek, and this label in conjunction with a knowledge of the general adjustment specification, constitutes a particular mechanical specification for that relay.

When a relay carries a green label, it indicates that 12 mil springs are equipped and the relay must therefore be adjusted to figures given in the general specification applicable to springs of this thickness.

In cases where a relay must operate over a long junction line or satisfy some difficult circuit condition, it is generally necessary to control the functions of the relay between close current limits, and in such instances it is not possible to produce the required adjustment by working to the general specification only. A red label in place of the white or green is used therefore to indicate that the relay requires some special attention, and a study of the individual relay adjustment card must be made before re-adjustment is commenced.

Impulsing Relays
Relays which have to respond to dial or other trains of impulses are fitted with a special type of armature which is made in such a way as to form an isthmus shaped link between the core and yoke. This degradation of the magnetic circuit, by causing saturation at a lower flux value, makes for more constant release lags when the relay has to work over lines of varying length.

Impulsing relays forming part of a transmission circuit, or any relays similarly situated, have three nickel iron sleeves fitted over the core before the winding is put on. This produces a coil which has high impedance to speech currents without affecting to any marked extent its sensitivity on direct current.

Slugged relays for use in impulsing and other circuit elements are catered for, and various sizes of slug are available for either fore-end or heel-end purposes to meet the requirements of various circuit timing conditions.

From the foregoing comments it will be appreciated that the 3,000 type relay has been designed on very sound lines, particularly in respect to minimum contact pressures of approximately 20 grammes, twin contacts, and straightforward methods of adjustment. Sound design and high standards of production guarantee equipment that gives trouble-free service for practically unlimited periods.

Last revised: October 14, 2019