The principles of construction of the P0 3000 Type Relay are shown below.

Code Labels                                                             Springs and tag numbering

The essential components of the relay are as follows:-

  1. The CORE consisting of a soft-iron rod with an enlarged end called the "pole face".
  2. The WINDING consists of many turns of fine insulated copper wire, wound in layers on the core.
  3. The COIL is the name given to the complete assembly of the core and windings. 
  4. The YOKE is the L shaped piece of soft iron attached at one end to the coil.  It provides a low reluctance path for magnetic field created by the current carrying coil.
  5. The ARMATURE is the L shaped piece of soft iron, pivoted at the end of the yoke on a knife edge. When current flows, the armature is attracted towards the pole face. The lever portion of the armature moves the lifting pins and operates the spring-sets.
  6. The BUFFER BLOCK is a block of moulded synthetic material mounted on the yoke of the relay. It is grooved so that it engages with lugs formed on the sides of the make and break springs to give correct tension and limit "follow" when the associated moving spring moves out of contact with them.
  7. The CONTACT UNITS can be supplied in 4 types which can be accommodated in a spring-set; a make, a break, a change over, and a make before break.  In addition a contact unit may be designed to operate before or after all the other contact units on the relay. These are known as "X" and "Y" contact units respectively.
  8. The RESIDUAL STUD OR SCREW is made of non magnetic material and is inserted or screwed into the armature.  Its purpose is to prevent the armature from making contact with the pole face, and completing a magnetic circuit of very low reluctance consisting entirely of magnetic material.  Thus, the residual stud or screw prevents a "closed circuit", and allows the armature to release correctly.  The residual stud, or screws is non-magnetic.

Main parts of the 3000 Type Relay
The coil and contact units are not shown


Magnetic Circuit

The magnetic circuit consists of the core, yoke, armature and the air-gap between the armature and the pole-face.

The operate current which flows in the coil must create sufficient flux in the magnetic circuit to produce a tractive force on the armature strong enough to overcome the force exerted by the contact springs.  When the current is disconnected from the coil, the flux in the magnetic circuit starts to decay; when the force exerted by the contact springs overcomes the holding force on the armature, the relay will release.

The resistance of the magnetic circuit to the creation of flux is known as the reluctance and to ensure a low reluctance the following steps are taken:-

  1. The core, yoke, and armature are made of high quality soft-iron and then annealed.  The components then have a low reluctance and under normal conditions the flux drops very nearly to zero when the magnetizing current ceases.  Any flux that remains when the current ceases, known as residual flux, will retard and in the extreme, prevent the release of the armature.
  2. The air-gap is made short because it forms the greatest proportion of the total reluctance of the magnetic circuit.
  3. A pole-face is formed at the armature end of the core.  The pole-face enlarges the cross-sectional area of the core adjacent to the armature, hence the area of the gap is increased, and consequently its reluctance is decreased.  The distance through which the armature moves is known as the travel of the relay; this is always less than the air-gap to prevent the armature making intimate contact with the pole face and so form a closed magnetic circuit of very low reluctance.  Such a closed circuit would retain a considerable amount of magnetism even when the magnetizing current was disconnected and this would prevent the correct release of the armature.  A stud or screw of a non-magnetic material, known as a residual is fixed to the armature so that an air-gap is maintained when the armature is operated.  The portion of the air-gap which remains when the armature is operated is known as the 'residual air-gap'.

The Coil
The winding is generally wound directly on to the thin insulating material which is wrapped around the core and contained within coil cheeks fitted at each end of the core.  The coil may consist of one or more separate windings depending on the function of the particular relay, which may be inductive, non-inductive or balanced.  The number of turns and the resistance of each winding is also in part dependent on the function of the relay, but the resistance is usually made as high as possible to keep the current value low.  The maximum power that may be dissipated in a major relay coil is approximately 7 watts, but this figure should be derated at higher ambient temperatures than 20C.

The ends are brought out on two, four or five tags.  The range of winding resistance is from 0.1 ohm to 42,000 ohms, but with a maximum of 10,000 ohms when a copper slug is fitted over either end of the core to give slow operation or release.

When high impedance is required, as in transmission battery-feed bridge relays, the core is enclosed in nickel-iron sleeves.

The Spring-set
A spring-set is a set of contact springs mechanically clamped together in the form of a pile to make one or more contact units.  The springs are of three types:-

  • lever springs which are actuated directly by the armature,
  • make springs with which the lever springs make contact when the armature operates, and
  • break springs from which the lever springs break contact when the armature operates.

The four types of contact unit in general use and the associated diagram symbols are shown Figure 1 below:-

Figure 1

'MAKE' CONTACT UNIT (M) is a combination of two contacts which make connexion when the armature operates.

'BREAK' CONTACT UNIT (B) is a combination of two contacts which break connexion when the armature operates.

'CHANGE-OVER' CONTACT UNIT (C) is a combination of three contacts in which a connexion is broken between two contacts, and a connexion is made between one of these two and the third contact, when the relay is operated.  There is normally a short period, known as the transit time, between the break and make actions.

'MAKE-BEFORE-BREAK' CONTACT UNIT (K) is a change-over unit in which the second connexion is made before the first is broken.
The number of contact units which can be incorporated in one spring-set and the number of spring-sets which may be fitted to a relay vary according to the circuit requirements and mechanical restrictions of the particular relay.  The 3000 type relay considered in this note provides for not more than two spring-sets.

The contact springs are made from nickel-silver because this metal possesses the flexibility and durability required to allow the contacts to bear firmly against each other when closed.  Electrical contact between these springs is made via twin dome contacts as shown in figure 2.  The Springs are designed to rub against each other when making contact to ensure that they "self clean" on operation.

The standard contact material, for currents of less than 300mA, is pure silver, contacts carrying more current use special materials.  Where special materials are used the diagram symbols are marked to indicate the material, the codes are:-
Palladium - Pd.
Platinum - Pt.
Mercury - Hg.
Tungsten - W.
Special alloys - Sp ALLOY (see figure 3).

Where palladium contacts are fitted the springs have semi-circular notch while platinum have a vee notch. (see figure 4).

As a further safeguard, relays should be mounted with the springs in the vertical plane, to minimise the possibility of dust settling on the contacts.

Spring Combinations
Spring combinations can be built up with any number of springs from two to eighteen (two units of nine each), with twin-contact spring arrangements of make (M), break (B), change over (C) and make-before-break (K).  Single, large silver and silver-nickel contacts (heavy-duty) are restricted to (M) and (B), with a maximum of twelve springs (two units of six each).

Armature Residuals
To prevent possible non-release of a relay armature caused by retained magnetism in the core, a residual air gap is left between the armature and the core when the relay is operated.

The requisite residual gap is ensured by means of a fixed stud or adjustable screw with lock-nut and is generally within the limits of 4 to 20 mils.  GEC advise that fixed stud residuals are manufactured in lengths of 4, 6, 12 or 20 mils (1 mil = 0001 in. = 00254 mm.).

Isthmus Armature
Relays intended to respond to trains of pulses are fitted with an isthmus armature.  This has its sides cut away to limit the saturation of that part of itself opposite the core, thereby restricting the total flux in the magnetic field and ensuring high speed and uniformity of operation and release under varying conditions, such as may be met on a telephone line when leak resistances etc., may occur.

Relay with Isthmus Armature
The springset is of the make before break type (K)

The operating characteristics of a relay can be altered by the addition of a thick ring of copper, placed at the armature or heel end of the coil.  This ring of copper is termed a "slug".

The Characteristics can be:-

  1. Slow Operating - A delay in the armature operating of up to approx 70 rn/secs after current is connected to the coil, can be obtained by the use of an armature end slug.  This will also give a delay in the release time.
  2. Slow Releasing - The provision of a heel end slug will give a delay in the release time of up to approx 20 rn/secs, without appreciately affecting the operating time.
The amount of delay depends on the length of the slug.  Slugs can come in lengths of 0.5", 1" and 1.5" and are factory fitted.
Relay with an Armature end slug


A relay which is to be connected across the speech path of a telephone circuit, must offer high impedance to alternating currents of speech frequency, thereby reducing the shunting effect on speech transmission.  At the same time, however, the resistance to the flow of direct current must be relatively low and the operate and release times normal.

Details of the information printed on the labels are shown later in this document.

The labels are coloured white, green or red, the colour indicating the type of adjustments to be applied to the relay.  The white and green labelled relays have standard adjustments applied to them.  The white label indicates that the relay has springs which are 14 mils thick, the green labelled relay has 12 mil thick springs.  The red label indicates that the relay is a special one, and that adjustments applied to it must be strictly in accordance with the relay adjustment card.

Prior to about 1938, residual values for red label relays were always shown on the code labels but since that date residual values have been omitted from the labels, except on relays to which restricted tolerances apply.

Residual values of white and green label relays are always shown on their respective code labels.

The following differences in markings on relays with respect to adjustable residuals apply. (The code number used is for illustration purposes only):-


A Nominal residual value quoted - White, green and red label relays. Standard residual tolerances apply.
= 3000 12
B Residual value not quoted - Red label relays only. Refer to relay-adjustment card for nominal residual value. Standard residual tolerances apply, unless otherwise indicated on the relay-adjustment card. = 3000
C  "Marked" residual value (within brackets)
White, green and red label relays. Restricted tolerances apply.
= 3000 (12)
D Empty brackets - Red Label relays only. Refer to relay-adjustment
card for nominal residual value. Restricted residual tolerances apply.
= 3000 (  )

This is a GEC document and 3000 type relays are called "major relays", whilst 600 type relays are called "minor relays".




Taken from the GPO London Telecommunications Engineering Training Centre Note for Students and GEC documentation


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Last revised: April 28, 2020