- 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:-
'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
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).
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.).
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:-
- 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.
- 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
HIGH IMPEDANCE RELAYS
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.
LABELS ATTACHED TO CORE CHEEK
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.
METHODS OF INDICATING ADJUSTABLE RESIDUALS
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
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):-
||Nominal residual value quoted - White, green
and red label relays. Standard residual tolerances apply.
||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.
|| "Marked" residual value (within
White, green and red label relays. Restricted tolerances
||Empty brackets - Red Label relays only.
Refer to relay-adjustment
card for nominal residual value. Restricted residual
3000 ( )
RELAY WING AND SCHEMATIC CONVENTIONS
This is a GEC document and 3000 type relays are called "major
relays", whilst 600 type relays are called "minor relays".