3. Relays
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Sri Lanka

Relays are electro-mechanical devices used for switching. Relays are used to make the signaling logic circuits in the interlocking plants. They consists of one or two magnetic coils (electro magnets) and a set of contacts.  

Magnetic System

The magnetic system of the relay illustrated below (JRK 10 type) consists of a cylindrical iron core with coil (pale blue near the bottom), two pole pieces and an armature. Larger relays (JRK 11) have two iron cores united at the rear with a yoke and the front end being provided with pole pieces. The armature extends across both pole pieces.

Iron core, pole pieces and armature are made out of iron with excellent magnetic properties. The armatures are so balanced that the vibration on the unit will not affect the relay operation. [Move the mouse over the relay to activate it!]


The relay contacts can be classified into four types.  A relay unit will contain a combination of these types

Front contact - NO

Back contact - NC

Front/ Back contact

Make before break contact

The contact springs are made out of nickel and the contacts tips are silver. The front contacts are of twin contacts and the back contacts are single contact type. The rear end of the  contact springs are fixed between two blocks of transfer molded carbonate plastic reinforced with glass fibre.  The stationary contact springs are supported at their free ends by a strip with notches, which limits the spring movement. The lower end of this strip is attached to the magnet support.

The movable contact springs are guided by an actuating strip which at the lower end attached by bearings to the armature and at the upper end to the upper most movable contact spring. The front edge of the actuating strip provided with slots, which lock the spring and guide the movement of the contacts.

The rear end of every contact spring has eight forked terminals.  This provides a very dependable connections to the plug board terminals, when the relays are plugged in.


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R. Jayanthan
Last updated on 01 January, 2002