The last thirty years have seen enormous changes in relay technology. The electromechanical relay in all of its different forms has been replaced successively by static, digital and numerical relays, each change bringing with it reductions and size and improvements in functionality.
At the same time, reliability levels have been maintained or even improved and availability significantly increased due to techniques not available with older relay types.
This represents a tremendous achievement for all those involved in relay design and manufacture.
1. ELECTROMECHANICAL RE LAYS
These relays were the earliest forms of relay used for the protection of power systems, and they date back nearly 100 years. They work on the principle of a mechanical force causing operation of a relay contact in response to a stimulus. The mechanical force is generated through current flow in one or more windings on a magnetic core or cores, hence the term electromechanical relay. The principle advantage of such relays is that they provide galvanic isolation between the inputs and outputs in a simple, cheap and reliable form – therefore for simple on/off switching functions where the output contacts
have to carry substantial currents, they are still used. Electromechanical relays can be classified into several different types as follows:
a. attracted armature
b. moving coil
c. induction
d. thermal
e. motor operated
f. mechanical
However, only attracted armature types have significant application at this time, all other types having been superseded by more modern equivalents.
2. Attracted Armature Relays
These generally consist of an iron-cored electromagnet that attracts a hinged armature when energised. A restoring force is provided by means of a spring or gravity so that the armature will return to its original position when the electromagnet is de-energised.
Typical forms of an attracted armature relay are shown in Figure 1. Movement of the armature causes contact
