The good old electromechanical protection relay

History of the relay
This is the oldest first-generation relay system in use for many years. They have earned a well-deserved reputation for precision, reliability, and reliability.
There are two basic types of operating mechanisms:
Electromagnetic attraction relay and
Electromagnetic induction relay

Principles of measurement
The electromechanical protection relay converts voltages and currents into magnetic and electrical forces and torques which exert pressure against the tensions of the springs in the relay.
The spring tension and the engagement of the electromagnetic coils in the relay are the main processes by which a user establishes a relay.

Relay function
These relays are generally instantaneous in action, with no intentional delay, closes as soon as possible after pickup if mechanical movement permits. We can add a delay by means of a bellows mechanism, dashpot, or escape mechanism.
However, the timing accuracy is considerably less accurate than that of induction type relays.
As such, users rarely choose these relays with a delay in switching applications.
Electromechanical relays can operate with either AC or DC on the coils. Therefore, the DC component of an asymmetric fault definitely affects these relays using this principle.
Most relays are enclosed in a flush-mounted semi-pull box. Installers typically install relays typically on the door of the distribution cabinet. They bring the sensor and control wiring to the connections on the box. The relay fits into the housing and connects using small switches or a jumper plug, depending on the manufacturer.
As such, we can disconnect it and remove it from the case without disturbing the wiring. When the relay is disconnected, the current transformer (CT) connections in the housing are automatically shorted to short circuit the CT secondary winding and protect the CT from overvoltage and damage.

Operation of the electromagnetic attraction relay
The figure shows a typical electromechanical relay. An input voltage is applied to the coil mechanism. The input voltage magnetizes the core which pulls the arm towards it. This action causes contact of the output contacts, closing the load circuit.
When the input voltage is removed, the spring-loaded lever will push the contacts away from each other, thus breaking the load circuit connection.

Operation of electromagnetic induction relay
Induction relays are available in many variations: providing accurate collection and update responses for a wide range of simple or complex systems.
They are actually like induction motors. On the relay, the moving element (rotor) is usually a metal disc, although sometimes it is a metal cylinder or cup. The stationary part (stator) is one or more integrated electromagnets, with current or potential coils inducing currents in the disk, causing it to rotate.
As long as the rotational forces are not sufficient to rotate the disc and bring its movable contact against the fixed contact, a spring prevents movement of the disc.

electromechanical protection relay
electromechanical protection relay

This closes the circuit controlled by the relay. The larger the fault detected, the greater the current in the coils and the faster the disc spins.
A calibrated setting called the time dial is used to define the spacing between the moving and fixed contacts. this varies the operating time of the relay from fast ( contacts slightly open ) to slow ( contacts almost a full revolution of the disc outside ).
The reset action begins when removing the rotation force, either by closing the contact of the relay that trips a circuit breaker or by eliminating the malfunction detected by the relay. The retaining spring returns the disc to its original position. The time it takes to reset depends on the type of relay and the timer setting (contact spacing).
Most electromechanical relays are generally designed for minimum input-output isolation voltages of 1500 to 2000 VAC.
Limitations of electromagnetic relays
Low operating speed.
Change in characteristics over a period of time due to the effect of aging.
Component failure leading to relay failure.
Relay is Bulky: Because there are internal mechanical components with physical size restrictions, the housing size of an electromechanical relay can limit the size of a printed circuit board design. Excessive power consumption.
Imposes a heavy burden on the CT
No-fault data is available except phase indication.
By design, the electromechanical relay must make mechanical contacts in order to switch a load. At the point of these contacts, oxidative degradation occurs over extended life cycles ( typically 106 operations ), and the relay will need to be replaced.
When an electromechanical relay is activated, rebound occurs at the contact site. Bounce creates a window of time during which the load circuit wobbles between open and closed, a condition to be considered in load design.
Isolation voltage is another area where electromechanical relays are limited.

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