Advantages and Disadvantages of Electro-Mechanical Relays

Electromechanical Relays are the oldest type of relays.  Electromechanical, have been gradually been replaced by Static and then by Numerical Relays. 
However, Electromechanical Relays are still being used.
The Advantages of Electromechanical relays are-

1. They are quick acting and can be reset fast.
2. They are simple in construction.
3. They are reliable. 
4. The values can be easily set.  No special programming device is required.
5. People can be trained on these relays easily

The disadvantages of Electro-mechanical relays are

1. The VA burden of these relays is higher than static and numerical relays.  Hence, the Potential and the Current Transformers should have a higher capacity.
2. These relays require to be calibrated periodically and tested.
3. These relays suffer from the effects of age.  As time passes, the springs and the linkages inside the relay grow weak.  This causes the setting values to drift.  This can result in maloperation and false trips.
4. These relays do not have the directional feature. 
5. The speed of operation is limited by the mechanical inertia of the moving components. 
6. Multifunctioning is not possible.  One relay can perform only one function.