Functional parts of rp and a devices and conventional symbols used in them

Latching relays

This type of relay operates exactly like the R-S flip flop. It has two different coils instead of one. When the first coil is actuated, the relay goes to the SET position and it remains there no matter if this coil is kept actuated. It will only change state (to RESET position) only if the other coil is actuated.
This type of relay is widely used in applications where the state of the relay needs to be kept as is, even after a power failure or a restart.

Fig 3.3. Latching relays

Almost everyone knows these relays. They do not actually have an electromagnetic coil. Instead, they remain armed all the time. Two electromagnets are placed one opposite the other. Between them, there is the armature. This armature is magnetized from both electromagnets.
The first electromagnet is placed in series with the Phase, while the other is connected in series with the Neutral. If the current that flows through both electromagnets is equal, then the armature is kept in balance. But if the current that flows through the second electromagnet is less than the current that flows through the first electromagnet, then the armature is pulled to the first electromagnet that has greater magnetic force! And how can this happen? Easy, if somehow an amount of current flows to the ground of the installation.

Fig 3.4. Relay for household electrical installation

These relays can (and SHOULD) be found in every household electrical installation, right after the main switch. Look at the following illustration:
The light bulb is turned on because the magnetic power from both coils is equal. Now look what happens if “somehow”, the current on the neutral is less than the current on the phase.
The magnetic power of the electromagnets is not equal, thus the relay will cut the power supply and our friend will be saved. For safety reasons, if this happens, the relay can only be restored mechanically, if someone pulls the lever of the relay up again:

b. Protective relays – overload
Very common relays in motor applications, as well as in all electrical installations. These relays wave no electromagnetic coil to move the armature. Instead, they have a bimetallic strip that the current flows within.
The material and the thickness of this strip is carefully selected, so that it will be heated (and thus bended) above a specified current value.
When the bimetallic strip is bended, the relay will cut-off the power supply. For security reasons, the relay can only be restored mechanically by moving the lever by hand.

Fig 3.4 Relay for overload protection

This is the basic idea of the overload protection relay figure below