A PLC illustrated With Relays - Seal-in Circuit

Many relays also have multiple outputs (throws) and this allows an output relay to also be an input simultaneously. The circuit shown in below Figure is an example of this, it is called a seal in circuit.

A PLC illustrated With Relays

In this circuit the current can flow through either branch of the circuit, through the contacts labelled A or B. The input B will only be on when the output B is on.

If B is off, and A is energized, then B will turn on. If B turns on then the input B will turn on, and keep output B on even if input A goes off.

After B is turned on the output B will not turn off.

Ladder Logic Seal-in Circuit