When relay contacts open and close, small sparks called arcs can eventually cause them to stick together. Also some types of corrosion can cause them to stick. Sticking relay contacts can be cleaned with a burnishing tool or with fine grit emery paper, if you can get to them. Some relays are sealed and cannot be opened. If you can't get to the contacts, you can't clean them.
Turn off the equipment you are working on to avoid electric shock.
Cut the emery paper into small strips and fold them in half so that the grit is exposed on both sides.
Insert the emery paper between the contacts to be cleaned. If you have a burnishing tool, use that instead.
Rub the emery paper or burnishing tool back and forth several times until the contacts are clean. Badly pitted contacts will take more work.
Wet part of the cotton cloth with alcohol and slip it between the contacts. Rub it slightly to remove any grease or dirt.
Wipe any excess alcohol off the contacts with the dry part of the cotton cloth.