When plugs become cracked or loose, or their prongs are damaged, it's time to replace them. Choose a replacement plug that's similar to the old one: round-cord plugs for heavier-duty round cords, and flat-cord plugs for (you guessed it) lighter-duty flat cords.
There's a quick fix for flat-cord plugs, called a quick-connect plug. Cut the old plug off the cord. Squeeze the new quick-connect plug's prongs together, and pull out its inner portion. Thread the old cord through the back of the new plug. Spread the prongs on the inner portion of the plug apart so that you can push the cord into the back of it. Squeeze the prongs together again, and push the inner portion of the plug back into the plug itself. You're done!
These are a little more difficult than quick-connect plugs, but still an easy fix. Cut the old plug off the cord, and strip about 1 inch of the outer insulation and ¾ inch of the insulation that covers the two or three individual wires within the outer insulation. Pry off the new plug's cover plate to expose the screw terminals inside the plug. (Some plugs come in two pieces that screw together. If this is the case, you'll need to take the two pieces apart to expose the screw terminals.) Thread the old cord through the back of the new plug.
Tie the black and white wires together using an Underwriters' knot. An Underwriters' knot is a looped knot tied in two wires that helps prevent them from slipping apart and is a safety measure. Make a loop in each wire, pass the other wire through that loop, and pull to tighten, as illustrated.
Use long-nose pliers to form hooks in the exposed copper ends of the wires. Then, connect the wires: black wire to brass screw; white wire to silver screw; and green wire (if present) to the green, or ground, screw. The copper parts of the wires must not touch each other. Tighten down the screws and put the plug's cover plate back on. Tighten the screws on the cord clamp that many round-cord plugs have at their back.
An Underwriters' knot is a safety technique used when wiring lamps and plugs to help the wires stay in place.
A Although plug design may vary, connecting the plug to the cord is simply a matter of attaching each wire to the plug's screw terminals.