If you closely examine a standard household electrical wall switch or outlet receptacle, you will notice that in addition to the screw terminals on the side of the device, there is a series of slots or holes in the back of the body. These are push-in wire connections, designed so that you can connect wires simply by inserting the bare end of the wire into the opening, where a spring clamp holds the wire. This is a quick and easy method, for sure, but it's not the best way to make connections. Standard screw terminals are better.
The Problem With Push-in Wire Connections
Professional electricians almost never use push-in connectors, and there's a reason why. No pro wants to be called back to a job to correct a bad connection, and push-in wire connections are much, much more likely to fail than the standard screw terminal connections.
Each push-in connector includes a very small, spring-tension, metal clamp inside the body of the switch or receptacle. This clamp is designed to grip the bare wire securely once it is inserted in the hole, and it can be released only if you insert the tip of the small screwdriver into the release slot next to the opening.
However, this connection is notoriously insecure, and it doesn't really provide the degree of metal-on-metal contact necessary for a good electrical connection. Wires in push-in connectors can easily turn in the hole and gradually work their way loose. And the sheer amount of metal-on-metal contact is very, very small with push-in connections, which can lead to overheating of the device. As overheating occurs, it loosens the connection even more, eventually causing a total breakdown of the connection. Once the device is disconnected and reconnected once or twice, the spring clamps tend to wear out and lose their ability to grip the wires.
Any electrician or homeowner who makes regular use of push-in wire connections very soon learns to avoid it in the future. This is one instance in which the old way is the best way.
Screw Terminals Are Better
The other standard option for making wire connections is the traditional way: to secure the wires under the terminal screws at the sides of the switch or outlet. This method takes a little longer and requires a bit of practice to master. Usually, it means forming the bare end of the wire into a clockwise loop around the screw terminal, then tightening the screw down firmly to grip the wire. On some types of receptacles and switches, the bare wires are inserted into a small bracket, then the screw is tightened down to clamp the wire securely.
Either type of screw terminal connection can be unfastened and reconnected as many times as you like—they will never wear out. Screw terminals are capable of making a tighter connection with the wires, which not only prevents the wires from coming loose, it also creates a better electrical connection. Electricity passes through a tight connection more freely than a loose connection.
Push-in Terminals With Clamping Screws
Some switches and outlets offer the best of both worlds: push-in fittings that have screw clamps to tighten the connection. These allow for quick installation without having to bend the wires, and they add a second layer of protection by letting you clamp down on the inserted wire with a screw. While they seem like a great option, push-in terminals with clamping screws do not provide the same secure connection as a clockwise loop does. Often, folding and pushing wires back into the box loosens the connection.