A switch is a mechanical device made up of a plastic toggle on the outside and a rocking lever on the inside that operates one or multiple spring-loaded contactors, making it possible to establish an electrical connection, or to cut it off.
Switches endure repeated manipulation over the years, so many of their parts can eventually break down and become unreliable. As a result, some problems might come up in their circuitry that will require troubleshooting.
Safety Note: Turn your breaker OFF before starting any wiring work.
1. Broken Toggle
One mechanical failure easy enough to recognize is an internally broken toggle, where the lever inside is unable (or only occasionally able) to activate the contactors. This problem can be felt by touch, as the plastic piece is usually loose in the switch without any spring tension.
2. Bad Connection
If you hear a buzzing or frying noise, or detect the scent of of ozone, you might be dealing with either a corroded contactor with carbon buildup or a loose terminal screw that leaves a wire “floating” within its connection. This can create intermittent contact, burning the wire at the connection points and causing even more carbon buildup, which can worsen the bad connection and build up heat that will further loosen the terminal screw.
3. Fixture Issue
A third problem that might be blamed on the light switch can actually caused by the light fixture itself. You might notice this type of issue if the light bulb flickers while turned ON. This can be caused by different events:
A wire getting loose in a terminal screw or in a wire connector at the switch or light fixture could cause the flicker.
A corroded contactor inside the switch could cause the flicker but would also probably cause the switch to feel warm at the touch.
The Hot Contact plate at the center of the light socket and that normally springs back out when removing a light bulb may have lost its stiffness, preventing a positive contact with the end of the light bulb and causing it to flicker.
The return path for the light socket can also be compromised by its connection between the metal threaded male part of the light bulb and the threaded part of the socket itself, especially if it’s just made through a narrow metal strip running up the side of the socket with the remaining surface of the threads only plastic. Figure 1 shows both types of sockets.
4. Socket Issue
A fourth problem encountered, especially when dealing with low-quality sockets, is a replacement light bulb failing to turn on. The likely culprit in this case is a hot contact at the bottom of the socket that doesn't have the flexibility to spring back out and stays collapsed or remains bent-in, preventing it from touching the end of the light bulb.
Safety Note: Again, the breaker should be OFF before pulling out on the socket contact.
Types of Switches
There are many different kinds of switches to accommodate different needs and requirements. The most common household switches are the single pole switch, the three-way switch, and the double pole switch. A person intending to do any troubleshooting should be able to identify what they're dealing with, since the wiring is very different for each one.
Single Pole Light Switches
The Single Pole switch shown in Figure 2 is the easiest to recognize, having only two brass terminal screws, and sometimes a third one green colored screw for the ground connection.
If the power line feeds through the switch’s junction box with the light fixture’s wires also coming from the same box, then the hot wire (black) from the power feed hooks up to one of the terminals on the switch, and the black wire from the light fixture to the other terminal, with the two white wires pigtailed through a wire connector.
If the power feed line goes to the light fixture first and then another wire feeds to the switch, the black wire from the power feed (hot) and the black wire going to the switch are to be pigtailed at the light fixture’s junction box. Then the black wire feeding the switch’s junction box is connected to one of the switch’s brass terminals, and the white wire left to the other terminal.
But since the white wire at the switch will be carrying power from the hot side to the fixture, a piece of black tape should be wrapped around it close to the connection to indicate this.
By the same logic, a piece of black tape should also be wrapped on the other end of the same white wire at the light fixture to indicate where the hot line comes from, and that wire connected to the hot contact in the center of the socket. The last white wire or neutral from the power line is then hooked to the other terminal of the socket connected to the threaded sleeve. Finally, all ground wires should be grounded where required.
The 3-Way switches are used in pairs and are required when the light fixture needs to be controlled from two different locations. They have three terminal screws plus possibly another green for ground (Figure 3).
The hot wire always connects to the darkest or the black screw called common (COM), the other two are called travelers and are interchangeable, so it’s impossible to get them wrong, although it’s important to note that there’s always one of the travelers that carry the line voltage. When replacing a 3-Way switch or removing wires from it, the wire going to the COM terminal should be wrapped with a piece of tape near the end to identify it from the travelers.
From the first 3-Way switch through a 3-wire cable, the two travelers from the first switch are connected to the travelers on the second switch through the black and the red wires. Then the black or darkest terminal is connected to a black wire that feeds to the light fixture to get connected on the hot contact on the fixture while the white goes to the socket sleeve of the switch.
Going back to the switches, all the white wires in each junction boxes are pigtailed with a wire connector, and the ground wires properly grounded to the junction boxes and the switches if possible.
And with those tips, fixing a light circuit is a very possible task for even a beginner DIYer. Be Safe!