Flickering lights in your home are a normal occurrence. In fact, the cause of flickering lights is usually benign. However, there are some instances in which this could be a symptom of a major electrical problem in your home, so it is important you know the differences between major and minor lighting issues.
Flickering You Can't Avoid
All wires have a level of natural resistance, meaning that when electric current flows through them, some amount of voltage gets used to force the current flow against the wire's resistance. This means that when the current reaches a destination, there is slightly less voltage for your various loads (lights, toaster, etc.). If you have loads that turn on and off, like lights, then the voltage supplying your lights will change, and a flicker will occur. Many loads have inrush current, which draws a lot more current when first turned on. On startup, they will cause a significant voltage drop in the supply.
Flickers of this nature are inevitable. With a properly designed electrical system, you can minimize flickers but not fully eliminate them. For example, large loads (washing machines, refrigerators, etc.) should be on their own 240V circuits. Since your main supply wires have some resistance, you will still get some flicker even when these loads are on different circuits. The better the design of your electrical system, the less noticeable the flicker will be, but it will still be there.
Benign Culprits for Flickering
Sometimes an appliance with rotary components has a bearing failure, which can cause the motor to work harder on startup, drawing more current and causing a noticeable light flicker.
Similarly, a flicker could occur when a load is activated unexpectedly. For example, a sump pump could result in a noticeable dimming or flicker even if it is properly wired.
The electrical loads at neighbors' houses can also impact your own lights. In most cases, it's likely you share a transformer with several neighbors, and because you share a circuit with them, their loads will impact your electrical supply.
Flickering That Indicates a Problem
If you're noticing a lot more flickering than normal when no intentional changes in the electrical system have been made, this could be an early indication of a more serious electrical problem.
For example, when a circuit connection becomes loose, it creates a high resistance point. This is one of the most dangerous problems that a flickering light could be symptomatic of because it will generate heat at this point, and consequently, the heat will create more movement of the holding screw and wire by the expansion and retraction of the components in the circuit, thus further increasing the amount of slack and eventually further worsening the problem.
Another serious issue that could manifest as a flickering bulb is a switch failure. This is dangerous as well, but the danger is confined to just the switch, unlike the loose circuit connection which endangers the whole system.
Finding the Culprit
If the flicker is significant or has changed in unexpected ways, you should investigate. Some of this investigation is easy enough and safe enough to do on your own. You can simply attempt to correlate the flicker with a particular appliance by listening for buzzing or crackling sounds at your breaker panel when loads are activated. This may allow you to troubleshoot the specific circuit or appliance causing the flicker.
However, if you are annoyed by the problem or believe it is getting worse, consider contacting a professional electrician. A professional can measure what sort of voltage drop you are seeing when various loads turn on.
Once you know where the flicker is coming from, you can decide whether it needs to be fixed and whether it should be done by you or an electrician. Just make sure you possess a good understanding of circuit breaker basics before you attempt to make any changes to your system.