If you have read the information on the Electrical Terminology page, you will understand that a reduction in circuit resistance will cause an increase in current flow in that circuit and also in the upstream feeder and main service. To try to understand what goes on when a breaker trips or a fuse blows, we can consider a practical situation. We will use a normal 15 Amp circuit at 120 Volts as an example. If this circuit has a load present that would normally draw 12 Amps, (like a kettle) we could conclude that the effective circuit resistance is 10 Ohms. We divided Voltage by Current to find this Resistance.
Of this 10 Ohms, about 9.9 Ohms is in the kettle, while the cable for the circuit has about 0.1 ohms. Therefore, if the cord connecting the kettle to the receptacle were to develop a fault that allowed current to pass from one wire to the other without passing through the kettle element (commonly known as a short circuit), this would cause the current to reach a value that could be 1200 Amps. This time we divided the Voltage by the cable resistance to find the current. As the cable size increases (gets thicker) to carry more current, the cable resistance decreases....and the available fault current increases. You may see a 10,000 Amp fault on a 100 Amp circuit breaker if the fault occurs on the main cables.
If 1200 Amps appears on a 15 Amp circuit breaker, the circuit breaker should trip instantly. However, we must remember that the 1200 Amps is also present on the main circuit breaker for the panel. This may be a 100 Amp circuit breaker. In such cases you may find that the main circuit breaker trips more quickly than the 15 Amp circuit breaker.
The same can be said for fuses. In some cases, the main fuse operates near it's maximum value for a long period of time and becomes "tired" or "stressed". In such cases the "tired" fuse may blow before the 15 Amp fuse.
So, how do you find the cause of tripping breakers or blowing fuses? You should start by paying attention to which circuit breaker tripped. If it is only the 15 Amp breaker that tripped you have already isolated the problem to that circuit.
If the main circuit breaker has tripped, you need to find out which individual circuit caused the problem. Ideally, you will have a megohm meter to test each circuit. If you rely on resetting the breaker to find the problem, this could create more damage so this should be avoided. You should begin by switching off all of the individual circuit breakers and ensuring that the main circuit breaker is completely OFF. Test the main bus of the panel for a fault to ground and also for a fault between the two "live legs" (red to black). With all breakers off, the megohm meter should show infinity. With this done, you know that the main breaker can be turned on. So switch it on to make sure it doesn't trip with all individual breakers off.
Assuming that the main circuit breaker stayed on, you now know that the problem is in an individual circuit....but which one? You now test each circuit looking for a very low resistance. Some of the circuits may have a low resistance due to the nature of the load. For example, lamps may have a very low resistance when cold. so begin by finding those circuits that have relatively high resistance. By switching on these circuits you may get some light to work by since you will likely be working by flashlight at this point.
Eventually, through the process of elimination, you will get to the offending circuit.
Once you have isolated the problem to an individual circuit, you need to look at how many receptacles, lights, etc. are on that circuit. In the case of the kettle example, you would unplug the appliance to see if this clears the fault. If the fault clears, this means that the kettle has a problem. In the case of a circuit with several lights, you should switch off individual room lights. Again, through the process of elimination, you will get to the source of the problem. If there are receptacles in the circuit and portable items plugged in, you should unplug all of these and inspect each of them.
If you follow these steps, you should get to the root of the problem. As with most things, you will work more quickly as you gain experience.
If you need to replace a circuit breaker, a light fixture, a switch, etc. to solve the problem, remember that you should avoid used equipment. With used electrical products, you have no way of knowing how reliable these will be and this could present safety concerns.
The process above deals with a short circuit. However, in some cases a circuit breaker will trip or a fuse will blow due to an overload condition. With an overload, there will be a period of time between each breaker tripping or fuse blowing episode. Usually, the overload will be on an individual circuit but it can also happen on a main circuit breaker or a main fuse. You can use a clip on ammeter to test the current being drawn on the main circuit breaker. This can often take some time to do, especially if heating loads are present. For example, it may happen only at meal times on cold days so you need to pay attention to the circumstances present when the problem occurs. You solve the overload by re-distributing the load on individual breakers and by upgrading the service if the main breaker is overloaded.
You can help solve problems by taking time to clearly lable your electrical panel to identify each circuit. When you have some spare time, use it to write down in a notebook, which breaker controls each light, receptacle, etc. By doing this you can avoid overloads by not using an already loaded circuit to plug the steam iron (air conditioner, etc)into.