Circuit Load – Circuit Resistance
Written on May 22, 2023 By Ronald Stewart in Blog
This Blog was posted for you By Your Electrician in San Antonio – Good Electric
Circuit Load – Circuit Resistance
Suppose you have read the information on the Electrical Terminology page. In that case, you will understand that a reduction in circuit resistance will cause an increase in current flow in that circuit and the upstream feeder and main service. We can consider a practical situation to understand what goes on when a breaker trips or a fuse blows. We will use a normal 15 Amp circuit at 120 Volts as an example. If this circuit has a load 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 these 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.
If the fault occurs on the main cables, you may see a 10,000 Amp fault on a 100 Amp circuit breaker. If 1200 Amps appear 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. Sometimes, the main fuse operates near its maximum value for a long 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 only the 15 Amp breaker tripped, you would have already isolated the problem to that circuit. If the main circuit breaker tripped, you need to find out which 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 the ground and 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 ensure it doesn’t trip with all individual breakers off. More on this website
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 meager resistance. Some of the circuits may have a low resistance due to the nature of the load. For example, lamps may have a meager 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 since you will likely be working by flashlight. Eventually, you will get to the offending circuit through the process of elimination. 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 to avoid used equipment. With used electrical products, you have no way of knowing how reliable these will be, which could present safety concerns.
The process above deals with a short circuit. However, sometimes, a circuit breaker will trip, or a fuse will blow due to overload. With an overload, there will be a period between each breaker-tripping or fuse-blowing episode. Usually, the overload will be on an individual circuit, but it can also happen on the main circuit breaker or the 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, 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 upgrading the service if the main breaker is overloaded. You can help solve problems by taking the time to label 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.