Text-only reference. Published from the recorded official FAA General Chapter 12 PDF. Diagrams, photographs, and figure artwork are not reproduced here; use the official FAA PDF for those materials.
12-83 + − Black lead Open circuit Red lead 0 Simplified ohmmeter ∞ + − Black lead Shorted circuit Red lead 0 Simplified ohmmeter ∞ + − Black lead R1 Red lead Isolate from circuit connect across resistor 0 Simplified ohmmeter ∞ R2 R3 + − Troubleshooting Shorting Faults in a Series Circuit An open fault can cause a component or system not to work, which can be critical and hazardous. A shorting fault can potentially be more of a severe nature than the open type of fault. A short circuit, or “short,” causes the opposite effect. A short across a series circuit produces a greater than normal current flow. Faults of this type can develop slowly when a wire bundle is not properly secured and is allowed to chafe against the airframe structure or other systems, such as hydraulic lines. Shorts can also occur due to a careless technician using incorrect hardware when installing an interior. If screws that are too long are used to install trim, it is possible to penetrate a wire bundle immediately causing numerous shorts. Worse yet, are the shorts that are not immediately seen but “latent” and do not show symptoms until the aircraft is in service. Another point to keep in mind is when closing panels. Wires can become pinched between the panel and the airframe causing either a short or a latent, intermittent short. The simplified circuit, shown in Figures 12-179 through 12-182 is used to illustrate troubleshooting a short in a series circuit.
In Figure 12-179, a circuit is designed to light a lamp. A resistor is connected in the circuit to limit current flow. If the resistor is shorted, as shown in the illustration, the current flow increases and the lamp becomes brighter. If the applied voltage were high enough, the lamp would burn out, but in this case the fuse would protect the lamp by opening first. Usually a short circuit produces an open circuit by either blowing (opening) the fuse or burning out a circuit component. But in some circuits, there may be additional resistors which do not allow one shorted resistor to increase the current flow enough to blow the fuse or burn out a component. [Figure 12-180] Thus, with one resistor shorted out, the circuit still functions since the power dissipated by the other resistors does not exceed the rating of the fuse.
Tracing Shorts with the Ohmmeter The shorted resistor can be located with an ohmmeter. [Figure 12-181] First the switch is opened to isolate the 12-84 Break V + − Black lead Needle moves full-scale (short) Conventional current Initial charging of capacitor behaves like a shorted circuit. Red lead 0 Simplified ohmmeter ∞ + − +− Black lead Needle moves full-scale (open) Full charge no current Once charged, the capacitor behaves like an open circuit. Red lead 0 Simplified ohmmeter ∞ circuit components. In Figure 12-181, this circuit is shown with an ohmmeter connected across each of the resistors.
Only the ohmmeter connected across the shorted resistor shows a zero reading, indicating that this resistor is shorted. Tracing Shorts with the Voltmeter To locate the shorted resistor while the circuit is functioning, a voltmeter can be used. Figure 12-182 illustrates that when a voltmeter is connected across any of the resistors that are not shorted, a portion of the applied voltage is indicated on the voltmeter scale. When it is connected across the shorted resistor, the voltmeter reads zero. Troubleshooting Open Faults in a Parallel Circuit The procedures used in troubleshooting a parallel circuit are sometimes different from those used in a series circuit. Unlike a series circuit, a parallel circuit has more than one path in which current flows. A voltmeter cannot be used, since, when it is placed across an open resistor, it reads the voltage drop in a parallel branch. But an ammeter or the modified use of an ohmmeter can be employed to detect an open branch in a parallel circuit.
If the open resistor shown in Figure 12-183 was not visually apparent, the circuit might appear to be functioning properly, because current would continue to flow in the other two branches of the circuit. To determine that the circuit is not operating properly, a determination must be made as to how the circuit should behave when working properly. First, the total resistance, total current, and the branch currents of the circuit should be calculated as if there were no open in the circuit. In this case, the total resistance can be simply determined by: RT = R N Where RT is the total circuit resistance N is the number of resistors R is the resistor value