Fault condition protective device for aircraft fuel gauging system

In an aircraft fuel gauging system (18) having at least one probe (14) disposed in an aircraft fuel tank (10) for at least partial immersion in the fuel, a generator (20) for applying an input signal at a predetermined frequency to the probe (14), the generator (20) being located exterior of the fuel tank (10) and being electrically coupled to the probe (14) via wiring (26, 36) extending through a wall (28) of the fuel tank (10), and instrumentation (24) for receiving an output signal from the probe (14) and for determining the quantity of fuel in the fuel tank (10), the instrumentation (24) being located exterior of the fuel tank (10) and being electrically coupled to the probe (14) via wiring (30, 36) extending through the wall (28) of the fuel tank (10), the improvement comprising:

• a resonant circuit (46) located adjacent the wall (28) of the aircraft fuel tank (10) and electrically coupled to the probe (14);
• said resonant circuit (46) having a resonant frequency substantially equal to the predetermined frequency of the input signal;
• said resonant circuit (46) providing a low series impedance at said resonant frequency;
• and
• said resonant circuit (46) providing a high series impedance at signal frequencies corresponding to fault conditions so that current flowing in the probe (14) is limited.

In an aircraft fuel gauging system (18) having at least one probe (14) disposed in an aircraft fuel tank (10) for at least partial immersion in the fuel, a generator (20) for applying an input signal at a predetermined frequency to the probe (14), the generator (20) being located exterior of the fuel tank (10) and being electrically coupled to the probe (14) via wiring (26, 36) extending through a wall (28) of the fuel tank (10) and instrumentation (24) for receiving an output signal from the probe (14) and for determining the quantity of fuel in the fuel tank (10), the instrumentation (24) being located exterior of the fuel tank (10) and being electrically coupled to the probe (14) via wiring (30, 36) extending through the wall (28) of the fuel tank (10), the improvement comprising:

• a fault condition protective device (48) electrically coupled to the probe (14) at a location adjacent the wall (28) of the fuel tank (10), said device (48) having a voltage clamp (50) and resonant circuit (46);
• said voltage clamp (50) having a resistor (R1) and transient suppressor (D1) adapted to limit the voltage applied to the probe (14);
• said resonant circuit (46) including an inductor (L1) and a capacitor (C1) connected in series and resonating at a frequency substantially equal to the predetermined frequency of the input signal such that said resonant circuit (46) provides a low series impedance when the input signal frequency is the predetermined frequency and a high series impedance at signal frequencies corresponding to fault conditions to limit the current flowing in the probe (14).