Voltage Drop on a 28 V DC Feeder
During a scheduled inspection of a Part 23 twin-engine aeroplane, you measure the 28 V DC feeder to a 15 A continuous cabin cooling fan. The feeder is 6 m of 2.5 mm² copper cable, routed in a conduit at an ambient temperature of 55 °C, protected by a 20 A thermal circuit breaker. With the fan running and all other loads off, the bus is at 28.2 V and the fan terminal is at 26.4 V. The fan manufacturer permits 24–29 V. The cable manufacturer’s data gives 20 A free-air rating at 30 °C and a 0.70 derating factor for the conduit and ambient. A previous shift replaced the fan connector but did not record a voltage-drop test. The aircraft is due for release in two hours. You have a calibrated milliohm meter and a clamp meter.
1. Calculate the expected voltage drop at 15 A and 55 °C for the 6 m, 2.5 mm² copper feeder, and compare it with the measured drop. What does the result indicate?
Read the worked answer
R20 = 0.0175 × 6 / 2.5 = 0.042 Ω. At 55 °C, using α ≈ 0.00393/°C: R55 = 0.042 × [1 + 0.00393 × (55 − 20)] = 0.0478 Ω. Expected drop = 15 × 0.0478 = 0.72 V. Measured drop = 28.2 − 26.4 = 1.8 V, which is 6.4% of bus voltage, not 2.5%. The extra 1.08 V corresponds to about 0.072 Ω beyond the cable’s expected resistance. That is too large to be explained by normal copper temperature rise. It points to a high-resistance connection: contaminated or loose terminal, poor crimp, partially broken strands, or a corroded splice. Because the previous shift disturbed the connector, that is the first suspect. The feeder should not be released on the basis that the fan still runs; locate and correct the resistance before certification.
2. Assess the suitability of the 2.5 mm² cable and 20 A breaker for this 15 A continuous load at 55 °C, using the manufacturer’s derating data. Show your reasoning and state the design options.
Read the worked answer
Derated cable ampacity = 20 A × 0.70 = 14 A. The continuous load is 15 A, so the cable is at 107% of its derated rating before any margin for breaker tolerance or bundle heating. The 20 A breaker is also above the derated cable ampacity, so it does not provide the intended protection against cable overheating. A 5% voltage-drop design limit on 28.2 V is about 1.41 V; the measured 1.8 V exceeds that as well. Therefore the installation is not satisfactory as a 15 A continuous circuit, even if the bad connection is repaired. Options are: reduce the continuous load, increase conductor size, for example to 4 mm², reroute the cable to a cooler zone, or obtain an approved engineering change that documents the thermal and voltage-drop case. Fitting a larger breaker would worsen cable protection and is not acceptable.
3. List the immediate maintenance actions and records you would complete before signing a maintenance release.
Read the worked answer
Isolate the DC bus and lock off. Remove and inspect the fan connector; check crimp height, pull test, contact retention, corrosion, and heat damage. Measure resistance from bus to fan terminal with a milliohm meter, then re-run the loaded voltage-drop test. Repair or replace the terminal/connector with approved parts using a calibrated crimp tool. If the cable is found undersized, raise a defect or engineering request rather than releasing it. Record the before-and-after voltages, resistance, ambient temperature, meter serial number and calibration, part numbers, and crimp tool calibration. Complete the work order description and reference the approved maintenance data. Do not sign the certificate of release to service until the voltage drop and derated ampacity are both acceptable. Note the time pressure as a human-factors risk in the shift handover.