Cabin Lighting Feeder: High-Resistance Joint or Undersized Cable?
A 28 V DC essential bus supplies a cabin lighting load through a 6 m feeder run of two conductors, each rated at 5 mΩ per metre. The load is three identical lamp units in parallel, each drawing 4 A at 28 V. With all three selected on, a technician measures 24.6 V at the lamp junction box; with the lamps switched off the same point reads 27.9 V, and the busbar holds 28.0 V under load. The feeder was replaced six months ago. The terminal studs at the junction box carry a light grey-green deposit and one nut has a discoloured washer. The maintenance manual permits a maximum feeder drop of 1.0 V at rated load. Available: a calibrated digital multimeter, a four-wire milliohm meter, a torque wrench and the wiring diagram. No spare feeder is in stock and the aircraft departs in four hours.
1. Calculate the drop the feeder conductors alone should produce at rated load, and determine whether the measured value is within the manual limit.
Read the worked answer
Total load current = 3 × 4 A = 12 A. The loop is two conductors, so loop resistance = 6 m × 2 × 5 mΩ/m = 60 mΩ. Expected drop = 12 A × 0.060 Ω = 0.72 V, giving 27.28 V at the box - comfortably inside the 1.0 V limit.
Now compare with measurement: drop from bus to box = 28.0 − 24.6 = 3.4 V. That is roughly 4.7 times the calculated conductor drop and more than three times the certified limit.
Equivalent loop resistance under load = 3.4 V / 12 A = 0.283 Ω, against 0.060 Ω expected. The excess, about 0.22 Ω, sits somewhere other than the cable - terminals, studs, socket contacts. The limit expressed as resistance is 1.0 V / 12 A = 83 mΩ, so the measured 283 mΩ is a clear failure. Conclusion: cable size is not the issue; the joint is.
2. The no-load reading at the same point is 27.9 V. Explain what that observation proves about the nature of the fault, and estimate the resistance at the suspect joint.
Read the worked answer
With no current flowing, V = IR gives zero drop across any series resistance, so an open-circuit reading shows only that the circuit is continuous and that there is no gross parallel leakage. A series fault is invisible at zero load - this is why load testing, not voltage probing on a dead circuit, finds these defects.
Fault discrimination:
- Not an open circuit: current flows and the lamps light.
- Not a short or partial short: that would lower loop resistance, raise current above 12 A and brighten the lamps.
- A high-resistance series joint: current is essentially normal, voltage at the load sags, and the joint dissipates heat.
Estimate: R_total = (28.0 − 24.6) / 12 = 0.283 Ω; subtract the 0.060 Ω of cable, leaving about 0.22 Ω at the junction. Dissipation there = I²R = 144 × 0.22 ≈ 32 W in a terminal stud - enough to discolour the washer, cook the corrosion-inhibiting film and create a fire risk. The grey-green deposit is consistent with corrosion that has grown resistive; the discoloured washer is the thermal fingerprint.
3. State the corrective action and the verification steps required before the aircraft departs.
Read the worked answer
Do not simply re-torque the nut: the resistive film would remain and the joint would degrade again.
- Isolate the bus and disconnect the terminal. Inspect stud threads, terminal barrel and insulation for blueing, pitting, arcing or melted sleeving. If the stud or terminal is heat-damaged or pitted, the junction box or terminal must be replaced - that is not deferrable, because the joint is already a fire risk at 32 W dissipation and will worsen.
- Clean per the maintenance manual (contact cleaner plus the approved mild abrasive method), renew corroded hardware, apply the specified corrosion-inhibiting compound to the mating faces, and torque to the manual figure with a calibrated wrench.
- Verify with a four-wire milliohm measurement of the joint alone; a sound joint reads in the sub-milliohm range, orders of magnitude below 0.22 Ω.
- Re-energise all three lamps and confirm at least 27.0 V at the box (28.0 V minus the 1.0 V limit) at 12 A, check each lamp current is equal, and after 10–15 minutes confirm no hotspot by touch-safe thermal check.
- Record the measurement in the technical log. If the correct part is unavailable and the drop cannot be brought within 1.0 V, the aircraft does not depart.