Landing Light Dimming - Locating a High-Resistance Joint
A 28 V DC bus supplies a 250 W landing light through a single-conductor cable. The run from the bus to the lamp is 12 m, and the return path through the structure has the same resistance, 0.0026 Ω per metre per conductor. The lamp filament, at normal operating temperature, measures 2.93 Ω. With the lamp lit, a clamp meter on the supply cable reads 9.0 A and a voltmeter directly across the lamp terminals reads 26.4 V. The bus is confirmed at 28.0 V. The previous shift had checked the same circuit with a bonded ohmmeter from the bus to the lamp base and recorded 'continuity good, no fault found'.
1. Calculate the current and the lamp terminal voltage that a healthy circuit of this type would produce, and compare with the measured figures.
Read the worked answer
Circuit resistance in a healthy installation:
- Cable loop length = 12 m out + 12 m return = 24 m
- R_cable = 0.0026 × 24 = 0.0624 Ω
- R_lamp = 2.93 Ω
- R_total = 2.9924 Ω
Predicted values:
- I = 28.0 / 2.9924 = 9.36 A
- V_lamp = 9.36 × 2.93 = 27.4 V
- Cable drop = 28.0 − 27.4 = 0.58 V
Comparison with measurement:
- Measured 9.0 A and 26.4 V. The lamp resistance implied by the measurement is 26.4 / 9.0 = 2.93 Ω, which matches the rated hot resistance - so the lamp itself is behaving correctly.
- The circuit is losing 28.0 − 26.4 = 1.6 V, not the expected 0.58 V. Roughly 1.0 V is unaccounted for.
The lamp is not the problem; the circuit is.
2. Quantify the unwanted resistance and the power dissipated in it, and explain why the ohmmeter check did not find it.
Read the worked answer
Total circuit resistance from the measurement:
R_total = V / I = 28.0 / 9.0 = 3.111 Ω
Subtract the known good elements:
- Lamp: 2.93 Ω
- Cable: 0.0624 Ω
- Remaining (foreign) resistance = 3.111 − 2.93 − 0.0624 = 0.118 Ω
Power in that resistance:
P = I²R = 9.0² × 0.118 ≈ 9.6 W
Nine watts in a connector, a terminal or a bonding point is a local hot spot, a fire risk and a progressive failure - the resistance rises as it heats, so the lamp gets dimmer and the joint gets hotter.
Why the ohmmeter missed it:
- 0.12 Ω is at or below the practical resolution of a hand-held ohmmeter once lead resistance and contact resistance of the probes are included.
- 'Continuity' checks pass at any resistance up to the meter's beep threshold - they prove a path exists, not that it is adequate.
- A marginal joint often only shows high resistance when current flows and the joint warms up, so a cold, no-load measurement can look perfect.
The correct test is a voltage drop measured under load, which is what the 1.6 V figure is.
3. Describe how you would localise and rectify the fault, and state the acceptance criterion on completion.
Read the worked answer
Localisation - split the circuit under load:
- Measure from the bus positive terminal to the lamp supply terminal. Any significant reading is drop in the positive leg.
- Measure from the lamp body/ground return to a clean structure point near the bus earth. Drop here points to the ground return or bonding.
- The leg showing the bulk of the 1.6 V contains the fault. Work backwards joint by joint (connector pins, splices, terminal lugs, bonding straps, earth stud) until the drop disappears across one item.
Likely causes: loose or under-crimped terminal, corroded or oxidised pin, copper-to-aluminium contact, undersized or damaged splice, paint or sealant under a bonding point, or a partially broken strand bundle.
Rectification:
- Remove and inspect the terminal; cut back and re-terminate with the correct crimp tool and terminals to the approved standard.
- Clean to bright metal, apply the approved corrosion protection, and re-bond to the specified resistance.
- Do not simply tighten and retest - a hot joint that has been arcing is already degraded.
- If a conductor has lost strands, replace the cable section, not just the lug.
Acceptance: with the lamp lit at rated bus voltage, total circuit drop must be within the figure stated in the wiring diagram/manual (of the order of the 0.58 V calculated), the lamp terminal voltage restored to about 27.4 V, and no detectable heating at any connection after a soak run. Record the before/after voltage drops on the paperwork.