12 V Bench Lighting Voltage Drop and RCD Nuisance Tripping
An electronics repair shop is adding 12 V LED task lighting over four benches. The owner buys a 12 V, 5 A enclosed power supply and a 5 m LED strip rated 36 W. The strip is mounted 14 m of cable run away from the PSU because the only free socket is near the door. The installer uses 18 AWG two-core cable and measures 12.1 V at the PSU terminals and 10.4 V at the strip; the far end is visibly dimmer and flickers when a soldering iron on the same bench is switched on. The 230 V bench circuit is protected by a 30 mA RCD. The iron is 60 W and the bench PSU draws about 150 W. The RCD occasionally trips when the iron is switched off, not when it is switched on. The owner wants a cheap fix: turn up the PSU to 13.5 V or replace the strip with a higher-power one.
1. Calculate the LED strip current at 12 V and estimate the voltage drop in the 14 m run using typical 18 AWG copper resistance. Should the installer solve the dimming by setting the PSU to 13.5 V? Show your reasoning.
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The nominal strip current is I = P/V = 36 W / 12 V = 3 A. For 18 AWG copper, use about 20.9 mΩ per metre per conductor. The current travels out and back, so the loop length is 14 m × 2 = 28 m. Loop resistance is about 28 × 0.0209 = 0.585 Ω, giving a drop of I × R = 3 A × 0.585 Ω = 1.76 V. With 12.1 V at the PSU, the strip would see about 10.3 V, which matches the measured 10.4 V and explains the dimness.
Turning the PSU to 13.5 V is not a sound fix. It raises the strip voltage only partly because the strip current also rises, increasing the drop. It also overdrives a 12 V strip and stresses the PSU, and the 5 A supply has little margin if current rises. Better options are to move the PSU close to the strip, use a larger conductor, or use a 24 V strip and driver. For example, 14 AWG has about 8.29 mΩ/m, so 28 m gives 0.232 Ω and only about 0.70 V drop at 3 A. The installer should also confirm the PSU voltage at its terminals under load and derate the cable for bundling and temperature.
2. The RCD trips when the soldering iron is switched off. Outline a safe diagnostic sequence and explain why replacing the 30 mA RCD with a 100 mA unit is unacceptable.
Read the worked answer
The RCD responds to an imbalance between live and neutral current, so a trip at switch-off points to a leakage path or a switching transient exposing a fault, not to a normal load. Isolate and lock off the bench circuit, then inspect the iron's plug, cable, strain relief and switch for damage or moisture. Test the iron's insulation resistance with an approved insulation tester after disconnecting it, and check the element for moisture ingress, which can change as the iron cools. Test the bench PSU and other equipment separately to see whether leakage is cumulative, and verify the RCD with a proper trip-time tester on both half and full test current.
Replacing the 30 mA device with 100 mA is unacceptable because it reduces protection against electric shock and does not remove the fault. It may simply hide a deteriorating iron or a neutral-to-earth fault. The correct action is to repair or replace the faulty item, correct any wiring defect, and confirm the RCD still trips within its rated time. If the iron cannot be made safe, remove it from service and tag it.