Mixed Lighting Circuit on a Pontoon
A floating pontoon has a 230 V shore supply feeding a consumer unit with a 30 mA RCD. The lighting circuit serves six LED bollard luminaires rated 12 W each, two 150 W halogen floodlights, and a 300 W transformer-fed strip of 12 V deck lights. The cable run is about 45 m of 2.5 mm2 three-core armoured cable buried in the pontoon structure. The owner reports that the 6 A MCB for the lighting circuit trips after about 20 minutes when all lights are on, and that one bollard has water inside its lens. The halogen floodlights are on the same circuit as the LED bollards, and the 12 V transformer is plugged into a nearby socket on a different circuit.
1. Calculate the approximate load current of the lighting circuit and assess whether the 6 A MCB rating is appropriate.
Read the worked answer
LED bollards: 6 x 12 W = 72 W. Halogen floods: 2 x 150 W = 300 W. Total connected load = 372 W. At 230 V, current is about 372/230 = 1.62 A, ignoring power factor and driver losses. Even allowing for inrush and driver inefficiency, steady current is well below 6 A. The 12 V transformer is on another circuit, so it does not add to this MCB. A 6 A MCB is therefore not obviously undersized for steady load, so the trip after 20 minutes points to a fault or thermal issue rather than simple overload. Possible causes include a partial insulation fault that worsens as cables warm, a failing driver or lamp drawing excess current, a loose termination heating up, or a borrowed neutral or shared circuit causing unexpected current. The water-ingressed bollard is a prime suspect and should be isolated and inspected before further energisation.
2. Explain why mixing LED, halogen, and transformer-fed loads on one circuit complicates fault-finding, and outline a staged isolation test.
Read the worked answer
Different loads have different signatures. Halogen lamps have high inrush and heat; LED drivers may have capacitive input and leakage to earth; transformers can have inrush and magnetising current. A 30 mA RCD may trip on accumulated leakage from several electronic drivers, while an MCB trips on overcurrent. The reported 20-minute delay suggests thermal or moisture-related deterioration rather than instant short circuit. A staged test: isolate the circuit, disconnect all luminaires, and megger the cable with the 12 V transformer confirmed off this circuit. Reconnect loads one group at a time, run for at least 30 minutes, and record current with a clamp meter. Test the wet bollard separately and inspect its gland and seal. Check terminations for heat discolouration. If the RCD trips rather than the MCB, measure protective conductor current from each driver group. Do not simply uprate the MCB; that masks the fault and may reduce protection.
3. Recommend the corrective actions and state what documentation should be updated.
Read the worked answer
Replace or re-seal the water-damaged bollard and verify its IP rating suits the location; check all bollard glands and drain paths. Confirm the halogen floods are suitable for the environment and that their heat does not damage adjacent cable. Verify the 2.5 mm2 cable is adequate for volt drop over 45 m at the actual load; at 1.6 A the drop is small, but check the whole installation. Confirm the RCD is functioning and that the circuit is correctly segregated from the 12 V transformer supply. Consider separate switching or a separate circuit for the floods so a single fault does not darken the whole pontoon. Update the circuit schedule with connected loads, protective device ratings, RCD test results, insulation resistance readings, and details of the replaced luminaire. Record the date and any altered cable route.