Wet-Weather Breaker Trips on a Street Lighting Circuit
A 120/240 V single-phase street lighting circuit feeds eight 100 W LED cobra-head luminaires on concrete poles along a half-mile section. The 20 A breaker at the service panel trips about once a week, usually during or after rain, and resets without trouble. The circuit is not GFCI-protected, and the poles are bonded to a bare copper equipment ground run with the circuit. A megohmmeter test with the circuit de-energized and the luminaires disconnected at each handhole gives 0.4 MOhm from the load-side conductor to ground at pole 3; poles 1, 2, and 4 through 8 read above 200 MOhm. The pole 3 handhole contains standing water and the fixture whip connector shows green corrosion. The service ground resistance measured 18 ohms. The crew wants to dry the handhole, reset the breaker, and move on.
1. Interpret the 0.4 MOhm reading. Is it a failure, and does it explain the breaker trips?
Read the worked answer
0.4 MOhm is a real defect, but it is not the explanation for the breaker trip. Do the arithmetic: at 120 V, 0.4 MOhm passes about 0.3 mA. A 20 A breaker will not trip on that. The megohmmeter has found a moisture path that must be repaired, while the actual trip cause is still unidentified.
Use the comparison as the first test: seven poles read above 200 MOhm and one reads 0.4 MOhm, a difference of nearly three orders of magnitude. Common practice treats insulation below 1 MOhm as failed and requires repair; 0.4 MOhm is clearly in that range, and standing water plus a green connector is direct evidence of the mechanism.
Wet-weather tripping with a marginal insulation reading usually means one of three things: an intermittent hard fault the megger did not catch because the conductor moved when the handhole was opened, a failing breaker, or a neutral or grounding problem on the shared circuit. The low reading tells you where to start, not what tripped the breaker.
2. What is the repair sequence, and what safety steps come first?
Read the worked answer
Safety first: lock out and tag the lighting circuit at the service panel, verify absence of voltage at the handhole with a meter rated for the circuit, and treat the neutral as a current-carrying conductor. Do not work alone, and do not stand in standing water.
Repair sequence:
- Pump and clear the handhole, then find why it holds water, such as a blocked drain, a missing cover gasket, or grade, and correct it. Otherwise the repair will fail again.
- Replace the corroded fixture whip and connector and any damaged conductor. Use wet-rated connections and reseal the handhole.
- Megger the repaired pole again, conductor to ground and conductor to neutral, and compare with the other poles.
- Inspect and torque the breaker connections, and check for a shared neutral or crossed circuit before re-energizing.
Drying the handhole and resetting the breaker is not a repair. The water will return with the next storm, and the next trip may occur with a crew in the handhole.
3. What else should be checked before re-energizing, and what would you document?
Read the worked answer
Confirm the equipment grounding conductor is continuous from the service to every pole, because pole bonding is what keeps a future fault from energizing the pole. The 18-ohm ground reading meets the NEC 25-ohm alternative for a single electrode, but check the agency specification. Many lighting specifications require 5 or 10 ohms, and if so, 18 ohms is nonconforming and should be improved with additional electrodes or a better connection.
Also check the breaker's trip curve and age, and look for a hard fault that appears only when the circuit is energized and warm. Document the before-and-after megohmmeter readings, the moisture source, the parts replaced, and the ground resistance.
If the breaker trips again after the repair, the next step is a recording ammeter and an insulation test while the circuit is energized and flexed, not another reset. Repeated resets without diagnosis are how a lighting circuit ends up with a disabled protective device.