Breaker tripping on Northern Lights generator

[CWester]

I have a Northern Lights M773LW3 7/9KW generator on our catamaran. This season, the 2-pole AC breaker on the generator is tripping regularly. The generator engine runs great and shows normal Temps. It continues to run after the breaker trips. I run the genset each evening to power our watermaker. After about an hour or two of normal operation, the breaker trips (usually right when a little extra load is added - but nowhere near what the generator used to easily handle). I do not see any wiring with obvious signs of overheating. I do notice a slight burning smell from the unit (no smoke and not the smell of burning oil, and again, temps all look normal).

So far, I have:
- Checked for loose electrical connections in the box that houses the AVR, breakers and wiring harness connections.

- Replaced the breaker itself (I originally thought perhaps it had gone bad)

- Removed the top from the insulating box the genset is in and opened the locker that houses it while running to improve airflow. I wondered if maybe an abnormally high ambient temperature might be contributing to the tripping.

What am I missing? Any suggestions would be greatly appreciated!

[skipperpete]

Start with the easy stuff, take off the generator end plate and see if the brushes are ok. This can be harder than it sounds if the genset is installed with the live end close to a bulkhead or in a sound shell but if the hours are high, this is a wearing part, easy to replace. And yes , run it with the lid off the sound shell, if it makes a difference, pursue the cooling option and put a fan on the control box to cool it.

[CarlF]

The genset doesn’t cause a breaker to trip. A load causes the trip. Replacing the breaker was a good idea but you ruled that out.

So I think something in the boat is intermittently drawing high amperage for a few seconds. Perhaps a wire with frayed insulation shorting to a piece of metal. Or a seizing motor on your watermaker. These can (and should) trip the breaker quickly enough that there’s no smoke or hot wire.

I’d try to figure out which pieces of equipment are always running when this happens. Then turn things off one at a time until you isolate the culprit.

[CWester]

Originally Posted by CarlF

The genset doesn’t cause a breaker to trip. A load causes the trip. Replacing the breaker was a good idea but you ruled that out.

So I think something in the boat is intermittently drawing high amperage for a few seconds. Perhaps a wire with frayed insulation shorting to a piece of metal. Or a seizing motor on your watermaker. These can (and should) trip the breaker quickly enough that there’s no smoke or hot wire.

I’d try to figure out which pieces of equipment are always running when this happens. Then turn things off one at a time until you isolate the culprit.

... thanks CarlF... forgive my ignorance, but wouldn't it trip the breakers on the circuits the of the devices that are causing the amperage spikes if that were the case? For example, the Watermaker is almost always on when the genset breaker trips, but the breaker for the WM pump isn't tripped.

[GordMay]

Originally Posted by CWester

... thanks CarlF... forgive my ignorance, but wouldn't it trip the breakers on the circuits the of the devices that are causing the amperage spikes if that were the case? For example, the Watermaker is almost always on when the genset breaker trips, but the breaker for the WM pump isn't tripped.

Yes, a lower rated, downstream circuit breaker should trip, prior to any higher rated upstream breaker. This is called Selective Coordination.

However, two circuit breakers, with instantaneous trips, can not be selectively coordinated, for fault currents, above a certain level. That level is
the fault current at which the upstream circuit breaker operates in its
instantaneous trip region.
When a fault above the instantaneous trip level occurs, the lower
circuit breaker (Device/Equipment) unlatches. However, before it clears the circuit, the upstream circuit breaker(s) [Generator] also unlatches.
Once a circuit breaker unlatches, it will open, thereby disconnecting the circuit from the power source.
For any fault current, greater than where the two circuit breaker curves intersect, the upstream circuit breaker does not necessarily coordinate with the down stream circuit breaker.

Circuit breaker coordination is a very technical subject, and I don't remember much detail, from 30+ years ago.