We have an AC unit installed on our 480DB flybridge that has worked without issues at our old marina, as well as when running on the genset. Recently we relocated to a new marina and when connected to shore power if we engage the compressor on the flybridge AC unit the master breaker on the dock trips - NOT the 50A breaker in our pedestal, but the 120A breaker in the main dock box. None of the other AC units cause this problem, despite larger compressors and bigger starting surge current. So it seems more likely to be a phase-to-phase or ground-fault issue, but I cannot figure out where to look for the problem. All of the wiring appears to be intact at the AC unit, C, S & R conductors are all properly insulated, and the same unit continues to operate properly on the genset. Any ideas?
Bridge AC Unit Ground Fault?
- Thread starter Benedetta
- Start date
Wanderer 290
Well-Known Member
Does the a/c still operate off generator?
It seems your new marina has implemented the NEC requirements for Ground Fault Protection which requires a GF device at the dock main power distribution panel and a low 30ma leakage trip.
So, if you have a device or equipment on your boat that is not returning all current within 30ma on the power line conductors the circuit breaker will trip.
If all other circuits on your boat are nominally operating and not tripping the ground fault device then either the AC unit is wired incorrectly or their is a short to the ground and bonding network. The short could be a chaffed wire, or an actual short in the equipment, like within the compressor.
An electrician can easily isolate and determine the fault.
So, if you have a device or equipment on your boat that is not returning all current within 30ma on the power line conductors the circuit breaker will trip.
If all other circuits on your boat are nominally operating and not tripping the ground fault device then either the AC unit is wired incorrectly or their is a short to the ground and bonding network. The short could be a chaffed wire, or an actual short in the equipment, like within the compressor.
An electrician can easily isolate and determine the fault.
Spark600
Well-Known Member
- Dec 28, 2013
- 460
- Boat Info
- 2015 Sundancer 410
Onan Generator
Mercury dinghy
- Engines
- Cummins QSB HO 380 HP
Zeus Pods
I am with ttmott it is usually the motor ( Compressor ).
Wanderer 290
Well-Known Member
What Ttmott states is what I was leading to. Our marina ungraded to the new shore power system 2 years ago and about 25 vessels began tripping the shore power system.
Yes - works fine on generator.
Thank you for the excellent reply and insights - I will engage an electrician to check the AC unit. Given that all of the conductors appear to be intact I am guessing that the compressor is the culprit. If so, is there any danger to operate the unit when on the genset?
Your generator (alike the utilities entrance to a home or business) ties the neutral and grounds and bonding together for 120VAC in the US. 240VAC is different as the generator puts out two legs of 120VAC out of phase in that there are two "hot" legs and the ground and bonding network on your boat isn't used other than grounding the equipment chassis. So, if you have low level current leakage to the AC unit's chassis and consequently it's going to ground (like a high resistance short in the compressor motor) and the current isn't enough to trip a 20 amp circuit breaker the potential risk to getting electrocuted is still there depending the condition of your ground and bonding network. Plus, consider the marina's GFI system a "boat saver"; any significant current leakage into the water around the boat is eroding the metals on your boat....
Also, you might ask if there is a swimming risk around the boat? In fresh water and connected to a marina's power without GF protection and leaking current through the boat's bonding system, the risk is very high. When generating power on the boat the risk is low as there isn't a closed loop through the water for the electrical current.
