AC is tripping the RCD at the panel.

[Mixman1]

I have 4 Cruisair AC units on board. All getting power from the same distribution panel. The panel INPUT has a 40A RCD (or GFCI) with a 35mA rating. All the breakers for the AC units are 2 pole breakers (line and neutral). AC unit #1 trips the RCD as soon as it kicks on. Not when power is applied to the circuit, but when the unit actually turns on to run. None of the other 3 units cause this. I have tried with them all running, and all off, and every combination in between. None of the circiout (overload) breakers trip. Just the RCD. Obviously there is a leakage from neutral to ground somewhere in unit #1. This just started happening a week ago. Been running the units a lot for the past year with no issue. Also, same behavior whether it is the generator or shore power.
I am pretty electrically savvy, but I need some help with troubleshooting this one. Any ideas would be greatly appreciated.

[ttex]

Could be a hundred different things.

Might try hooking the faulting ac unit up to a different known working gfci and see if it trips. Could be a bad gfci. They can go bad.

Check connections. Loose connections. Corrosion on bus bars if any, etc.

Could be a bad motor in the unit.

A bunch of other stuff...

[Mixman1]

My first thought was the RCD breaker. But all 4 of the units breakers are fed from that master RCD breaker. And I can run all 3 other units simultaneously through that breaker and it doesn’t trip. Then I shut all units off and only turn on unit #1 and it trips right away.

[Coolerking]

2 pole breakers?
With a L1 and N?
Or L1 and L2?
115vac or 208/230?
My point is to start at the beginning.
then we can talk about the unit itself.

[Mixman1]

230v vessel. L1-230v, Neutral, earth. All breakers on the boat, mains and branch circuits, are 2 pole. Both L1 and N are breakered.

[Mixman1]

Just to clarify. Modern EU boat built to modern EU standards. 230v. One hot leg (L1) one neutral leg (N) and one earth leg (G). Shore power onto boat, breaker, to electrical distribution panel.
At the panel, in this order, RCD differential breaker (also known as GFCI) as a main breaker. Will trip on over current or on a 35mA ground fault. Both N and L1 pass through ALL breakers, main and branch circuits.
After the RCD main breaker, there are 4 branch circuit breakers for the 4 AC units. Each one is an over current protection 2 pole breaker. When tripped it breaks both N and L1.
When AC unit #1 is turned on and all other units are off, it trips the main RCD. Not it’s branch circuit breaker. So it can’t be an over current situation.
When unit #1 is off and all 3 other units are on, it never trips.
This tells me clearly there is a ground leakage somewhere in the unit #1 path.
What I need from the good people here on the forum is ideas on steps to troubleshoot this. Hoping someone has been through this scenario and will share with me.

Side note, this is a recent problem. All has been working excellent for a year. Last week this happened out of the blue. No new work has been done.

[Jammer]

Hello Mixman


You are correct that the likely cause is a neutral to ground short however it is possible that it is unrelated to the air conditioner. The presence of a large load can make problems show up that otherwise cause too little leakage to trip the RCD. This is because the ground-to-neutral voltage increases somewhat when a large load is operating. It is also possible that your problem is cumulative with a little leakage here and a little leakage there.


Since you have double pole breakers the first step is to shut off all the breakers for everything except the air conditioner and see whether the problem resolves.


Then you can try disconnecting all power sources, shutting off all breakers, and checking the neutral-to-ground resistance with an ohmmeter to isolate the problem. If it's less than 10 kohms anywhere you'll want to investigate further. Then check the hot-to-ground resistance the same way, anything under about 100 kohms hot-to-ground can pose problems.


Good luck, let us know how it turns out

[flyingfin]

Sounds like locked rotor on compressor, or shaft bearing
Does that unit have its own blower? Is blower shaft connected to compressor shaft? Can you spin by hand?

[sinnerman]

So if water pump is running OK and does not trip the breaker it is not the water pump.

Try running it in "Fan Only" (AC OFF). If breaker does not trip it is not the blower.

Next would be the compressor / controller.

GFCI's look for leakage to ground. So if you have a multimeter with a high (megaohm) setting, with power OFF (breakers off) start measuring between ground (green) and the controller, compressor wires etc. You will have to disconnect some wires in the process.

The only things that should be "grounded" (green wire) are the metal compressor case, controller metal case etc. If you get low readings to ground on the compressor wires, you found your culprit.

Please note that breakers, GFCI's / ELCI's can get weak with age. I had an issue with one where an ELCI would constantly trip. Replacing the actual breaker fixed the problem (it had a capacitor go bad internally...)

Cheers

[Cadence]

Any water intrusion in or on that unit?

[Mixman1]

No water intrusion at all.

[sailon46]

On me Bene 51.4 had the same system whenever one of the units had air in the cooling line it will trip the breaker only if I primed the unit with sea water everything went back to normal,
Fyi Ernie on the Loremar

[Orin]

Sounds like an insulation problem with one of the motors, or the wiring to the motors. With all power off, I would start disconnecting motors and the wires to those motors and check for neutral to ground impedance. Hopefully you find your problem straight away. Those things can be a little difficult to troubleshoot without being there to observe the behaviors. I would pay special attention to any place on the AC unit where the wiring crossed a metal portion of the AC. It’s possible that the AC chaffed thru the insulation of a neutral wire and now it’s contacting a grounded component for instance.

[ozsailer]

Hi Mixman. Trouble shooting from afar is almost impossible with so many variations. Having an electrical background I am also loathe to give advise over the next simply due to some people having absolutley no idea what they are doing and the subsequent dangers of working with electricity and miscommunication. Without to much drama by turning the power off I would then disconnect the main elecrical feed into the A/C unit and terminate the ends. I would then turn the power back on and see if the RCD trips. If the unit triips as least you know the that the issue is now between the RCD and terminated end. RCD are notorious for tripping out if there is just a small amount of dust/dirt/hair across the terminals. Being an EC unit in a boat this would be my second area of inspection. Once again ensure all power is disconnected. . Other than those suggestions I would suggest a qualified person do the checking as most boaties do not have the required meters to undertake the next line of checking. Just my two cents worth.


Ozsailer
lagoon 440

[Orin]

Originally Posted by ozsailer

Hi Mixman. Trouble shooting from afar is almost impossible with so many variations. Having an electrical background I am also loathe to give advise over the next simply due to some people having absolutley no idea what they are doing and the subsequent dangers of working with electricity and miscommunication. Without to much drama by turning the power off I would then disconnect the main elecrical feed into the A/C unit and terminate the ends. I would then turn the power back on and see if the RCD trips. If the unit triips as least you know the that the issue is now between the RCD and terminated end. RCD are notorious for tripping out if there is just a small amount of dust/dirt/hair across the terminals. Being an EC unit in a boat this would be my second area of inspection. Once again ensure all power is disconnected. . Other than those suggestions I would suggest a qualified person do the checking as most boaties do not have the required meters to undertake the next line of checking. Just my two cents worth.


Ozsailer
lagoon 440

I forget most people don’t have a Fluke 87 V in their glovebox... and also totally forget that plenty of yachties will think that’s some sort of anchor.

In all practicality it’s cheaper and safer to have a qualified person come check it out. 230V can kill you if you don’t know what you’re doing.

Thanks for the sanity check here.