Inverter case charged

[tkeithlu]

I just got the !@#$%^ shocked out of myself from touching my inverter case.

Here's the set-up: Steel boat. Three ways to supply 120VAC - shore, genset, inverter (AIMS 3000). Each has an outlet socket at the wheelhouse panel; you plug the boat into one of the three. Then it goes to a 1:1 transformer for isolation followed by the circuit breaker panel. All sources show correct orientation coming out of the transformer.

The inverter case is not grounded, but its output has a ground going to the common ground. The inverter sits on a wood shelf in the engine room. The case is live to about 75VAC.

Have I got a fault in the green wire leaving the inverter? Should the case have a separate ground, and if so, directly to the AC,DC, bond?

Any thoughts on what's going on here?

[smac999]

the case should have a chassis ground to DC ground. same size as the inverter battery cables. although since it's live you'll just going to blow fuses or breakers when you add it. you need to find the problem and fix it. then add chassis ground.


your "plug" sounds ridiculous. you should have have a switch.

also I don't know why you'd have a transformer for gen or inverter. only shore needs it.

you say the inverter ground goes to a common ground? it should not if you have it feeding an iso transformer. it would only go the the transformer input ground. which would be isolated from the common boat ground.


does your inverter have a netreal to ground bond (should be on a boat) or is it floating?

[tkeithlu]

Thank you. That gives me some guidance on how to approach it. I know the "plug the boat in" system is strange, but it actually works rather well.

[chris95040]

I suggest getting some actual expert advice on this.

My $0.02...

I believe the output ground of the isolator should indeed be bonded to your "common" AC ground, along with every other AC ground.

Basically you want a regular AC grounding system, with this isolation in between only the shore power's ground and your boat.

I think you end up accomplishing this with your plug strategy, and it just needlessly takes the inverter or genny output and puts it through that isolation transformer with (maybe?) no ill effect, and hopefully your inverter case issue is just that its not properly tied to the boat's AC ground.

Certainly don't just experiment, other than ohming out wires to make sure they actually connect what you think they are connecting with no unexpected breaks or shorts, look at the wiring diagram in the manual for your inverter. Have that ready even if you hire an expert.

[tkeithlu]

Thank you, Chris. I'm coming to suspect the grounding from the inverter to the AC common, but I'm going to ask AIM also. I'm also going to check the orientation at the inverter socket for the boat, since the transformer might well straighten out the orientation before power proceeds to the breaker box. You're right - unless this turns out to be very simple, it's time for a professional. At least I'm not worried about the other two circuits - shore power and genset - or the AC after it leaves the breaker panel.

[Boatwright]

As far as theory goes: Inverters create a voltage neutral the same way the power company does - by using a neutral voltage tap in the AC transformer circuit. Ground is bonded to neutral and the inverter case. The external inverter ground should be bonded to the boat's ground system. Note the instructions that came with your inverter that call for a [U]heavy[U], #6 or #4 bonding wire. If you have AC voltage at the inverter case, there are several possibilities: First you need to isolate the inverter from the rest of the system. Disconnect everything on the output side of the inverter: recepticals, hard wired appliances, you mentioned an isolation transformer, everything. Directly connect the inverter to the battery and bond the inverter to boat ground. If, with the inverter properly grounded the case is still hot, the inverter is defective - because of the 75 volt reading possibly an internal short in the inverter transformer windings. If the inverter tests ok, you have a serious problem with the AC wiring and the inter-connections you seem to have. IN EITHER CASE, CALL A PROFESSIONAL CERTIFIED MARINE ELECTRICIAN! PLEASE! AC KILLS!

[Boatwright]

One more: Inverters sharing a common AC distribution bus MUST have a shore power isolation relay that prevents interconnection when shore power is hot. This is a different function from and in addition to an isolation transformer.

[Boatwright]

The more I ponder this query the more I worry.

PLEASE IMMEDIATELY DISCONNECT YOUR BOAT FROM SHORE POWER. DISCONNECT YOUR INVERTER FROM THE BATTERY. BECAUSE OF STRAY AC VOLTAGE TO GROUND THERE IS A POTENTIALLY LETHAL SHOCK HAZARD. CALL AN ELECTRICIAN PLEASE.

[chris95040]

Originally Posted by Boatwright

One more: Inverters sharing a common AC distribution bus MUST have a shore power isolation relay that prevents interconnection when shore power is hot. This is a different function from and in addition to an isolation transformer.

If I'm understanding correctly, they maintain this isolation in way that's even better than a relay: Only one can be plugged into the distribution bus at a time.

But I agree, consult an expert.

[Boatwright]

To explain the function of a shore power isolation relays vs an isolation transformer vs a battery isolator/charge controller :

When shore power is energized the relay automatically disconnects the inverter output from the AC bus. Some $1,000+ marine inverters include an isolation function built in.

The isolation transformer provides a physical separation between the boat's AC bus and the shore power supply. The power is transmitted via the magnetic field of the transformer. This eliminates other problems such as stray galvanic currents, hot neutrals, etc..

A battery isolator separates the batteries from each other relative to the input from the source of charging current.

As long as I am on the subject, it is clear from many of the electrical threads here that there are way too many unqualified amateur electricians working on boats. It is possible to study and educate oneself on how to safely wire a boat. However, I have NEVER seen an older boat that didn't have potentially dangerous, un-workmanlike amateur wiring rats' nests. AC kills. DC starts fires, blows up electronics, overheats pumps, etc., etc..

[tkeithlu]

I'm inclined to agree with your position on amateur wiring, even though I am a guilty party. The problem in my case, though, was begging for months for a licensed electrician of any sort, much less marine, to come do the AC job, and finally having to sit down with the books and do it myself. Even now, in a port with an excellant boat yard, it will be very difficult to find expert help.

[barnakiel]

The case should be grounded. Someone did a (...) job on your boat.

As others said: STOP TOUCHING IT NOW. Then call a pro.

The inverter will have to be removed and likely replaced, maybe can be fixed. Make sure whoever does the job is a pro and the new or fixed unit gets proper grounding from the case.

You are lucky to be alive.

b.

[transmitterdan]

What is the brand and model # of the inverter? Many cheap non-marine inverters are designed so that the two A/C power terminals are "hot" relative to ground at 1/2 the line voltage (about 65V in the US). A marine inverter will not be like this. It will have a real neutral side (same voltage as the case) and a "hot" side. If someone connects the case of one of those cheap "trucker" inverters to system ground then some interesting problems can arise.

A wiring diagram of your system would help understand the problem better. A verbal description leaves out some important details most of the time.

[smac999]

if you read his post he says his iso transformer is between the inverter / shore / gen plug setup and the AC panel. so it's isolating all 3. which is not needed and leading to wrong.

if you ground the inverter chassis to boat ground you'd be bypassing the iso transformer.

as it's currently wired the gen, shore, and inverter AC green ground should go to transformer ground input. normally only the shore ground would go there.

but in this ase there is really no way to currently connect his inverter chassis ground

he should rewire it so the transformer is between shore plug and his on board plug switching setup.

[donradcliffe]

I'm not sure that you are not confusing the neutral wire with the grounding wire when you talk about 'boat ground'.

If you are not confused, I'd take a wire from the case of the inverter to the green grounding wire. It should either eliminate your problem or blow up the inverter. If the inverter blows up, you really needed a different one. If you still have a hot case, better call in the electrician.

I've use the plug-in system on motorhomes, and it actually works pretty well.