AC to DC grounding

RandyBC

Seaman
Joined
Jul 31, 2012
Messages
66
I'm installing shore power on my Trophy 2352 with 5.7 Mercruiser to power an on board battery charger. My question is, where is the best place to ground the onboard ac side to the dc side. I have a negative bus bar in the immediate location. Is this acceptable to ground here?
 

RandyBC

Seaman
Joined
Jul 31, 2012
Messages
66
Re: AC to DC grounding

Thanks Bubba. I would think that it would all be tied back to the engine at some point. There is only one DC system on the boat.

I have a fibreglass hull and the boat is out of the water almost always so I don't think a galvanic isolator is warranted at this point.

Instead of using the bus bar, I could run a wire directly to the engine. Do you know where the grounding post is located on the Mercruiser 5.7?
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Re: AC to DC grounding

It's called the engine block. If you insist on a true common point, that would be the same bolt that secures the NEGATIVE battery cable to the engine block
 

Grandad

Lieutenant Commander
Joined
Jun 7, 2011
Messages
1,504
Re: AC to DC grounding

I'm installing shore power on my Trophy 2352 with 5.7 Mercruiser to power an on board battery charger. My question is, where is the best place to ground the onboard ac side to the dc side. I have a negative bus bar in the immediate location. Is this acceptable to ground here?
Hey Randy. Why do you want to connect the AC ground to the DC system? I think you're just opening a can of worms that could end with galvanic/electrolytic corrosion of your drive. Assuming that the charger is a certified safe product, it's already bonded in the supply cord or is double insulated. If that's your only electrical device, I wouldn't do it. I didn't with mine. - Grandad
 

Grandad

Lieutenant Commander
Joined
Jun 7, 2011
Messages
1,504
Re: AC to DC grounding

OK, easy Bubba. I asked what the OP's intention is so we could determine whether he is installing an entire AC system on his boat or just installing the single battery charger as the only load. Yes, an AC system on a boat is required to include grounding/bonding of the engine and any other metallic parts that could become alive. Complete systems require accommodations for corrosion, etc as well as safety. The OP's installation will fall somewhere between a complete system that might require an isolation transformer/isolator and the installation of a single appliance, the charger.

Under ABYC's standard A-20 for chargers, the input and output of a charger is to be isolated by various means. One of the methods is by using a metallic shield that is required to be connected to the AC system ground conductor. Installing a conductor between the AC ground and the DC negative creates an obvious connection between the input and output. Perhaps this is not a concern if you have an isolated AC system on your boat, but may have consequences if the system is not isolated.

And yes, electrical fatalities occur at marinas regularly. Most marinas post "No Swimming" signs, though unfortunately they don't explain why. I certainly wouldn't allow anyone in my family to enter the water in an electrically serviced marina. The distinction for me is that if a cord connected charger is the only appliance, the likelihood of a damaged cord electrifying the engine on a fibreglass boat is remote. On the other hand, if you're going to install a thru-hull receptacle, a distribution system and permanent wiring to the charger, you should consider using an isolated system and grounding the AC system to the DC negative. - Grandad
 

RandyBC

Seaman
Joined
Jul 31, 2012
Messages
66
Re: AC to DC grounding

Thanks for the replies fellas. Ideally I would add a galvanic isolator which I believe would be more important if I kept the boat in the water and plugged in most of the time. The truth is that most of the time it will be out of the water when plugged in. If you see an advantage to using a galvanic isolator in that scenario I would be interested in your thoughts.
 

Grandad

Lieutenant Commander
Joined
Jun 7, 2011
Messages
1,504
Re: AC to DC grounding

Thanks for the replies fellas. Ideally I would add a galvanic isolator which I believe would be more important if I kept the boat in the water and plugged in most of the time. The truth is that most of the time it will be out of the water when plugged in. If you see an advantage to using a galvanic isolator in that scenario I would be interested in your thoughts.
Hi Randy. The safest (and probably the most expensive) installation of shorepower to a boat involves the use of an isolation transformer at the first point of connection. When this is wired properly into the circuit, the boat now has its own 120 volt system, isolated from power on shore with no "desire" to short to ground through the water. The boat now has its own system where any onboard 120 volt shorts "desire" to return only to the onboard isolation transformer. All metal parts on the boat that could become alive are then bonded together with a grounding wire providing a safe path for any shorts back to the transformer.

Your charger has as its main component, an isolation transformer. The irony of your installation is that if you're installing only the charger, by putting the additional ground wire as suggested, you're going to bypass the safety provided by the transformer and introduce a "desire" for stray currents to flow from your outdrive back through the water to the shore. This opens the door to corrosion and actually reduces the level of safety inherently provided by the charger's isolation transformer. So, ironically, you're considering installing a ground isolator to prevent the potential of corrosion that you just created.

By now, you must be confused. I'm hoping that someone who understands isolation transformers and chargers can chime in here and either confirm what I'm saying or tell us both where I'm wrong. - Grandad
 

RandyBC

Seaman
Joined
Jul 31, 2012
Messages
66
Re: AC to DC grounding

Grandad, I am a bit confused but that is ok. By pointing out alternatives you've stimulated me to do some more research on isolation transformers and thus hopefully I will become more educated.
 

RandyBC

Seaman
Joined
Jul 31, 2012
Messages
66
Re: AC to DC grounding

BTW Grandad, this is the only AC device that I am adding at the moment as you've deduced. In this case, am I correct to say that it would actually be safer not to bond the AC to the DC system? The charger that I have is a Charles SP series that was pulled off my sailboat when I had an inverter installed. It was made for marine use.

Eye opening about marinas and why they don't allow swimming. I never really thought of it. Now when I dive my boat to clean the bottom I have something else to think about. Yikes.
 

RandyBC

Seaman
Joined
Jul 31, 2012
Messages
66
Re: AC to DC grounding

I looked at the literature for the Charles 5000SP and it I don't see that has a built in isolation transformer.
 

smokeonthewater

Fleet Admiral
Joined
Dec 3, 2009
Messages
9,838
Re: AC to DC grounding

pretty simple.... what is the input amperage of said charger? under 20 amps?yes? then a simple 12/3 cord (IE extension cord) wired direct to it will suffice... black hot white neutral and green grounded to charger. If the boat is ONLY to be out of the water when using I wouldn't bond to dc system... If you will ever use it while in the water then a simple jumper to any bolt on the engine block (usually a lug on the intake manifold and or bellhousing) from the green ground circuit will do the job.

MANY bass boats have on board chargers and none of them are bonded.
 

Grandad

Lieutenant Commander
Joined
Jun 7, 2011
Messages
1,504
Re: AC to DC grounding

I looked at the literature for the Charles 5000SP and it I don't see that has a built in isolation transformer.

Charles doesn't likely want to tell the world too much about things that they may consider proprietary. In my past life as an electrical equipment approvals inspector, I required manufacturers of equipment to provide evidence that their products contained components that were built to acceptable standards before I would apply a field approval label making it legal for an owner to connect and use previously unapproved equipment. Now retired, I no longer have ready access to standards that must be purchased (and no authority to force a manufacturer to validate that their equipment meets a given standard).

From what I've been able to find about the standards to which chargers are built, in 2005, ABYC standards A20 for chargers (that I quoted previously) and A25 for inverters were combined into ABYC Standard A31. ABYC clause 31.5.2.1 states that all chargers must meet UL Standard 1236 (as did the old A20 document).
Charles says that they now have UL 1236 Certification for their chargers, though I don't know if this covers your model.
See http://www.charlesindustries.com/Press_Releases/032312_IMCUL.pdf
http://www.charlesindustries.com/Press_Releases/061212_IMCfullUL.pdf
If Charles won't divulge whether their product has an isolation transformer, one could confirm that UL 1236 requires an isolation transformer. ABYC clause A31.5.2.5 requires that there be isolation between the AC input and the DC output, but Charles doesn't say that their chargers meet ABYC standard A31. Retirement is good.

In any event, since you've said you're not likely going to power the charger while the boat is in the water, it's really moot at this point. Let me summarize my thoughts. Under no circumstances should you disconnect or not connect a ground at the supply to the charger whether it's in the water or not. If you are installing only this charger and not a complete AC electrical system on the boat, I think that there is no hazard in not connecting a ground jumper from the charger to the DC negative. (This would effectively jumper out the isolation between the AC and DC required by ABYC and introduce the possibility of corrosion). If however, you were to supply the charger from either an isolation transformer or a galvanic isolator, there would be no safety problem connecting the ground jumper and no likelihood of stray current corrosion. Sorry to be so long winded. - Grandad
 

RandyBC

Seaman
Joined
Jul 31, 2012
Messages
66
Re: AC to DC grounding

Wow Grandad! Thanks for the excellent info! Let me peruse the links you gave a bit more so that I can fully understand what you are telling me. I was a bit confused because Charles does not mention this safety characteristic in the brochure for their 5000SP (40amp) charger. I'll try to read between the lines as you suggest.

I won't say that I'll NEVER use the charger while in the water. Occasionally I may if staying in a marina during a fishing trip. I don't know if that changes your recommendations.

BTW, I see you are from Barrie. I grew up in Brampton (Bramalea actually but that got swallowed up many years ago) and spent lots of time in Midland and the muskokas (Point au Baril) growing up. Good memories. I feel homesick now.

All the best.
Randy
 

Auger01

Petty Officer 1st Class
Joined
Aug 15, 2009
Messages
201
Re: AC to DC grounding

If you really wanted to know, a quick check with a multimeter would verify weather or not the input is isolated from the output.
 

cjjjdeck

Petty Officer 2nd Class
Joined
Apr 22, 2012
Messages
165
Re: AC to DC grounding

I have a DualPro Pro Series 3 bank on board charger accessed with a shore plug near the stern. It is not grounded to the boat nor does the installation instructions direct you to do so (it is a fully waterproof charger). It is dedicated to the operation of this charger only and will not connect any other devices to it.

I have a question for Grandad and/or Bubba:

Since a charger by design is converting AC current to DC current, doesn't the transformer that makes that happen isolate already or is that transformer actually an isolation transformer?
 

dingbat

Supreme Mariner
Joined
Nov 20, 2001
Messages
15,416
Re: AC to DC grounding

Charles doesn't likely want to tell the world too much about things that they may consider proprietary. In my past life as an electrical equipment approvals inspector, I required manufacturers of equipment to provide evidence that their products contained components that were built to acceptable standards before I would apply a field approval label making it legal for an owner to connect and use previously unapproved equipment.
Ontario Hydro?
 

Grandad

Lieutenant Commander
Joined
Jun 7, 2011
Messages
1,504
Re: AC to DC grounding

pretty simple.... what is the input amperage of said charger? under 20 amps?yes? then a simple 12/3 cord (IE extension cord) wired direct to it will suffice... black hot white neutral and green grounded to charger. If the boat is ONLY to be out of the water when using I wouldn't bond to dc system... If you will ever use it while in the water then a simple jumper to any bolt on the engine block (usually a lug on the intake manifold and or bellhousing) from the green ground circuit will do the job.

MANY bass boats have on board chargers and none of them are bonded.
Hi Smoke, Stray voltages exist between ground points such as ground rods, even those connected by grounding conductors. Just ask a dairy farmer whose cattle won't drink from a properly grounded electrically heated water bowl while they stand in a wet barnyard. By extending the ground conductor from the power distribution system on shore to connect to your boat's negative conductor and the outdrive, you've actually extended the ground grid of the shorepower to include your boat. Your outdrive is now another ground rod and fair game for any ground potential differences. The voltage is low, usually referred to as "tingle voltage", so the current flow is low, but even low current can cause substantial corrosion damage.

To take it one step further, what happens if the ground connection in the shorepower distribution gets broken, let's say in the green conductor in the branch circuit feeding the pedestal that you or your neighbours are plugged into. Now your outdrive is the only ground path back to the mainland for any short circuits in any equipment plugged into that pedestal. Don't swim in a marina. The only way to avoid this is to create isolation between your boat's ground system and the shorepower's ground system. - Grandad
 

Grandad

Lieutenant Commander
Joined
Jun 7, 2011
Messages
1,504
Re: AC to DC grounding

If you really wanted to know, a quick check with a multimeter would verify weather or not the input is isolated from the output.
Hi Auger, a multimeter doesn't cut it. It's output voltage is very low and although it can read the resistance at say 9 volts, the resistance changes entirely at higher voltages as the voltage breaks down the resistive barrier. What is used to test such isolatioin is a "dielectric strength tester". It is adjusted to whatever the standard test condition is for UL 1236. Most equipment is tested at 1500 volts DC for a period of time while the tester measures the leakage current in milliamps, sorta like a ground fault circuit interrupter. It's a pass or fail test and can be destructive. I used a portable dielectric tester to test hundreds of pieces of equipment. I always got the equipment owner to sign a waiver acknowledging that the test could destroy the equipment if its isolation failed. - Grandad
 

Grandad

Lieutenant Commander
Joined
Jun 7, 2011
Messages
1,504
Re: AC to DC grounding

To be honest, the cost to ground the system is so minimal why take chances? I know you intend to run only the batt. charger but you know how that goes, at some point you or someone will plug something into it. (I was just going to drill a couple holes so I plugged the drill in and.....)
I hear you Bubba. What might be the best solution is to not install any on board distribution system, perhaps not even a thru-hull. Just put a good quality supply cord on the charger that's accessible for regular inspection and protect it from damage. Can't plug anything else in. - Grandad
 

Grandad

Lieutenant Commander
Joined
Jun 7, 2011
Messages
1,504
Re: AC to DC grounding

I have a DualPro Pro Series 3 bank on board charger accessed with a shore plug near the stern. It is not grounded to the boat nor does the installation instructions direct you to do so (it is a fully waterproof charger). It is dedicated to the operation of this charger only and will not connect any other devices to it.

I have a question for Grandad and/or Bubba:

Since a charger by design is converting AC current to DC current, doesn't the transformer that makes that happen isolate already or is that transformer actually an isolation transformer?
cj, that's it exactly. The standards for a certified charger (eg UL, cUL, CSA, etc) require isolation. So the discussion stems from the opinions on whether to add a ground jumper from the supply side of the charger to the output side. Many diagrams, including some from ABYC show such a ground jumper. However, all these diagrams show the ground jumper to the DC negative connecting to the boat's isolated ground, not the shorepower ground. So, if you intend to add the ground jumper, do it only from an isolated ground derived from either an isolation transformer or a ground isolating device.

Most transformers are typically called isolation transformers as most have no connection between the input and output windings as there is in an "autotransformer". However, the particular type of transformer that complies with the standards for chargers usually has a grounded metal foil or shield between the primary and secondary windings. The intention is of course that should either winding fail, there's no way that the primary voltage can leak across the barrier and enliven the secondary with an overvoltage. Other types of isolation barriers are sometimes acceptable subject to rigorous testing.
- Grandad
 
Top