Single pole and 2 pole conflict

Can anyone advise if there is a potential problem installing single pole diesels in an alloy hull with 2 pole house battery, AC genset/shore power systems.
I'm concerned with the possibility of stray current damage caused by one system - isolated engines and the other AC/DC system having separate earth grounding.

 

What is a single pole diesel?

 

An engine or any equipment that uses its chassis as earth will destroy your boat with stray electricity. Isolate earth..its easy to do...starter motor, alternator and engine sensors

 

Almost all engines and equipment use the chassis as earth. Some Volvo Pentas have a floating earth for the electronics.

 

Thanks Michael
It's the issue of stray current that concerns me. This particular project, an alloy CAT reqs twin Yanmar 6LY3 fitted only with single pole start/electronics, engines I would not normally use in this circumstance.

I thought if we separate the start batteries from the house DC/AC system incl. battery charger I could eliminate possible stray current, then as you suggest I only need to isolate the engine electronics/start grounding.

Have you any experience with DC blockers in this situ.

 

The normal way is that the engine has two alternators .I for the strting battery which is neg earth to the engine and a second for the house supply which has a + and - terminal ..ie insulated for the house batteries ..what you are calling 2 pole
You would be unwise to try to isolate the eletronics /starting from the engine just isolate the whole engine via rubber mounts/coupling from the hull.

 

Very interesting topic. I am just finishing a survey on a steel motorsailer where the owner/builder has completely isolated the AC and DC systems from each other and the hull. Propulsion engine also isolated from hull and propeller shaft. No generator but a 30 amp shore cord and inverter. The purpose is to protect the hull from stray current. I have explained the risks of stray AC current getting to the hull and DC system and the hazard that creates for humans.

My suggestion is that he provide a connection between the AC green ground and DC negative buss then install a lead from there to the hull through a galvanic isolator. The isolator will allow his AC and DC electrical systems to float unless hazardous current was present. Might be a way to for you to go too but isolating the systems as this fellow has takes a lot of care and planning.

 

Michael Kasten provided some interesting reading about bonding (electrical), in metal boat quarterly. A bit old, but valid.

A general statement wheather or not, is not made though.

Regards
Richard

 

Isolate - Isolate - Isolate, then use a isolating transformer for the ground.
and be careful of using appliances the at connect common to ground directly.

 

Thanks Richard, MK material is interesting reading

 

Stray currents are defined as electrical currents following a path deviating from the designated one. There must also be a source and a destination (or multiples thereof).

The phenomenon is common in metal parts of buildings, industrial installations like cranes and in rail transport systems using electrical power. An easy to understand example is an electric train using overhead supply and rails as ground. The loc draws a lot of current, causing the supply voltage to drop and the rail potential to rise. The rails are somewhere connected to the power station's ground or neutral terminal, but the resistance between that point and the train allows a voltage differential between the rails and the railbed, causing the clamps to corrode and the rails to dislodge.

Another train, pulled by a diesel electric loc, for obvious reasons cannot and does not cause any stray currents.

Your alloy cat, with self contained electrical systems, does not qualify for stray currents as long as the water is only on the outside of the hulls.

Galvanic corrosion yes, stray currents no.

Does this answer your question?

 

Hmm ??..I dont understand the statement... "Your alloy cat, with self contained electrical systems, does not qualify for stray currents as long as the water is only on the outside of the hulls. "

 

Starting from scratch an isolation transformer is a good way to prevent problems coming aboard through the green ground and insuring the system is always polarized properly. DC negative, green ground and hull are all connected to the isolator ground to protect people aboard and with no direct connection to the dock stray DC current cannot come aboard via that conduit.

The problem I see most are concerned with is DC current leaking to the hull via the grounding system. The AC current is dangerous when it leaks but does not cause corrosion of the hull. An isolation transformer or galvanic isolator elminiates the low voltage DC current path that causes galvanic corrosion (unless it is a really big flow). Prevents yours and other boats from becoming part of a big marina battery. There can be flow between the metal parts of the hull exterior from your own system just as CDK describes on the rail system due to variations in the potentials of different parts but that is what protective anodes guard against.

 

An isolation transformer is a AC component . Ive never seen an Isolation transformer on the DC side. The AC isolation transformer does prevent copper to copper conductor connection with the shore, but it doesnt prevent copper to copper ,earth wire , connection to the shore and as a result is a common earth, you are wired to your neighbors boat, your neighbor electrical leak problem. . Ive often asked marine electricians why I cant disconnect the Earth wire at the shore AC powerpoint to gaurd against stray DC voltage. Never seem to get the same answer.

 

Polarization transformer hull is earthed to shore. Isolation transformer hull is not (transformer becomes earth for hull). Cheapest solution is galvanic isolator. It blocks earth to shore unless current flows that is hazardous to people.