Should an Engine Isolator Switch be in the +ve or -ve feed?

Hi,

This is my first post, and have searched but not found an answer.

Let's make 2 assumptions:
1) As soon as I connect my boat batteries to a battery charger connected to the shore supply there will be some spurious AC and DC voltages and currents present at the battery terminals. All the galvanic isolation devices I install can only mitigate, but not eliminate, these spurious voltages.

2) Connecting my engine - and through that my propeller, to spurious voltages is a bad thing. Of course, my propeller is bonded to a nearby anode.

My Yanmar diesel wiring diagram specifies a battery isolation switch in the +ve feed.

Is this what "everybody" does? If so, I disagree!

I plan to rewire my engines with the battery isolation switch in the -ve feed. My reasoning is simple: When I disconnect the switch, my propeller is also disconnected from spurious shore induced voltages.

Is my simple reasoning faulty? Can some boat wiring / electrolysis expert correct my logic?

Thanks in advance,

David

 

What do you call a spurious voltage? There can be stray current, voltage biased by DC and AC component on a badly regulated charger. I think that you need to study electrical definitions to explain yourself better.

 

Hi Gonzo,
Thank you for your prompt response. To clarify the context of my post: my definition of a spurious voltage or current is one that results in unwanted electrolysis of the propeller. It includes all the sources of stray currents you listed, as well as many others.

David

 

Voltage and current are two different things. The current is a function of voltage and resistance. I don't understand why you believe there will be AC at the battery terminals when you connect a charger.

 

Voltage and current are inseparable, except for superconductors. Propeller electrolysis (the subject of this post) will occur whether the propeller "sees" either a voltage or a current. You will see an AC ripple if you connect an oscilloscope to a battery being charged by an AC charger.

 

Assumption #2 is correct, but #1 is wrong.

A battery charger has at least 2500 V isolation between input and output, so even without any additional devices the "spurious voltages" do not exist.
The need for an engine isolation switch is questionable, but if you fear an electrical failure in the alternator or flooding of the engine bay, the switch should be in the positive wiring.

 

Does anyone have measurements?

Has anyone got any measurements of the effect on the boat DC wiring when shore power is connected to the boat?

 

With an oscilloscope connected across the battery terminals during charging, you will see a 100 Hz ripple with an amplitude equal to the difference between the average battery voltage and the charger's peak voltage. For a simple (non regulated) charger that can be anything between 0.2 and 3.0 volts.

This is not AC, just a DC ripple.
With a properly wired installation, shore ground must be isolated from DC ground to avoid any galvanic action in case the shore ground is "dirty" (it usually is).

 

Hi CDK, you say, quite rightly, "this is not AC, just a DC ripple". My question is will a 3 volt 100 Hz "ripple", applied to the propeller via a permanently connected -ve feed from the battery aggravate propeller electrolysis? Any measurements will be welcome.

 

"A battery charger has at least 2500 V isolation between input and output, so even without any additional devices the "spurious voltages" do not exist."

Has anyone measured the leakage from a battery charger after it has spent a few years in a damp salt laden atmosphere?

 

"if you fear an electrical failure in the alternator or flooding of the engine bay, the switch should be in the positive wiring."
Can anyone explain how an engine room isolator switch in the +ve feed gives greater electrical failure protection than if the switch is in the -ve feed? Thanks.

 

<My question is will a 3 volt 100 Hz "ripple", applied to the propeller via a permanently connected -ve feed from the battery aggravate propeller electrolysis?>

The voltage exists between the battery terminals only. To measure any voltage on the prop you need an electrode of the same material dangling in the water as a reference. You will measure 0 volts there unless the charger isn't isolated.

<Has anyone measured the leakage from a battery charger after it has spent a few years in a damp salt laden atmosphere?>

Yes, I did.
My battery charger has a sealed aluminum enclosure. Inside is a potted toroid transformer, there is no leakage voltage except for some capacitive coupling that can only be measured when the output is floating. The current involved is in the nano amp region.

Most commercial marine chargers have an EI transformer with separate primary and secondary windings, vacuum impregnated. Only the cheap Chinese ones may have or develop current leakage; you shouldn't use these in a boat.

<Can anyone explain how an engine room isolator switch in the +ve feed gives greater electrical failure protection than if the switch is in the -ve feed?>

It makes little or no difference.

It is a convention to disconnect the +ve. Any electrician will assume the - to be permanently connected and uses metal parts in the engine room as a reference when measuring voltages. If you do your own fault finding and have an accurate wiring diagram, do as you please.

With an automatic bilge pump you cannot completely disconnect the -ve in the engine bay.

 

Hi CDK, thanks for all the info.

 

Voltage and Amperage are separable. Only when there is a path between points with an electrical potential (voltage) can there be a current (amperage). Otherwise they are electrically isolated.

 

This thread is creeping back to Zincs. Planesail there are several excellent do it yourself books available on marine wiring systems. (The Twelve Volt Bible) by Miner Brotherton (Managing 12 Volts by Harold Barre) and of course the king of references (Boatowners Illustrated Handbook of Wiring) by Charlie Wing. I have all three plus a few more for older craft but I recommend the latter. It's an easy read/understand with good diagrams and you can post any questions you run into therein as most active members here will have a copy to refer to.
One important point I think CDK mentioned and that is do not skimp on your marine charger as a proper one will be equipped with the proper isolation transformer. For those connecting a scope to a charger on this side of the pond you will see 120cycles pulsating DC not 100. 60cycle verses 50 cycle AC primary supply is the determining factor. Basically what happens is a full wave rectifier places the -ve. half cycle part of the AC cycle waveform above the 0 line or in other words converts it into a +ve. half cycle.
The other concern you had re electrolis corrosion of your prop, thru hulls and even the hull itself if metal, will be protected from from self induced stray AC and Pulsating DC currents if you install a proper charger in your own boat but there is no guarantee it will be protected from the same as a result of improper wiring or shore charging systems on other peoples boats. This is where properly installed zincs give you max. but not necessarly total protection. For the ultimate info on that see that never ending post on this forum. ---Geo.

P.S. Gonzo-correct, Voltage(E) a measurement of the difference of electrical potential will cause current (I) electrons to flow (measured in amps.) only when the conductive circuit (wiring and associated devices) is a closed circuit(power switch on, a path for current to flow from the -ve. thru the circuit to the +ve side of the source in a DC system. If the power switch is off no current exist therefore it's measurement amperage is also non existant or zero.

A yacht is not defined by the vessel but by the care and love of her owner.