Battery isolation on -ve side?

[steverow]

Re: Do you believe everything you read in the papers, too?

Yes but you are assuming isolated (floating) electrical system.
Not sure whether bluedragon mentioned this.
I think we've all assumed negative ground.
Perhaps we've all been barking up the wrong tree?

I agree, isolated is to be preferred always, at least for engines.
It becomes more complicated when running stuff like HF radios when RF grounds come into play.
My boat has two 24v diesels, individually isolated....each to it's own anode.
Only path that exists is between the anodes.
Domestic system, for lights and fridge only is off stbd bank only.
12v instrumentation system is again totally separate and isolated...apart from any stray current all systems should be totally separate.

Im sending you a PM 'bout something else entirely.

Steve.

 

[Anonymous]

Re: Do you believe everything you read in the papers, too?

John,[ QUOTE ]
..... Boat owners insisting that their interpretation of Calder be followed - usually they know so little about the subject that they have totally screwed up what Calder is saying, or what Calder is saying does not apply to their boat...

[/ QUOTE ]I agree...and that certainly applies to a lot of what has been written in this thread! The wisest counsel for anyone who doesn't have an excellent grasp of electricity is to forget everything they have read in this thread because it would take too long to correct all the wrong information. As I'm sure you'll agree, this is not a safe thread. David

 

[boatmike]

NAS You left out the reasons........

I agree with your posts.
I will put it in words of one syllable, as there appear to be a plethora of opinions expressed conflicting with good practice and common sense.
All classification societies,that is Lloyds, DNV, etc. agree that batteries should be grounded on the negative and "live " on positive. Batteries should always be isolated on the positive (live) side as close to the batteries as possible. This is therefore the convention adopted as standard. Independent isolation of the negative terminal is optional. i.e. either positive only or both. Never just negative.
One reason for this is the negative terminal goes to a common ground. That means all batteries are joined together on the negative side. The positive goes through various routes to every part of the boat. If the boat was damaged and flooded in such a way that the damaged stability is not compromised i.e. boat is still afloat, it is possible that both terminals of a battery could end up under water. In any eventuality this will short the batteries and kill the system so that its inoperable. This is therefore unavoidable however the boat is wired. However if the batteries are not compromised but water enters some other part of the boat a short circuit of the live power to earth through the water can be avoided by isolating the positive terminal of the battery concerned leaving all other batteries in circuit. Not so if the negative side is isolated because here all batteries have a common link to the same earth.
In practice therefore, as an example, a short circuit caused by any eventuality (not just water) on the domestic side, can be taken off line by isolating the service batteries leaving the cranking battery on line for engine starting. Many will argue that the same could be achieved by isolating the negative side if each battery was switched independently. This is theoretically true but it does not stop the current flowing through the live circuits of the boat just waiting for the water to rise to the battery negative terminal reinstating the circuit and causing a short that could start a fire. Isolating the positive prevents that happening and any "sparks" will be confined to the battery compartment.
In other words it is safer.

 

[boatmike]

Re: Do you believe everything you read in the papers, too?

For clarification John, my last post was supposed to answer NAS post not yours. I agree that the negative side of the battery system should not be grounded to the hull or anodes but in many cases it is indirectly as the engine block forms the negative side of the starter motor circuit and is bonded to the propeller anode.
It as possible to wire up to avoid this but I have yet to see a small diesel that is not this way. I had a client with a problem lasdt week that thought his engine was NOT grounded until I pproved to him that a circuit in fact existed through his seawater intake......
Still think DNV Lloyds etc are quite correct therefore to insist that the positive side of the battery is isolated (at least they do in Europe)

 

[Ships_Cat]

Re: NAS You left out the reasons........

Hi Mike

Just for the sake of others and clarity in case anyone misinterprets the comment All classification societies,that is Lloyds, DNV, etc. agree that batteries should be grounded on the negative and "live " on positive, that does not mean that the classification societies forbid the non grounding of the negative, but is a statement of what they require should they be grounded.

If the negative is not grounded to the hull/ac earth, should be noted that class (I think all of them) requires double pole breakers at the switchboard ie both the negative and positive are broken at the board (our own boat is so done which means quite a bit bigger switchboard due to each breaker being twice as big).

{EDIT: Whoops we did some overlapping posting there Mike so read the above in that light. On the isolation of small pleasure diesels, I don't know how many are but they certainly are available - our own Volvo (circa 1996) is isolated completely including cranking and both alternators. As you say that isolation can be compromised through incorrect installation. Also, agree that is best to always isolate the positive, if only for the sake of conformity so taking one wildcard out of the boat for the case of future electrical work making the wrong assumption.}

Regards

John

 

[oldharry]

The vast majority of small boat engines use the block as negative return for engine electrical circuits, which include the alternator, starter and engine instrumentation.

Small boat electrics have a tough time living in a damp and salt atmosphere, which will readily create a current leakage, which in turn sets up all sorts of corrosion problems in switchgear, terminals and contacts.

I once measured over 2 volts from the CASING of a battery that was damp and dirty.

Any dampness, accelerated and attracted by salt, will cause current leakage, which rapidly leads to corrosion of contacts and components that are live, so that if the circuitry is permanently 'live', ANY leakage that links it back to the negative battery pole will cause problems. It only needs a fraction of a volt to cause serious corrosion over a period of time.

If there is no voltage potential in the electrics to start with, then there can be no risk of damage.


.

 

[pvb]

Sorry, that\'s wrong too...

Sorry, it's wrong to say that the "circuitry is permanently live" if the battery is isolated by disconnecting at the negative terminal. There would only be voltage in the circuit relative to the isolated negative terminal, not to anything else. Any situation in which dampness creates current leakage would be equally problematical whichever battery terminal was isolated.

 

[Ships_Cat]

I once measured over 2 volts from the CASING of a battery that was damp and dirty.

But people should not panic if they measure DC voltages in unexpected places - it does not mean that there is a problem.

Taking as an easy example a metal boat on which all the DC is isolated from the hull, you will still usually measure 12v between the DC positive bus and the hull. As you say, due to dampness there is not perfect isolation between the electrics and the hull, but the resistance should be very high - as long as you measure the voltage between the positive bus and the hull with a meter of much higher resistance than the leakage resistance from the dampness you will measure 12 v, but if you measure the current flow you will find it to be zilch (12v potential difference and very high resistance means effectively zero current).

If the voltmeter is not of much higher resistance than the leakage path, then a voltage lower than 12 v may be measured as the meter effectively parallels the leakage resistance, but the voltage over the leakage path is actually 12v. It could be, for example, when you measured 2 volts that this is what was happening and the voltage was 12 volts.

To see it another way, if you only have 12 volts on the boat it is not possible to have only a 2v potential difference between any two components on it unless there is a very low resistance between those two components such that it results in a 10 volt voltage drop. As we know from ohm's law for there to be a 10v voltage drop there has to be a very big current (for example, even the low resistance of a the engine starter motor does not result in a 10v drop).

What happens is your meter is "lying" and the message is that just because one measures DC voltages around the boat in unexpected places, that usually does not give any information at all and is usually of no concern. Measurement of current and/or resistance is the thing that matters. If there is no/zilch current, there is no problem for the boat.

John

 

[hightech]

Re: Do you believe everything you read in the papers, too?

Buy a new breaker which switches both, but get a make before break model, so you don't disconnect your alternator from the battery while the engine is running.

That way with both side disconnected no-one gets electrocuted, nothing gets fried and no chance of a frie.

 

[bluedragon]

Wow!!! this question of mine certainly raised some comments (and differing opinions). I do have a conventional -ve ground system on board and I believe the anodes are linked-in via the -ve strap to the starter motor / engine. Logic tells me that if that the -ve pole of the battery is disconnected and electrically isolated then unlike shore based AC systems, there is no way the "live" current can leak anywhere (unless the boat was flooded to battery level). However, am I certain of this...NO...so it's probably best to be on the safe side and go with convention by re-wiring the isolators to +ve. Fascinating discussion though. Many thanks to all who contributed.

 

[boatmike]

Re: Everything that has gone before

As usual John I don't think we disagree but just to avoid confusing others I would like to restate what I think is the position.
It is best to isolate the battery completely by having double pole switches
Where only one switch is used it should be on the positive terminal (there is in fact a less than 12 passenger code for small craft that says precisely this)
This is regardless of whether the neg side is bonded to the hull or not.
One thing I think is being missed here is that it is virtually impossible to isolate the engine block from the hull when the thing is seawater cooled. Seawater is a good conductor of electricity and effectively makes a connection to the "oggin" whether you like it or not....... This means that the earth return of the generator and starter motor through the block connects the battery earth of the cranking battery to the hull. It is of course possible to isolate the service batteries from the hull and ideally one should but if the battery earths are common as they often are to be able to change over to the service battery as a back up for starting the whole chebang gets earthed to the hull through the cooling water, at least when the engine is running.......

 

[oldharry]

Re: Sorry, that\'s wrong too...

Sorry PVB, I disagree. Leaving the +ve connected means that a large part of the circuitry is at 12 volt potential, 'seeking' by any means to complete the circuit, damp being the ideal means of doing so. Cutting the +ve supply means that there is no risk of this happening.

One old boat I had was negative switched. The electrics were a constant source of entertainment as things fizzed and sputtered. As my (then 12 yr old) son commented, volts were escaping all over the place. Making sure the volts were not escaping from the battery in the first place, by using a +ve isolater, largely solved the problem. (Didnt stop the boat leaking both above and below though! /forums/images/graemlins/crazy.gif)

Ideally the battery should be isolated at both terminals.

 

[bluedragon]

As a curious person (hasn't killed the cat or me yet!), I decided to do an experiment this afernoon on the boat. The replies to my original question broadly fall into two camps:

a) it doesn't matter which side of the battery is isolated. If the circuit is not complete, then nothing is going anywhere.
b) not isolating the +ve side of the battery is dangerous as the circuit is "live" and could travel to earth through the boat or a person

So, I just did the following:

1.Hung a piece of copper wire over the side into the water
2.Disconnected the -ve poles on the batteries (+ve still attached)
3.Connected the -ve terminal of my digital multimeter to the wire in the water
4.Connected the +ve terminal on the multimeter to +ve on each battery in turn, then measured a) any potential difference between battery and water b) any current flowing in the circuit (ie. battery +ve to water).

The results:

Engine start battery: 0.2V to water
Service battery: 1.5V to water

In both cases no current was registered on the 200 microamp scale.

So it seems the "aint goin nowhere" camp is correct...or are they??
Thoughts anybody?? Was this a valid experiment?

I should point out that Cardiff Bay behind the barrage is fresh water, not salt, so the conductvity of the water will be much lower. But I'm not sure salt water would change anything.

 

[pvb]

Easy experiment you can try at home...

In the true "Blue Peter" spirit which used to characterise every issue of Practical Boat Owner, here's a little experiment you can try at home. /forums/images/graemlins/wink.gif

First disconnect the connection from the negative terminal of your car battery. The battery is now isolated but, using your theory, a large part of the car's wiring is at 12 volt potential. Try switching something on, and note the results. Now take a piece of wire, connect one end to the positive terminal, and touch the other end against the engine, and again note the results.

Don't worry, you'll find that nothing happens. The 12 volt potential only exists between the 2 terminals of the battery. Disconnecting either terminal isolates the battery. Volts can't "escape from the battery", current can only flow if both battery terminals are connected. Don't know whether this makes things any clearer for people, but it's all I can think of.

 

[halcyon]

Re: Easy experiment you can try at home...

If you split the circiut before the the load, and you have leakage, current will flow.
If you split it after the load , and have leakage current will flow. Were ever you fit your isolater one side will be positive and the other side negative. The only way to be safe is to do it the Lloyds way, fit two pole isolater in both live and earth, thus totally isolating the battery.
The other point, how many have held two wires on battery terminals to jump start another engine, and lived to tell the tall. From the back of my mind 72 volt is the highest safe voltage, which is why it's used on fork trucks etc, 110 volt is around the danger area. But 12 and 24v are safe, the rest are at your own risk.

Brian

 

[Krusty]

Re: Do you believe everything you read in the papers, too?

Wonderful stuff, sarcasm; reminds me of advice from the wisest man I ever met: "Every man, without exeption, is limited by his experience".

 

[Ships_Cat]

Re: Everything that has gone before

Hi Mike

I think your comment on isolation of the engine is only if the alternator and cranking motor are not themselves isolated. As you say the block/heat exchanger is always short circuited to the hull to the extend depending upon the length of the saltwater path to the hull (assuming not short circuited almost completely by metal sea water piping) and the coating resistance and closeness of exposed metal on the outside of the hull.

As an aside, both our engine cranking battery and our service batteries are isolated with double pole switches so both positive and negative are broken and agree with you that is the best. We also open the switch whenever the engine is not in use, even if only stopped for an hour or so.

Regards

John

 

[Ships_Cat]

As OldHarry, Mike, etc have said, both the negative and the positive should be isolated.

Regarding your experiment one has to keep in mind that the moment you put one probe of a voltmeter or any metal attached to that into the water you are immediately introducing galvanic cell voltages if there is any path (even if of high resistance) to the water from the other probe too eg back through a bonding system.

Some years ago I was helping an insurer defend a case where a boat owner maintained that a poor electrical installation had caused severe corrosion of his aluminium hull. His technical "expert" in his brief of evidence stated that he had measured x volts between the hull and the seawater when he placed the other probe of his meter into the sea and claimed this showed that there was electricity "leaking" to the sea from the boat's electrical system through the hull /forums/images/graemlins/crazy.gif.

The insurer and their lawyers were aghast and almost immediately thought they had no case. So I set up a demonstration for them with an aluminium pie dish in some salt water put one probe of the voltmeter on the pie dish and the other into the water. Of course the meter measured the galvanic cell volts between the aluminium pie dish and the metal of the other salt water submerged meter probe and I asked them where they thought the electrical "leakage" was coming from because in this case there was no electrical system. I have to say they were entirely confused then and suspected that some form of "black magic" was involved - they had no idea whatsoever as to what was happening.

John

 

[bluedragon]

A good thought...I expected no voltage to show, and wondered why the two batteries gave different values. Now there should be no direct link from the battery +ve poles to the bonding system and hence to the water via the anode. I think everything was off, but I am re-wiring at present. Perhaps this signifies a high resistance leakage path? But as you said in an earlier post, if no current then why worry. I'll try this again when all is back together out of (more) curiousity.

 

[steverow]

Just to Recap then...

Just to recap then....
Put the isloator into the positive line as close as possible to the battery.
If you want to do a proper job, then switch both leads from your battery bank.
This is all it really amounts to, and the concensus of opinion.
We engineers eh?? we do love a good theoretical argument.
Great stuff...

Steve.