Earth return ?

[sjc]

The use of an earth return for onboard electrical equipment is discouraged as it is a potential source of hull corrosion ? YET most of us use high power alternators with exactly that, so what's the difference ?

If a few mA from a radio will cause damage, think what 100A will do !

[Guest]

The use of an earth return for onboard electrical equipment is discouraged as it is a potential source of hull corrosion ? YET most of us use high power alternators with exactly that, so what's the difference ?

If a few mA from a radio will cause damage, think what 100A will do !

A big difference. The negative side of my battery bank is earthed to the hull. All circuits on my boat have their own negative cable. The alternator earth return (as you point out) is earthed over a very short distance in my case; not enoguh to matter. Somebody with a better siganl and a keyboard will explain better.

[Chewbacka]

It's the voltage an not the current that causes electromigration and corrosion.

You are correct in that the alternator is earthed to the engine, but there should be a heavy earth/negative cable between the engine or engine bearers to the battery. Therefore very little current will flow in the hull. So while it would be better to avoid this current path, the small risk associated with using a standard alternator is considered by most as acceptable.

[Paul C]

As above, the difference is the alternator's earth return is via the engine, not the hull.

[nicknorman]

There should be no current path through the hull. The alternator may well be earthed to the engine (not always, some have an insulated -ve) but in that case there should also be a cable from the engine to the battery -ve.

 

It is also normal to connect the hull to -ve (although chances are it already will be via the prop shaft, exhaust pipe, control cable outers etc) but this should be done by means of a wire from the engine to the hull. This still means that there is no circuit and hence no current through the hull, except in the case of a wiring short etc which would hopefully blow the fuse.

[Guest]

There should be no current path through the hull. The alternator may well be earthed to the engine (not always, some have an insulated -ve) but in that case there should also be a cable from the engine to the battery -ve.

 

It is also normal to connect the hull to -ve (although chances are it already will be via the prop shaft, exhaust pipe, control cable outers etc) but this should be done by means of a wire from the engine to the hull. This still means that there is no circuit and hence no current through the hull, except in the case of a wiring short etc which would hopefully blow the fuse.

That ^^

[MtB]

The use of an earth return for onboard electrical equipment is discouraged as it is a potential source of hull corrosion ?

 

 

It's also not done due to the difficulty in making a good electrical connection to the hull next to whatever you are installing. Running a pair of cables is no more difficult than a single cable and a lot less confusing to other technicians following on.

 

Besides, there are plenty of boats out there with correctly installed electrics and extensive hull corrosion, I suspect!

[Scholar Gypsy]

 

 

It's also not done due to the difficulty in making a good electrical connection to the hull next to whatever you are installing. Running a pair of cables is no more difficult than a single cable and a lot less confusing to other technicians following on.

 

Besides, there are plenty of boats out there with correctly installed electrics and extensive hull corrosion, I suspect!

 

Unless - as happened to me - when the boat was stretched the polarity for the horn cables got switched somewhere amidships, so the red wire has to be fixed to the horn bracket ....

 

It took me several blown fuses to work out what was going on, after I replaced the horn a few years ago.

[sjc]

Well, on my high output alternator about 10% of the current returns via the hull.

I do have a -ve from the battery to the engine, and the alternator is not of the isolated type.

But there is another -ve from the alternator to the hull, which I was told was normal.

( I believe this is to avoid high currents through the propshaft, or mechanical cables etc ? )

The clamp meter typically shows say 40A output on the alternator +ve, and 4A on the -ve from the hull,

therefore 36A returning via the intended -ve via engine / alternator mounting.

I have wondered whether to try and reduce / eliminate this 4A but wondered what the general situation was ?

[nicknorman]

Well, on my high output alternator about 10% of the current returns via the hull.

I do have a -ve from the battery to the engine, and the alternator is not of the isolated type.

But there is another -ve from the alternator to the hull, which I was told was normal.

( I believe this is to avoid high currents through the propshaft, or mechanical cables etc ? )

The clamp meter typically shows say 40A output on the alternator +ve, and 4A on the -ve from the hull,

therefore 36A returning via the intended -ve via engine / alternator mounting.

I have wondered whether to try and reduce / eliminate this 4A but wondered what the general situation was ?

There is nothing wrong with having the second -ve from the alternator to the hull, but you must have another hull connection eg battery -ve to hull, such that a circuit is made and some of the current goes that way. I would ensure that there is only one DC connection to the hull, ie the one from the alternator.

[by'eck]

The use of an earth return for onboard electrical equipment is discouraged as it is a potential source of hull corrosion ? YET most of us use high power alternators with exactly that, so what's the difference ?

If a few mA from a radio will cause damage, think what 100A will do !

 

Even if the device doesn't have an isolated return (e.g. automotive alternator) there should still be a negative return cable from its casing, such that only under fault conditions will any current flow through the hull.

 

In any case its a myth that a negative earthed DC system will cause hull corrosion. See here under DC system with no AC onboard.

[nicknorman]

 

In any case its a myth that a negative earthed DC system will cause hull corrosion. See here under DC system with no AC onboard.

It says : "From the point of view of galvanic corrosion it makes no difference whether or not the DC system is bonded to the hull (but the hull must never be used as a return path in the manner of vehicle wiring)."

 

I'm not quite sure what you mean by "-ve earthed" but having a single connection between the electrical -ve and the hull is one thing, having multiple connections results in using the hull as (part of, at least) the return path.

[by'eck]

It says : "From the point of view of galvanic corrosion it makes no difference whether or not the DC system is bonded to the hull (but the hull must never be used as a return path in the manner of vehicle wiring)."

 

I'm not quite sure what you mean by "-ve earthed" but having a single connection between the electrical -ve and the hull is one thing, having multiple connections results in using the hull as (part of, at least) the return path.

 

I mean having a single strap from battery negative to hull. Maybe -ve bonded would have been a better term.

 

ETA: I think we agreed some time ago on your recommendation that the best place for the strap to run was from starter motor negative to hull, assuming there was also a cable direct from starter battery common negative direct to this point on starter as well.

Edited September 1, 2015 by by'eck

[ditchcrawler]

So if I have a length of copper wire in parallel with a great big lump of steel and there is 100 amps flowing from one end to the other no current will flow through the steel?

[nicknorman]

So if I have a length of copper wire in parallel with a great big lump of steel and there is 100 amps flowing from one end to the other no current will flow through the steel?

 

No, the current will be shared between the copper wire and the great big lump of steel according to the relative resistances of the two paths. That is why there shouldn't be a circuit through the great big lump of steel aka the hull.

[sjc]

To Quote the last post :-

So if I have a length of copper wire in parallel with a great big lump of steel and there is 100 amps flowing from one end to the other no current will flow through the steel?

 

That's what I am struggling with, I could ( and probably will ) run a dedicated -ve from the battery bank to the alternator body ( where it is currently strapped to the hull ). I think this will reduce the current through the hull but not eliminate it, and even if it is a small percentage ... out of 100A that is still a lot ! Surely other people have a current flowing from their "earth straps" ? Which still leaves me confused about the answer to the original question ! Having the boat blacked at present and the anodes look fine, with minimal pitting ( for the age ) so it does seem to be harmless - hmmmm

[ditchcrawler]

To Quote the last post :-

So if I have a length of copper wire in parallel with a great big lump of steel and there is 100 amps flowing from one end to the other no current will flow through the steel?

 

That's what I am struggling with, I could ( and probably will ) run a dedicated -ve from the battery bank to the alternator body ( where it is currently strapped to the hull ). I think this will reduce the current through the hull but not eliminate it, and even if it is a small percentage ... out of 100A that is still a lot ! Surely other people have a current flowing from their "earth straps" ? Which still leaves me confused about the answer to the original question ! Having the boat blacked at present and the anodes look fine, with minimal pitting ( for the age ) so it does seem to be harmless - hmmmm

Probably most of them do. There are a few proper marine engine installations where they have insulated returns from the starter motor and the alternator, some even have a bid relay in the return so that is isolated as well so no the starter motor is completely isolated. Nearly all modern alternators unless specified are earth return, they will flow to the engine block and then back to the battery via the return from the starter motor but anything connecting the engine to the hull will give a parallel path. You can only reduce it as far as possible. Most of the instruments on a modern engine are earth return.

Edited September 1, 2015 by ditchcrawler

[nicknorman]

Probably most of them do. There are a few proper marine engine installations where they have insulated returns from the starter motor and the alternator, some even have a bid relay in the return so that is isolated as well so no the starter motor is completely isolated. Nearly all modern alternators unless specified are earth return, they will flow to the engine block and then back to the battery via the return from the starter motor but anything connecting the engine to the hull will give a parallel path. You can only reduce it as far as possible. Most of the instruments on a modern engine are earth return.

I disagree. There should be no other parallel path from battery -ve to the engine via the hull. The "earth return" instrument senders you mention will return via the engine case and the wire from engine to battery -ve.

[Paul C]

So if I have a length of copper wire in parallel with a great big lump of steel and there is 100 amps flowing from one end to the other no current will flow through the steel?

 

What conductive returns are there in addition to the engine-hull earth wire? Engine mounts are rubber, propellor shaft goes through a cutlass bearing so doesn't contact hull, control cables connect to plastic control levers, fuel line has a flexible rubber section, coolant lines similar, etc.

[nicknorman]

What conductive returns are there in addition to the engine-hull earth wire? Engine mounts are rubber, propellor shaft goes through a cutlass bearing so doesn't contact hull, control cables connect to plastic control levers, fuel line has a flexible rubber section, coolant lines similar, etc.

Exhaust? And probably the prop shaft since it's a metal shaft in a metal tube. The cutlass bearing is surely just the seal at the end?

 

Our engine is certainly connected to hull via a few bits and bobs, but the important thing is that the battery -ve is not seperately connected to hull so there is no circuit.

[ditchcrawler]

Exhaust? And probably the prop shaft since it's a metal shaft in a metal tube. The cutlass bearing is surely just the seal at the end?

 

Our engine is certainly connected to hull via a few bits and bobs, but the important thing is that the battery -ve is not seperately connected to hull so there is no circuit.

So does it not bother you that if you lose your -ve your starter current will flow through your exhaust and prop shaft?

[nicknorman]

So does it not bother you that if you lose your -ve your starter current will flow through your exhaust and prop shaft?

But it won't. This is the point I'm trying to make. Whilst the engine is connected to the hull, inevitably, there is no other connection from the hull to battery -ve thus if I lose the -ve cable (seems unlikely, but not impossible of course) the starter will be dead. There is no parallel circuit.

[sjc]

I believe the engine "earth strap" is to avoid the current finding a return path that perhaps wouldn't be obvious ( such as those mentioned above ). I remember years ago it would be the choke cable on a car engine that would get hot. So I am now thinking I should remove the -ve from the alternator to the hull and run it to the battery -ve directly. Previous posts asking about the potential return path, what about DC bonding to the hull ? I am unsure how the mains earth, 12v -ve, and GI combine to connect to the hull.

[ditchcrawler]

But it won't. This is the point I'm trying to make. Whilst the engine is connected to the hull, inevitably, there is no other connection from the hull to battery -ve thus if I lose the -ve cable (seems unlikely, but not impossible of course) the starter will be dead. There is no parallel circuit.

I know this is reaching unlikely circumstances but are your batteries bonded to the hull and is your exhaust welded to the hull , if so you have a return path from the starter motor case. You yourself mentioned the prop shaft.

[nicknorman]

I know this is reaching unlikely circumstances but are your batteries bonded to the hull and is your exhaust welded to the hull , if so you have a return path from the starter motor case. You yourself mentioned the prop shaft.

No, my battery -ve is definitely not bonded to the hull other than via the cable to the engine, so there is no circuit. The hull is connected to the engine, but one connection doesn't make a circuit!