Should I have a switched negative ground?

[ErnestV]

Hi gals and guys, I'd ask for your opinion here. I am currently planning the refit of the main electrical system of an owner-built Van de Stadt I recently bought. This boat had three master switches:

• house/auxiliary: one switch for positive and one for negative/ground

• starter bank: only one switch for negative/ground, the positive wire going directly to the starter motor.

I am quite sure I will change this so that both positive wires can be switched but I'm hesistant to include master switches for the negative side as I don't see any reason... What is your take on this?

Both banks didn't share a common ground but were grounded on different spots, the starter at the backside of the engine block, and the house/auxiliary at a stringer of the steel hull. Is it only me having my hair stand on end?

Thanks for your input!

[smac999]

A switched battery ground. (All batteries being cut off on one switch). Is common in Europe. But all pos is also switched.

In North america switched grounds are frowned on and I often remove them. They are normally done wrong anyways by only cutting some things off. and cause more problems then solve with weird ground loops still powered.. Should defiantly not be only switching ground. Either Pos only. Or both at same time.

[noelex 77]

Van de Stadt designs are constucted in fibreglass, steel and aluminium. What is the construction material in your case?

[ErnestV]

Originally Posted by noelex 77

Van de Stadt designs are constucted in fibreglass, steel and aluminium. What is the construction material in your case?

My VdS 36 Seal is built in 4/3mm steel (0.16/0.12")

[noelex 77]

Originally Posted by ErnestV

My VdS 36 Seal is built in 4/3mm steel (0.16/0.12")

For a steel boat there are two principal options. An isolated ground system with dual pole switching (both positive and negative circuits) is the most expensive and best option, but most steel boats opt for the less expensive system where there is one (and only one) ground point.

Sometimes these systems are combined with the aim of achieving the advantages of an isolated ground system without significant extra expense. In this case there is a single ground point, but with the option of better isolation by disconnecting the main negative supply and sometimes the engine negative supply via a battery switch. This sounds like the system adopted on your boat. This disconnection of the negative supply is also required for boats to meet some marine standards.

You need to be very careful about modifying this wiring on a metal hulled boat. The option of disconnecting a negative battery supply to circuits can be essential for many metal hulled boats. Do not remove this without being very sure about the effects. However, the lack of switching of the positive supply to the engine battery sounds very strange. I suspect this is a bodge incorporated by a previous owner.

[mitiempo]

Originally Posted by ErnestV

Both banks didn't share a common ground but were grounded on different spots,.......... the house/auxiliary at a stringer of the steel hull. Is it only me having my hair stand on end?

Thanks for your input!

That is most definitely incorrect.

One solution on a metal hull is to switch the negative to the starter/block off when not running the engine, as it can be difficult to isolate an engine and shafting from the hull. This can be accomplished with a solenoid wired to the start switch's run position. This way the connection is only made when the engine is running isolated the rest of the time.

Ideally all wiring on a metal boat avoids the hull connection. As noelex points out the ideal is all DC on double pole breakers, disconnecting both positive and negative.

All pumps etc should be isolated from the hull as their cases are typically negative grounded.

[Oceanride007]

My back ground was in commercial shipping, I was horrified to find the Negative attached to my hull at engine block. I inserted a switch as suggested above, and to ensure that my alternator was not interrupted I ran a negative from the isolated negative of the alternator, but not sure if this necessary.
Main reason for my horror was definitely did not want circulating currents in the hull. As this would cause corrosion problems in hull and cost Ahrs. If a hull is isolated and if a contact to any phase occurs it is not really a issue unless you have another contact from another phase, on ships that first contact is treated with priority it must be cleared.
An interesting "Yes but" issue is a nearby lightening spike, you see a dangerous to equipment spike can occur and on the negative side because its wired to everything, everything will get zapped, would switching negative to hull give some protection? I have no answer, but so far neither does anybody else.

[adjo]

Aboard my 35yo aluminium yacht I have a negative switch between the battery and the motor. Since the rest of the system is "above ground" why link it to the hull?

It has never been a problem and cannot do any harm.

I have only had to change my hull anodes 4 times in 35 years (prop anodes more often). This probably has nothing to do with the switch and much more to do with the paint system.

[PirateGuy]

Interesting discussion.
Thank you fellow Cfuisers.

[ErnestV]

Thank you all for your insights, recommendations and experience! Your input is very important to me, and I sincerely hope I'll be able to "pay it back" eventually.

Based on your experiences I'm currently inclined to design the system without connecting the battery negative to the hull - with the exception when ignition is switched on which will be controlled by a power relay. As for shore power I believe I should have the PE (ground) connected to the steel hull (using an galvanic isolator), but no connection between PE and battery minus should be necessary IMHO.

[hlev00]

Originally Posted by smac999

A switched battery ground. (All batteries being cut off on one switch). Is common in Europe. But all pos is also switched.

In North america switched grounds are frowned on and I often remove them. They are normally done wrong anyways by only cutting some things off. and cause more problems then solve with weird ground loops still powered.. Should defiantly not be only switching ground. Either Pos only. Or both at same time.

I can attest to the weird ground loops the switched negatives can set up. I have a Beneteau, US built but French design with a switched negative. When we leave the boat we normally turn all negative and +12 switches off. One day the house +12 got switched on but the negative was left off. I attempted to start the engine and nothing. I went below and saw the problem. After switching the negative on, the boat started fine. However later we noticed our bilge pump switch / breaker indicator light was dimmer than normal. The stereo was behaving oddly too. So in addition to the switched negative there are a few always on circuits like the bilge pump. Tracing the wires down, the fuse on the negative that feeds the DC panel had blown. I always thought a fuse on the negative was weird but I now see they were trying to prevent what happened when I turned the key with the negative switched off. That is the boat tried to supply the ground through its only available path, the relatively tiny negative wire for the DC panel.

There is still something amiss. My guess is that the bilge has a ground going to both the boat ground as well as an always on ground. It is on my list this Spring to trace all this down. I think we'll just leave the negative switch on from now on if it is really acceptable in the US to not have a switch.

Harry

[Stu Jackson]

Harry,


This unusual, but typical Beneteau wiring has been addressed many times by Maine Sail.


Here is a link to his "correction"


Beneteau Wiring Diagrams - Maine Sail's corrections - Just in case you have a friend with a Beneteau and want to help him with his snafued wiring Reply #8

https://forums.sailboatowners.com/in.../#post-1456512

Originally Posted by hlev00

I can attest to the weird ground loops the switched negatives can set up. I have a Beneteau, US built but French design with a switched negative. When we leave the boat we normally turn all negative and +12 switches off. One day the house +12 got switched on but the negative was left off. I attempted to start the engine and nothing. I went below and saw the problem. After switching the negative on, the boat started fine. However later we noticed our bilge pump switch / breaker indicator light was dimmer than normal. The stereo was behaving oddly too. So in addition to the switched negative there are a few always on circuits like the bilge pump. Tracing the wires down, the fuse on the negative that feeds the DC panel had blown. I always thought a fuse on the negative was weird but I now see they were trying to prevent what happened when I turned the key with the negative switched off. That is the boat tried to supply the ground through its only available path, the relatively tiny negative wire for the DC panel.

There is still something amiss. My guess is that the bilge has a ground going to both the boat ground as well as an always on ground. It is on my list this Spring to trace all this down. I think we'll just leave the negative switch on from now on if it is really acceptable in the US to not have a switch.

Harry

[seabreez]

Originally Posted by Oceanride007

My back ground was in commercial shipping, I was horrified to find the Negative attached to my hull at engine block. I inserted a switch as suggested above, and to ensure that my alternator was not interrupted I ran a negative from the isolated negative of the alternator, but not sure if this necessary.(EDITED...) An interesting "Yes but" issue is a nearby lightening spike, you see a dangerous to equipment spike can occur and on the negative side because its wired to everything, everything will get zapped, would switching negative to hull give some protection? I have no answer, but so far neither does anybody else.

The ground should not be switched. It is dangerous. There should be one grounding point and all grounds should go to that point and not through the hull for the return to the ground and to the battery. The engine should be grounded to that point. The grounding point could be on the engine block. (This is the case on fiberglass boats) On Steel boat, the ground is one point on a frame and all grounds go to that point even both engines to that point. Both engine are also structurally grounded to the frame. And the starter also uses the block for grounding and the return to the battery. In my opinion, the grounding point in a steel both will be very close to the prop shafts, struts and rudders. Now here is the danger of switching OFF the ground wire... If a lightning strike happens, I want the electrical charge to dissipate in the water via the steel, the shaft and all other paths, even the ground wire. If the ground wire is disconnected, the electrical charge will choose it own route to eliminate the charge and might... yes might go through other grounding routes through wiring not of the proper size to handle the current surcharge and might cause fire and / or burn electrical equipment. I just completed rewiring a steel tug and a another 70 foot steel vessel... My two cents worth...

[hzcruiser]

Originally Posted by ErnestV

[...]

Both banks didn't share a common ground but were grounded on different spots, the starter at the backside of the engine block, and the house/auxiliary at a stringer of the steel hull. Is it only me having my hair stand on end?

While updating and modifying the wiring on my steel sloop, I'm thinking about the best approach, too.
Notwithstanding any regulations out there, from an electrical perspective, disconnecting the starter battery ground terminal might actually not be a bad idea after all: it disconnects the batt from the starter but also removes the common ground between house and starter system. The engine block and starter motor are connected to the hull anyway, nothing we can do there (within reason).

As a current can only flow in a complete loop, it then doesn't matter where the positive wire from the starter batt is connected. You could now short the starter wire anywhere to the hull, the motor, or any house system and no sparks will fly.

We basically have six "grounds" on the boat to be aware of:

• starter battery bank
• house bank
• shore neutral
• shore protective earth (PE)
• radio shield and antennas
• lightning ground

I'd argue the starter batt can be taken care of as mentioned above and done on your boat.
The house bank includes consumers like radios. Some radios, in particular for HF, need a ground on top of the positive and negative power supply, as they use it as their counterpoise (the second part of a dipole antenna). This makes it difficult to completely isolate the hull from the house battery, unless you have a double breaker for it, which you do.
Hence it seems the PO had thought about a few things, rather than trying to cut corners.
A double breaker on the starter bank might look even better, yet it might be a belt and suspenders solution and you need to make absolutely sure that ground is connected when you turn on the ignition. If not, the loop will be closed through the house system's ground, with the results as described by hlev00 in post #11 above.

As for shore neutral (and the inverter output): it should go with the AC live wire into dedicated conduits around the boat to keep it separate from boat ground.
Shore PE should also be connected to the steel hull, otherwise it's pretty pointless. As mentioned above-thread, a galvanic isolator here protects from stray currents. Also note that since the PE should trip way below 100mA, the galvanic isolator can be very small, too.

Lastly, I'm tempted to say "don't worry about lightning ground, it will find its way" . But in the real world, a good conduit from the mast to water can save your boat on a direct strike. I had a small direct strike a few years ago which jumped from the isolated (antenna) backstay to the MOB pole which was loosely tied to it without actually touching the stay. A black mark on the deck right at the foot of the pole gave it away (and the HF radio was a write-off, too).

[Wotname]

Originally Posted by hzcruiser

............ Some radios, in particular for HF, need a ground on top of the positive and negative power supply, as they use it as their counterpoise (the second part of a dipole antenna). This makes it difficult to completely isolate the hull from the house battery, ..........

FWIW, The RF ground for a HF need not be a DC ground; it can be RF (AC) ground achieved with capacitors. The RF will flow through the capacitor but DC will be blocked.