Metal Chassis Equip. grounding?

[Karl in NY]

Many things like stereo head-units, amplifiers, VHF radios, etc. have metal chassis and normal mounting results in electrically using the aluminum hull as a ground return, even if a separate ground wire is run to the battery neg. or to a grounding block. It would seem that the metal hull ground would be far lower resistance than a small gauge grounding wire, thus carrying most of the return current, probably resulting in stray-current corrosion of the hull.

Should such electronics be electrically isolated from the hull, using plastic spacers, shoulder washers, a wood mounting plate, nylon machine screws,etc.?

I have seen no discussion about this issue. But, it seems like most marine products have their metal cases electrically connected to the grounding wire, not an issue with plastic boats, obviously.

Opinions?

[Chaps]

I think it becomes an issue if and when the ground wire for a device becomes compromised and the device begins to utilize its hull ground for actual completion of the circuit. I'm working on a boat right now that is showing signs of stray current corrosion. Bottom paint and epoxy primer is coming off in chunks and the bare aluminum underneath is showing a swirled etch that looks suspiciously like a Tesla tornado (my terminology).

[Karl in NY]

Why wouldn't the hull be used as the primary ground, even if there's a separate (but not isolated) grounding wire for the device, whatever it might be? The return current will be distributed between both the hull and grounding wire, and the hull would offer far lower resistance and thus carry the bulk of the return current to the battery, much like what typically happens in a car, even when things like stereos do have a separate grounding wire, it's primarily for redundancy, or for when the stereo is mounted to a plastic dashboard.

I don't think most marine electrical/electronic items supply things like plastic mounting brackets, because both aluminum and steel hulls are such a small fraction of the boats they get installed on.

I would think mounting hardware should electrically isolate the device chassis from the hull, if an ohmmeter shows continuity between the grounding wire and the metal chassis/enclosure of the device, otherwise, stray current corrosion would seem to be inevitable. It's a given that marine differs from automotive, hence the need to double the length of the wire run when calculating wire gauge for a given voltage drop.

Even simple things like rubber well-nuts would isolate the mounting bracket from a panel that's welded to the hull...

I would certainly isolate all devices, to insure that the only return ground path is via the ground wire, but that is apparently seldom done on aluminum boats, despite the potential devastating damage that could result.

[Chaps]

Well then the first thing you need to isolate is the engine as it is grounded as soon as you attach it to the hull . . .

[kmorin]

Karl, Chaps,

the DC has two legs and unless there's s a power supply failure the chassis bond is not even used; ever. Its sole purpose in life is to satisfy UL/NEMA that IF>>>> IF mind you, a power supply failed then chassis ground conductor would act as the DC-Neg in terms of safety.... There is no need in today's potted, sealed, solid state power supplies to bond the chassis to anything. There just ain't gonna be any DC on that metal box even if the device is immersed in water! IF a power supply fails now days it just goes open, and there's no chassis charge to 'drain'- the idea of the chassis bond is a largely a holdover from the 'old days' and the types of 'risk' that is attributed to that series of instruments no longer used!

So on metal boats the DC Power Leg (Plus) and the DC Return legs (negative) just need to go to their respective buses and all's good.

Karl metal boats are so NOT LIKE cars that there is no way to begin to address this misconception. If you charge the hull you'll begin to dissolve the hull; end of story. Want to keep the hull(?) ; don't charge the hull. All circuits need to be two wire, "out and back" and there is no need for a chassis bond. The ABYC has diagrams, but they cost to get inside the paywall, and posting them is sort of frowned on by the copyright issues involved. But http://newboatbuilders.com/pages/links.html Ike's site has lots of links to ABYC and CG regs and some research there will help you get more familiar.

The entire DC and AC systems should be bonded to the hull (in one place only) and that is easiest done at the engine starter DC Neg terminal if inboard and at the battery Neg Bus if the power is outboard. For the AC, the third wire (bond/ground/not L1 or L2) is bonded to the same point as the DC neg.

A dedicated, well terminated, single conductor of #14 stranded copper will have less resistance than the aluminum hull- considering a potential to DC Neg (battery negative) not to the water. But if the hull is used as a charged conductor it will become its own anode in locations not near the sacrificial zincs.

There are some good articles here in the archive that may add more info to this repeat thread?

Cheers,
Kevin Morin
Kenai, AK

[Karl in NY]

I was under the impression that with 12vdc devices (where there is no shock hazard) that the metal chassis of the device was bonded to the negative lead for purposes of noise and interference rejection (shielding).

What do you suggest if an ohmmeter shows zero ohms between the black lead and the mounting bracket of the device?

In that case, wouldn't the hull be acting as the primary ground return to the central grounding point?

[Chtucker]

Mounting brackets, enclosures (DC refrigerator, AIS transponder etc.), engines, antenna brackets through the coax ground... all show continuity between the battery return and ground. Same as I said on THT.

The wire provides a lower resistance to battery return. Disconnect the engine ground wires and disaster strikes with electrolysis. The heavy gauge return wires on the engines prevent this.

[Karl in NY]

I guess I'm just missing the logic of that. Assume that a device has a 14ga. black lead that runs to the central grounding point, yet is also connected to an aluminum hull via the device chassis or mounting brackets. If I were 12vdc electricity, I would choose the hull return path to get to ground...the hull has like zero ohms and zero voltage drop, compared to a 14ga. wire. And, depending on the current-draw of the particular device, its black wire may be much smaller than 14ga.

I guess I'll have to locate my copy of Nigel Calder's book which I read years ago...I believe he discusses this issue.

I still have a plastic boat, so it's not an immediate issue for me, however.

[MacGyver]

Many things like stereo head-units, amplifiers, VHF radios, etc. have metal chassis and normal mounting results in electrically using the aluminum hull as a ground return, even if a separate ground wire is run to the battery neg. or to a grounding block. It would seem that the metal hull ground would be far lower resistance than a small gauge grounding wire, thus carrying most of the return current, probably resulting in stray-current corrosion of the hull.

Should such electronics be electrically isolated from the hull, using plastic spacers, shoulder washers, a wood mounting plate, nylon machine screws,etc.?

I have seen no discussion about this issue. But, it seems like most marine products have their metal cases electrically connected to the grounding wire, not an issue with plastic boats, obviously.

Opinions?

Just because a chassis is connected to the hull does not mean it is also grounding the
dc negative to the hull, it may be floating. Measure the resistance between the bracket and the positive and negative power wires. With the power switch ON to find out what is going on. If there is resistance between wires and the hull. That does not mean is there enough current leakage/voltage to cause you a problem? You need to do further testing.

[Karl in NY]

I have used an ohmmeter on several devices, and found exactly zero ohms between the (disconnected) black grounding wire and the device chassis and metal hulls and its metal mounting brackets. As I stated before, I think the intent is to provide noise/interference shielding, whether it's a stereo head-unit, amplifier, or a VHF radio. You can make this test as soon as you remove the device from its shipping box. In my opinion, zero ohms between the black wire and the device chassis is potential trouble, especially considering how simple it is to isolate the device chassis from the hull electrically.

I don't see how that shielding would be compromised by electrically isolating the device from a metal hull, and thus providing only a single possible return path to common ground, via the black lead, and eliminating any chance of stray-current corrosion caused by the device seeking ground via the hull, the norm with automotive devices, whether or not they even have a black lead. Most do, because of mounting on plastic dashboards, where a chassis ground-return is unreliable, but even with car electronics, mounting brackets generally still screw into metal which is attached to the car frame, an undesirable situation in an aluminum-hulled boat, in terms of stray-current corrosion.

Try temporarily disconnecting the black lead on any device, and measure resistance to any part of the aluminum hull, resistance between the floating black lead will likely show zero ohms to any part of the hull, due to the device chassis itself being connected to the hull and the black lead.

[Chtucker]

How would you isolate the engines from the hull?

If you look at any Furuno equipment the power cables are shielded. There is also a dedicated connection point on their equipment for those shields. All in the interest of reducing interference. That connection point IS chassis ground as well.

Another "what if?" My radar dome (Furuno DRS4A) is fed from a Furuno PSU-012 power supply with a 3 conductor cable (Shield, battery + and battery return) The cable is pre stripped and terminated so that the shield is clamped inside the dome for strain relief. The dome is also bonded to the hull through its mounted bolts. How would you isolate the radar dome from the hull?

VHF radio- I have a 3 VHF radio ratchet mounts on my boat, all with the rubber gasket behind them, but mounted with stainless bolts. ALL three show continuity between coax braid/radio chassis and the hull. How would you isolate the antennas?

My Furuno FA-150 AIS system is a large metal box. It is mounted on HDPE Starboard. Again, it is bonded through the hull by way of the antenna.

I haven't checked the metal brackets for my chart plotter, but I am going to assume they are chassis ground as well.