[PSUBS-MAILIST] Electrical Question

Tue Jun 30 13:37:27 EDT 2015

James,

I was looking into building an aluminum sailboat some years ago and
everything I read said to never ground anything to the hull, has to go back
to the negative side of the battery/engine block and then there would be a
dielectric spacer in the propeller shaft . I decided to build a sub instead
but I can't tell you how many power/sail boats I have had to fix because
they had stray current running threw their hull which acted like an anode?
There would be multiple circles ranging from a dime to a half dollar that
had eroded almost all the way threw the hull below the water line and I
would have to  fill them back up with Tig welding.  I don't understand
electricity like some of the others here but my 2 cents worth would be to
never use the hull as a ground and put a small section of non conductive
hose in your supply and return lines for LP or HP air.

Rick

On Mon, Jun 29, 2015 at 11:02 PM, James Frankland via Personal_Submersibles
<personal_submersibles at psubs.org> wrote:

> Hi Guys,
>
> There is of course a fuse inline.  There is for all the electrical items.
>
> But ive realised that even if I insulate the body of the compressor from
> its mounting bracket, it wont make any difference as the pipework is still
> grounded to the hull.   There's no way I can insulate that, so I cannot
> isolate the earth unless I can find how its connected inside the motor and
> isolate it there.
>
> But in the meantime, I am going to put a switch on the negative lead as
> Keith suggests and just click it on when I need it.
>
> Regards
> James
>
> On 29 June 2015 at 21:43, via Personal_Submersibles <
> personal_submersibles at psubs.org> wrote:
>
>> James,
>>
>> How about putting a battery switch in the negative lead of the
>> compressor. Blue Sea makes high current units, that I use to
>> connect/disconnect the battery banks on my boat. You can get tham at most
>> marine supply stores (Westmaine, Defender, etc.).
>>
>> That way you can kill the compressor with the twist of the wrist,
>> instantly.
>>
>> Keith T.
>>
>> Alan James via Personal_Submersibles <personal_submersibles at psubs.org>
>> wrote:
>>
>> >James,in the mean time, why not just put a suitable fuse betweenthe hull
>> & - battery connection. The biggest issue, as Sean says, is if apositive
>> contact comes off any electrical item on the sub, & makes contact with the
>> hull.Then you are shorting the battery & can cause a fire or blow up the
>> battery etc.Better still put the compressor switch, or an additional switch
>> (or solenoid) in the wiring from the hull tothe negative terminal. Then if
>> there is any shorts they won't happen continuously, onlywhen you have the
>> compressor on.Alan
>> >      From: James Frankland via Personal_Submersibles <
>> personal_submersibles at psubs.org>
>> > To: Personal Submersibles General Discussion <
>> personal_submersibles at psubs.org>
>> > Sent: Tuesday, June 30, 2015 3:45 AM
>> > Subject: Re: [PSUBS-MAILIST] Electrical Question
>> >
>> >Agreed, that would be better.  Not sure if its possible.  I suppose I
>> could try and find the negative brush and insulate it.  I can only assume
>> the negative brush is in contact with the chassis. This is the compressor
>> here.. https://www.google.com/patents/US2450468
>> >On 29 June 2015 at 16:00, Sean T. Stevenson via Personal_Submersibles <
>> personal_submersibles at psubs.org> wrote:
>> >
>> >
>> >
>> >Would it be possible to disconnect the compressor motor negative lead
>> from its chassis, instead wiring it to your negative/common bus, and then
>> ground the chassis? If it's easy to do, that would be the better solution
>> as it preserves incidental contact safety.Sean
>> >
>> >
>> >On June 29, 2015 8:52:58 AM MDT, James Frankland via
>> Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>> >The battery negative is not connected to the hull.   But, the battery
>> negative is connected permanently to the negative bus, which is then
>> connected to the negative of all electrical items.  So far this has been
>> fine as the electrical items are all insulated.  However, in the case of
>> the compressor, as the compressor is connected to the hull, and the
>> negative connects to the compressor chassis, its opening up a hull
>> connection to the negative terminal.  I suspect that if I put my voltmeter
>> across the positive battery terminal and the hull, I will get a reading.
>> It still wont create a circuit as the positive is of course insulated, but
>> I don't want any connections to the hull at all. Im going to insulate it.
>> Ive ordered some nylon bolts which I think will do the job, along with some
>> plastic washers.
>> >On 29 J! une 2015at 15:38, Sean T. Stevenson via Personal_Submersibles <
>> personal_submersibles at psubs.org> wrote:
>> >
>> >Perhaps I misunderstand. Chassis continuity to hull is a good thing. You
>> just don't want to tie your power supply common (battery negative) to
>> it.Sean
>> >
>> >
>> >On June 29, 2015 7:56:32 AM MDT, James Frankland via
>> Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>> >Hi sean,
>> >The battery negative is NOT connected to the hull.  Only the chassis of
>> this new item. But thinking about it, I suppose it does create a permanent
>> connection to the negative terminal.   The negative wire on the compressor
>> is connected to the negative bus on the boat, which is creating a permanent
>> connection to the hull via the mounting bracket.   Damn, i'll have to
>> insulate it somehow, before Sat. RegardsJames
>> >On 29 June 2015 at 14:46, Sean T. Stevenson via Personal_Submersibles <
>> personal_submersibles at psubs.org> wrote:
>> >
>> >Your electrical system should not be neg! ativegrounded, meaning that
>> the battery negative should not be common with the hull / chassis as it
>> would be in an automotive system, for example. The reason for this is in
>> part to do with galvanic corrosion, since this avoids regular currents and
>> nonzero potentials through structural elements, and in part to do with arc
>> safety, since a single fault or operator error which connects either
>> battery potential to the chassis will not produce a short circuit current
>> through the battery in this case. That said, SAFETY grounds, which include
>> AC ground and most chassis ground and cable shield connections which are
>> confirmed not in common with the supply DC negative, should indeed be
>> connected through the hull (either locally or through a dedicated ground
>> point) in order to serve their intended function: providing a short path to
>> earth-ground potential in the event of a fault that might otherwise
>> energizeequipment / chasses that could be hazardous to personnel, and
>> serving as a connection to an "infinite" charge sink to reference cable
>> shields to for effective noise rejection.A negative connected chassis on a
>> DC powered compressor can be accommodated, as you surmised, by isolating
>> that chassis from the hull, and additionally should be isolated from the
>> operator / cabin (via enclosure?), because the chassis in that case does
>> not represent the safety ground potential, and is thus akin to a large bare
>> conductor at the battery negative potential.  Alternatively, you could look
>> at modifying the unit to break the negative-chassis connection, running
>> that negative to the battery and grounding the chassis.Sean
>> >
>> >On June 29, 2015 6:48:15 AM MDT, James Frankland via
>> Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>> >Hi All, Ive made a real point of making sure that nothing at all is
>> electrically connected to the hull of my boat.  Everything is wired to and
>> from the battery and insulated from the hull. However, ive recently fitted
>> a new item, a Cornelius compressor which Hank gave me.  Ive realised that
>> the negative terminal on the unit is the chassis of the compressor, which
>> is bolted to brackets on the hull, so its actually connected to the hull of
>> the boat as well.   The hull of the boat is not connected to the negative
>> battery terminal. I don't think there is an issue there with galvanic
>> corrosion, but im not sure.
>> >I could insulate the compressor from the mounting bracket and hence
>> insulate it from the hull, but it will be a bloody pain and im diving this
>> weekend so don't really want to start changing it now. Thanks
>> >James
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