[PSUBS-MAILIST] Electrical Question

Tue Jun 30 14:31:41 EDT 2015

There seems to be some confusion arising from terminology.

DO use the hull for grounding (safety/earth ground, equipment chasses, cable shield reference, antenna ground plane).

DO NOT use the hull as a battery negative or DC power supply common bus connection.

There should be no currents in your hull in the course of normal operations, but in the event of a serious electrical fault, you want to tie down the potentials of any exposed equipment rather than energizing it.

Sean

On June 30, 2015 11:37:27 AM MDT, Rick Patton via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>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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>>> >
>>> >
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>>
>>
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>
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