[PSUBS-MAILIST] Electrical Question

[Sean T. Stevenson via Personal_Submersibles]

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.
>
>Regards
>James
>
>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 negative grounded, 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
>> energize equipment / 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
>>>
>>>
>>> ------------------------------
>>>
>>> Personal_Submersibles mailing list
>>> Personal_Submersibles at psubs.org
>>> http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
>>>
>>>
>> _______________________________________________
>> Personal_Submersibles mailing list
>> Personal_Submersibles at psubs.org
>> http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
>>
>>
>
>
>------------------------------------------------------------------------
>
>_______________________________________________
>Personal_Submersibles mailing list
>Personal_Submersibles at psubs.org
>http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.whoweb.com/pipermail/personal_submersibles/attachments/20150629/a7c3acd0/attachment.html>