[PSUBS-MAILIST] Electrical Question

Mon Jun 29 11:00:24 EDT 2015

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 June 2015 at 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.
>>>
>>> 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 neg! ative 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
>>>
>>>
>> _______________________________________________
>> 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/e66254fa/attachment-0001.html>