[PSUBS-MAILIST] Electrical Question

[James Frankland via Personal_Submersibles]

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
>>>>
>>>>
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>>>>
>>>>
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