[PSUBS-MAILIST] Electrical Question

Mon Jun 29 11:45:46 EDT 2015

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 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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>>>>>
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