[PSUBS-MAILIST] Electrical Question

[James Frankland via Personal_Submersibles]

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
>>
>>
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>>
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