[PSUBS-MAILIST] Electrical Question

[Sean T. Stevenson via Personal_Submersibles]

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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