[PSUBS-MAILIST] wiring problem

[Sean T. Stevenson via Personal_Submersibles]

Then that is not a "ground". It is a DC power supply common / negative.

In automotive systems, these are at the same potential, because the battery negative is tied to the vehicle chassis. In marine systems, they must be isolated. The consequence of this is that the potential of both battery terminals can then float with respect to the safety ground - the potential difference is instantaneously indeterminate. This is referred to as "common-mode" voltage, as both positive and negative DC rails float at the same time, so that their difference is constant. Your installed equipment will likely have a common mode voltage rating (or if not a rating, it still has a physical limit) beyond which it cannot insulate between one of the power terminals and the safety ground, which is at earth potential. One possible solution to this is to tie the power supply common (battery negative) to the hull / earth ground as in an automotive system, but to do so through a very high resistance, such that the battery gains a reference potential, limiting the common-mode drift,
but will still not permit a short-circuit current to flow in the event of e.g. connecting your positive battery terminal to the hull.  My preference is to maintain complete isolation, but if that corrects the problem it is diagnostically useful.

Assuming that there is no continuity between either supply terminal and your actual "ground" (the hull), I would check the polarity as you suggested, and also check the open circuit resistance of both devices at their power supply inputs.

Sean


On June 9, 2016 7:26:35 AM MDT, hank pronk via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>The compressor ground goes to negative on battery and compressor
>chassis is bolted to the hull.
>
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