<html><head></head><body><p dir="ltr">There seems to be some confusion arising from terminology.</p>
<p dir="ltr">DO use the hull for grounding (safety/earth ground, equipment chasses, cable shield reference, antenna ground plane).</p>
<p dir="ltr">DO NOT use the hull as a battery negative or DC power supply common bus connection.</p>
<p dir="ltr">There should be no currents in your hull in the course of normal operations, but in the event of a serious electrical fault, you want to tie down the potentials of any exposed equipment rather than energizing it.</p>
<p dir="ltr">Sean<br>
</p>
<br><br><div class="gmail_quote">On June 30, 2015 11:37:27 AM MDT, Rick Patton via Personal_Submersibles <personal_submersibles@psubs.org> wrote:<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
<div dir="ltr">James,<div><br /></div><div>I was looking into building an aluminum sailboat some years ago and everything I read said to never ground anything to the hull, has to go back to the negative side of the battery/engine block and then there would be a dielectric spacer in the propeller shaft . I decided to build a sub instead but I can't tell you how many power/sail boats I have had to fix because they had stray current running threw their hull which acted like an anode? </div><div>There would be multiple circles ranging from a dime to a half dollar that had eroded almost all the way threw the hull below the water line and I would have to fill them back up with Tig welding. I don't understand electricity like some of the others here but my 2 cents worth would be to never use the hull as a ground and put a small section of non conductive hose in your supply and return lines for LP or HP air.</div><div><br /></div><div>Rick</div></div><div
class="gmail_extra"><br /><div class="gmail_quote">On Mon, Jun 29, 2015 at 11:02 PM, James Frankland via Personal_Submersibles <span dir="ltr"><<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>></span> wrote:<br /><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div>Hi Guys,</div><div><br />There is of course a fuse inline. There is for all the electrical items. </div><div> </div><div>But ive realised that even if I insulate the body of the compressor from its mounting bracket, it wont make any difference as the pipework is still grounded to the hull. There's no way I can insulate that, so I cannot isolate the earth unless I can find how its connected inside the motor and isolate it there. </div><div> </div><div>But in the meantime, I am going to put a switch on the negative lead as Keith suggests and just click it on when I need it.
</div><div> </div><div>Regards</div><div>James</div></div><div class="gmail_extra"><br /><div class="gmail_quote">On 29 June 2015 at 21:43, via Personal_Submersibles <span dir="ltr"><<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>></span> wrote:<br /><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">James,<br />
<br />
How about putting a battery switch in the negative lead of the compressor. Blue Sea makes high current units, that I use to connect/disconnect the battery banks on my boat. You can get tham at most marine supply stores (Westmaine, Defender, etc.).<br />
<br />
That way you can kill the compressor with the twist of the wrist, instantly.<br />
<br />
Keith T.<br />
<br />
Alan James via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>> wrote:<br />
<br />
>James,in the mean time, why not just put a suitable fuse betweenthe hull & - battery connection. The biggest issue, as Sean says, is if apositive contact comes off any electrical item on the sub, & makes contact with the hull.Then you are shorting the battery & can cause a fire or blow up the battery etc.Better still put the compressor switch, or an additional switch (or solenoid) in the wiring from the hull tothe negative terminal. Then if there is any shorts they won't happen continuously, onlywhen you have the compressor on.Alan<br />
<span>> From: James Frankland via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>><br />
> To: Personal Submersibles General Discussion <<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>><br />
> Sent: Tuesday, June 30, 2015 3:45 AM<br />
> Subject: Re: [PSUBS-MAILIST] Electrical Question<br />
><br />
>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.. <a href="https://www.google.com/patents/US2450468" rel="noreferrer" target="_blank">https://www.google.com/patents/US2450468</a><br />
>On 29 June 2015 at 16:00, Sean T. Stevenson via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>> wrote:<br />
><br />
><br />
><br />
>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<br />
><br />
><br />
>On June 29, 2015 8:52:58 AM MDT, James Frankland via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>> wrote:<br />
>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. <br />
</span><span>>On 29 J! une 2015at 15:38, Sean T. Stevenson via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>> wrote:<br />
><br />
>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<br />
><br />
><br />
>On June 29, 2015 7:56:32 AM MDT, James Frankland via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>> wrote:<br />
>Hi sean,<br />
>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. RegardsJames<br />
>On 29 June 2015 at 14:46, Sean T. Stevenson via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>> wrote:<br />
><br />
</span>>Your electrical system should not be neg! ativegrounded, 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 energizeequipment / 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<br />
<span>><br />
>On June 29, 2015 6:48:15 AM MDT, James Frankland via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>> wrote:<br />
>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.<br />
>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<br />
>James <br />
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