<div dir="ltr"><div>Hi Hank,</div><div> </div><div>Your right, there wont be any current flow as the positive is switched. However, without a switch, the negative would be permanently grounded. So, in the event of a fault somewhere else, there is a return path for any stray current. I don't want that. So if I put a switch on the earth as well, it eliminates the issue, more or less.</div><div> </div><div>I had an accident while building where I had inadvertently left a spanner on the main positive battery terminal which turned and contacted the hull. I then grounded another spanner from the negative against the hull and the resulting hail of sparks as the spanner disintegrated in my face has made me very conscious of electrical shorts.</div><div> </div><div>regards</div><div>James</div><div> </div><div> </div></div><div class="gmail_extra"><br><div class="gmail_quote">On 30 June 2015 at 13:14, hank pronk 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>
I am not getting why you need a switch on the negative side? The positive is switched. If the hull came into contact with the negative side of the batteries the current won't do anything because the positive is switched. Or am I missing something.<br>
Hank<br>
--------------------------------------------<br>
<span>On Tue, 6/30/15, James Frankland via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org">personal_submersibles@psubs.org</a>> wrote:<br>
<br>
Subject: Re: [PSUBS-MAILIST] Electrical Question<br>
</span><span> To: "Personal Submersibles General Discussion" <<a href="mailto:personal_submersibles@psubs.org">personal_submersibles@psubs.org</a>><br>
</span><span> Received: Tuesday, June 30, 2015, 5:02 AM<br>
<br>
</span><span> Hi<br>
Guys,<br>
There is of course a fuse<br>
inline. There is for all the electrical items. <br>
But ive realised that even if I<br>
insulate the body of the compressor from its mounting<br>
bracket, it wont make any difference as the pipework is<br>
still grounded to the hull. There's no way I can<br>
insulate that, so I cannot isolate the earth unless I can<br>
find how its connected inside the motor and isolate it<br>
there. But in the meantime, I am<br>
going to put a switch on the negative lead as Keith suggests<br>
and just click it on when I need it. <br>
</span><div><div class="h5"> RegardsJames<br>
On 29 June 2015 at 21:43,<br>
via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org">personal_submersibles@psubs.org</a>><br>
wrote:<br>
James,<br>
<br>
<br>
<br>
How about putting a battery switch in the negative lead of<br>
the compressor. Blue Sea makes high current units, that I<br>
use to connect/disconnect the battery banks on my boat. You<br>
can get tham at most marine supply stores (Westmaine,<br>
Defender, etc.).<br>
<br>
<br>
<br>
That way you can kill the compressor with the twist of the<br>
wrist, instantly.<br>
<br>
<br>
<br>
Keith T.<br>
<br>
<br>
<br>
Alan James via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org">personal_submersibles@psubs.org</a>><br>
wrote:<br>
<br>
<br>
<br>
>James,in the mean time, why not just put a suitable fuse<br>
betweenthe hull & - battery connection. The biggest<br>
issue, as Sean says, is if apositive contact comes off any<br>
electrical item on the sub, & makes contact with the<br>
hull.Then you are shorting the battery & can cause a<br>
fire or blow up the battery etc.Better still put the<br>
compressor switch, or an additional switch (or solenoid) in<br>
the wiring from the hull tothe negative terminal. Then if<br>
there is any shorts they won't happen continuously,<br>
onlywhen you have the compressor on.Alan<br>
<br>
> From: James Frankland via<br>
Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org">personal_submersibles@psubs.org</a>><br>
<br>
> To: Personal Submersibles General Discussion <<a href="mailto:personal_submersibles@psubs.org">personal_submersibles@psubs.org</a>><br>
<br>
> Sent: Tuesday, June 30, 2015 3:45 AM<br>
<br>
> Subject: Re: [PSUBS-MAILIST] Electrical Question<br>
<br>
><br>
<br>
>Agreed, that would be better. Not sure if its<br>
possible. I suppose I could try and find the negative<br>
brush and insulate it. I can only assume the negative<br>
brush is in contact with the chassis. This is the<br>
compressor here.. <a href="https://www.google.com/patents/US2450468" target="_blank" rel="noreferrer">https://www.google.com/patents/US2450468</a><br>
<br>
>On 29 June 2015 at 16:00, Sean T. Stevenson via<br>
Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org">personal_submersibles@psubs.org</a>><br>
wrote:<br>
<br>
><br>
<br>
><br>
<br>
><br>
<br>
>Would it be possible to disconnect the compressor motor<br>
negative lead from its chassis, instead wiring it to your<br>
negative/common bus, and then ground the chassis? If<br>
it's easy to do, that would be the better solution as it<br>
preserves incidental contact safety.Sean<br>
<br>
><br>
<br>
><br>
<br>
>On June 29, 2015 8:52:58 AM MDT, James Frankland via<br>
Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org">personal_submersibles@psubs.org</a>><br>
wrote:<br>
<br>
>The battery negative is not connected to the hull. <br>
But, the battery negative is connected permanently to the<br>
negative bus, which is then connected to the negative of all<br>
electrical items. So far this has been fine as the<br>
electrical items are all insulated. However, in the case<br>
of the compressor, as the compressor is connected to the<br>
hull, and the negative connects to the compressor chassis,<br>
its opening up a hull connection to the negative terminal. <br>
I suspect that if I put my voltmeter across the positive<br>
battery terminal and the hull, I will get a reading. It<br>
still wont create a circuit as the positive is of course<br>
insulated, but I don't want any connections to the hull<br>
at all. Im going to insulate it. Ive ordered some nylon<br>
bolts which I think will do the job, along with some plastic<br>
washers. <br>
<br>
>On 29 J! une 2015at 15:38, Sean T.<br>
Stevenson via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org">personal_submersibles@psubs.org</a>><br>
wrote:<br>
<br>
><br>
<br>
>Perhaps I misunderstand. Chassis continuity to hull is a<br>
good thing. You just don't want to tie your power supply<br>
common (battery negative) to it.Sean<br>
<br>
><br>
<br>
><br>
<br>
>On June 29, 2015 7:56:32 AM MDT, James Frankland via<br>
Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org">personal_submersibles@psubs.org</a>><br>
wrote:<br>
<br>
>Hi sean,<br>
<br>
>The battery negative is NOT connected to the hull. <br>
Only the chassis of this new item. But thinking about it,<br>
I suppose it does create a permanent connection to the<br>
negative terminal. The negative wire on the compressor<br>
is connected to the negative bus on the boat, which is<br>
creating a permanent connection to the hull via the mounting<br>
bracket. Damn, i'll have to insulate it somehow,<br>
before Sat. RegardsJames<br>
<br>
>On 29 June 2015 at 14:46, Sean T. Stevenson via<br>
Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org">personal_submersibles@psubs.org</a>><br>
wrote:<br>
<br>
><br>
<br>
>Your electrical system should not be neg!<br>
ativegrounded, meaning that the battery negative should not<br>
be common with the hull / chassis as it would be in an<br>
automotive system, for example. The reason for this is in<br>
part to do with galvanic corrosion, since this avoids<br>
regular currents and nonzero potentials through structural<br>
elements, and in part to do with arc safety, since a single<br>
fault or operator error which connects either battery<br>
potential to the chassis will not produce a short circuit<br>
current through the battery in this case. That said, SAFETY<br>
grounds, which include AC ground and most chassis ground and<br>
cable shield connections which are confirmed not in common<br>
with the supply DC negative, should indeed be connected<br>
through the hull (either locally or through a dedicated<br>
ground point) in order to serve their intended function:<br>
providing a short path to earth-ground potential in the<br>
event of a fault that might otherwise energizeequipment /<br>
chasses that could be hazardous to personnel, and serving as<br>
a connection to an "infinite" charge sink to<br>
reference cable shields to for effective noise rejection.A<br>
negative connected chassis on a DC powered compressor can be<br>
accommodated, as you surmised, by isolating that chassis<br>
from the hull, and additionally should be isolated from the<br>
operator / cabin (via enclosure?), because the chassis in<br>
that case does not represent the safety ground potential,<br>
and is thus akin to a large bare conductor at the battery<br>
negative potential. Alternatively, you could look at<br>
modifying the unit to break the negative-chassis connection,<br>
running that negative to the battery and grounding the<br>
chassis.Sean<br>
<br>
><br>
<br>
>On June 29, 2015 6:48:15 AM MDT, James Frankland via<br>
Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org">personal_submersibles@psubs.org</a>><br>
wrote:<br>
<br>
>Hi All, Ive made a real point of making sure that<br>
nothing at all is electrically connected to the hull of my<br>
boat. Everything is wired to and from the battery and<br>
insulated from the hull. However, ive recently fitted a new<br>
item, a Cornelius compressor which Hank gave me. Ive<br>
realised that the negative terminal on the unit is the<br>
chassis of the compressor, which is bolted to brackets on<br>
the hull, so its actually connected to the hull of the boat<br>
as well. The hull of the boat is not connected to the<br>
negative battery terminal. I don't think there is an<br>
issue there with galvanic corrosion, but im not sure.<br>
<br>
>I could insulate the compressor from the mounting<br>
bracket and hence insulate it from the hull, but it will be<br>
a bloody pain and im diving this weekend so don't really<br>
want to start changing it now. Thanks<br>
<br>
>James <br>
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</div></div> -----Inline Attachment Follows-----<br>
<div class="HOEnZb"><div class="h5"><br>
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</div></div></blockquote></div><br></div>