[PSUBS-MAILIST] Electrical Question

Tue Jun 30 07:56:48 EDT 2015

James,
My whole sub is grounded to the batteries, not ideal, but worked since 1971.  
Hank   --------------------------------------------
On Tue, 6/30/15, James Frankland via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

 Subject: Re: [PSUBS-MAILIST] Electrical Question
 To: "Personal Submersibles General Discussion" <personal_submersibles at psubs.org>
 Received: Tuesday, June 30, 2015, 5:02 AM

 Hi
 Guys,
 There is of course a fuse
 inline.  There is for all the electrical items. 
  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.   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. 
  RegardsJames
 On 29 June 2015 at 21:43,
 via Personal_Submersibles <personal_submersibles at psubs.org>
 wrote:
 James,

 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.).

 That way you can kill the compressor with the twist of the
 wrist, instantly.

 Keith T.

 Alan James via Personal_Submersibles <personal_submersibles at psubs.org>
 wrote:

 >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

 >      From: James Frankland via
 Personal_Submersibles <personal_submersibles at psubs.org>

 > To: Personal Submersibles General Discussion <personal_submersibles at psubs.org>

 > Sent: Tuesday, June 30, 2015 3:45 AM

 > Subject: Re: [PSUBS-MAILIST] Electrical Question

 >

 >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.. https://www.google.com/patents/US2450468

 >On 29 June 2015 at 16:00, Sean T. Stevenson via
 Personal_Submersibles <personal_submersibles at psubs.org>
 wrote:

 >

 >

 >

 >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

 >

 >

 >On June 29, 2015 8:52:58 AM MDT, James Frankland via
 Personal_Submersibles <personal_submersibles at psubs.org>
 wrote:

 >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.  

 >On 29 J! une 2015at 15:38, Sean T.
 Stevenson via Personal_Submersibles <personal_submersibles at psubs.org>
 wrote:

 >

 >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

 >

 >

 >On June 29, 2015 7:56:32 AM MDT, James Frankland via
 Personal_Submersibles <personal_submersibles at psubs.org>
 wrote:

 >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. RegardsJames

 >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 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

 >

 >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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