[PSUBS-MAILIST] SubConn BH4F and IL4M Connectors

Fri Apr 12 18:14:46 EDT 2019

It isn't strictly necessary to fuse both terminals under ordinary conditions.  Doing so just extends the fault tolerance of the electrical system.  A single fuse or breaker on either leg at the battery will protect against a DC high current short provided the fuse is in series somewhere in that short circuit.  In a grounded electrical system, such as most automotive systems which have the battery negative terminal connected directly to the vehicle chassis, you actually can't fuse the negative side because if that negative fuse blows before the positive side fuse, then you still have a return current path through installed equipment such as radios etc., and that smaller wiring could start a fire if subjected to short circuit current.  In the floating system (marine systems), the second fuse is permissible.

Fusing both legs provides protection for shorts which might occur at the terminals themselves, or at the bus bars or main feeders which exist between the battery bank terminals and the circuit breaker / fuse box.  If your battery shifts, or your sustain a seawater leak, for example, which produces a fault right at the positive terminal in off-board battery pods, the negative side fuse would protect the vessel in the event of a second fault.  The only condition which is not protected by this configuration is a fault directly between terminals or bus bars / mains between the battery and the protection devices.  If you are using circuit breakers instead of fuses, you want a breaker which interrupts both leads in response to a failure of either one.

A non-grounded, or floating, electrical system is inherently single-fault tolerant, because any short between one battery terminal (positive or negative) and your hull will not cause current flow.  You actually need two simultaneous failures (or a bridge between potentials) for a high current fault to occur, which provides for the ability to continuously monitor continuity between each potential and the hull, and throw an alarm to indicate the presence of such a single fault.  The fuses or circuit breakers provide protection for the double-fault, and if you have a triple fault, you're going to have a bad day.

Sean

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Friday, April 12, 2019 2:35 PM, Rick Patton via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

> Sean,
> I have been very careful not to use the hull as a ground for obvious reasons but was just wondering if there was any drawbacks to having the breakers for the battery's attached to the lugs as soon as they inter the hull? I would prefure them to be inside because they would be easier to replace or reset plus I have a lot more room for them.
> Why do both pos and neg need to be protected with fuses?
> Rick
>
> On Fri, Apr 12, 2019 at 10:05 AM Sean T. Stevenson via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>
>> Remember, on boats, and in submersibles, you want to have an ungrounded electrical system (no continuity between battery negative and hull / equipment chassis protective earth), and you will want main fuses / breakers on both the positive and the negative leads coming from the battery. Ideally, you want these protective devices as close to the battery bank(s) as possible, but accessable so that you can replace fuses or reset the breakers if you happened to cause the short or it is otherwise repairable before going live again.
>>
>> Sean
>>
>> -------- Original Message --------
>> On Apr 12, 2019, 13:50, Rick Patton via Personal_Submersibles < personal_submersibles at psubs.org> wrote:
>>
>>> Was wondering if it is better to have the main fuse/circuit breaker inside the battery pod or just inside the hull or does it make any difference. I really lack in the electrical side of things.
>>> Rick
>>>
>>> On Wed, Apr 10, 2019 at 9:34 AM Jon Wallace via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>>>
>>>> I called MacCartney and talked to the manager there who happens to be the wife of the previous manager that set us up with the discount.  She said a competitor went out of business which has created a huge draw on their manufacturing capacity.  She assured me that our small orders are not preempted by orders for 20,000 units.  She did say most items are on a 12-16 week back order.
>>>>
>>>> I will offer the following if anyone thinks it might help for the future...perhaps we could take some of our member dues money and use it to purchase some units that we keep locally to avoid this in the future.  Only problem is, I really do not want to be responsible for shipping.  Someone we know and trust would have to volunteer to hold the stock and ship items when our members order them.
>>>>
>>>> Otherwise, we will just have to wait patiently.
>>>>
>>>> Jon
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>>>
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