[PSUBS-MAILIST] Earth Fault

[Marc de Piolenc via Personal_Submersibles]

Lots of carbon black in tire rubber.

Marc

On 7/27/2015 8:28 PM, via Personal_Submersibles wrote:
> James,
> I've found that some rubber compounds are conductive. For instance when
> I tried to electrically insulate with a piece of inner tube from a tyre,
> it still passed current.
> Jim
> In a message dated 7/27/2015 7:14:28 A.M. Central Daylight Time,
> personal_submersibles at psubs.org writes:
>
>     No.  Just sat on the trailer.
>
>     On 27 July 2015 at 11:58, Alan via Personal_Submersibles
>     <personal_submersibles at psubs.org
>     <mailto:personal_submersibles at psubs.org>> wrote:
>
>         James were they in the water?
>
>         Sent from my iPad
>
>          > On 27/07/2015, at 10:33 pm, James Frankland via
>         Personal_Submersibles <personal_submersibles at psubs.org
>         <mailto:personal_submersibles at psubs.org>> wrote:
>          >
>          > Hi All
>          >
>          > I have a very peculiar issue.  I have an earth fault on the boat.
>          >
>          > My test consists of holding one probe of the meter on the
>         positive battery terminal and the other to the hull somewhere.
>         I was showing a reading of 24v.  So obviously a negative
>         connection somewhere to the hull.
>          >
>          > I went around everything taking things off and have tracked
>         the fault down to the lights.
>          >
>          > The lights are the trustfire ones and the negative connection
>         is grounded to the chassis of the light.  However, when I fitted
>         the lights, I was aware of this and so insulated the mounting
>         bracket from the  light casing itself with a piece of rubber.
>         So theoretically, there is no physical connection from the case
>         to the hull.  Only the internal wire.
>          >
>          > Anyway, if I disconnect the lights and leave them dangling on
>         their wires, there is no earth fault.  The lights all work, and
>         the hull is clean of current.
>          >
>          > So the lights must be leaking back through the connection
>         somehow, but I cant see how.  The case is insulated from the
>         mounting bracket with rubber and the brackets are connected to
>         the fibreglass faring, so it shouldn't leak back?
>          >
>          > Anyway, ive fixed it by insulating the mounting bolts with
>         delrin washers, but I cant see how the earth could return
>         through a piece of rubber and then glass fibre.  A mystery
>         unless anyone can see something obvious?
>          >
>          > Regards
>          > James
>          >
>          >
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