Jump to content
Strawberry Orange Banana Lime Leaf Slate Sky Blueberry Grape Watermelon Chocolate Marble
Strawberry Orange Banana Lime Leaf Slate Sky Blueberry Grape Watermelon Chocolate Marble
Sign in to follow this  
Craig Shelley

Electron Leak

Featured Posts

Yesterday, while tidying up some cabling, we discovered the current supply to the radio is higher than its current return.

Current supply: ~1A

Current return: ~400mA

 

The remainder has been traced to be going down the aerial cable screen to the hull, and then back to battery negative via the engine .

This may be unrelated, but when we recently serviced the prop shaft seal and discovered one of the brass components was bright green and pitted, indicating galvanic corrosion.

 

Currently, there is no bonding from battery negative bus bar to the hull. I'm going to install a bond wire with sufficient captivity to handle parasitic and fault currents. It makes a lot of sense to have a bond wire from the negative bus bar to the hull. There is another forum thread which discusses the need for a bond wire in detail.

 

The supply to the radio is shared for TV, and several cigarette lighter sockets. A percentage of the current from these devices is also returning via the radio.

 

Does anyone have any suggestions to prevent current escaping to the hull via the radio aerial?

Edited by Craig Shelley
Removed ref to current via prop seal

Share this post


Link to post
Share on other sites

Yes, cut the braid in the aerial lead and fit a capacitor across the cut. Several posts on the forum about this giving a suitable value.

 

As the engine and domestic negatives are normally/should be linked by heavy cable normal practice is to simply extend the negative engine block negative connection down to the steel engine bed and hence the hull. No need to run an extra bond cable from the 12V negative domestic bus bar.

  • Greenie 1

Share this post


Link to post
Share on other sites

Thanks for the quick reply.

Would this work for powered DAB aerial?

The aerial has two coax cables, and an amplifier power supply wire. The DAB coax  is terminated with SMA/SMB connectors.

Share this post


Link to post
Share on other sites
38 minutes ago, Craig Shelley said:

This may be unrelated, but when we recently serviced the prop shaft seal and discovered one of the brass components was bright green and pitted, indicating galvanic corrosion.

I really wouldn't have expected it to have caused pitting on brass components as Brass is WAY up the Galvanic scale, virtually everything else on the boat (Steel, stainless steel, etc) are bellow Brass on the galvanic chart.

I think you may have another problem.

 

 

 

 

Galvanic Order of Metals A - C 2.jpg

Share this post


Link to post
Share on other sites

If the aerial amplifier is being powered through one of the coax leads, then a capacitor across the shield will stop the amplifier working. Better to install a different aerial with no built in amplifier and isolate that with a cap, then have an amplifier between it and the radio. Is the aerial one designed for cars? If so, then they often use the metal roof of the car as part of the aerial design and rely on an electrical bond with the roof to work most effectively. If this is the case with yours, then to carry on using it and not get the stray return current through the hull, you'd have to have a separate metal plate, isolated from the hull on to which to mount the antenna.

 

Jen

Share this post


Link to post
Share on other sites

Yes a small value rf capacitor in series with the aerial cable(s) braid is the thing to do. Fitted after any amplifiers as Jen says. DAB is on a higher frequency than FM so an even smaller value capacitor will do. You just need a capacitor that has low impedance at 300MHz. Around 100pF - 1nF should do it.

 

Be careful with earth/0v straps. If you put a strap between battery negative and hull and if, like most people, you have an alternator and starter whose case is connected to 0v and an engine block that, by various means, is connected to hull (exhaust, control cables, fuel lines, prop shaft etc) then by adding another path between battery negative and hull, you provide a return current path between alternator and starter negatives, and battery negative, via the hull. Which is not what you want! Any 0v strap for the DC system is best taken from the engine to hull, for this reason. But obviously, since you already have a proven path between hull and battery negative (due to items mentioned above) there may not be much point in  adding another dedicated one.

Edited by nicknorman
  • Greenie 1

Share this post


Link to post
Share on other sites
1 hour ago, Craig Shelley said:

Thanks for the quick reply.

Would this work for powered DAB aerial?

The aerial has two coax cables, and an amplifier power supply wire. The DAB coax  is terminated with SMA/SMB connectors.

Some dab antennas don’t bond to earth (eg mag mt with plastic base) some amplified antennas don’t Earth through their mount, but have a separate earth wire which should not earth to the hull.

 

For a passive antenna, if you don’t fancy cutting the braid and soldering in a little rf capacitor then one of these might do the trick.  It uses f connectors so suitable for a tv antenna or dab with an adapter if required.

https://www.amazon.co.uk/Axing-TZU-10-01-eliminator-F-Connectors-0/dp/B002MQJQG4/ref=sr_1_4?keywords=Ground+loop+isolator+f&qid=1572774979&sr=8-4

 

  • Greenie 1

Share this post


Link to post
Share on other sites
1 hour ago, Alan de Enfield said:

I really wouldn't have expected it to have caused pitting on brass components as Brass is WAY up the Galvanic scale, virtually everything else on the boat (Steel, stainless steel, etc) are bellow Brass on the galvanic chart.

I think you may have another problem.

 

Sorry, there's an error in my original post.

The prop shaft actually has a flexible coupling, which is electrically isolating it. The prop shaft at the seal is at about +0.5V from the hull. I imagine this is due to the metal the prop is made from.

The lost return current can be measured at the engine -ve wire so it must therefore be returning via some other route on the engine e.g. exhaust.

Share this post


Link to post
Share on other sites

If anything happens to split the atom or a neutron of this wayward electron we could have a bomb to look forward to. I'm gonna start digging a nuclear shelter. :closedeyes:

Share this post


Link to post
Share on other sites
1 hour ago, nicknorman said:

Be careful with earth/0v straps. If you put a strap between battery negative and hull and if, like most people, you have an alternator and starter whose case is connected to 0v and an engine block that, by various means, is connected to hull (exhaust, control cables, fuel lines, prop shaft etc) then by adding another path between battery negative and hull, you provide a return current path between alternator and starter negatives, and battery negative, via the hull. Which is not what you want! Any 0v strap for the DC system is best taken from the engine to hull, for this reason. But obviously, since you already have a proven path between hull and battery negative (due to items mentioned above) there may not be much point in  adding another dedicated one.

Very good point. Engine to hull would certainly be a much better place for a bonding point.

Also often overlooked is fusing the negative charging wire to bow thruster batteries, in case of short from +ve to hull at the battery.

Share this post


Link to post
Share on other sites
4 hours ago, Craig Shelley said:

Also often overlooked is fusing the negative charging wire to bow thruster batteries, in case of short from +ve to hull at the battery.

How would that work then?  Short the +ve charging cable to the hull and the fuse in the +ve cable would blow. 

Share this post


Link to post
Share on other sites
Just now, WotEver said:

How would that work then?  Short the +ve charging cable to the hull and the fuse in the +ve cable would blow. 

The fuse in the +ve is 250A to the motor controller.

The fuses in the +ve and -ve charge cables are 50A.

The charge cables are 10mm2, and route through the cabin. The negative terminates at the busbar. The +ve goes via a charge relay contact to the starter alternator.

If "somehow" the +ve bow thruster battery terminal were to be shorted to the hull, the fault current would circulate via the hull and engine to the negative busbar, then back along the charging cable -ve to the bow thruster battery.

Without a fuse in the -ve there is a possibility that the -ve charge cable might burn up. At 10mm2 I'm doubtful it would go that far, but the cable does run through the cabin. There's no harm in fusing it close to the battery.

Share this post


Link to post
Share on other sites
1 hour ago, Craig Shelley said:

The fuse in the +ve is 250A to the motor controller.

The fuses in the +ve and -ve charge cables are 50A.

The charge cables are 10mm2, and route through the cabin. The negative terminates at the busbar. The +ve goes via a charge relay contact to the starter alternator.

If "somehow" the +ve bow thruster battery terminal were to be shorted to the hull, the fault current would circulate via the hull and engine to the negative busbar, then back along the charging cable -ve to the bow thruster battery.

Without a fuse in the -ve there is a possibility that the -ve charge cable might burn up. At 10mm2 I'm doubtful it would go that far, but the cable does run through the cabin. There's no harm in fusing it close to the battery.

Gotcha. I thought you were referring to the +ve charging cable shorting to the hull. :)

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.