Possible dock power leakage, or something even stranger.

[GILow]

Here’s one for the electrical enthusiasts.

My boat is, for practical purposes, completely grid-independent when it comes to power. All systems on board are either 12 volt and run directly from the battery bank, or, in a few minor cases, run from the battery-bank-powered 240 volt AC inverter which is only switched on when needed, otherwise it remains electrically isolated from the batteries through the use of one of those chunky plastic-key type rotary switches. Power to the batteries is supplied either from solar (95%) or wind (5%). (One of these days I’ll get around to tapping the new engine alternator for power, but I haven’t needed to yet.). I have no battery charger aside from a little 15 amp smart charger that sits in a locker for emergencies.

The boat is fibreglass and all skin fittings are unbonded. This has never caused a problem for the boat in 40 years, so I’m happy to leave things as they are. Zincs protect the drive shaft, propellor and rudder bits.

The 12 volt system is not bonded to the boat in any meaningful way, though the presence of things like the wind generator may allow some electrical pathway to, say the lifelines, standing rigging etc.

This is all background.

So, when I say completely grid-independent, I lied slightly. While I am refitting the boat, I have run a power cable into the boat to run the horribly power-hungry shop vac which I use to keep things moderately clean. (The vac draws 2400 watts, so I just don’t see any reason to put my batteries through that kind of punishment. )

The power cable is a good quality 15 amp lead with a 15 amp IP67 fitting at the dock end and a good clean single female 10 amp socket at the boat end. There is absolutely no connection to the boat from the dock power cable. It just arrives and sits on a bench on the main saloon to be used when needed.

Stay with me... I’m getting there...

So, yesterday I went to solder up some new cabin lighting, and in fit of laziness, I plugged the 240 volt soldering iron into the dock power lead. (Otherwise I would have had to climb off the settee, dig out the key for the inverter, power it up, run a lead from the inverter outlet to the soldering iron... you get the picture.). The iron is the earthed sort.

Well, as usual, I was working with the 12 volt lighting circuit switched on. Makes sense to me, easier to see if the soldered circuit is working, plus I had plenty of light to work by.

As I brought the iron to the power lead, I got a very big spark. The sort that makes a good loud “crack” sound and jumped a good 2 or 3 mm between the line and the iron. I thought “oh sh-t!” (but without the dash), “that’s gotta have fried something...”. But everything was ok. Solar regulators were still happy, stereo kept going... no smoke.

So, I thought, “what the heck” and kept working. And every time I brought the iron into contact with the circuit I got the same effect, on both the positive and negative leads. (Later I decided maybe that was a dumb idea, but nothing has died yet...)

So, any suggestions? Where on earth was that power coming from and going to? Some kind of capacitance build up from emf induction on the rigging? Leakage from other boats? Aliens?

It’s got me stumped.

Matt

[Fendell]

Note: Im not a sparkie, so just my intrigued thoughts..

So best guess is the vessel has a different potential to the lead coming in from the shore power box, and when you go to solder its bringing the vessel to the same potential as the marina earth? but it could be more serious and be earth leakage in a boat around you, or the marina.

Maybe stay out of the water until a sparkie has given their opinion.

[CharlieJ]

There is AC leakage between the resistive heating element in the tip and the tip. In other words, there is some amount of AC present on the tip. That is the easy part. If you were using the soldering iron on a boat without the degree of isolation you describe, then the AC on the tip would find a path to vessel ground via the B- of the component you were soldering.
The cause of the spark is some variation of this but I can't work out how to complete the circuit as you described your set up. That's all I have.

[Dsanduril]

You have an alternator, which presumably means you have an engine, which presumably means you have a start battery. And the negative of the start battery is connected to the engine block? And thence to "earth" by your prop shaft? (unless you have isolation deliberately installed this is pretty much the case).

If you had planned to charge with your alternator then the house bank and the engine electrics negative have to be connected. Are they? Or has this never been done? Because that then gives you a path to "earth" from the house bank.

Soldering tips are notorious for leakage because the insulation around the heating element breaks down. There's a whole market of solder tip leakage testers. And a whole part of the electronics industry centered around getting leakage voltage to less than 2mV (which is now considered too high).

So, if your house battery negative is connected to the engine start negative then pretty easy to see a path for your tip voltage to earth.

Bottom line, you probably need a new soldering iron - but it will also probably work fine for years, or until you shock yourself.

[CharlieJ]

@Dasanduril
I agree that he probably needs a new soldering iron.

@GILow
In my original post, I got it in my mind that you were on the hard. Since a re-read show that you are afloat, Dasandrul #4 is spot on on how the circuit is completed from the B- conductor to the engine block, through the propeller shaft, into the water and finally back to the ground of the transformer supplying your shore power pedestal.

[GILow]

Well, I admit, a new soldering iron is a cheap test and you guys have presented a convincing argument.

But, for the record, the engine is entirely separate from the house bank. Literally completely isolated.

But the house bank has any number of possible paths to earth, so I’ll go with the faulty soldering iron approach. And at the very least, I’ll end up with a spare iron if it turns out not to have been the problem.

[Wotname]

Crook soldering iron - use you multimeter to confirm!

[Wotname]

BTW, are the negatives of the house and starter banks completely isolated even at the engine?

[dfelsent]

From the OP. “ the iron is the earthed sort. “
The iron tip is likely tied to the ac plug earth.
So the dockside ground is not the same as the vessel ground.
The vessel ground is kinda sorta controlled by the local seawater potential, marina leakage and all. Maybe zero maybe not.
But what is clear to me is that either the dockside ground is not right or the water is hot. Or the iron is busted.
Possibly but unlikely all three.

Check the resistance between the iron tip and the earth prong. Should be zero or close.

If so, chase the marina issues.
If not replace the iron and try again.

Well worth chasing down.

[GILow]

Originally Posted by Wotname

BTW, are the negatives of the house and starter banks completely isolated even at the engine?

Absolutely.

[GILow]

Originally Posted by Wotname

Crook soldering iron - use you multimeter to confirm!

Alas, apparently not. Perfect resistance between both active/neutral and the earth pin. At least while the iron is cold. Just heating it up ow to see if that changes.

[GILow]

Originally Posted by GILow

Alas, apparently not. Perfect resistance between both active/neutral and the earth pin. At least while the iron is cold. Just heating it up ow to see if that changes.

Still 100% resistance with the iron hot.

[Wotname]

What is the PD between the tip and the job?

This is the metric you should be checking

[GILow]

Originally Posted by Wotname

What is the PD between the tip and the job?

This is the metric you should be checking

Well... that’s even weirder.

13.13 volts between the positive bus and the soldering iron tip. This while the battery bank was at 13.45. (Bus relatively positive)

But then, here’s the odd bit.. 0.305 volts between the negative bus and the tip at the same time. (Iron relatively positive)

Both cases were DC. No AC readable.

36 mA from positive bus to iron earth. 0.1 mA from iron earth to negative bus.

[Wotname]

Originally Posted by GILow

Well... that’s even weirder.

13.13 volts between the positive bus and the soldering iron tip. This while the battery bank was at 13.45. (Bus relatively positive)

But then, here’s the odd bit.. 0.305 volts between the negative bus and the tip at the same time. (Iron relatively positive)

Both cases were DC. No AC readable.

36 mA from positive bus to iron earth. 0.1 mA from iron earth to negative bus.

You are on the right track, keep hunting it down . Start disconnecting stuff; i.e. seperate and isolate with your best friend (voltmeter) in your other hand. Don't forget to use some load resistor when measuring these voltages, something like a 10K resistor or similar.

I am looking forward to hearing about how you resolve this!

Do remember that no matter how weird it seems, the electricity will always conform to Ohm's and Kirchhoff's Laws. When one is sure it doesn't, one is wrong and one has to keep digging deeper.